SCOPE 21 -The Major Biogeochemical Cycles and Their Interactions 

ABSTRACT

Material transport occurs over very wide ranges of space and time, and is an intrinsic part of biogeochemical cycles. The transport aspects of the carbon, nitrogen, phosphorus, and sulphur cycles are examined in this paper. This examination has three principal sections. The first is a broad review of the many processes underlying transport of these elements, ranging from intercellular translocation within organisms to global atmospheric circulation. The processes are divided into mobilization, transport itself, and deposition of materials. The second section is a more detailed analysis of atmospheric transport of the several forms of these four elements. The third section summarizes the conclusions.

Throughout this paper, special attention is given to interactions between C, N, P and S cycles. Such interactions are most pronounced in five areas. (1) biological mechanisms of mobilization; (2) human-caused mobilization employing energy derived from fossil fuels; (3) photochemical reactions in the atmosphere; (4) biological mechanisms of deposition; and (5) influences of anthropogenically derived acids.

5.1 INTRODUCTION 

Material transport is, by definition, an intrinsic part of biogeochemical cycles. There can be no cycles without movement of matter. Movement occurs over a huge range of spatial scales. It can range from microns, in the circulation of ions within unicellular organisms, to tens of thousands of kilometers in global atmospheric circulations. As units of matter cross critical boundaries, the scale of their movement can shift abruptly. An ion or molecule may escape from a very small-scale cycle like an intraplant system and become incorporated in a global-scale transport process, and thereby a larger-scale cycle. The reverse must also happen at approximately equal rates. These shifts in scale of movement underlie the loss and input rates of specific cycling systems. At the same time, these shifts represent the interconnectedness of cycles of all scales.

In this paper, transport of matter is examined in the context of the cycles of carbon, nitrogen, phosphorus, and sulphur. The objectives of this paper are to: (1) broadly review transport processes for these elements; (2) highlight critical interactions between these elements in the transport aspects of their cycles; and (3) illustrate and suggest approaches that may be useful in modeling and predicting transport of these elements on a regional or larger space scale.

5.2 TRANSPORT PROCESSES

Transport can be separated into three phases: mobilization, transport perse, and deposition. Mobilization is a necessary part of moving a substance from one place to another; it involves freeing the substance from an immobile, fixed form so that it can be moved by a carrier force. Physical and chemical weathering, so-called biological mineralization, soil deflation, and gas production through combustion are examples of mobilization.

The transport phase entails the actual movement of a substance from one point to another. The deposition phase includes those processes that cause immobilization of a transported substance. These include, for example, chemical precipitation, gas absorption, and inertial impaction.

With this broad view of transport, a very large list of processes can be generated. I have attempted to arrange many of the possible processes into categories in order to more easily make comparisons of their importance for specific elements, spatial scales, or geographic areas (Table 5.1).

I have divided mobilization processes into three major classes: (A) natural biological-caused, (B) natural physical-caused, and (C) human-caused processes. As with all classifications, this approach is arbitrary to some degree, so that some overlaps between some classes occur and important interactions between processes are obscured.

A. Mobilization Processes and Products
Biological Processes
	Plant processes
		`Mineralization' of carbon compounds by respiration (CO2)
		Production of volatile organics (terpenes)
		Movement of chemical substances from internal tissues to surfaces (HCO3- , NH4+, NO3-, PO43-, SO42-)
	Animal processes
		`Mineralization' of organic materials (CO2, CH4, urea, NH4+)
		Incorporation of food and ions into mobile animals
	Microbial processes
		`Mineralization' (CO2, CO, NH3, PO43-, SO42-)
		Nitrification (NO2-, NO3- )
		Denitrification (NO, N2O, N2)
		Sulphate reduction (H2S, dimethylsulphide)
		Organic carbon reduction (CH4)
		Phosphate reduction (P2H4)
Natural Physical Processes
	Mineral weathering
		Physical weathering (inorganic and fossil organic particulates)
		Chemical weathering (S2-, SO42-, PO33-)
	Soil processes
		Cation exchange (NH4+)
		Entrainment by wind and moving water (particulate organic and inorganic forms)
		Ammonia volatilization (NH3)
	Glacial acquisition (organics and inorganics)
	Marine aerosol formation from evaporated spray (HCO3- , PO43-, S2-, SO42-)
	Volcanic exhalations (CO2, SO2, NH3)
	Atmospheric electrical discharges (NO3)
	Wildfires (hydrocarbons, CO2, CO, NH3, NOx, PO43-, SO2, COS)
	Suspension of particulates on leaf surfaces (HCO3-, NH4+, SO42-)
Human-caused Processes
	Wood fuel combustion (see above)
	Land management fires (see above)
	Fossil fuel combustion (CO2, CO, NOx, N2, SO2)
	Agricultural land tillage (organic and inorganic soil and crop particulates)
	Food and fibre processing (dissolved and particulate organics, soil)
	Fertilizer manufacture and application (CO2, NH3, NO3-, PO43-, SO42-)
	Mining (organic and inorganic particulates, S2-, SO42-, PO43-)
	Smelting (S2-, SO2, CO2)
	Petrochemical extraction, refining and transport (volatile hydrocarbons)
	Industrial processing (miscellaneous)
B. Transport Mechanisms and Relative Ranges of Transport
Biological Transport Mechanisms
	Plant uptake from soil and translocation (very local)
	Canopy leaching to throughfall-stemflow (very local)
	Plant litterfall to ground (very local)
	Animal uptake and subsequent movement, excretion, death (very local to continental)
Mass-wasing Transport Mechanisms (very local to local) Glacial Transport (very local to continental)
Fluvial Transport Mechanisms (particulate and dissolved, organic and inorganic)
	Saturated and unsaturated flow in soil (very local)
	Ground-water flow (very local to regional)
	Surface-water flow (very local to continental)
	Oceanic currents (hemispheric to global)
Atmospheric Transport Mechanisms (gases and aerosols)
	Saltation (very local to mesoscale)
	Suspension (mesoscale to global)
	Solution (mesoscale to global)
	Human Transport (food and fibre products, ores, industrial products (mesoscale to global))
C. Deposition Processes and Materials
From Mass-wasting Transport
	Deposition at base of slope (massive organics, inorganics)
From Glacial Transport
	Deposition as till, kames, moraines, out-wash, etc. (organics, and especially inorganics of all sizes)
From Fluvial Transport
	Uptake of dissolved substances of sessile organismse.g., vascular aquatic plants, corals (HCO3-, NO3- , PO43-)
	Nitrogen fixation by sessile organisms (N2)
	Physical sedimentation (organic and inorganic particulates)
	Chemical sedimentation (CO32-)
From Atmospheric Transport
	Wet deposition
		Incident precipitation via rainout and washout (HCO3-, NH4+, NO3-, PO43-, SO42-)
Dry deposition
	Sedimentation (large particles)
	Gaseous absorption
		Active: photosynthesis (CO2), N fixation (N2)
		Passive absorption to chemical sinks by foliage, soil, water (CH4, hydrocarbons, NH3, NOx, SOx, H2S)
		Inertial impaction (soil particulates, HNO3, NH4Cl, H2SO4, (NH4)2SO4, MPO4 aerosols > 1 µm)
		Molecular diffusion (HNO)3, NH4C1, H2SO4, (NH4)2SO4 aerosols < 0.1 µm)

 A. Biological Mobilization Processes

Biological mobilization processes are subdivided into those promulgated by plants, animals, and micro-organisms. Four general comments can be made about these biological processes. First, all of them are globally ubiquitous, but between ecosystem types there are very definite differences in the strength of their activities. Fresh-water wetlands, coastal marshes, and shallow shelf areas are particularly important for the critical reduction activities listed under microbial processes. With closer analysis, at least a graded series of importance by ecosystem type could be developed.

