SCOPE 21 - The Major Biogeochemical Cycles and Their Interactions, Chapter01, Interactions of Biogeochemical Cycles

An overview of contributions and discussions at the SCOPE
Workshop on the Interaction of Biogeochemical Cycles,
Örsundsbro, Sweden, 2530 May 1981.

1.1 GLOBAL BIOGEOCHEMICAL CYCLES AND MAN

The elements of carbon, nitrogen, phosphorus and sulphur, and of course hydrogen and oxygen, are needed for the formation of the fundamental molecules on which life depends. Their availability, circulation and interaction in nature has therefore been decisive for the development of life on earth and also for the maintenance of the present global ecosystem. On the other hand, the development of living organisms on earth and their ability to utilize their immediate environment has modified the primordial distributions and circulation of these fundamental elements. The hypothesis has even been advanced (Lovelock, 1979) that the biosphere has developed in such a manner that close to optimum conditions for life on earth prevail and are being maintained by the life processes themselves. Regardless of the extent to which this may be the case, the biogeochemical cycles are crucial for maintenance of life on earth in its present form and biological processes largely determine the main features of these cycles.

Man today influences the major biogeochemical cycles on earth significantly. The annual release of carbon dioxide to the atmosphere by burning fossil fuels is about ten percent of the amount being used in primary production of organic matter by plants (Bolin et al., 1979). During the last few hundred years about ten percent of the land surface, primarily forests and grasslands, has been transformed into agricultural land. The formation of fixed nitrogen by combustion and fertilizer manufacturing is currently about half of what is produced naturally (Söderlund and Svensson, 1976; Rosswall, Chapter 2, this volume). The land transformations that man has initiated also have resulted in a major translocation of nutrients, particularly nitrogen compounds, from the soils to the rivers and lakes and ultimately to the sea. Emissions of sulphur dioxide to the atmosphere, primarily by fossil fuel combustion, probably exceed the natural emissions of gaseous sulphur from land and oceans; in industrial regions S emissions by far exceed those due to natural processes (Cullis and Hirschler, 1980; Ivanov, 1981; Freney et al., Chapter 2, this volume; Crutzen, Chapter 3, this volume). A significant eutrophication of lakes and possibly coastal waters has been caused by the increased release of phosphorus in waste material into rivers and estuaries; erosion of agricultural lands adds to the problem (Richey, Chapter 13, this volume; Wollast, Chapter 14, this volume).

The main features of the natural cycles of carbon, nitrogen, phosphorus and sulphur (C, N, P, S) have been described during the last decade but many aspects of their dynamics are not well known. It is therefore difficult to determine what the effects of man's intervention may have been and may become in the future. For example, it is easy to predict that the amount of carbon dioxide in the atmosphere will increase if man continues to emit this gas in significant amounts by fossil fuel burning. It is more difficult to foresee how the overall carbon cycle will change and how the natural quasi-equilibrium of the global ecosystem will be disturbed. Additionally, cycles of the basic elements interact in the biological processes and a disturbance of one of them will, in all likelihood, affect the others (Melillo and Gosz, Chapter 6, this volume; Cook, Chapter 12, this volume).

We have a limited knowledge of the character, magnitude and possible on-going changes of the global cycles of C, N, P, and S. World wide measurements with good accuracy are needed. A further difficulty is the mosaic structure of the biosphere, particularly on land. The local variations usually obscure those on larger scales, which vary only slowly with time. Also the state of the global ecosystem may be changing due to natural causes. Thus climate variations on time scales greater than a decade probably affect the biogeochemical cycles and ecosystems. It is not enough to establish that possibly large-scale and long term changes are taking place. We must also try to understand whether they are due to natural processes or brought about by man.

Another complication is our lack of familiarity with the scale of problems confronting us. We can design extensive field experiments that help us in understanding the short term dynamics of a limited, homogeneous community. It is more difficult to grasp the cumulative changes that occur from one year or one decade to another. On the other extreme we can measure global-scale pool sizes of CO₂ in the relatively well-mixed troposphere, but we find it more difficult to deal with heterogeneities in the distribution of more rapidly turning over chemical species in the atmosphere, soil, or oceans. The kind of information required to build from local studies to a regional-scale understanding of element interactions is largely lacking, and it is only this information that will allow us to develop more realistic global models. The importance of such regional-scale studies is well illustrated by the gradual anthropogenic acidification of precipitation in industrial regions (Odén, 1968; Cook, Chapter 12, this volume).

We are steadily increasing the use of the natural resources of the world. It is obvious that this increase must not go beyond the level at which the global ecosystem will be seriously damaged and a decrease in the renewable resources of the earth will occur. It is disturbing that we do not yet adequately understand the global ecosystem and its dynamics to assess what this statement implies. How wide-spread will the impact of acid deposition be if the use of fossil fuels expands as energy-use scenarios predict and a corresponding increase of sulphur emissions occurs? How will the quality of soils used extensively in agriculture change? Will there be a significant change of the ozone layer due to increasing emissions of nitrogen oxides and chlorine containing gases?

It is likely that the ultimate impact of man's interventions will not be adequately deduced until the global processes as manifested in the steady flow of C, N, P, and S in the air, the waters, and the soil are well understood. In such an effort to determine the main pathways and rates of exchange for these major elements we will also learn about the flows of the minor constituents in nature that are also essential for the life processes. Further, the fate of some toxic substances emitted by man that find their way into the environment and are globally dispersed (e.g. PCB), can be studied with similar methods. A readiness for action can thereby be created that will be of use if and when new threats to the environment are discovered in the future.

With the aim of furthering our basic knowledge of the global ecosystems the SCOPE Workshop on Interactions of Biogeochemical Cycles was arranged. On the following pages of this first chapter we shall give an overview of the papers that were presented to the workshop and that will be found in the remainder of this report. We shall also summarize the discussions held during the workshop. The most important conclusions will be given at the end of this chapter. We hope that in this way a more comprehensive account of the full workshop will be available.

1.2 BIOLOGICAL PROCESSES AND INTERACTIONS OF BIOGEOCHEMICAL CYCLES

As emphasized in the introduction, the formation of the organic compounds in life processes requires the availability of the elements C, N, P, and S and a number of trace elements in distinct proportions. These biochemical processes occur on a spatial scale from the size of the simple organic molecules to that of the individual cell, but they have far-reaching consequences at all ecosystem levels both in marine and terrestrial systems.

Redfield (1958) synthesized a number of ideas regarding the chemistry of planktonic organisms and the chemistry of the marine environment into a scheme that suggested extensive control by the biota over the chemistry of some elements in sea-water and oxygen in the atmosphere. These general relationships form a basic stoichiometric model for the control of C, N, O, and P chemistry and a major part of the chemical control of Si (Broecker,1974) and Al (Mackenzie et al., 1978; Stoffyn, 1979) in sea-water. The biologically mediated stoichiometry of C, N, and P in organisms and sea-water is often referred to as the 'Redfield Ratio'. Schindler (1977) extended the Redfield model to fresh-water systems, thus enlarging the model's generality to a broader range of systems. The use of this approach is fundamental to the modelling of the circulation of these elements in the rivers, estuaries and the open ocean as described by Richey (Chapter 13, this volume), Wollast (Chapter 14, this volume) and Fiadeiro (Chapter 17, this volume).

Because the cycles of C, N, O, P, and to a large extent S, are so strongly mediated and driven by the biota, there has been a tendency for those studying biogeochemical cycles to adopt the Redfield scheme too indiscriminately. There are deviations from this scheme in other systems. As our knowledge increases, we will be able to consider the specific biochemical processes in more detail and more complex models will evolve. Still the principle problem of how biochemistry and physical transfer interact is well illustrated by the following simplified overview. Table 1.1 shows typical C, N, S, and P ratios in emissions from fossil fuel combustion, the burning of plant material and in various natural reservoirs of organic matter.

Table 1.1 Carbon, nitrogen, phosphorus and sulphur in several fluxes and natural reservoirs


	C	N		S		P

Fossil Fuel Emissions¹	9300	36		130		1
Combustion of Biomass¹	920	20		1		1
Land Plants¹	790	7	.6	3	.1	1
Land Animals¹	78	11		1	.1	1
Terrestrial Bacteria²	43	4	.3	0	.2	1
Terrestrial Fungi²	188	11	.7	0	.8	1
Soil Organic Matter¹	54	3		1	.2	1
Ocean Plants¹	129	12		2	.9	1
Ocean Animals¹	93	12		0	.7	1
Ocean Sediments (surface)³	212	16				1

¹From Delwiche and Likens (1977) and Likens et al. (1981).
²From Ausmus et al. (1976) and assuming a P:S ratio of 5.7 for bacteria and 1.25 for fungi (Bowen, 1979).
³From Jørgensen (Chapter 18, this volume).

The oceanic system has a discrete water-air interface, a very deep water column and a relatively thin sediment layer. The major producers of organic material are single-celled algae, especially diatoms and dinoflagellates. The biomass of these organisms is largely decomposed in the water column, but some variable fraction sinks to the sediments, which may or may not be aerobic depending on the flux of organic matter and the circulation of water which determines oxygen availability (Jørgensen Chapter 18, this volume). Where sediments are anoxic, nutrient elements such as nitrogen and phosphorus are preferentially liberated to return to the water column, and denitrification and sulfate reduction are usually important processes. Also, in some marginal waters (eg. Baltic Sea, Black Sea and various fjords and estuaries), anoxia may extend into the water column overlying anoxic sediments.

Soils in themselves do not have a carbon source so the equivalent terrestrial system must be expanded to include plants and the atmosphere. To some extent plant biomass is consumed and elements regenerated above ground, an equivalent to decomposition in the surface waters of the ocean. Most plant products fall to the ground (shoots) or upon death are mixed in the soil (roots). In most terrestrial systems the major part of decomposition occurs on and in the soil, the equivalent to decomposition in marine sediments. As in oceans, the degradation of organic material in soils is mainly accomplished by micro-organisms. In this sense there is a direct analogy between the two systems. In soils and in the ocean the micro-organisms generally have the same biochemical composition and thus the same resource needs as far as synthesis of microbial biomass is concerned, although the predominance of fungi in soils shift this stoichiometry somewhat (Table 1.1). The final decomposition products are, however, basically identical. In addition, microbes are responsible for N-fixation, whether in root nodules or free-living, and N-fixing organisms, like all other microbes, have a very definite need for phosphorus and sulphur. So there is a tie between phosphorus availability and accumulated nitrogen in soils as well as ocean water.