The second comment is that microbial processes, in particular, give rise to mobile products that can become involved in long range transport as well as local, intrasystem cycles. This is particularly true for the  microbiologically mediated reduction processes leading to formation of highly mobile gaseous forms of these elements.

The third point is that the strength of all of these processes is roughly proportional to the cycling rate or resource flow-through rates of ecosystems. Ultimately, all of these functions depend on cell numbers and cellular metabolic rates. These, in turn, are dependent on resource supplies. Cells require C, N, P, and S in particular ratios for accumulation of mass and numbers and in different ratios for maintenance. The required proportions of these elements can be highly variable, depending on the specific system; but, in the last analysis, all of these four elements, and other elements, can be rate-limiting resources. This is a fundamental, underlying interaction between C, N, S, and P for all biologically driven portions of biogeochemical cycles.

The fourth point regarding biological processes is that virtually all of them are sensitive to pH. Thus, there are important interactions between biological rates and the concentrations of acids of C, N, S, and P. The most interesting of these from a perturbation viewpoint involves the effects, both direct and indirect, of anthropogenically derived sulphuric and nitric acids. Cook (Chapter 12, this volume) reviews many of these effects.

B. Natural Physical Mobilization Processes

The second class of mobilization processes are physical-chemical phenomena occurring whether or not man is a dominating influence. This class is further subdivided into mineral weathering, soil-centred, and miscellaneous other processes.

Mineral weathering as a direct, liberating process is much more pertinent for C, P, and S than for N, because rocks generally do not contain much N. Nevertheless, there are important relationships between N and weathering, as will be discussed later. Rates of mineral weathering are highly variable over the face of the earth. Weathering potential definitely has predictable, geographic patterns in that it is roughly correlated with temperature and precipitation (Strakhov, 1967; Loughnan, 1969; Birkeland, 1974). On the other hand, this potential can be approached only to the extent that weatherable minerals occur in a weathering zone. In large part, this availability is related to rock type, relief, and youthful terrain (Gibbs, 1967; Johnson et al., 1968; Reynolds and Johnson, 1972). All of these geological factors have complex geographic patterns and are not usually characteristic of ecosystem types. For example, deglaciated terrainyouthful in terms of exposed, weatherable mineralsis located under arctic tundra, boreal forest, deciduous forest, grasslands, and mountain complexes. On the other hand, one ecosystem type, such as the tropical forest biome, covers a wide range of relief (Gibbs, 1967). Thus, a geographic model of weathering rate as a mobilization process for transport is not likely to be feasible using climatic or ecological criteria for geographic units. Rather, such a model would have to be based on specific conditions, especially geological characteristic, including relief and rock type, in conjunction with climatic and biotic factors.

Some of the most interesting interactions between C, N, P, and S relevant to weathering occur through the effects of biota. Plants can contribute to weathering through mechanical effects, but the most important influence of all the biota is probably through the excretion of hydrogen ion and the generation of acids (carbonic, organic, nitric, and sulphuric) (Gorham et al., 1979). Thus, weathering is influenced by acid production, which is a product of the amount and kind of plant cover, organic detritus, and microbiota present. These, in turn, are influenced by the supply of limiting resources, including C, N, P, and S. Another interaction involves the reciprocal effects of rock-weathering rate and N-fixation rate in primary succession or on impoverished substrates (Reiners, 1981). Nitrogen fixation is apparently dependent on an adequate P supply and to a lesser extent on pH, especially for rhizobia (Granhall, 1981). Thus, the weathering rate of apatite and metallic cations may influence the N-fixation rate. To the extent that N supply limits biological activity, it can, in turn, influence weathering rates by limiting acid production, so that there is a mutual feedback between weathering and N fixation under these circumstances.

Mineral acids of anthropogenic origin that are not neutralized by the soil should contribute to mineral weathering (Gorham and McFee,1980; Turk and Peters, 1979) or be transferred to stream ecosystems. Enhanced weathering effects caused by such mineral acids have been undetectable or are of very local extent (Johnson et al., 1972; Johnson et al., 1981) or are confused by soil leaching effects. In some ecosystems, disturbance may lead to enhanced nitric acid production via nitrification (Vitousek et al., 1979), possibly enhancing weathering potential temporarily.

Glacial acquisition of materials from overridden plants, soils, and rocks can represent enormous mobilization rates for local environments or particular periods of history. While most of the burden of glaciers is mineral, and therefore mainly of importance for inorganic forms of C, P, and S, some of the burden can be organic and thus include N.

Three processes are included under the soil category in Table 5.1. Ammonium is the only cationic form of all four elements, and thus only nitrogen can be mobilized by cation exchange. It can be mobilized for leaching transport through replacement by other cations, especially hydrogen. Hydrogen can be supplied by airborne nitric and sulphuric acids, as described above, and by organic, carbonic, and nitric acids formed by natural processes in the soil.

Entrainment is the process by which loosened material is acquired by wind and water as transport agent. Natural wind and water erosion are associated principally with arid and semi-arid climates, but these zones can be enlarged by human cultivation of former grasslands. The relationship between mean annual precipitation and wind and water erosion is shown in Figure 5.1 (from Marshall, 1973). Curve A indicates that under natural vegetation, the peak erosion potential occurs in areas that receive about 300500 mm precipitation per yearsemi-arid environments. Under these conditions precipitation is inadequate to allow formation of complete canopy cover. The relationship between canopy cover and wind and water erosion is shown in Figure 5.2. Curve B of Figure 5.1 shows the erosion potential from bare grounda condition that exists at least for short periods of time following cultivation or some fires. Curves C and D of Figure 5.1 show the relationship between precipitation and wind erosion potential under natural vegetation and bare ground. Wind erosion decreases as vegetation cover increases due to increased water available for plant growth. Wind erosion from bare ground decreases with increased precipitation, presumably because moist soils are less susceptible to wind erosion. Clearly, when soils are dry and plant cover is low, potential erosion is high from both wind and water. Excellent reviews of these processes are found in Statham (1977), Foster (1977), and Branson et al. (1981).

Atmospheric transport of deflated soil material is of great importance at local, regional, and global scales. Idso (1976) reviewed the meteorology and world geography of dust storms. Most aspects of wind erosion in the Sahara are presented in more detail by Morales (1979). The geography of deflation in the United States can be seen in Fletcher et al. (1978). For this process, a geographic model for dust source areas might well be associated with the desert and grassland biomes, but better resolution could be gained from other geographic criteria, as demonstrated by the wind erosion map in Fletcher et al. (1978).

Figure 5.1 Relationships of water erosion (continuous lines) and wind erosion (broken lines) with increasing mean annual precipitation. The curves for water erosion indicate the relationships with mean annual precipitation for (A) areas of natural vegetation cover and (B) bare ground (after Schumm, 1969). The curves for wind erosion indicate the relationships with mean annual precipitation for (C) areas of natural vegetation cover and (D) bare ground. These curves are based on what would be expected from the relationship of wind erosion to vegetation cover and to moist soil (from Marshall, 1973). Reproduced from Drought, edited by J. V. Lovett by permission of Angus & Robertson Publishers, Sydney, Australia

There are only weak interactions between elements involved in the mobilization of detached particulates, but there are definite chemical associations for the material transported. Wind- and water-transported material, especially that transported long distances, is enriched in lighter fractions of the surface soil horizons (Branson et al., 1981). These fractions are often rich in organic matter and thus richer in C, N, S, and P than deeper soil horizons or soil parent material. This enrichment may be minimal when the transported material is derived from desert soils that have not accumulated much organic matter (Rahn et al., 1979) and may be maximal for former grassland soils that have been turned to the plough. Areas undergoing desertification might produce dust and sediments richer in C, N, S, and P than traditional desert sources do. Dust derived from recently ploughed grasslands of Oklahoma, U.S.A., had a loss-on-ignition percentage of 7.26, indicating a carbon percentage of 3.6; nitrogen, phosphorus, and sulphur percentages were 0.19, 0.08, and 0.07, respectively (Kilmer,1979).

Figure 5.2 Interrelationships between drought, plant cover, and soil erosion by wind and by water. As drought severity increases, plant cover decreases. The markedly nonlinear nature of the relationship of wind and of water erosion to plant cover is noteworthy (from Marshall, 1973). Reproduced from Drought, edited by J. V. Lovett by permission of Angus & Robertson Publishers, Sydney, Australia

The process of ammonia volatilization is mainly related to ammonia supply and soil pH. Thus, interactions between the biogenic elements are not particularly critical. Söderlund and Svensson (1976) indicate that most naturally derived ammonia comes from animal excrement, but broad geographic areas with alkaline soils, such as grasslands (Woodmansee et al., 1981), may be important sources of ammonia. Fertilization of circumneutral soils with urea and anhydrous ammonia, coupled with extensive periods of little or no crop cover, would seem to be a very important ammonia source (J. Freney and R. Woodmansee, this workshop, personal communication). Atmospheric ammonia concentrations over the ocean, tropical land areas, and `other' land areas (Table 5, Söderlund and Svensson, 1976; Figure 5.3a) suggest that the land is a more important source than the oceans and that tropical regions may be especially strong sources. The same regions producing high levels of ammonia may also be major sinks, as indicated in Table 14 of the same reference. Junge's map (1958) of ammonia in precipitation in the United States strongly suggests a Great Plains origin for much of the ammonia returned in wet deposition further east (Figure 5.3b).

Figure 5.3 Maps indicating distributions of NH₃ in the United States and north-western Europe. (Above) Average NH₄⁺ concentration (mg/litre) in rain over the United States in JulySeptember 1956 (from Junge, 1958). Reproduced by permission of American Geophysical Union. (Below) Mean annual surface concentrations of NH₄ compounds and NH₃ gas for the years 19681972 (from Söderlund, 1977). Reproduced by permission of the Royal Swedish Academy of Sciences

The increased use of ammonia fertilizer and development of feedlots in that region may have significantly increased the emission levels since Junge's data were collected in 1955. Most of the ammonia that combines with sulphate further downwind over the more industrialized parts of the country may be derived from natural soil processes augmented by agricultural practice. In the North-western Europe case (Figure 5.3b), the largest contributory source is believed to be domestic animal waste, followed by coal combustion and fertilizers (Söderlund, 1977). A good potential may exist for producing geographic models of ammonia sources for global modelling. Grasslands, croplands, and tropical regions may compose large natural sources that might be overlain with specific industrial sources. There are no obvious chemical interactions between C, N, P, and S aside from those general ones mentioned earlier concerning nutrient limitations for biological processes and mineral acid effects.

Aerosols created by ocean spray jets contain organic C, organic N, organic and inorganic phosphate, nitrate, and especially sulphate (Söderlund and Svensson, 1976; Pierrou,1976; Granat et al., 1976; Graham et al., 1979; Duce, Chapter 16, this volume). Six to ten percent of the aerosols so generated fall on land (Granat et al., 1976), so there is a net transport by this means from the sea margin to several hundred kilometers over land. Such transport obviously represents a transfer from oceans to terrestrial ecosystems but does not discriminate between terrestrial ecosystem types.

Volcanic emissions have been extremely important in the geochemical development of the earth and its atmosphere. Estimates of contemporary contributions vary widely; most recently they have been raised for sulphur to 29 Tg/yr (Ivanov, 1981; Freney and Rodhe, Chapter 2, this volume). These emissions are major sources for remote areas and primary sources of temporal variation over larger areas. The distribution and activity of volcanoes are readily mapped for geographic modelling, but the distribution of active volcanoes does not coincide with biome distributions.

Wildfires are very important sources of various compounds of C, N, S, and even P. Fires have consistently been considered in the major SCOPE papers of chemical cycles (Svensson and Söderlund, 1976; Bolin et al., 1979a), and details on emissions have been reviewed (e.g., Raison, 1979; Crutzen et al., 1979; Crutzen, Chapter 3, this volume). A particularly good set of reviews on the geography of fire is in Mooney et al. (1981), and a good review on secondary effects of burning on chemical cycles is provided by Woodmansee and Wallach (1981). Wildfires are more wide-spread than commonly is thought, but a mapping pattern of emissions in time and space from biomes having seasonal climates is definitely feasible. Chemical interactions are especially interesting for this process, both in increasing the probability of fire (e.g., N limitations in soil), in chemical conversions in the atmosphere, and in post-fire effects (Woodmansee and Wallach, 1981).

The physical process of suspending particles from plant canopies into the air is directly linked to the mobilization of materials from the interior of plant tissues to the surface. This suspension, and resuspension, is very difficult to measure in the field and to treat in a theoretical way (Fish, 1972; Slinn, 1976; Beauford et al., 1977). The chemical nature of such suspended particulates is not well known, but C, S, and P are certainly involved. Tropical forests may be especially strong sources of such particulates (Crozat, 1978; Hogan and Mohnen, 1979; Lawson and Winchester, 1979). Much of this suspended material may be resuspended locally, but some may escape for intercontinental transport (Hogan and Mohnen, 1979). Such local transport also greatly complicates the measurement of atmospheric inputs for watershed budget studies because of contamination of bulk precipitation collections.

C. Human-caused Mobilization Processes

All the aspects of human-caused mobilization are familiar to most readers and are covered in one form or other in the major SCOPE publications on the global cycles of C, N, P, and C (Svensson and Söderlund, 1976; Bolin et al., 1979b). Human activities can largely be viewed as local accelerations of natural processes, and involve the same chemical interactions. The effects of man on erosion can be seen in Figures 5.1 and 5.2. Where natural vegetation occurs and is not seriously overgrazed, erosion losses are generally low compared with sites that have been denuded by cultivation or other land-clearing activities. Wood fuel and land management fires will differ in seasonality and secondary effects from wildfires, but will produce similar emission products. Fossil fuel combustion, fertilizer applications, mining, and smelting are very much like intensive weathering processes. Agricultural tillage and food and fibre processing are exaggerated versions of soil processing by meso- and micro-fauna and flora. Petrochemical extraction and processing is a vast enhancement of reactions resulting from natural petroleum or asphalt seepage losses. Industrial processing, on the other hand, includes syntheses of materials that are totally exotic in the natural environment, such as some halogen-containing organic substances.

The major difference between human-caused and natural mobilization processes is in the geographic distribution, extreme localization, and very high intensity of the human-caused processes. All three of these properties are well known from economic and marketing data. Just as the rate of fossil fuel combustion is the best-established datum for any flux in the C cycle, so should these other human-caused mobilization processes be easily estimated compared with natural processes. In order to establish a geographic model of sources and sinks, these estimates can be set as overlays on more diffuse natural processes.

One very important chemical interaction underlying these human-caused processes is the intense relationship between hydrocarbons and other mobilized substances. Except for fires and non-industrialized agriculture, the bulk of these processes is driven by fossil-fuel-(hydrocarbon) derived energy. 

5.2.2 Transport Processes

Following mobilization, substances can be transported by a variety of agencies. Remaining as inclusive as possible, I have listed four classes of transport mechanisms representing all scales of transport distance (Table 5.1).

A. Biological Transport Mechanisms

The mechanisms listed in this category are significant almost exclusively on a short range basis. The three plant processes listed are of little consequence on even an inter-ecosystem spatial scale, but they are essential in the maintenance of local, or `internal', cycles (Rosswall,1976). Animal transport is occasionally an important medium range phenomenon, as in the cases of marine birds creating guano deposits, anadromous fish fertilizing small streams, or , migratory herds of ungulates moving across vast areas. 