Three major chemical differences lead to pronounced contrasts between soil systems and the marine systems. The first is that terrestrial plants are markedly enriched in carbon relative to other elements (Table 1.1) by the extensive production of structural compounds of carbon (cellulose, lignin, waxes, etc.). These structural components are relatively resistant to degradation by micro-organisms. Thus the organic material entering a soil system is much different from sedimenting marine material in the type of compounds present and the relative proportion of carbon to other elements.

The second major difference is the strong role of the inorganic soil matrix and of inorganic phosphorus chemistry in aerobic soils. The matrix provides some protection against the degradation of organic matter. Most phosphorus in soils is present in a form that is unavailable to plants or microbes at any given time. As a result, the ratio of total phosphorus to C and especially N in soils is quite different from the ratios of these elements in marine plankton and microbial biomass (Table 1.1).

The third difference between the terrestrial and marine (or aquatic) systems arises from the first two. Due to the abundant flux of organic carbon to soils, C cycles more rapidly than N and P and the soil retains N and P relative to C. Also, due to the importance of non-available pools of phosphorus in soil, a large part of the P cycle within the soil is regulated by non-biological processes. To a large extent its cycle is uncoupled from those of C and N.

Redfield did not incorporate S in his total scheme in terms of fixed ratios because sulphate is so enormously abundant in sea-water. It is, however, related to the C, N, P, and O cycles in at least two ways. First, S is an essential element for all life. Second, sulphate is the major oxidizing agent for organic matter in anaerobic marine sediments, thereby regenerating oxygen in the form of carbon dioxide (Jørgensen, Chapter 18, this volume). On geological time scales reduced S is an oxygen sink that helps set oxygen levels in the water and atmosphere (Holland, 1978). The semi-independent cycle of S in the sea is in some ways a counterpart of the semi-independent cycle of P on land.

Even though on first consideration it may seem that soils and sea sediments are quite different in terms of their redox potential, the sulphur cycle in the soil compares with that in the sea in that all the above mentioned processes are possible. The contrast lies in a relative scarcity of S on land (except for reduced sulphur in the form of pyrite) because sulphate is highly soluble. S is largely stored in organic form in soils so that in fact there may be a more direct relationship between S and C and N on land than in the sea. This coupling is manifested in those cases in which S is a limiting nutrient, and additions of S via fertilizer, dry or wet deposition produce a positive growth response.

The biological aspects of the ocean-based Redfield model are definitely central to cycles of these same elements on land. All four cycles intersect where biological processes occur; each element can limit the cycling of another. Comparison, however, cannot be made at the level of stoichiometric equivalency. Nevertheless, from agricultural management studies we will gain a better understanding of the types of relationships that can occur between C, N, S, and P. So far we have merely begun to understand these key processes quantitatively and to include them into regional or global models of the biosphere. It will be difficult to unravel the conditions that prevailed before the industrial revolution. It is even more difficult to deal with the evolution of the present biosphere and how man may modify it in the future.

We will be following the fates and interactions of the key compounds from the atmosphere, where changes usually are first detectable, to the sea, where in most cases the regulation of man's activities ultimately will occur on time scales of centuries to millenia or even longer. The main features of the C, N, P, and S cycles are presented separately in Chapter 2, which will serve as a reference.

1.3 THE ATMOSPHERE

The atmosphere plays a decisive role in regulating the temperatures at the earth's surface. Without the presence of the atmosphere, average surface temperatures would be about 18°C, given an albedo of the earth-atmosphere system of 30%. In reality, the mean surface temperature is about 35°C higher due to the trapping of a large fraction of the out-going infrared surface radiation in the atmosphere. Clouds and the radiatively active gases H₂O, CO₂ and O₃ are of special importance in this respect. A doubling in the atmospheric carbon dioxide content according to the best present knowledge is expected to result in a mean surface temperature increase of 1.54°C (Manabe and Stouffer, 1980; Kellogg and Schware, 1981; Thompson and Schneider, 1981). Recently, attention has also been drawn to the climatic importance of the minor constituents N₂O, CH₄ and the industrial gas CF₂Cl₂ at volume mixing ratios as low as a few ppbv (Ramanathan, 1975; Wang et al., 1976).

The atmosphere is also the transport medium for many biologically or photochemically active gases such as O₂, N₂, H₂O, CO₂, CO, CH₄, O₃, NO, NO₂, HNO₃, NH₃, N₂O, SO₂, H₂S, CH₃SCH₃, and COS as well as airborne particulate matter containing C, N, S, P and other nutrient elements. Several of the listed compounds are emitted or taken up by the biosphere and the reduced compounds are oxidized in the atmosphere by photochemical reactions. A key role in their oxidation is played by the OH radical, which is formed when ozone is photolysed by ultraviolet solar radiation in the presence of water vapour (see Crutzen, Chapter 3, this volume). Several compounds interact in the atmosphere to form particulate matter. The nutrient elements C, N, and S are returned to terrestrial ecosystems and the oceans by precipitation scavenging and by direct gas phase and particulate matter transfer at the earth's surface (Reiners, Chapter 5, this volume; Liss, Chapter 15, this volume; Duce, Chapter 16, this volume). The global ecosystem must have developed over geological time scales in response to climate and to the supply of nutrient elements from the atmosphere and the lithosphere. Agricultural and industrial activities have changed the balances and quantities of inputs and outputs of nutrient elements in many ecosystems, which cause the ecosystems to change according to their degree of resilience (see further Reiners, Chapter 5, this volume; Houghton and Woodwell, Chapter 11, this volume; McGill and Christie, Chapter 9, this volume).

During the industrial era the atmospheric transfer rates of many C, N and S containing gases have changed greatly. Consequently, the atmospheric composition also has changed. What was the atmospheric composition prior to man's impact on it? There has been remarkably little thought given to this question.

By direct measurements we know that the amount of CO₂ in the atmosphere has increased by 10% during the last 25 years. The abundances of other important trace gases also have been modified both in response to climate changes and owing to man's emissions. No data on their historical abundances are, however, available. Much of what will be given in the following is therefore derived from extrapolations backwards using our limited knowledge of the processes that govern the composition of the present atmosphere. In the future, analysis of air bubbles in polar ice may provide important information on the history of atmospheric composition. Certainly, changes have occurred during the past century in the atmospheric concentration of the oxides of nitrogen (NO, NO₂, N₂O, HNO₃), carbon monoxide, sulphur dioxide, methane and other hydrocarbons. Also the organic chlorine content of the atmosphere has changed dramatically during the past decades.

Extrapolating backwards from the known trends in the atmospheric CO₂ concentrations during the past decades and accepting the analysis that the terrestrial ecosystems have contributed 125 ± 50 x 10¹⁵g C during the past 150 years, a pre-industrial CO₂ volume mixing ratio of 265 ± 30 ppmv (parts per million by volume) is derived, compared to 338 ppmv in 1981 (Brewer, 1978; Bolin, 1983). This lower value may roughly have represented a natural, atmospheric equilibrium value maintained by the more pristine lands and oceans before the agricultural and industrial eras. However, even this value of the atmospheric CO₂ concentrations has probably not been constant. Analyses of the air trapped in Antarctic and Greenland glaciers show that the atmospheric CO₂ concentration seems to have varied considerably in association with climatic conditions. For instance, during the last ice age, between 15,000 and 30,000 years ago, atmospheric CO₂ concentration seems to have been as low as 200 ppmv (Berner et al., 1980; Delmas et al., 1980). Changes in atmospheric CO₂ have possibly contributed to the climatic changes that have occurred during the last thirty millenia. From the available data, however, it is more likely that atmospheric CO₂ responded to climate. The observed correlation between CO₂ and mean surface temperatures cannot be used to derive the likely climatic response to the on-going increase in the atmospheric CO₂ concentrations (Thompson and Schneider, 1981).

The composition of the atmosphere is strongly influenced by the photochemistry of ozone. Its photolysis by ultraviolet solar radiation leads to the production of the hydroxyl (OH) radical, which reacts with many atmospheric compounds that would otherwise be much more inert and abundant. Tropospheric reactions dominate the budgets of most gases. Tropospheric ozone therefore influences the concentrations of many atmospheric trace compounds. The oxides of nitrogen (NO and NO₂) play an important role in these processes. Above a critical NO concentration (>0.01 ppbv), ozone is created in the troposphere by the photochemical oxidation of hydrocarbons to carbon monoxide and further to carbon dioxide. Since the anthropogenic sources of oxides of nitrogen are estimated to be larger than the natural ones, it may be inferred that the abundance of tropospheric ozone might have increased during the past few centuries. Unfortunately, it is not possible to directly prove or disprove this supposition due to the lack of sufficient ozone records. Despite larger surface destruction rates in the northern hemisphere, the higher prevailing concentration of ozone there, compared to that of the southern hemisphere could be explained by higher anthropogenic inputs of NO in the northern hemisphere (Fishman et al., 1979). However, a larger downward transfer of ozone from the stratosphere to the troposphere in the northern hemisphere may be a contributing factor (Viezee and Singh, 1980; Husain et al., 1981). We note further that the influence of NO on the ozone distribution leads to ozone depletion in most regions of the stratosphere, but to an ozone increase in the troposphere (see Crutzen, Chapter 3, this volume).

There is some observational evidence that the atmospheric loading of N₂O has increased by about 0.2% annually during the past 15 years (Weiss, 1981). This increase may be due to a combination of the combustion of oil, coal and biomass and man's impact on the nitrogen cycle, even though the role of artificial fertilizers seems less than originally estimated. Assuming that the trend is mostly due to the production of N₂O in fossil fuel combustion, Weiss (1981) derived a pre-industrial atmospheric N₂O volume mixing ratio of 270 ppbv, which is 10% less than present levels. As the oxidation of N₂O is the main source of NO_x (NO + NO₂) in the stratosphere and as both NO and NO₂ act as catalysts in the destruction of stratospheric ozone, a 10% increase of N₂O during the past century would have led to a 1.5% reduction in total ozone, according to our present understanding of the photochemistry of the atmosphere. No observational records are available to confirm this hypothetical depletion, which furthermore may have been masked by the increase in tropospheric ozone due to the industrial NO input described above.