B. Mass-wasting Transport Mechanisms

Mass-wasting is the gravitational transfer of material. It includes a number of typescreep, solifluction, mudflows, landslides, avalanches, and subsidences. Naturally, such movements are usually associated with substantial relief in the terrain, whether terrestrial or submarine. Mobilization for mass-wasting can arise from weathering processes, but other factors may be the causes, such as volcanism or tectonic movements. In any case, all four elements are usually involved, and the range of movement generally is less than 10 km. For an excellent estimate of the role of mass-wasting in material transfer on mountainous watersheds, see Swanson et al. (1981).

C. Fluvial Transport Mechanisms

Water movement over the earth's surface is an obvious and primary mechanism for the transport of nearly all substances. This process might actually be considered to begin with atmospheric wash-out, but that is classified later as a depositional mechanism. Mechanisms in Table 5.1 begin with transport through the solumwhat is commonly termed leaching. This is a very critical process and includes a number of interesting chemical interactions. For example, the leachability of cations like ammonium is partly a function of the concentration of other cations like hydrogen. Hydrogen can be derived from acids of C (carbonic), N (nitric), and S (sulphuric). The last two can be of natural or anthropogenic origin. Leachability is also a function of the concentration of soluble anions such as bicarbonate, nitrate, and sulphate. Although anions are not retained against the mass flow of water by exchange reactions to the extent demonstrated by cations, there can be a variable amount of anionic absorption, depending on the mineralogy and other chemical attributes of the soil (Nye and Tinker, 1977). Soil leaching need not lead to total removal from the soil profile. Much of the transfer can be over short distances, simply to lower horizons. Soil solutions can even become diluted as they pass through B, C, and D horizons (Cronan and Schofield, 1979).

Ground water is 18.8% of all the water on earth and 95.7% of all fresh water (Garrels et al., 1975). The flux rate through this reservoir is highly variable, ranging from very fast in karst regions, to exceedingly slow in the more massive aquifers, to functionally zero in cases of connate water. The chemistry of ground water in a wide variety of materials in the United States is thoroughly reviewed in White et al. (1963). The importance of ground water as a transport medium is as variable as the world's geology. Certainly, fluxes could not be generalized on a biome or even ecosystem basis; transport would have to be estimated on an individual region basis.

The movement of dissolved and solid materials from land to lakes, reservoirs, and the sea by surface run-off is one of the most important fluxes for all biogeochemical cycles. The hydrologic cycle has the net effect of collecting a diffuse source of energy, precipitation, into a concentrated form, running water (Statham, 1977). During this concentrating process, soil material is transported from the upland positions of watersheds to lowland positions as dissolved ions, suspended substances, and bedload. In addition, the greater energy of flowing water in the lower reaches of drainages can cause channel cutting, thereby greatly increasing the entrained material in the water. The importance of these entrained materials in streams and rivers is addressed by Richey (Chapter 13, this volume). As the energy of water is reduced in downstream channels, deposition occurs, as discussed below. The importance of such fluxes for individual element cycles is calculated in Chapter 2 (this volume) and the references contained therein. For more detailed geographic analysis, hydrological data are available from world atlases, while world river water chemistry is reviewed by Livingstone (1963). While in transit, waterborne substances may undergo degassing, biological uptake, and sedimentation (Hynes, 1970) that will lead to temporary re-assortment in sediments or permanent distillation to the atmosphere. These processes are particularly important at the mouths of rivers, where deposition can be prominent and freshwater meets saltwater (Wiley, 1977; Wollast, Chapter 14, this volume).

Once in the ocean, water mixes according to fairly well-known and understood patterns (Neumann, 1968; Stern, 1975; Fiadeiro, Chapter 17, this volume), and re-assortment of water contents will take place according to the many biogeochemical processes of recirculation, degassing, and differential sedimentation (Broecker,1974; Gibbs, 1977; Wollast, this volume). Transport in the ocean is treated by Fiadeiro (Chapter 17, this volume).

D. Atmospheric Transport Mechanisms

The atmosphere is the principal medium for fast, long range transport. For carbon and nitrogen, the atmosphere is the principal source for land ecosystems and many aquatic systems. The atmosphere is also the principal source of sulphur for many unpolluted land ecosystems (e.g. maritime zones). This importance has expanded over wider regions with the increase in emissions of anthropogenic sulphur.

Saltation is only of local importance in moving soil materials. In contrast, suspension of particles is extremely important as a long-distance mode of transport. Truly suspended particles range from 10^-3 µm to 1 µm in diameter. Below 0.1 µm, particles are subject to deposition by Brownian movement and are little affected by gravitational settling. Between 0.1 and 1 µm, particles are little affected by either factor and have the longest survival time in the atmosphere. Particles from 1 to 10 µm diameter have a significant sedimentation rate but are maintained in the atmosphere by turbulence (Twomey, 1977). These large particles are readily deposited to surfaces through inertial impaction.

Aerosol concentrations range from very low values in hundreds/cm³ in the cleanest air of the troposphere to tens of thousands/cm ³ in urban areas. This is a large number, but it generally represents a relatively small mass, comparable with the mass of trace gases. The total mass of suspended particulate matter in the entire atmosphere is estimated by Twomey (1977) to be about 10¹¹ g. This may be low by about an order of magnitude; an estimate of about 10¹² g seems more consistent with emission rates and turn-over times. This can be compared with 2 x 10¹⁵ g N for N₂O and 1.1 x 10¹² g N for NH₃ (Söderlund and Svensson, 1976).

Aerosols are derived from soil deflation, sea spray jets, volcanoes, suspension of particles on canopies, and condensation from various gases. Table 5.2 gives an estimate of the magnitudes of production by several natural and human sources. Clearly, C, N, and S are prominent among the chemicals involved in aerosol formation. Interactions between compounds of these elements are extremely important in the formation of aerosols (Taylor et al., Chapter 4, this volume). Phosphorus is of trivial importance from the viewpoint of total mass of aerosols, but atmospheric transport of phosphate aerosol is a major part of the short term phosphorus cycle (Richey, Chapter 2, this volume) and may, in the long run, be a critical source for oligotrophic land and aquatic ecosystems.

Source		Size
Natural
Soil and rock debris*		100500
Forest fires and slash-bruning debris*		3150
Sea salt		(300)
Volcanic debris		25150
Particles formed from gaseous emissions:
	Sulphate from H2S	130200
	Ammonium salts from NH3	80270
	Nitrate from NOx	60430
	Hydrocarbons from plant exudations	75200
Subtotal		7732200
Man-made
Particles (direct emissions)		1090
Particles formed from gaseous emissions:
	Sulphate from SO2	130200
	Nitrate from NOx	3035
	Hydrocarbons	1590
Subtotal		185415
Total		9582615
*Includes unknown amounts of indirect man-made contributions.

The mixing and subsequent movement of gases in the atmosphere is the most important means of transport for carbon, nitrogen, and sulphur in the atmosphere. Gaseous contents of the atmosphere are given in Svensson and Söderlund (1976) and Freyer (1979), and sources and lifetimes in the atmosphere are given by Crutzen (Chapter 3, this volume). The chemical interactions between these gases are described by Crutzen (Chapter 3, this volume), and the relationships between the gases and particulates is exceedingly intimate (Taylor et al., Chapter 4 this volume). In terms of the transport aspect of biogeochemical cycles, the most important chemical interactions between carbon, nitrogen, and sulphur occur in the biological aspects of mobilization and deposition and in atmospheric chemistry occurring while in transit.

The geography of atmospheric transport is a function of emission source areas, lifetimes of chemical species and physical forms in the atmosphere, movement of air masses, and distribution of factors contributing to deposition. Further discussion of atmospheric transport will follow later in this paper.

E. Human Transport

The spatial scale and variety of material transfer affected by human activities is immense. An analysis of such transport would require a major review in economic geography. Man undoubtedly accomplishes more transfer by releasing waste materials into the atmosphere or into rivers and the sea than he does by actually moving material itself. Such activity is covered indirectly as a result of human-caused mobilization (Table 5.1).

5.2.3 Deposition Processes

A. Deposition from Mass-wasting Transport

Deposition of material moved downslope through the force of gravity simply occurs when the transported mass reaches a stable position, generally at the base of the slope, although momentum can carry material some distance up opposing slopes or across flat ground. This is almost entirely a physical process, involving no interactions between critical elements and is entirely of local significance.

B. Deposition by Glacial Transport

The manner of deposition by glaciers is quite variable in every respect. Usually deposition is most prominent around glacial margins, where glacial advance is balanced by ablation, or by iceberg calving into lakes or the sea. Deposition can also cover broad areas where down-wasting creates vast till plains or out-wash carries debris over extensive drainage basins. Persistent deposits are, of course, mainly terrestrial and have very long term effects on the nature of ecosystems developing on such deposits (Jenny, 1980). Material delivered to rivers or directly to the sea will undergo the same sorting and sedimentation as other fluvial solids.

C. Deposition from Fluvial Transport

Eroded material from upland positions of watersheds is progressively deposited downhill as gradient changes occur (Branson et al., 1981). For example, some eroded soil material from hill slopes may be deposited on lower-slope positions or in low-order drainage basins, and some may be carried further downstream, depending on the energy of the flowing water. As a consequence of this progressive deposition, the total amount of material eroded from the upland positions of watersheds during storms or snow melting is greater than the amount measured as entrained substances in water flowing out of the watershed.

The ecological impacts of the deposition of sediment on toe slopes and low-order drainages are increased soil depth, increased soil water-holding capacity caused by each, greater proportion of fine-textured soil material and organic matter, and, consequently, often increased storage of C, N, P, and S and in cycling rates of N, P, and S. The impact of deposition on high-order drainages is often the formation of fertile floodplains (i.e., many of the important agricultural centres of the world, especially in the semi-arid and arid regions). However, deposition of sediment, especially that resulting from erosion caused by man's activities, can have serious economic and environmental impact: (1) detrimental deposition on land and crops; (2) alteration of water quality of fresh-water, estuarian, and marine communities (see Ritchie, Chapter 13, this volume); and (3) aggradation of river channels, increasing flood hazards and reducing reservoir storage capacity.

As noted earlier, some material in water is transferred to the atmosphere through gas escape or aerosol suspension (see Liss, Chapter 15, this volume and Duce, Chapter 16, this volume). The reverse is true, as well. Nitrogen gas and carbon dioxide for example, do dissolve from the atmosphere into water and become biologically fixed. Deposition processes in this category include those for which dissolved or suspended substances are temporarily, at least, removed from the moving water column and fixed in place. The first two processes include biological fixation by sessile organisms on benthic or reef substrates. Floating organisms can also fix nitrogen and carbon; take up nitrate, phosphate, and sulphate; and, by their death or excretion of faecal pellets, lead to sedimentation of these elements in a position below this zone of synthesis (Broecker, 1974). Besides organic matter, calcium carbonate and many other minerals can be sedimented in this manner, as well (Lowenstam, 1981). Much of the organic matter and calcium carbonate thus sedimented is redissolved and recirculated in deep ocean currents (Jørgensen, Chapter 19 this volume). However, some of it will ultimately be deposited for the long term and either up-thrust as new sedimentary rock on land or, less likely, lost to metamorphosis in crustal subduction (Kempe, 1979).

D. Deposition from Atmospheric Transport

The mode and rate of deposition of substances from the atmosphere depends on the form of the substance, meteorological conditions, and the nature of the surface. The various modes of deposition have been described in many places; one of the most succinct has been the description by Fowler (1980), which will be followed here.

It is simplest to first consider wet deposition. Rain and snow are responsible for approximately half of the deposition of nitrogen and sulphur in well-watered industrial countries (Galloway and Whelpdale, 1980; Grennfelt et al., 1980). The rate of deposition on a particular locale depends on the chemistry of the air in which droplets are formed and on the annual rainfall over the locale. Fowler (1980) calculated that 6070% of the sulphur and nitrogen in wet deposition comes from the cloud condensation nucleus pathway, and about 20% comes from the solution and oxidation of gaseous species in droplets that will become raindrops. This oxidation may not go to completion (Dana, 1980). Again, heterogeneous reactions between chemical species of carbon, nitrogen, and sulphur and with other elements are critical for controlling rates at which gas in the atmosphere can be swept out by wet deposition. While aerosols impacted on falling drops comprise only a small part of the chemical load of raindrops, this is a very important mechanism for removing small aerosols from the atmosphere. Butcher and Charlson (1972) reiterate Junge's (1963) approximation that 8090% of particulate removal occurs by incorporation of aerosols into hydrometeors and delivery by wet deposition.

Strictly speaking, all other forms of deposition are classified as dry deposition, although one formsedimentationis generally measured together with wet deposition in `bulk precipitation' collectors. The other anomaly is that cloud droplet impaction is classified as dry deposition even though it is quite a `wet' process. Sedimentation is the deposition of larger-sized particles (> 10 µm diameter). Soil dust, volcanic ash, and locally transported organic fragments probably make up most of this category, which is usually a minor source, except for downwind of deflation areas. Kilmer (1979) gives an example of dust deposition from a single storm that originated in the TexasOklahoma Panhandle in 1937. Dust was carried over 800 km, leaving deposits of 10 g/m² in Ames, Iowa; 5 g/m² in Marquette, Michigan; and 4 g/m² in New Hampshire.

I have divided gaseous absorption into active and passive processes. This seems appropriate in that CO₂ and N₂ fixationtwo of the major driving forces in the biosphereare rate limited by distinct physiological processes and not usually by physical ones. Wherever physiological processes come into consideration, the basic requirement for a proper balance of limiting elements becomes a germane chemical interaction for C, N, P, and S in the manner described by Redfield (1958).

Passive gaseous absorption is not entirely passive, either. Deposition rates will ultimately depend on partial pressure differentials in the absorbing material. All of the substances listed in this category in Table 5.1 must undergo a chemical reaction to reduce this back pressure. Some reactions are at least partly biologicale.g., microbial decomposition of methane and stomatal absorption of SO₂. Assuming these sinks are functioning, then gas absorption is largely limited by physical processes.

Figure 5.4 The dry deposition process (from Fowler, 1980). Reproduced by permission of Norwegain SNSF Project

Figure 5.4, from Fowler (1980), describes the dry deposition process as a series of mechanisms for penetrating two boundary layers and ultimate sorption on the surface. These mechanisms are somewhat different for gases than for particles, but the principles of measurement are the same for each, so inertial impaction and molecular diffusion processes can be treated simultaneously: Inertial impaction is the major depositional mechanisms for particles > l µm and < 10 µm in diameter; molecular diffusion is the depositional mechanism for particles < 1 µm in diameter. A convenient means of estimating flux from the atmosphere to a surface is to convert diffusion rates across boundary layers to resistance as an analog to the analysis of electrical circuits. In Table 5.3, resistances to gaseous deposition are shown for a variety of circumstances for vegetated surfaces The resistances representing the turbulent boundary layer and laminar boundary layer are pooled and termed atmospheric resistance. Surface sorption characteristics are described as canopy resistance in this table. Atmospheric resistance is inversely related to canopy roughness, which is crudely proportional to height. It is also inversely related to wind speed. Hence, other factors being equal, dry deposition will be least in still air over a smooth surface. It will be greatest in windy conditions over forest vegetation (Figure 5.5). Looking back at Table 5.3, we see that for nitrogen and sulphur gases, resistances are least for wet surfaces, low for open stomata, higher for closed stomata, and highest for senescent surface tissues. Perception of these factors in terms of deposition rates is made easier by the deposition velocity column in Table 5.3. Deposition velocity can be viewed as a coefficient for capture. Deposition is the product of deposition velocity times exposure (concentration).