Atmospheric methane and carbon monoxide play important roles in the photochemistry of the atmosphere due to their influence in establishing ozone and hydroxyl concentrations (see Crutzen, Chapter 3, this volume). The photochemical oxidation of methane also leads to the formation of carbon monoxide. Measurements during the past years show an annual increase in atmospheric concentrations of methane of about 2% (Rasmussen and Khalil, 1981; W. Seiler, personal communication). There are many atmospheric sources in the environment, such as release from wetlands, rice paddy fields, ruminants, termites, biomass burning, and natural gas leaks. Several of these have been increasing with time due to human activities. Their relative contributions to the global methane cycle are, however, rather uncertain. Nevertheless, it is likely that pre-industrial methane concentrations were appreciably lower than present ones.

The past temporal changes of carbon monoxide are more difficult to estimate. Its increased atmospheric formation from methane oxidation and from growing industrial and biomass burning activities has probably been countered by reduced atmospheric formation from photochemical oxidation of hydrocarbon gases that are emitted by forest tree foliage. Expect for the industrial carbon monoxide sources, the magnitudes of other sources of CO are not well enough known to allow even a guess about whether CO has increased or decreased with time.

The atmospheric input of gaseous SO₂ has increased much over the past centuries, especially close to the main industrial centres of the world. According to present estimates (Granat et al., 1976; Ivanov 1981; Freney and Rodhe, Chapter 2, this volume; Crutzen, Chapter 3, this volume) the input of industrially produced SO₂ to the atmosphere may well exceed the natural formation rates of SO₂ from volcanoes and from the oxidation of biogenic reduced sulphur species, such as H₂S, CH₃SCH₃, CH₃SH, CH₃SSCH and COS. SO₂ is not produced directly by the biosphere. For the earth as a whole the atmospheric circulation of sulphur oxides and sulphuric acid has at least doubled, while in the industrialized parts of the world the emission of SO₂ and the deposition of SO₂ and sulphuric acid has increased manyfold. This has caused disturbances in the quantities and balance of nutrient supplies from the atmosphere and the acidity of soils and lakes (Cook, Chapter 12, this volume). Detrimental effects have occurred in the lakes of Scandinavia, Canada and the north-east of the U.S.A., where acidification of the waters has increased to levels that threaten lake life. These lakes are examples of ecosystems with a low resistance against environmental changes.

Besides sulphuric acid, nitric acid has also contributed to the increased acidification of precipitation. The increased input of ammonia, which has taken place simultaneously as a result of growth in agriculture and increased use of nitrogen fertilizer, has only partially balanced the increased acidification of clouds and rain resulting from sulphuric and nitric acid. According to FAO statistics (FAO, 1979) present global use of nitrogen fertilizers amounts to 5060 Tg N/year, of which possibly 10% is volatilized as NH₃, while the industrial production rates of SO₂ and NO_x amount to about 100 Tg S (Granat et al., 1976; Cullis and Hirschler, 1980; Ivanov 1981), and 20 Tg N per year (Söderlund and Svensson, 1976). Due to the growing acidification of aerosols and precipitation by HNO₃ and H₂SO₄ that has occurred during the industrial era, it is likely that the solubility of NH₃ in water phases has increased (Taylor et al., Chapter 4, this volume) and, therefore, the residence time decreased.

An important consequence of the increased atmospheric emissions of especially SO₂ and the resultant increase in the concentration of sulphate, its oxidation product, has been the observed change in visibility in the industrial parts of the world. During stable meteorological conditions in these regions the visual range is frequently reduced to less than 5 km for hundreds of kilometers downwind of pollution centers, covering areas of several hundred thousand square kilometers (Husar and Patterson, 1980). Together with the photochemical smog produced from urban automotive emissions they constitute a drastic deterioration in man's ability to enjoy his natural environment. In addition, the climatic implications of the increased loading of the atmosphere by sulphate particles may be of significance (Bolin and Charlson, 1976).

Carbonyl sulphide (COS), with a concentration of about 0.6 ppbv (Torres et al., 1980) and a relatively long residence time, is the most abundant sulphur containing gas in the atmosphere. COS is important because during periods with little volcanic activity its photo-oxidation in the stratosphere is the main source of the sulphate that is contained in the stratospheric sulphate aerosol layer at about 20 km altitude (Crutzen, 1976; Whitten et al., 1980; Meixner, 1981). In this way, the gas indirectly plays a role in the earth's radiation budget (Turco et al., 1980). The sources and sinks of COS have not been established quantitatively, but as this gas is also produced in the burning of coal and biomass, its atmospheric concentrations may be increasing although much more observational work is needed to verify such a trend (Hoffmann and Rosen, 1981). In contrast to the climatic effects of most gaseous emissions (such as CO₂, N₂O, CH₄ and CF₂Cl₂) to the atmosphere, increased atmospheric emissions of COS tend to cool the earth's surface, because the enhanced sulphate aerosol layer in the lower stratosphere increases scattering of incoming solar radiation back to space.

From available information (Singh et al., 1979; Berg et al., 1980) we conclude that until a few decades ago methyl chloride (CH₃Cl) was the only significant chlorine containing organic compound in the air, with a mixing ratio of about 0.6 ppbv (Lovelock, 1974). At present the organic chlorine content of the atmosphere is almost five times higher due to the presence of many additional industrial organic chlorine gases, especially carbon tetrachloride (CCl₄), the fluorocarbons F-11 (CFCl₃) and F-12 (CF₂Cl₂) and methyl chloroform (CH₃CCl₃). All these compounds are important because they are the only significant carriers of chlorine into the stratosphere (Berg et al., 1980). Their photochemical breakdown in the stratosphere leads to the production of photochemically active Cl and ClO radicals that catalytically remove some ozone in the stratosphere (Molina and Rowland, 1974). While the chlorine species did not play a dominant role in the ozone balance of the natural atmosphere (NO and NO₂ are much more important), their influence is now becoming increasingly significant, affecting in particular the ozone concentrations between 30 and 45 km (Crutzen, Chapter 3, this volume). Some reduction in the ozone concentrations at these levels has probably already occurred. Significantly larger changes may be expected in the next half-century if the production of industrial, organic chlorine compounds continues at about the present rates. The total ozone content of the atmosphere may then decline by 310%, allowing more ultraviolet radiation to affect the biosphere (WMO-NASA, 1982).

Many of the changes that occurred in the environment over the past 150 years will continue, and additional ecosystems will be affected in the future, especially in the tropics. There is little doubt that the concentrations of important constituents in the atmosphere will change significantly during the next hundred years. The following summary emphasizes problem areas that should be given particular attention in this regard. Man is expanding his activities on earth in many ways and it is becoming increasingly clear that we must consider these simultaneously to foresee the likely future changes in atmospheric composition. In particular the expansions of agriculture and forestry will have to be considered carefully (cf. section 1.4.3). The need for a proper understanding of the interactions of biogeochemical cycles is obvious from the preceding presentation.

The total area covered by tropical rain forest and seasonal forest is at present about 11 x 10¹² and 5 x 10¹² m² respectively (Bolin et al., 1979). A significant fraction of these remaining tropical forests may well disappear during the next 50 years (section 1.4.3). Table 1.2 shows the present rate of deforestation in tropical America, Africa and Asia and the annual emissions of carbon, nitrogen, sulphur and phosphorus (based on Lanly and Clement, 1979). Burning of biomass in the process of deforestation and also in shifting cultivation and dry grass fires in the savannas produces many important trace gases such as CO, CH₄, N₂O and NO_x (Crutzen et al., 1979). Emissions of particulate matter cause a large aerosol loading in the atmospheric boundary layer above the tropical savanna. A substantial fraction consists of organic matter and elemental carbon (soot) (Seiler and Crutzen, 1980) containing an important submicron component. The extent to which it may influence the heat balance of the atmospheric boundary layer is not well-known (Watson, 1979).

Deforestation in the tropics leads to smaller emissions of hydrocarbons to the atmosphere, especially isoprene and terpenes. The natural production of carbon monoxide is also reduced. On the other hand, the increased CO₂ concentrations in the atmosphere might enhance the productivity of the remaining forests (see section 1.4.4.A) and, furthermore, fast-growing trees, such as eucalyptus or casuarina, which are prodigious terpene and isoprene producers, will be planted for commercial use. Industrial activities emitting nitric oxide may have a detrimental impact on local and regional air quality in the tropics in that photochemical smog may be formed during stable meteorological conditions.

When more biomas is cleared in the tropics, more material will become available for decomposition and for entrainment in run-off. This will probably result in an increase of organic carbon in aquatic systems (see section 1.5.3). As eutrophication is increased in these systems anaerobic conditions will become more prevalent and more methane will be produced. The release of methane to the atmosphere will probably further increase due to expansions in the sources of methane, such as livestock and rice paddy fields. The use of nitrogen fertilizers has been shown to lead to enhanced CH₄ production in rice paddy fields (Cicerone and Shetter, 1981).

There is growing observational evidence that industrial pollution is affecting the composition of the air in the Arctic. During winter and spring, pollutants may travel thousands of kilometres from the industrial areas of the Northern Hemisphere to form haze containing sulphate, trace metals and soot of which about 80% is of anthropogenic origin (Rahn and Heidam, 1981; Rahn 1981; Rosen et al., 1981). The build-up can be explained by the inefficient scavenging of the pollutants due to infrequent precipitation in winter. In contrast the Arctic is much less polluted during the summer. Black soot in the aerosol is a most efficient absorber of solar radiation. A significant heating of the lower Arctic atmosphere in the spring may occur (Rosen and Novakov, 1980).

Table 1.2 Present annual rate of deforestation in the tropics and the annual release of C, N, S, P from forest clearing based on fixed proportionalities between these elements in vegetation and soils (Lanly and Clement, 1979)

The demand for more food in the world will lead to a further increase in the demands on land used for agriculture and the use of more fertilizers. According to some recent projections, the emphasis will be on intensifying land use, but new land will also be brought under cultivation (FAO, 1919). To bring out possible future implications of expanding and intensified agriculture, let us consider the use of 25 x 10¹² m², which is close to the maximum possible, rather than 15 x 10¹² used today and application of nitrogen fertilizer at an annual rate of 10 g N/m² (100 kg N/ha) to such an increased area. The global use of nitrogen fertilizer would reach 250 Tg N per year, which is about a five-fold increase. With the trend towards the use of urea as the main form of fertilizer, the volatilization of ammonia to the atmosphere may be increased by 25 Tg N annually, if typically 10% of the applied nitrogen is lost in this way. The release of ammonia to the atmosphere may be further increased by 40 Tg N per year, if the cattle population continues to grow by about 2% annually during the next three to four decades, and by still by another 25 Tg N per year if the use of coal takes place according to the projections of Rotty and Marland (1981) during this time. The total release of ammonia from the land to the atmosphere may, under these conditions, increase by almost 100 Tg N.