Taken together, estimates in Table 5.3 enable us to assess the kinds of conditions and surfaces that would lead to more or less dry deposition for a given exposure. Wet climates will enhance not only wet deposition but also generally dry deposition, as well. Vegetated surfaces are more efficient collectors than non-vegetated surfaces. This suggests not only ways of rating ecosystem types for deposition but also ways of rating seasonal effects. Cropland ought to be superior to urban areas, forests ought to be superior to croplands and grasslands, and tall, moist forests ought to be the best sinks for dry deposition. Except under special meteorological conditions, dry deposition of gases, and especially particles, is generally higher on land surfaces than on lake or ocean surfaces (Sheih et al., 1979). The potential of this method for estimating sink characteristics on a geographic basis will be discussed further in a later section.

5.3 INTER-BIOME TRANSPORT VIA THE ATMOSPHERE 

The foregoing discussion included processes involved in transport at all scales: at the local ecosystem level, along soil catenas, within watersheds, over landscapes, in regional land systems, and on a global scale (Woodmansee and Adamsen, 1981). One of the conclusions of this survey is that mobilization is critical for transport at any scale. The same processes that mobilize for small-scale (intra-ecosystem) transport are necessary for long range transport. On the other hand, high rates of mobilization within an ecosystem are necessary but not sufficient for long range transport at the regional to global scale. There may be cases in which the system having the highest mobilization rate may also have the highest deposition rate. For example, the generation rate of suspended aerosols may be highest in tropical forests, but deposition velocities for these aerosols may be highest there also. Because mobilization is high in an ecosystem system, it does not mean that deposition, as well, may not be high there.

Table 5.3 Rates of dry deposition of SO₂, NO₂, and HNO₃ on vegetation (from Fowler, 1980). Reproduced by permission of Norwegian SNSF Project

	Atmospheric		Canopy resistance ‡			Deposition velocity*
	resistance*†		rc(s m-1)			Vg (mm s-1)
Vegetation height (m)	r(a + b)(s m-1) SO2, NO2, HNO3	Surface condition	SO2	HNO3	NO2	SO2	HNO3	NO2
0.1	90	Stomata open	100	100	200	5	5	3
		Stomata closed	300	200300	400600	3	4	2
		Surface senescent	500	300500	500+	2	2	1
		Wet	0	0	?	11	11	?
1.0	35	Stomata open	100	100	200	8	8	4
		Stomata closed	300	200300	400600	3	3	2
		Surface senescent	500	300500	500+	2	2	1
		Wet	0	0	?	28	28	?
10.0	5	Stomata open	100§	100	200	10	10	5
		Stomata closed	400500	300500	400-600	2	3	2
		Wet	0	0	?	200||	200	?
*r(a + b) and Vg reference height 1 m above surface at which a wind speed of 2.5 m s-1 is assumed.
†Small differences in the component rb due to different molecular diffusion coefficients for SO2, NO2, HNO3 are ignored.
‡rc values for 32SO2 assumed similar or slightly smaller for HNO3 values for NO2.
§Assuming Rc SO2:rc H2O is the same as ratio of molecular diffusion coefficients D SO2:D H2O (2) and taking a value for rc with stomata open for sitka spruce (Picea stichensis).
||Though very large, this value would rarely be found unless the water on the trees has not reached equilibrium with ambient SO2 concentration; in practice, this value would only be applicable for a very short time (minutes) unless there is a mechanism for removing S(IV) from solution and neutralizing the H+ generated by the solution and oxidation of SO2.

Figure 5.5 Variation of aerodynamic resistance with wind speed and vegetation height (from Fowler, 1980). Reproduced by permission of Norwegian SNSF Project

A second conclusion is an obvious one: that the most rapid, long distance transfers are via the very fluid atmosphere. Thus, mobilization processes that lead to production of gases or aerosols are most likely to lead to rapid, interregional transport.

The third general conclusion is that, whereas the greatest net transport of elements away from terrestrial systems may be via fluvial agencies, the return of material to the land from aquatic systems, particularly the sea, can be affected in the short term (<1 year) only through the intermediary of atmospheric transport. Thus, while rock weathering is a major, long term input to terrestrial systems, the atmosphere is the principal source of short term inputs for land as a whole. On a smaller spatial scale, fluvial transport from uplands to lowlands is very important, but on the larger spatial scale it is not important for transport to terrestrial systems.

The fourth general conclusion is that the influences of one terrestrial system on another as affected by material transfer must, in the short term, be mainly through atmospheric transport.

Given these generalizations, if we take a primarily terrestrial and short term focus on the role of transport in biogeochemical cycles, then it is logical to concentrate primarily on atmospheric transport processes. For the remainder of this section, I will review the general mobility of the major airborne compounds of C, N, P, and S. This review will set the scale at which geographic modelling of material transfers via the atmosphere is best attempted. For compounds that enter large atmospheric reservoirs and are long-lived, global mixing dictates global-scale modelling; for other compounds, regional modelling is more sensible (e.g., Rodhe, 1978). Direction and speed of atmospheric transport by global circulation patterns is a problem of climatology over long distances and time intervals and of meteorology over shorter space and time intervals. Deposition rates over these transport tracks depend on chemical transformation rates, precipitation rates, and the nature of the ground surface for dry deposition processes. Stochastic variation in all of these factors can, of course, affect deposition at any single place and time.

5.3.1. Residence Times and Travel Distances 

A. Carbon Compounds

Although carbon dioxide emissions vary in time and space, the carbon dioxide pool is so vast and well mixed that circulation is essentially global. The same is true for methane and unreactive, volatile hydrocarbons and essentially true for carbon monoxide (Table 5.4). Only C-containing particulates are deposited in reasonably short distances, so that the vectors of travel for these might be interesting.

The residence time of carbon dioxide was calculated from the reservoir size and the sum of inputs in the model developed by Bolin et al. (1979a). Residence time for dust came from Kempe (1979); all other values came from Freyer (1979).

B. Nitrogen Compounds

Nitrogen gas and nitrous oxide are essentially globally distributed due to the very large atmospheric turn-over times (i.e. reservoir content compared to inputs and output fluxes). NH₃, NO_x and HNO₃ all have residence times in the lower atmosphere of the order of only one day due to efficient transformation and removal processes. Nitrogen compounds in secondary aerosols (mainly NH₄⁺ and NO₃^-) have a residence time determined by that of the aerosol particles, which may vary from a few days to a few weeks depending on climatic conditions.

Table 5.4 Residence times and relative transport distances for major chemical species of carbon, nitrogen, phosphorus, and sulphur. Sources for residence times are described in the text. Relative transport distances are the same as those defined in Table 1

Compound		Residence time	Transport distance (km)
Carbon Compounds
CO2		Years	Global
CO		Weeks	Continental
CH4		Years	Global
Volatile Hydrocarbons
	Vapour phase	Variable	Global
	Particulate phase	Daysweeks	Regionalcontinental
Dust and spray particulates
	d > 1 µm	Days	Regional
	d < 1 µm	Weeks	Continental
Nitrogen Compounds
N2		2 x 107 years	Global
NH3		Days	Regional
N2O		< 2 x 102 years	Global
NOx (gas)		Days	Regional
NOx (solid)		Days	Regional
Organic particulate		Daysweeks	Regionalcontinental
Phosphorus Compounds
Dust and spray PO43- particulates
	d > 1 µm	Days	Regional
	d < 1 µm	Daysweeks	Regionalcontinental
Sulphur Compounds
SO2 (combined dry and wet)		Ca. 1 day	Mesoscaleregional
SO42- (combined dry and wet)		A few days	Regional
H2S (as H2S and converted to SO2, SO42-		Days	Regional
(CH3)2S (converted to SO2, SO42-)		Days	Regional
COS		> 1 year	Global
Dust and spray particulates
	d > 1 µm	Days	Regional
	d < 1 µm	Daysweeks	Regionalcontinental

Residence times for all N compounds were taken from Svensson and Söderlund (1976) or calculated from their data.

C. Phosphorus Compounds

Atmospheric transfer of P is an important part of the global phosphorus budget, although small quantities of matter are involved. Atmospheric deposition may be a critical input to very oligotrophic terrestrial and fresh-water ecosystems, where mineral weathering provides little P. No stable or abundant gases of P are known; Particulate P is the only important form. A global budget for atmospheric P, prepared by Graham and Duce (1979) and reviewed in Duce, Chapter 16, this volume, gives valuable perspectives on atmospheric transport of P. About 4.6 T g/yr are deposited from the atmosphere to earth, 70% falling on land. About 83% of this deposition is estimated to be of crustal origin (mostly dust from arid regions) and 7% from sea-salt particles. The remainder is anthropogenic, with the largest single fraction of that being derived from soil dust raised by man's activities (2850%). Travel distances calculated in Table 5.4 are divided in terms of particle size following the approach of Kempe (1979). Travel time and distances for particulates are very sensitive to size. If combustion-derived phosphate leads to small particles sizes, then such particles are carried long distances and probably highly diffused over vast areas. More interesting, from a modelling point of view, are the trajectories of coarser particles derived, mostly, from soil dust.

D. Sulphur Compounds

The behaviour and deposition of S compounds have been well studied due to the large volume of anthropogenic S emissions and their ecological importance. The regional transport of S has been estimated with considerable success by the OECD program LRTAP (Doveland and Semb, 1980) and others in Europe. Other programs for mapping atmospheric transport in North America are described by Pack et al. (1978), Perhac (1978), Wilson (1978), and Whelpdale (1978). Sulphur has been, perhaps, an ideal case for mapping trajectories of emissions due to the relatively short transit distances of some of the compounds but mostly because of the localized sources. `Hot spots' of intense anthropogenic emissions generate 60% of global S from 1% of the earth's surface (Husar and Husar, 1978). The approaches of these programs in modelling regional transfers (Figure 5.6) and global transfers (Figure 5.7) serve as examples for the modelling of other substances.

Except for the case of carbonyl sulphide (COS), residence times, and thus transit times, of S compounds are relatively short, so that intraregional transport would probably be the most reasonable scale for modelling. Residence times for sulphur dioxide and sulphate came from Rodhe (1978) and thus represent conditions in Northern Europe. Residence time for hydrogen sulphide was approximated from a variety of sources (Granat et al., 1976; Rodhe, 1978; Georgii, 1978). Residence time for dimethyl sulphide was extrapolated from Granat et al. (1976) for sulphur dioxide and sulphate together, with the assumptions of a slight dry deposition of dimethyl sulphide itself and a relatively slow conversion rate to sulphur dioxide. Data on carbonyl sulphide came from Graedel et al. (1981). Estimates for dust and spray particulates followed the approach of Kempe (1979).

Figure 5.6 Emissions puff advection and diffusion scheme used in EURMAP (from Johnson et al., 1978) Reproduced by permission of Pergamon Press Ltd.

5.4 SUMMARIZING CONCLUSIONS

1. Transport, as a part of biogeochemical cycles, can be effectively analysed in terms of three phases: mobilization, transport, and deposition. Each phase involves separate processes.
2. Collectively, all the processes involved in mobilization, transport, and deposition embrace a very broad spectrum of the natural sciences, necessitating a multidisciplinary approach to analysis. These processes can be organized into classes more amenable to generalization; but transport, in the broad sense, is not easily reduced to a few generalizations or principles.
3. Transport occurs on a continuum of scale in terms of space and time.
4. The same mobilization processes may lead to either short or long range transport, but processes leading to formation of gases (e.g. methanogenesis) are more likely to ensure long range transport.
5. Chemical interactions in the transport of C, N, P, and S are most pronounced in four areas:

1. Biological mechanisms of mobilization.
2. Mobilization processes engendered by human activity, especially those powered by fossil hydrocarbons.
3. Biological mechanisms of deposition.
4. Influences of anthropogenically derived acids.

6. The behaviour of P is remarkably conservative in all aspects of transport compared wtih C, N, and S. This can give P a special regulatory role in biological processes.
7. Mobilization and deposition rates may both be high for the same substance in the same system. High rates of mobilization are necessary but not sufficient for net flux from one system to another.
8. Atmospheric transport is of special importance for 

1. Rapid, long distance transport.
2. Deposition to terrestrial systems. 
3.  Land-to-land transfers and effects. 

9.  Fluvial transport is of special importance for

1. Massive land-to-sea transport. (Sea-to-land transport occurs only through the intermediary atmosphere.)
2. Involvement of huge pools.
3. Relatively slow transport rates.
4. Slow time scale and long term regulator role.

10. Biomes or other biologically defined units are not necessarily the best units for defining source and sink regions for material transfer. Furthermore, air mass trajectories, drainage systems, and ocean currents do not usually conform to ecological patterns of gradients or boundaries. Different geographic criteria are necessary for different substances and modes of transport for resolution below the land, sea, atmosphere scale of definition.

Figure 5.7 Global-scale mapping of SO₄^2-, the long-lived product of H₂S emissions (upper), and combined H₂S and SO₂ emissions (lower). The upper panel is a calculated distribution of SO₄^2- for July resulting from a release of 40 Tg H₂S S/year (pptv), more or less representing background emissions to the atmosphere. In the lower panel 80 Tg anthropogenic SO₂ S/year (pptv) for July has been added to the background H₂S. Numbers in the circles give percentage contributions of man-made sources to local concentrations of SO₄^2- at different heights and latitudes (from Rodhe and Isaksen, 1980) Reproduced by permission of American Geophysical Union.

ACKNOWLEDGEMENTS

5.5 REFERENCES

Birkeland, P. W. (1974) Pedology, Weathering, and Geomorphological Research, New York, Oxford University Press.

Bolin, B., Degens, E. T., Duvigneaud, P., and Kemp, S. (1979a) 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, Chichester, Wiley, 1-53.

Branson, F. A., Gifford, G. E., Renard, K. G., and Hadley, R. F. (1981) Rangeland Hydrology, 2nd Edn, Range Science Series No. 1, Dubuque, Iowa, Kendall/ Hunt Publishing Co.

Butcher, S. S., and Charlson, R. J. (1972) Introduction to Air Chemistry, New York, Academic Press.

Cronan, C. S., and Schofield, C. L. (1979) Aluminum leaching response to acid precipitation: Effects on high-elevation watersheds in the northeast, Science, 204, 304-306.

Crutzen, P. J., Heidt, L. E., Krasnec, J. P., Pollack, W. H., and Seiler, W. (1979) Biomass burning as a source of atmospheric gases CO, H₂, N₂O, NO, CH₃Cl, and COS, Nature, 282, 253-256.

Dana, M. T. (1980) SO₂ versus sulfate wet deposition in the eastern United States, J. Geophys. Res., 5, 4475-4480.

Duce, R. A. Biogeochemical cycles and air/sea exchange of aerosols, Chapter 16, this volume.

Fish, B. R. (1972) Electrical generation of natural aerosols from vegetation, Science, 175, 1239-1240.,

Fletcher, J. E., Sorensen, D. L., and Porcella, D. F. (1978) Erosional transfer of nitrogen in desert ecosystems, in West, N.E., and Skujins, J. (eds) Nitrogen in Desert Ecosystems, Stroudsburg, Pennsylvania, Dowden, Hutchinson & Ross, Inc., 171-181.