Region	CO₂-C Released		SO₂-S Released
	1975¹	2025	1975¹	2025

North America	1500	2300	30	45
Western Europe	1000	1100	21	22
U.S.S.R./Eastern Europe	1100	3400	22	68
Other Developed Countries	400	1000	8	20
China	500	2200	10	44
Latin America	200	1100	4	21
Middle East	50	800	1	16
Africa	50	400	1	9
South Asia	100	1300	2	27

Totals	4700	13600	100	270

¹The values in 1975 are calculated by assuming that S is emitted from each region in proportion to energy use and from the total S emission data of Cullis and Hirschler (1980).

A typical estimate of the future increase of fossil fuel combustion (Rotty and Marland, 1981), implies that the atmospheric CO₂ concentrations will be about 460 ppmv in the year 2025 and 680 ppmv soon after the middle of the next century (WMO, 1981). As mentioned before (section 1.3.2.B), an increase of atmospheric N₂O may also result. Significant climatic changes may occur (Manabe and Stauffer, 1980) and have an impact on the global ecosystem. This problem receives much attention at present and will not be discussed further here.

The emissions of SO₂ to the atmosphere will undoubtedly increase due to the expected rise in fossil fuel combustion, most of which will be coal combustion. The projection by Rotty and Marland (1981) implies nearly a three-fold increase in the global anthropogenic sulphur dioxide emissions to the atmosphere by the year 2025 (see table 1.3). As the SO₂ concentrations in air increase, cloud water becomes more acidified causing the solubility of SO₂ in water and its water phase oxidation to decrease (Penkett et al., 1979; Taylor et al., Chapter 4, this volume). This would perhaps lead to an increasing possibility for SO₂ to penetrate through clouds to higher levels and may be important for long range transport of sulphur compounds (Rodhe, 1981). The increased emissions might therefore affect such ecosystems that so far have not been markedly influenced by atmospheric SO₂ inputs, such as Siberia and various regions in the tropics. If industrial emissions are not regulated more efficiently than is now the case, environmental deterioration might occur in a manner similar to what so far has taken place in and around the most heavily industralized parts of the world, i.e. parts of North America and Europe. Continued industrialization in the tropics may particularly affect savanna regions (e.g. the cerrados of Brazil). Little is known about the natural atmospheric S deposition in the savannas, but it may amount to less than 1 g m^-2yr^-1 (i.e. totally only a few Tg S as an average for all tropical savannas) due to the low S/C ratio of vegetation, as compared to a possible 40 Tg S per year due to fossil fuel combustion (cf. table 1.3). How these ecosystems would respond to such an increase in sulphur deposition rates is unknown.

Through a variety of man's activities in the developed and the developing world, large amounts of nutrients (N, P, S) will be transported through the atmosphere and waters and accumulate in various ecosystems in amounts and ratios that differ markedly from pristine conditions. Ecosystems respond to such perturbations in various ways: an imbalance of minerals in the plants; higher nutrient content of senescent material returning as litter; and higher rates of decomposition; and change in soil acidity, etc. Knowledge of the C, N, P and S cycles of vegetation and soils must serve as a basis in assessing the effects of changes in nutrient deposition patterns and from this knowledge we will possibly learn how to avoid major deteriorations of the environment.

1.4 TERRESTRIAL ECOSYSTEMS

As already emphasized (section 1.2), living organisms and their metabolic products have an important influence on the biogeochemistry of all but the most extreme terrestrial ecosystems. For example, plants can decrease wind and water erosion of soil by a factor of 10 or more by adding litter to the soil and by binding the soil with roots. Plants and microbes increase chemical weathering and the transport of soluble compounds to deeper soil layers and to aquatic systems by raising the partial pressure of CO₂ in the soil, by converting part of the N and S they obtain from the atmosphere to mobile ions (i.e. nitrate and sulphate) and by releasing hydrogen ions in exchange for cations in the soil solution. Micro-organisms, plants, and animals produce many of the important trace constituents (or their precursors) in the atmosphere (Crutzen, Chapter 3, this volume).

Several interactions among the C, N, P, and S cycles under natural conditions can be illustrated by examining the changes in element supply, availability, and mobility that occur during the establishment and development of soil and vegetation on a previously unoccupied site. If a new land surface is deposited following a volcanic eruption or exposed by the melting of a glacier, the new substrate is at first nearly devoid of organic carbon and nitrogen. The development of biota and soils on such sites requires: (i) C-fixation from CO₂ by photosynthesis; (ii) N-fixation by symbiotic or non-symbiotic micro-organisms, or slow accumulation by the retention of small amounts of fixed N deposited from the atmosphere; (iii) P which generally is derived from the weathering of the substrate, and therefore is generally present in newly exposed material (Walker and Syers, 1976); and (iv) S weathered from the substrate or accumulated from the atmosphere. Symbiotic nitrogen-fixing plants are at a significant competitive advantage during the early stages of soil development because they can obtain N and thus C much more rapidly than other plants if adequate amounts of P and S are available. N-fixers add organic matter with a relatively low C to N ratio to the site; the organic matter accumulates and increases the water- and cation-holding capacity of the soil, and the level of available N in the site rises. The rates of N-fixation are eventually reduced as the availability of N increases in the soil or when P or S become limiting to the N-fixers. Once the availability of N in the soil is high, other plants without N-fixing capabilities can grow. Competition for light, water, and other nutrients becomes more important, and those plants that invest large fractions of their energy on structural tissue in stems or roots generally have a competitive advantage. Consequently, N-fixation peaks relatively early in soil development (Walker and Syers, 1976) and thereafter the relative availability of N is more closely coupled to that of P and S.

Within terrestrial ecosystems, organisms have evolved the ability to operate over a fairly wide range of C, N, P, and S contents (Melillo and Gosz, Chapter 6, this volume; Vitousek, Chapter 7, this volume; Stewart et al., Chapter 8, this volume; McGill and Christie, Chapter 9, this volume; Hunt et al., Chapter 10 this volume). Much of the variation between organisms can be explained by differential storage or retranslocation of C, N, S, P within the organisms or by differential availability of these elements in the soil; this variation can in turn cause differences in decomposition rates and nutrient availability. Consequently, the addition of one element to a system may bring about a change in the form of storage rather than the input or output of other elements.

Early in soil development, any change beyond the range of the metabolic flexibility of the organisms can still be adjusted by additional N-fixation, or by continued mineralization of soil P and some forms of S. With additional time and further soil development, however, the availability of P gradually declines

as P is lost or transformed into unavailable forms. This very slow loss is irreversible under natural conditions because there is relatively little replenishment of P from the atmosphere. Eventually, this means that not enough C (and N) can be fixed to replenish losses of those elements and the biomass and nutrient content of the system declines (Walker and Syers, 1976; Cole and Heil 1981; Melillo and Gosz, Chapter 6, this volume; McGill and Christie, Chapter 9, this volume).

This directional change in P availability in soil development, and the attendant changes in C, N, and S fluxes and storage, can be reversed by any process that brings unweathered material to the surface. Since the cycle of soil development is generally slow relative to the cycle of growth and wastage of continental glaciers, and slow relative to the frequency of volcanic activity in tectonically active regions, the final state of P deficiency can only be observed in geologically quiescent tropical and subtropical areas.

The natural biogeochemical cycles of C, N, P, and S do not interact only through this slow progression of soil development modified by changes in climate and landforms. Natural disturbances are important features of most natural landscapes (White, 1979) and can influence the natural biogeochemical cycling of C, N, P, and S. They are sudden in comparison with the slow changes of soil development discussed in section 1.4.1. The most important disturbances in grasslands and savannas are fire, grazing and severe droughts; in forests they are fire, windstorms, and insect outbreaks. These natural disturbances can occur with a frequency of one or more per year to one per several hundred years, and with a size from less than one to many millions m². They are integral components in determining the long term quasi-steady state features of most grasslands and many forests. Even without such catastrophic exogenous disturbances, tree falls occur frequently and can open large gaps in the forest canopy.

Different disturbances affect C, N, P, and S cycling differently. Fire volatilizes organic C, N, and S, producing a suite of major and trace atmospheric constituents including relatively large amounts of CO, NO, and aerosols, but fire converts organic P to a more biologically available form. Grazing by large herbivores concentrates nutrients into small patches of faeces and urine, which can then be sites for volatilization losses of N as NH₃, N₂O, and some amines. All of these disturbances can increase soil temperature and water content, thus increasing the mineralization of N, P, and S at least for short periods of time.

The retention of these mineralized nutrients (particularly N) against losses by leaching or volatilization after a disturbance is dependent in large part on the maintenance of the cycling of nutrients between soils, micro-organisms and plants within terrestrial ecosystems (Rosswall, 1976). When the cycles are interrupted by disturbance, losses of N are generally somewhat increased. Organisms that decompose structural material with a high C:N ratio are themselves likely to be N-limited as a consequence however (Melillo and Gosz, Chapter 6, this volume; Vitousek, Chapter 7, this volume; Hunt et al., Chapter 10, this volume), and they will take up available N from the soil solution, incorporate it in their metabolic machinery and retain it against loss. This structural material thus plays a stabilizing role in retaining nitrogen within disturbed systems.

The soil

plant

micro

organism cycle is eventually re-established by rapidly growing pioneer plants that invade disturbed sites and take advantage of the available light, water, N, and P to grow. The site then returns towards its predisturbance state, drawing C and some N and S from the atmosphere, and P and more N and S from the soil. This re-establishment is much more rapid (tens to hundreds of years) than that observed during soil development, due in part to the organic matter and nutrient stocks already present in the soil. Most natural terrestrial ecosystems, then, are a patchwork of different aged vegetation resulting from natural disturbance (Bormann and Likens, 1979). A few of the patches are likely to be losing C, N, and S at any given time; many more are likely to be in various stages of C, N, and S accumulation. Averaged over an ecosystem, the net result is no large loss or gain of C, N, P, or S.

Interactions of the biogeochemical cycles of C, N, P, and S thus occur on several time scales in terrestrial ecosystems: daily (or shorter) intervals involved in plant and microbial growth; annual cycles of community-level growth and decay; several month to several hundred year intervals of biological C, N, P, and S accumulation between natural disturbances; and several thousand to several hundred thousand year intervals of soil development and degradation.