Fowler, D. (1980) Removal of sulphur and nitrogen compounds from the atmosphere in rain and by dry deposition, in Drabløs, D., and Tollan, A. (eds) Ecological Impact of Acid Precipitation, Proceedings of an International Conference, Sandefjord, Norway, March 11-14, 1980, Oslo, SNSF, 22-32.

Freyer, H. D. (1979) Atmospheric cycles of trace gases containing carbon, in Bolin, B., Degens, E. T., Kempe, S., and Ketner, P. (eds) The Global Carbon Cycle, SCOPE Report No. 13, Chichester, Wiley,101-128.

Garrels, R. M., Mackenzie, F. T., and Hunt, C. (1975) Chemical Cycles and the Global Environment, Los Altos, California, William Kaufmann, Inc.

Gibbs, R. J. (1967) The geochemistry of the Amazon River system. Part I. The factors that control salinity and the composition and concentration of the suspended solids, Geol. Soc. Am., 78, 1203-1232.

Gorham, E., and McFee, W. W. (1980) Effects of acid deposition upon outputs from terrestrial to aquatic ecosystems, in Hutchinson, T. C., and Havas, M. (eds) Effects of Acid Precipitation on Terrestrial Ecosystems, New York, Plenum Press, 465-480. 

Gorham, E., Vitousek, P. M., and Reiners, W. A. (1979) The regulation of chemical budgets over the course of terrestrial ecosystem succession. Ann. Rev. Ecol. Syst., 10, 53-88

Graham, W. F., and Duce, R. A. (1979) Atmospheric pathways of the phosphorus cycle, Geochim. Cosmochim. Acta, 43,1195-1208.

Granat, L., Hallberg, R. O., and Rodhe, H. (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.

Grennfelt, P., Bengston, C., and Skarby, L. (1980) Estimation of the atmospheric input of acidifying substances to a forest ecosystem, in Hutchinson, T. C., and Havas, M. (eds) Effects of Acid Precipitation on Terrestrial Ecosystems. NATO Conference Series I, Ecology, New York, Plenum Press, 29-40.

Husar, R., and Husar, J. (1978) Forward, Atmos. Environ., 12, 3-5.

Idso, S. B. (1976) Dust storms, Sci. Am., 235(4),108-114.

Jenny, H. (1980) The Soil Resource. Origin and Behavior. New York, Springer-Verlag. 

Johnson, N. M., Likens, G. E., Bormann, F. H., and Pierce, R. S. (1968) Rate of chemical weathering of silicate minerals in New Hampshire, Geochim. Cosmochim. Acta, 32, 531-545.

Johnson, N. M., Driscoll, C. T., Eaton, J. S., Likens, G. E., and McDowell, W. H. (1981) `Acid rain', dissolved aluminum and chemical weathering at the Hubbard Brook Experimental Forest, New Hampshire, Geochim. Cosmochim. Acta, 45,1421-1437.

Jørgensen, B. Processes at the sediment-water interface, Chapter 18, this volume. 

Junge, C. E. (1958) The distribution of ammonia and nitrate in rain water over the United States, Trans. Am. Geophys. Union, 39, 241-248.

Kempe, S. (1979) Carbon in the rock cycle, in Bolin, B., Degens, E. T., Kempe, S., and Ketner, P. (eds) The Global Carbon Cycle, SCOPE Report No. 13, Chichester, Wiley, 343-378.

Kilmer, V. J. (1979) Minerals and agriculture, in Trudinger, P. A., and Swaine, D. J. (eds) Biogeochemical Cycling of Mineral forming Elements, Amsterdam, Elsevier Scientific Publishing Co., 515-558.

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

Loughnan, F. C. (1969) Chemical Weathering of the Silicate Minerals, New York, American Elsevier.

Marshall, J. K. (1973) Drought, land use and soil erosion, in Lovett, J. V. (ed.) The Environmental, Economic, and Social Significance of Drought, Sydney, Angus and Robertson, Publishers, 55-77.

Morales, C. (ed.) (1979) Saharan Dust. Mobilization, Transport, Deposition, SCOPE Report No. 14 Chichester, Wiley.

Nye, P. H., and Tinker, P. B. (1977) Solute Movement in the Soilroot System, Berkeley and Los Angeles, California, University of California Press.

Perhac, R. M. (1978) Sulfate regional experiment in northeastern United States: The SURE program, Atmos. Environ., 12, 641-647.

Nitrogen, Phosphorus and SulphurGlobal  Cycles, SCOPE Report No. 7, Ecol. Bull (Stockholm),

Rahn, K. A., Borys, R. D., Shaw, G. E., Schutz, L., and Jaenicke, R. (1979) Long-range impact of desert aerosol on atmospheric chemistry: Two examples, in Morales, C. (ed.) Saharan Dust. Mobilization, Transport, Deposition, SCOPE Report No. 14, Chichester Wiley, 243-266.

Redfield, A. C. (1958) The biological control of chemical factors in the environment, Am. Sci., 46,205-221.

Reynolds, R. C., and Johnson, N. M. (1972) Chemical weathering in the temperate glacial environment of the northern Cascade Mountains, Geochim. Cosmochim. Acta, 36, 537-554.

Richey, J. E. The phosphorus cycle, Chapter 2, this volume.

Rodhe, H. (1978) Budgets and turnover times of atmospheric sulfur compounds, Atmos Environ., 12, 671-680.

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

Slinn, W. G. N. (1976) Dry deposition and resuspension of aerosol particlesa new look at some old problems, in Englemann, R. J., and Sehmel, G. A. (coordinators) AtmosphericSurface Exchange of Particulate and Gaseous Pollutants (1974), CONF-740921, Tech. Information Center, ERDA, Springfield, Virginia, 1-40.

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.

Strakhov, N. M. (1967) Principles of Lithogenesis, Vol. 1, Edinburgh, Oliver and Boyd, (English translation).

Study of Man's Impact on Climate (SMIC) (1971) Inadvertent Climate Modification, Cambridge, Mass. M.I.T. Press.

Nitrogen, Phosphorus and SulphurGlobal Cycles, SCOPE Report No. 7, Ecol. Bull. (Stockholm),

Swanson, J. F., Fredricksen, R. L., and McCorison, F. M. (1981) Material transfer in a western Oregon forested watershed, in Coniferous Forest Biome Synthesis Volume, Stroudsburg, Pennsylvania, Dowden, Hutchinson and Ross (in press).

Turk, J. T., and Peters, N. E. (1978) Acid-rain weathering of a metasedimentary rock basin, Herkimer County, New York, in Izard, H. H., and Jacobson, J. S. (eds) Scientific Papers from the Public Meeting on Acid Precipitation, May 4-5, 1978, Lake Placid, New York, Science and Technology Staff, New York State Assembly, Albany, New York, 136-145.

Vitousek, P. M., Gosz, J. R., Grier, C. C., Melillo, J. M., Reiners, W. A., and Todd, R. L. (1979) Nitrate losses from disturbed ecosystems, Science, 204, 469-474.

Whelpdale, D. M. (1978) Large-scale atmospheric studies in Canada, Atmos. Environ., 12, 661-670.

Wiley, M. (1977) Estuarine Proceses, Vol. 2, Circulation, Sediments, and Transfer of Material in the Estuary, New York, Academic Press.

Wollast, R. Interactions in estuaries and coastal waters, Chapter 14, this volume.

Woodmansee, R. G., and Adamsen, F. J. (1981) Biogeochemical cycles and ecological hierarchies, in Todd, R. L. (ed.) Nutrient Cycling in Agricultural Ecosystems, Ann Arbor, Michigan, Ann Arbor Science Publishers, Inc. (in press).

Woodmansee, R. G., and Wallach, L. S. (1981) Effects of fire regimes on biogeochemical cycles, in Clark, F. E., and Rosswall, T. (eds) Terrestrial Nitrogen Cycles, Ecol. Bull. (Stockholm), 33, 649-669.