Man's influence on the C, N, P, and S cycles of terrestrial ecosystems is felt in most parts of the world. The natural forests, semi-deserts and grasslands of the temperate regions have long been disruptively exploited by humans for food and fibre. More recently, population pressures, industrialization, and the search for ways to expand the world's natural resource base have caused the tropical regions to become the foci of extreme and often destructive exploitation (Melillo et al., Chapter 6, this volume; Vitousek, Chapter 7, this volume).

Man's impacts on natural terrestrial ecosystems range from alterations of the timing and intensity of processes, such as fire and grazing, to the application of agricultural and silvicultural treatments that create fundamentally different ecosystems, such as permanent agriculture, shifting cultivation, timber plantations, or drainage and reclamation of wetlands.

The increased intensity of land use takes many forms, but historically almost all have the long term consequence of decreasing the reserves of C, N, P, and S in soils. Even though interactions between the cycles primarily are governed by biological processes, C, N, S, and P appear capable of cycling somewhat independently in ecosystems (section 1.2; Stewart et al., Chapter 8, this volume; McGill and Christie, Chapter 9, this volume).

The response of ecosystems to man's interventions occurs on time scales from days to centuries. Simultaneously, natural variations are present and we are confronted with a major difficulty in trying to separate these and to determine cause and effect (cf. sections 1.1 and 1.2). Examples of this basic difficulty will be given in the following discussion of the pathways and magnitudes of C, N, P, and S losses from managed sites.

Fire is nearly universally employed in the conversion of forest land to other uses. Such fires generally have the same effects as natural fires except that natural fires, like fires used in shifting agriculture, are followed by relatively rapid regeneration. The increase in the amount of organic material consumed as a consequence of human activities is substantial. According to one estimate among several, an average of about 5 x 10¹⁰ m² of tropical forest per year is expected to be converted to other uses in the next 20 years (cf. Table 1.2 and Lanly and Clement, 1979), equivalent to an annual amount of 0.3 % of the total area of tropical forests. Tropical forest conversion includes a variety of alterations ranging from shortening the fallow period in shifting agriculture to complete destruction of forest cover and replacement by cultivated fields.

The inputs of gaseous C and N to the atmosphere from the burning of forests and savanna have been discussed in section 1.4.4A. Frequent savanna and forest fires have occurred for hundreds or thousands of years and it is not known if the amount of material consumed has increased in recent years or if this burning causes a change in the net flux of C, N, and S from terrestrial ecosystems.

Upland cultivated soils in the temperate zone can lose up to 50% of their C and N, 3040% of their S, and 1030% of their P in the 5070 years following land conversion (Stewart et al., Chapter 8, this volume); similar changes occur more rapidly in the tropics. A portion of the C, N, and S and most of the P is lost to harvest and erosion (see below), but a larger fraction of the C, N, and probably S is lost to the atmosphere in gaseous form or to aquatic ecosystems in ionic form. Appropriate agricultural management can reduce these losses, but it is difficult to eliminate them entirely.

These losses of C, N, and S occur because the decomposition rate is increased in cultivated soils due to physical alterations and because less organic material is returned to the soil than in natural systems. When organic C flux to the soil is reduced, microbes have a reduced capacity to immobilize the increased levels of N and S released during decomposition (McGill and Christie, Chapter 9, this volume). The N and S released exceeds the amount that plants can take up (at least for part of the year). Some of the excess C, N, and S in cultivated soils is leached to stream-water or ground-water as bicarbonate, dissolved organic C, nitrate, dissolved organic N, and sulphate. The remainder is lost to the atmosphere as CO₂, other C gases, N₂, NH₃, N₂O, and various sulphur gases (see section 1.3.2). The relative importance of these pathways is largely unknown and probably variable among sites, except that most of the C lost goes to the atmosphere as CO₂.

After one to several years of cultivation, these losses reduce the ability of the soil to supply adequate amounts of N, P, and S to crops, and fertilization is necessary to maintain high levels of production. Much of the applied fertilizer is present in forms that can be easily leached or volatilized. Present management procedures have transferred the almost closed nutrient circulation of the pristine ecosystem (forest or grassland) into the open system of modern farm land.

The decline in soil organic matter content, the absence of a surface litter layer, and a decrease in perennial root biomass in cultivated systems combine to reduce the cohesiveness of the soil. Consequently, erosion by both wind and water is increased many-fold by human activities. Pimentel et al., (1976) estimated that erosion has severely damaged much land in the U.S. and high rates of erosion have been reported in other regions (Rapp, 1975). The material that reaches rivers and lakes increases sedimentation, decreases the lifetime of lakes and reservoirs and can decrease aquatic productivity by limiting light penetration.

In drier areas, increased grazing by domestic animals caused by increasing population pressure has led to a decrease in plant cover and significant increases in both wind and water erosion. This reduces infiltration rates of water, soil water holding capacity, and organic carbon content and thus plays an important role in the deterioration of arid shrublands and dry savannas. Even though the rate of anthropogenic land degradation is currently 0.05 x 10¹² m² yr^-1, 36 x 10¹² m² of land, which supports one-sixth of the world's population, may ultimately be at risk (United Nations, 1977).

Organic C, N, P, and S are removed from croplands in harvested products and concentrated into urban areas or into large-scale animal feedlots in developed countries. Organic material from large land areas is transported into small urban areas and processed or consumed. A large fraction of it is then discharged into waterways whether or not sewage systems are in use; the rest is generally disposed of in highly concentrated form on land. Urban areas thus represent a significant concentrated source of organic C and biologically available N, P, and S. The major cities of the world are located near major rivers and coastal areas, and water discharges into riverine and marine ecosystems will increase primary production and anoxia (see section 1.5.4).

Man has influenced C, N, P, and S interactions by managing grazing animals in various ways. Management can be extensive (animals maintained in losely restricted areas by fencing or herding) or intensive (pasture improvement by altering plant species composition, fertilization, irrigation and supplemental feeding). With proper extensive management, the only important alterations of any of the cycles is increased production of NH₃ from the animal waste products. Although such losses are of a low magnitude over large areas, they have important regional atmospheric chemistry implications (see section 1.4.3 B).

With intensive management, animal numbers can be greatly increased in small areas. Intensive management can cause increases in NH₃ evolution in relatively small areas (section 1.4.3 B) and significant inputs in nitrate into surface or ground-water (section 1.5.4).

The cultivation of terrestrial ecosystems may cause substantial losses of C, N, P, and S through all of the pathways discussed above. However, cultivated ecosystems can be managed so that loss of soil fertility is minimized and even so that the organic matter, nutrient content and productivity of once degraded sites are increased (Power, 1981). These practices are now relatively energy intensive if they are to be effective on the time scale of decades.

During the 50100 years of continuous cultivation in the North American Great Plains, approximately half of the C and N, 3040% of the S and 1030% of the P in the soil has been lost (Haas et al., 1957; Haas et al., 1961; and Stewart et al., Chapter 8, this volume). Similar losses have taken place in less than 10 years in tropical areas (Nye and Greenland, 1964). The loss of N, P, and S reduces the ability of the soil to supply these nutrients, and increasing levels of fertilization are required to maintain crop production. Without perennial plants or structural carbon in the soil, fertilizer nitrogen cannot be effectively retained from year to year. Loss of soil organic matter and continued erosion decreases soil fertility and can threaten the long term maintenance of productive agriculture on a site.

The fundamental concepts underlying alternative cultivation practices involve bringing into balance the net inputs of organic C (by photosynthesis, residue return, and reduced decomposition), N, P, and S, thus in some sense reconstructing the interacting nutrient cycles of natural terrestrial ecosystems. After land abandonment, the cycles of these elements can also be brought into balance by natural processes but these will often require tens or hundreds of years, even provided enough P remains in available form in the soil (Cole and Heil, 1981). In the light of the marked changes of the soils that are occurring throughout the world, the further development of long term strategies for soil management is most urgent.

The CO₂ released in fossil fuel combustion increases the atmospheric CO₂ concentration on a global scale (Bolin et al., 1979). The direct effects of elevated atmospheric CO₂ on terrestrial ecosystems are not well known, beyond the fact that both photosynthesis and growth increase in individual plants that are not strongly light, water or nutrient limited. The water use efficiency of plants (the ratio of net primary production to evapotranspiration) is also increased because the CO₂ concentration gradient between plants and leaves is increased. It is not clear, however, that either natural or cultivated plants in the field are CO₂ limited often enough so that plant growth can be substantially increased in this way. Even if the growth of individual plants was increased, competition would limit the possible increase in community-level biomass. Some increase in terrestrial productivity (due to water use efficiency) and some, but probably quite limited, increase in total biomass is the most likely result of elevated atmospheric CO₂ levels, and the latter will require considerable time.

The majority of the S and N released is at present deposited in either gaseous (SO₂, NO_x, HNO₃) or ionic form (SO₄², NO₃, NH₄⁺) within 1500 km of its origin (Rodhe, 1981). As the emissions increase, the dispersion of these elements will very likely be more wide-spread (see section 1.3.3D; Rodhe, 1981; Taylor et al., Chapter 4, this volume). Still, most of the N and S released during fossil fuel combustion will be deposited within a few thousand kilometers of where the fuel is consumed.

The overall effects of these changes on C, N, P, and S cycling in terrestrial ecosystems are largely unknown. SO₂ and other gases do reduce plant growth near major sites of fossil fuel combustion. Most terrestrial ecosystems are relatively well buffered, however, and direct toxic effects of increased acidity (comparable to the deleterious effects so clearly demonstrated in many lakes) have only been conclusively established in a few areas (Ulrich et al., 1980). Acid precipitation does cause increased leaching of sulphate and accompanying cations and trace metals to stream-water and ground-water in many soils, but sulphate adsorption in old, acid soils can reduce leaching to negligible levels (Johnson et al., 1980). When increased leaching does occur, the loss of cations could eventually decrease terrestrial productivity. Acid precipitation at current levels could also decrease rates of litter decomposition by up to 10% (Cook, Chapter 12, this volume).

On the other hand, the N and S in precipitation could fertilize the impacted areas, increasing production in N- or S-limited sites (Melillo and Gosz, Chapter 6, this volume). The effect could be significant over considerable areas for N and in fewer sites for S, but there is no conclusive evidence for any directional change in forest productivity attributable to changes in atmospheric deposition. If forest biomass is increased by these N and S additions, some of the CO₂ released by fossil fuel emissions could accumulate in the increased biomass. The maximum potential accumulation of C in this pool is estimated at 250500 Tg/yr, which is 510% of the C released by combustion (Melillo and Gosz, Chapter 6, this volume, and discussions at the workshop).

The emissions to the atmosphere due to fossil fuel combustion means the addition of very large amounts of C, N, and S to the natural circulation of these elements in nature. By continuing this practice at a high and even increasing level we will change the chemical climate on earth. It is urgent to establish more firmly what this will imply over the time scales of one or a few centuries.

1.5 THE FRESHWATER SYSTEM AND COASTAL WATERS

The circulation of C, N, P, and S is largely due to the motions of air and water on earth. The role of the atmosphere in this regard has been discussed (section 1.3). At this stage we are concerned with the transfer of matter by water from land to sea.

The hydrologic cycle begins with the evaporation of water due to solar radiation and the dispersion of water vapor about the atmosphere. Part of the water is deposited upon the continents and returns to the oceans via rivers, coastal run-off and subterranean discharge; part returns directly to the oceans. The atmospheric and terrestrial waters are continuously altered chemically by processes along their way back to the oceans. Gas dissolution in the water droplets of the atmosphere, biological activity in the rivers, and continental erosion are examples of such processes. Many chemistries interact to give each river a unique composition (Richey, Chapter 13, this volume).

Since the industrial revolution over a century ago, the combination of social, agricultural and technological activities of man have distorted this cycle. Pollutants have entered the atmospheric and the terrestrial waters. Rivers have been dammed and channelled; the natural chemistries of weathering have been altered. As a consequence, the waters entering the marine environment have many imprints of human society.

The world's rivers are the major transporting agents for continental weathering products to the oceans. However, they are also a dynamic system, and have active biogeochemical cycles that modify their dissolved and suspended loads (Richey, Chapter 13, this volume).

The sources of C, N, P, and S in rivers are precipitation, erosion, entrainment from flood plains, chemical weathering, slope run-off and leaching. The processes involved have generally produced C, N, P, and S species in oxidized states. The diversity of (1) various weathering regimes, (2) the involvement of flood plains, (3) the character of the vegetation in the drainage basin, (4) hydrographic conditions, and (5) the occurrence of episodic events such as floods, determine river compositions. Aerobic respiration is the most important process modifying the species, forms and amounts of C, N, P, and S transported by the rivers.

When lakes occur along a river path, flow is interrupted, leading to increased particulate deposition and longer transit times favouring the occurrence of phytoplankton blooms. In the hypolimnion (the cold, deep and undisturbed region of such lakes) there is a shift toward reduced chemical species (e.g. less NO₃and more NH₄⁺).

The waters of rivers mix with the waters of the sea in the estuarine zone. Chemical and biological processes may profoundly change the speciation and the transfer of elements to the adjacent coastal zone, depending to a large degree on the hydrodynamical properties of the estuary. Precipitation, flocculation and sedimentation remove part of the riverborne organic matter and some inorganic species essential for plant growth, such as iron. The decrease in turbidity associated with these processes promotes high productivities when, as is usually the case in estuaries, the nutrients are in high supply.

The nutrients are then transferred from solution to particulate organic matter and can be trapped by sedimentation in deeper waters or in sediments. On the other hand, when high productivity occurs, high turbidities can result from the large amount of phytoplankton in the photic zone. As a consequence, there can be a severe limitation on the depth of photosynthetic activity. This effect is enhanced by a thermal stabilization due to light absorption and to associated warming in the uppermost layer. With limited nitrogen availability, these stabilized conditions promote blue-green algae growth.

Another consequence may be increased sedimentation of organic matter that leads to high biological activity in the surface sediments (benthos). Nutrient regeneration to the waters overlying the sediments is controlled by the microbial degradation of organic matter originating from river input and primary production in the surface waters. C, N, and P compounds introduced into the pore waters from such processes re-enter the overlying waters by molecular diffusion, bioturbation, sediment resuspension, and ground-water seepage.

Depending upon the intensity of the primary production and the hydrography of a coastal area, there is a possibility that all of the oxygen in the sediments may be consumed through the microbial degradation of organic matter. Anoxia can also occur during summer months in the water column over these sediments. This condition can be expected in either a permanent or seasonal mode depending upon whether the system has a permanent stratification (e.g. a tropical river estuary) or a seasonal one (e.g. the Baltic Sea and some fjords).

Following the loss of oxygen from sedimentary strata, nitrate is utilized as the oxidizing agent yielding nitrogen gas and ammonia as products. With the depletion of nitrate, which generally occurs deeper in the sediment, sulphate becomes the oxidizing agent, and hydrogen sulphide is produced. Finally, when all of the sulphate is utilized, fermentation and carbon dioxide reduction are initiated with the evolution of methane (Jørgensen, Chapter 18, this volume).

The coastal zone, which we define as that part of the marine environment bounded by the 200 metre isobath and the continents, receives the material transported by the rivers and estuaries. This area produces about 25% of the primary productivity of the oceans and receives about 90% of the organic matter that reaches the ocean floor and is buried (Jørgensen, Chapter 18, this volume). Further, about 50% of the fisheries that provide society with food are located here.

The primary productivity of coastal waters can be limited by nitrogen deficiencies and in rare cases by those of phosphorus. In such cases the waters are generally clear and the nutrients N and P are rapidly taken up by the plankton.

In unpolluted rivers the dissolved organic carbon mixes with ocean water without disintegration and most of it is exported to the open ocean. Particulate and dissolved organic carbon in the estuary is formed from microbiologically labile material, which is extensively recycled.

The major perturbation in the weathering cycle brought about by man involves the increased fluxes of C, N, P, and S throughout the hydrosphere and, from increases in N and P, a consequent increase in photosynthetic activity. This tendency toward euthrophication is especially evident in lakes and coastal marine waters and in some rivers.

The nutrient concentration of rivers on a global basis and over a 30 years' period has increased by at least a factor of 2 for dissolved inorganic nitrogen and phosphorus (Wollast, Chapter 14, this volume). This is a result of the wide-spread use of fertilizers, detergents, and industrial chemicals containing N and P. In addition there is an increased transport of organic carbon in the weathering cycle due to deforestation and land-use changes as well as through the increased production of municipal, agricultural and industrial wastes. A part of these materials is delivered to the coastal zone by the rivers. Much will be modified within the rivers, especially near sources such as outfalls and manufacturing plants.

The increased biological activity due to the entry of N and P in rivers has led in some cases to anoxia and consequent fish mortalities. Water impoundment for flood control, hydroelectric power, or water storage has caused the retention of large amounts of particulate materials. Additionally, earth moving activities have yielded large amounts of particulates to the rivers (Richey, Chapter 13, this volume). Channelization has modified river transit by confining the river to its bed and preventing plain flooding (entrainment) from influencing river chemistry. Rivers may also carry elevated levels of substances antagonistic to primary productivity, such as heavy metals, e.g. copper (Morel and Morel-Laurens, 1982).

The changes in the ratios of nutrients entering the coastal zone over the past decades have brought about major changes as well. The entry of large amounts of P relative to N and Si have lead to dominant plankton communities of blue-green and brown algae that replace the silicon containing diatoms, which form the basis of many valuable food webs (Wollast, Chapter 14, this volume). This situation develops in those coastal areas not subject to extensive mixing with the open ocean.

Future changes in the food web are to be expected. Presently, a shortening of the food web, high mortality of the phytoplankton and a subsequent rapid turnover of organic matter is observed (Goldberg, 1979a; Wollast, Chapter 14, this volume).

The increased biological activity usually extends throughout the coastal region and probably beyond it. The increased supply of organic detritus from plant production in the coastal zone will shift the location of decomposition from the water column, which recycles organic matter on time scales of days to weeks, to the sediments, which recycles it on time scales of months to years (Martens and Jannasch, comment to Chapter 18, this volume).

The CaCO₃ budget of the rivers and coastal ocean can be disturbed by the increased productivity induced by man. Respiratory activity in some rivers has increased to such an extent that high CO₂ pressures, and the consequent decrease in pH, has led to undersaturation of CaCO₃ and to its dissolution. Such a situation has been observed in the coastal ocean with the potential dissolution of the more readily soluble magnesium calcites.

Although these changes in the major weathering cycle have been observed, a complete quantification of the changing nutrient budgets of C, N, P, and S for such major rivers as the Amazon, Ganges, Yangzi, Zaire and Missisippi has not been made. Such data are essential for understanding the carbon cycle, where large transient reservoirs might exist but at present are not accounted for, as well as for understanding the present and future vitalities of food chains with value for human society.

The fluxes of C, N, P, and S induced by man now exceed the natural fluxes in many parts of the world. There are a number of effects of this on rivers and on the coastal ocean. Primarily, there is the increased production of organic matter and its consequencesthe replacement of the algae at the base of important food chains with less desirable species; increasing areas of anoxia; and increased loadings of sediments with organic carbon.

This situation clearly has an impact upon the renewable resources of the coastal ocean. Besides the food from the sea, recreational areas and aesthetic attractions of the coastal environment can be threatened by extensions of anoxic areas. There is, in principle, a limit to the use of the oceans as a receptor for a part of the wastes generated by society.

The world population is increasing and the utilization of materials that are to be disposed of continues to rise. The coastal environment appears to many as the most convenient place for disposal. Additional entries of organic materials and of nutrients, such as nitrogen and phosphorus, would add to the existing problem of eutrophication.

It is thus essential that the assimilative capacities of the oceans be assessed with respect to waste disposal. At the present time there have been only a few studies dedicated to this end (see Goldberg, 1979b). These evaluations generally use any alteration of the structure of communities of organisms as a criterion for determining whether the carrying capacity of a coastal water has been exceeded. Such assessments are still in a primitive state.

Those responsible for maintaining the quality of the marine environment, at national or international levels, can be guided by this concern. Optional methods of waste management are to be assessed with the recognition of this problem of enhancing the eutrophication of waters in the weathering cycle.

1.6 THE OPEN SEA

The open sea is by far the largest reservoir for the basic elements of our concern. On time scales of many hundred years to millenia, the water and the sediments of the oceans will be the main receptor of most of the human emissions regardless of whether these are made to the air, the soils, the fresh-water system or directly into the oceans. The pathways of the emissions from these various spheres to the sea are, however, long and the oceans respond slowly to imposed disturbances. New quasi-equilibria between the various parts of the global ecosystem will therefore only be established slowly. Due to the size of the ocean reservoir, final equilibria may not be vastly different from those that existed during pre-industrial times. The transient period on the other hand is and will continue to be characterized by marked deviations from pristine conditions. The characteristic time scale of modern society is short compared to the characteristic times of adjustment of the oceans. To provide a first order picture of the role of ocean circulation in this regard, the interaction of the wind stress and the heat and salt (thermohaline) transport processes must be considered.

The global winds generate a system of horizontally circulating gyres in the surface layers of the oceans. Those occurring in the equatorial and subpolar regions are characterized by upwelling and those in the subtropics by downwelling.

Atmospheric warming creates a warm surface layer in the tropics and subtropics, extending north and south to 4045^° latitude. In most of the subtropical anticyclonic gyres this layer is thick due to the downwelling. In contrast, the surface layers in the equatorial regions are shallow, merely to 100 m depth, and they extend well into the subtropical gyres in both hemispheres. This shallow surface layer communicates with deeper layers by inclusion of upwelling water in the equatorial region and in coastal upwelling zones, such as those off the coasts of Peru, California and northwest Africa.

Moderately warm water occupies the layer down to around 1000 metres and equatorward of the West wind maxima at 4045° latitude. This layer is the permanent thermocline, and upwelling in low latitudes into the surface layer is limited to waters from this layer. The poleward half of the subtropical gyres on the other hand are characterized by pronounced seasonal variations and late winter convective activity. The volumes of the warm surface waters and the moderately warm thermocline region are about 1 and 10%, respectively, of the total ocean volume and have characteristic response times of a few years and a few decades (Fiadeiro, Chapter 17, this volume).

The upwelling subpolar gyres that are subject to surface cooling have little surface stratification and are sites of deep convective overturning. Although these high latitude regions represent but a small fraction of the total ocean surface area, they are the only regions where vertical communication by water motion occurs between the surface layers and the deep water. This is limited to the North Atlantic and the Antarctic waters in our present climate. From radiocarbon data, the gross residence time for the deep waters relative to formation at the surface is 5001000 years (Fiadeiro, Chapter 17, this volume). The intrusion of anthropogenic tracers as observed in the North Atlantic deep waters as far south as 30°N latitude is consistent with this long response time due to the sluggish interior circulation of the deep waters. The fraction of the deep water that carries these tracers is quite small.

The large volume of abyssal waters is not very well mixed. The microbial breakdown of sedimenting organic debris within this water mass and the resultant release of nutrients can significantly influence biogeochemical equilibria.

In studies of the role of the ocean in the global biogeochemical cycles rather simple models have been developed. From these studies it is possible to successfully reproduce the gross vertical distribution of, for example, radiocarbon and total dissolved inorganic carbon (e.g. Oeschger et al., 1975). Fiadeiro (Chapter 17, this volume) emphasizes, however, that simple models are incapable of depicting transient processes on time scales that are small compared with the transfer times that are resolved by the model. Because the time scale of development of modern industrial activity is a few decades to at most half a century, the traditional box-models of the oceans must be replaced by models that can properly depict the motions on time scales of less than 30 years. Such work is in progress (Bolin, 1983; Viecelli et al., 1981; Fiadeiro, Chapter 17, this volume). Ultimately, detailed general circulation models will be employed for such studies.

Photosynthesis in the sea is limited to the photic zone and therefore so is most life. Still some remarkable communities have developed in the deep sea, of which those found around deep sea vents are most spectacular (Jannasch, Chapter 19, this volume). The evolution of any such features of the global ocean ecosystem is a very slow process, but their destruction may occur much more quickly.

The turn-over of organic matter in the well mixed ocean surface layer (about 100 m deep) is rapid and well governed by the Redfield hypothesis of distinct ratios of the elements involved (see section 1.2). Concentrations of the major elements in the surface waters represent a balance between biological processes, exchange with the atmosphere (Liss, Chapter 15, this volume; Duce, Chapter 16, this volume) and with deeper layers of the ocean. Only a few percent of the dead organic matter settles out of this layer as detritus. Still, all nutrient inhomogeneities of the ocean below the surface layer stem from this material transfer into deeper layers and its decomposition and dissolution.

Due to the ability of some marine organisms to fix nitrogen, it appears that of the major nutrients, only phosphorus supply is truly critical. Thus, the removal of carbon in the form of organic matter from the surface layers is, to first order, totally controlled by phosphorus.

Primary production, which requires light, is limited to a fairly thin layer near the ocean surface. Photosynthetic organisms are very efficient in utilizing available nutrients and, in fact, they recirculate much of the nutrient materials at a rapid rate. The result is that most of the phosphorus is found in organic matter and very little inorganic phosphorus exists in solution. Some small fraction of the organic matter does settle out, most in partially digested faecal material and to some extent in dead settling organisms of a size large enough to provide significant settling rates.

The decomposition of this organic material in the deep water or in the surface layers of the sediments leads to a development of carbon dioxide supersaturation accompanied by nutrient regeneration into dissolved form approximately in the Redfield ratios. The excess nutrients and carbon dioxide are, on a long time scale, returned to the surface layers to partake again in the biogeochemical cycles except for possible reactions with surface sediments and burial of some materials in the sediment. The factors determining the difference in concentration of these nutrients between surface and deep waters are the turn-over rate of the ocean as a physical system and the internal dynamics of the biological system. Assuming the turn-over rate is fixed, we have a basis for a nutrient controlled biological transport mechanism for carbon dioxide. The system is likely to be nonlinear, since several trophic levels will be involved. It is also complicated by the presence of silica, which seems to control the balance between diatom and calcareous algae production. If, however, an increased phosphorus supply were to cause a substantial increase in the photosynthesis in surface layers, growths in grazer and predator populations would ultimately lead a new equilibrium in which practically all the extra phosphate would be present in the deep waters along with a significantly larger amount of excess dissolved CO₂. Enhanced corrosiveness of the deep waters would then lead to carbonate dissolution from sediments. Thus the oceanic PCO₂ and hence, the carbon dioxide concentration in the atmosphere would be controlled by the available phosphate supply in the oceans. Such interactions might conceivably occur on time scales associated with glacial and interglacial periods (Broecker, 1981). The importance of biogeochemical interactions of this kind during the coming centuries due to increasing perturbations by man needs to be carefully assessed.

Man's interference with the global ocean system is still small. Even though pollution of the ocean surface layer can be detected all over the world, the direct impact of man on the C, N, P, and S cycles has not yet been established by direct measurements in the open sea. The required accuracy of measurements can be achieved but the natural variability implies that long observation series are required. The expected change of total inorganic carbon in the surface layer since the beginning of the industrial revolution due to CO₂ transfer from the atmosphere is 12% and the present rate of changes less than 0.4% or 0.01 mmol/1 per decade. Similarly we can estimate that even if 20% of all phosphorus used by man reaches the open sea (which is most unlikely) the photosynthesis could increase by only 12% if all of this additional supply were used. Due to the size of the oceans, cumulative effects take place only very gradually. It is, however, important to monitor the oceans carefully in these regards, because small changes correspond to large net fluxes and are therefore of importance for the overall budgets for the elements concerned.

1.7 CONCLUSIONS

In the course of the survey given in the preceding sections, a number of conclusions have been reached; the contributed papers contain many additional specific results. An equally important outcome of the workshop is, however, a more integrated view of how the global ecosystem as a whole functions. Clearly, there are numerous ways whereby different parts of this system interact. We have learned that when disturbances to the system becomes significant in one region of the world, implications need to be carefully assessed for the globe as a whole. Much further research is required to fully grasp this global interplay with regard to both important detailed processes and the overall interactions. We shall briefly summarize the outcome of the workshop in this regard in a series of concluding statements.

A. Man's activities on earth today induce fluxes of carbon, nitrogen, phosphorus and sulphur that are of similar magnitude to those associated with the natural global cycles of these elements; in limited areas man's influence dominates the cycles. The likely increase of man's activities during the remainder of this and during the next century will undoubtedly mean significant disturbances of the global ecosystem.

C. The present rate of change of the global ecosystem in response to man's activities will increase further and this implies increased departures from the pre-industrial quasi-steady state. No part of the globe will be untouched by these changes.

E. The development and continuation of highly productive units in agriculture and forestry means an increasing dependance on technological advances that, to be properly directed, requires profound knowledge about long term modifications of the soil.

F. The long term implications of exploiting the natural resources of the earth are not well understood, nor do we understand what is permissible in order to guarantee that present or future (possibly higher) levels of productivity will not later decline. We need to develop a strategy for how to assess the long term carrying capacity of the earth. It should be obvious from the overview of this vast problem area that we are yet far from such an integrated view of the global ecosystem.

1.8 REFERENCES

Berg, W. E., Crutzen, P. J., Grahek, F. E., and Gitlin, S. N. (1980) First measurements of total chlorine and bromine in the lower stratosphere, Geophys. Res. Lett., 7, 937-940.

Bolin, B. (1983) Changing global biogeochemistry, in Brewer, P. (ed.) 50th Anniversary Volume, Woods Hole Oceanographic Institution, The Future of Oceanography, New York, Springer-Verlag, 305-326.

Bolin, B., Degens, E. T., Duvigneaud, P., and Kempe, S., (1979) The Global Biogeochemical Carbon Cycle, In Bolin, B., Degens, E. T., Kempe, S., and Ketner, P. (eds) The Global Carbon Cycle. SCOPE Report No. 13, New York, Wiley,1-53.

Bowen, H. J. M. (1979) Environmental Chemistry of the Elements. New York, Academic Press.

Brewer, P. (1978) Direct observations of the oceanic CO₂ increase, Geophys. Res. Lett., 5, 997-1000.

Broecker, W. S. (1981) Glacial to interglacial changes in ocean and atmosphere chemistry, in Berger, A. (ed.) Climatic Variations and Variability: Facts and Theories, Dordrecht, D. Reidel 111-121.

Cole, C. V., and Heil, R. D. (1981) Phosphorus effects on terrestrial nitrogen cycling, in Clark, F. E., and Rosswall, T. (eds) Terrestrial Nitrogen Cycles, Ecol. Bull. (Stockholm), 33, 363-374.

Crutzen, P. J. (1976) The possible importance of CSO for the sulphate layer of the stratosphere, Geophys. Res. Lett., 3, 73-76.

Crutzen, P. J., Heidt, L. E., Krasnec, P. J., Pollock, W. H., and Seiler, W. (1979) Biomass burning as a source of atmospheric gases CO, H₂, NO, CH₃Cl and COS, Nature, 282, 253-256.

Delmas, R. J., Ascencio, J. M., and Legrand, M. (1980) Polar ice evidence that atmospheric CO₂ 20,000 yr BP was 50% of present, Nature, 284,155-157.

Duce, R. A. Biogeochemical cycles and the air-sea exchange of aerosols (Chapter 16, this volume).

Fishman, J., Solomon, S., and Crutzen, P. J. (1979) Observational and theoretical evidence in support of a significant in-situ photochemical source of tropospheric ozone, Tellus, 31, 432-446.

Food and Agriculture Organization (FAO) of the U.N. (1979) Agriculture: Toward 2000, Conference C/79/27, Rome, FAO, 30-31.

Goldberg, E. D. (ed.) (1979a) Scientific Problems Relating to Ocean Pollution, Washington, DC, U.S. Dept. of Commerce (NOAA).

Granat, L., Rodhe, H., and Hallberg, R. O. (1976) The global sulphur cycle, in Svensson, B. H., and Söderlund, R. (eds) Nitrogen, Phosphorus and SulphurGlobal Cycles, SCOPE Report No. 7. Ecol. Bull. (Stockholm), 22, 89-134.

Haas, H. J., Grunes, D. L., and Reichman, G. A. (1961) Phosphorus changes in Great Plains soils as influenced by cropping and manual applications, Soil. Sci. Soc. Amer. Proc., 25, 214-218.

Holland. H. D. (1978) The Chemistry of the Atmosphere and Oceans, New York, Wiley-Interscience.

Hunt, H. W., Stewart, J. W. B., and Cole, C. V. A conceptual model for the basis of interactions among carbon, nitrogen, sulphur, and phosphorus transformations (Chapter 10, this volume).

Husar, R. B., and Patterson, D. E. (1980) Regional scale air pollution: sources and effects, in Kneip, T. J., and Lioy, P. J. (eds) Aerosols: Anthropogenic and Natural Sources and Transport. Ann. N. Y. Acad. Sci., 338, 339-417.

Jannasch, H. W. Interactions between the carbon and sulphur cycles in marine environments (Chapter 19, this volume).

Jørgensen, B. B. Processes at the sedimentwater interface (Chapter 18, this volume).

Lanly, J. P., and Clement, J. (1979) Present and future natural forest and plantation areas in the tropics, Unasylva, 31(123),12-20.

Liss, P. S. The exchange of biogeochemically-important gases across the air-sea interface (Chapter 15, this volume).

Lovelock, J. E. (1979) Gaia, A New Look at Life on Earth, Oxford, Oxford University Press.

MacKenzie, F. T., Stoffyn, M., and Wollast, R. (1978) Aluminium in seawater: control by biological activity, Science, 199, 680-682.

Martens, C. S., and Jannasch, H. W. Cycling of metabolizable C, N, P and S in organic-rich marine sediments. (Comment to Chapter 18, this volume).

Melillo, J. M., and Gosz, J. R. Interactions of biogeochemical cycles in forest ecosystems (Chapter 6, this volume).

Morel, F. M. M., and Morel-Laurens, N. M. L. (1982) Trace metals and plankton in the oceans, in Wong, C. S., Boyle, E., Bruland, K. W., and Burton, J. D. (eds.) Trace Metals in Sea Water, Proc. NATO Adv. Res. Inst. New York, Plenum Press (in press).

Odén, S. (1968) The acidification of air precipitation and its consequences in the natural environment. Bulletin from the Ecological Research Committee Vol. 1. Swedish National Science Research Council, Stockholm. Translation Consultants Ltd. Arlington, Virginia, USA, 117 pages.

Penkett, S. A., Jones, B. M. R., Brice, K. A. and Eggleton, A. E. J. (1979) The importance of atmospheric ozone and hydrogen peroxide in oxidizing sulphur dioxide in cloud and rainwater, Atmos. Environ., 13, 123-138.

Power, J. F. (1981) Nitrogen in the cultivated ecosystem, in Clark, F. E., and Rosswall, T. (eds) Terrestrial Nitrogen Cycles, Ecol. Bull. (Stockholm), 33, 529-546.

Rahn, K. A. (1981) The Mn/V ratio as a tracer of large-scale sources of pollution aerosol for the Arctic, Atmos. Environ., 15, 1457-1464.

Ramanathan, V. (1975) Greenhouse effect due to chlorofluorocarbons: climatic implications, Science, 190, 50-52.

Rasmussen, R. A., and Khalil, M. A. L. (1981) Atmospheric methane (CH₄): trends and seasonal cycles. J. Geophys. Res., 86, 9826-9832.

Reiners, W. A. Transport processes within and between biomes (Chapter 5, this volume).

Rodhe, H. (1981) Current problems related to the atmospheric part of the sulphur cycle, in Likens, G. E. (ed.) Some Perspectives of the Major Biogeochemical Cycles, SCOPE Report No. 17, Chichester, Wiley, 51-60.

Rosen, H., Novakov, T., and Bodhaine, B. A. (1981) Soot in the Arctic, Atmos. Environ., 15, 1371-1374.

Rosswall, T. (1976) The internal cycle between vegetation, micro-organisms, and soils, in Svensson, B. H., and Söderlund, R. (eds) Nitrogen, Phosphorus and SulphurGlobal Cycles, SCOPE Report No. 7, Ecol. Bull. (Stockholm), 22, 157-167.

Roddy, R. M., and Marland, G. (1981) Constraints on fossil fuel use, in Bach, W., Pankrath, J. and Williams, J. (eds) Interactions of Energy and Climate, Dordrecht, D. Reidel, 191-212.

Seiler, W., and Crutzen, P. J. (1980) Estimates of gross and net fluxes of carbon between the biosphere and atmosphere from biomass burning, Climatic Change, 2, 207-248.

Söderlund, R., and Svensson, B. H. (1976) The global nitrogen cycle, in Svensson, B. H., and Söderlund, R. (eds) Nitrogen, Phosphorus and SulphurGlobal Cycles, SCOPE Report No. 7, Ecol. Bull. (Stockholm), 22, 23-73.

Stewart, J. W. B., Cole, C. V., and Maynard, D. G. Interactions of biogeochemical cycles in grassland ecosystems (Chapter 8, this volume).

Taylor, G. S., Baker, M. B., and Charlson, R. J. Atmospheric interactions of C, N and S Cycles: The role of aerosols and clouds (Chapter 4, this volume).

Torres, A. L., Maroulis, P. J., and Bandy, A. R. (1980) Atmospheric OCS measurements on Project GAMETAG, J. Geophys. Res., 85, 7357-7360.

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Viezee, W., and Singh, H. B. (1980) The distribution of Beryllium-7 in the troposphere: implication on stratospheric-tropospheric air exchange, Geophys. Res. Lett, 7, 805-808.

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	1.1 Global Biogeochemical Cycles and Man
	1.2 Biological Processes and Interactions of Biogeochemical Cycles
	1.3 The Atmosphere
		1.3.1 AtmosphereBiosphere Interaction
		1.3.2 The pre-industrial and the present atmosphere
		1.3.3 Looking towards the future
	1.4 Terrestrial Ecosystems
		1.4.1 Interactions of C, N, S and P in Natural Terrestrial Ecosystems
		1.4.2 Natural Disturbances
		1.4.3 Human Influences
		1.4.4 The Impact of Fossil Fuel Combustion on the Global Ecosystem
	1.5 The Freshwater System and Coastal Waters
		1.5.1 Water as an Agent in Biogeochemical Cycles
		1.5.2 The Natural System
		1.5.3 Disturbances in the Weathering Cycle Created by Man
		1.5.4 Human Waste and the Waters of the World
	1.6 The Open Sea
		1.6.1 Response Characteristics of the Oceans
		1.6.2 Life in the Sea
	1.7 Conclusions
	References

	the atmosphere
	the terrestrial vegetation and soils
	the fresh-water system, rivers and coastal waters
	the oceans and ocean sediments.


		America		Africa		Asia		Total
		Rain	Seasonal	Rain	Seasonal	Rain	Seasonal	Rain	Seasonal
		forest	forest	forest	forest	forest	forest	forest	forest

Area
cleared		2.7	0.2	0.6	0	1.7	0.1	5.0	0.3
(10¹⁰ m²)

C	Veg	510	18	110		330	15	950	33
(Tg)	Soil	110	7	20		70	6	200	13
	Total	620	25	130		400	21	1150	46

N	Veg	3.4	0.12	0.7		2.2	0.10	6.3	0.22
(Tg)	Soil	8.5	0.56	1.8		5.4	0.46	15.7	1.02
	Total	11.9	0.68	2.5		7.6	0.56	22.0	1.24

S	Veg	0.34	0.01	0.07		0.22	0.01	0.63	0.02
(Tg)	Soil	0.90	0.06	0.19		0.57	0.05	1.66	0.11
	Total	1.24	0.07	0.26		0.79	0.06	2.29	0.13

P	Veg	0.34	0.01	0.07		0.22	0.01	0.63	0.02
(Tg)	Soil	0.90	0.06	0.19		0.57	0.05	1.66	0.11
	Total	1.24	0.07	0.26		0.79	0.06	2.29	0.13

	a gradual change towards a warmer climate, the details and implications of which we know very little about;
	the concentration of ozone will decrease in the stratosphere, due to the increased release of N₂O and chlorine compounds and increase in the troposphere, due to the increased release of NO_x and hydrocarbons;
	an increase of the areas affected by lake and stream acidification in mid-latitudes and possibly also in the tropics; the ion balance of the soils may be significantly disturbed, as is now being found with regard to aluminium;
	a decrease of the extent of tropical forests, which will enhance the rate of increase in atmospheric CO₂ concentration and release other minor constituents to the atmosphere; this may also contribute to soil degradation;
	due to loss of organic matter and nutrients, soil deterioration will occur and this implies a reduced possibility for the vegetation to return to pristine conditions; global mapping of ongoing soil changes is urgently needed;
	a trend toward the eutrophication of estuarine and coastal marine areas;
	more frequent development of anoxic conditions in fresh-water and marine systems and sediments.

1	Interactions of Biogeochemical Cycles
	B. BOLIN, P. J. CRUTZEN, P. M. VITOUSEK, R. G. WOODMANSEE,
	E. D. GOLDBERG AND R. B. COOK

	expanding agriculture and forestry and the associated use of fertilizers (nitrogen and phosphorus) significantly alter the natural circulation of these nutrients;
	increased exploitation of the fresh-water system both for irrigation in agriculture and industry and for waste disposal.