SCOPE 13 - The Global Carbon Cycle

ABSTRACT

A general survey is made on various terrestrial ecosystems with respect to carbon flow in and out of the individual system. Emphasis is placed on: (i) tropical, (ii) temperate, and (iii) boreal forests. It can be shown that modern forestry and agriculture techniques significantly reduce the standing crop of biomass in the world's forests; furthermore, oxidation of soil organic matter is accelerated inasmuch as this reduction is not matched by an increase in the rate of photosynthesis in, for example, virgin forests; thus the CO₂ released by man's `cultivating' activities should ultimately end up in the atmosphere or the ocean.

6.1 INTRODUCTION

Due to population explosion and industrialization, man is exploiting the natural resources of earth. There is a risk of man altering the equilibrium in nature due to, for example, the increasing combustion of fossil fuels.

Man's influence on the temperate forest has been far-reaching. These forests still constitute an important resource for human use. Several of the world's most densely populated areas are located within the temperate forest zone, for example large parts of Europe and the U.S.S.R., large parts of the United States and Canada, as well as much of Japan and China. In these densely populated areas, much of the land has now been transformed from forests into various manmade ecosystems. Yet there are vast areas of forest still in existence, for example in Eurasia, in North America, and also in southern Chile, Tasmania, and New Zealand.

Until a century ago, man used the forests of the temperate zone primarily for fuel and building material. Many forests were also grazed. Even though ancient civilization first developed in some subtropical and Mediterranean areas, there is evidence that man already exerted a strong influence on temperate forests several thousand years ago, during the stone age. Many of the forests in Europe were, for instance, subject to shifting cultivation during the late stone age. In addition to direct cultivation, land was cleared in order to increase pastureland. One of the consequences of this land use had been the formation of extensive heathlands in many of the countries around the North Sea, as well as in western France. An analogous development formed the `maquis' and `garrigue' of the Mediterranean countries (Tamm, personal communication).

In many areas, man took advantage of the forests' ability to store large amounts of organic material and nutrients in their biomass and litter layer. Litter was often used as a kind of manure, usually mixed with dung from stables. This was usual in large parts of central Europe. All these activities resulted in an increased liberation of CO₂ from the forests.

During this century, man has begun to exploit other parts of the world, for example the tropical rain forests: These forests are large pools of carbon, and an altered equilibrium here leads to an accelerated flow of carbon into the atmosphere and a decreased assimilation of CO₂ by the rain forests. Most of the nutrients in a tropical rain forest are bound up in the biomass; cutting and utilization of wood from this ecosystem will cause severe impoverishment of the soil. Mires and other peatlands are normally large sinks for carbon. An increased utilization of peat for fuel and soil improvement, as well as draining of bogs, can have severe ecological consequences. New agricultural methods can also alter the pool of carbon in the soil and increase the CO₂ liberation into the atmosphere. Combustion of garbage and sludge will have negative effects on the carbon balance, inasmuch as most of the carbon will be converted to CO₂.

6.2 POOLS OF CARBON IN THE TERRESTRIAL ECOSYSTEM 

6.2.1 Vegetation 

There have been many attempts to estimate the net primary production for the world. Whittaker and Likens (1975) estimate the net primary production for land biota at 105 x 10¹⁵ g dw*/year (Table 6.1). This corresponds to 47.9 x 10¹⁵g C. The estimations for sea production are about 55 x 10¹⁵ g dw/year. To obtain the values for carbon in Table 6.1, the values for dry matter were multiplied by 0.45. The values are net production, i.e. the amount of organic matter remaining after respiration by the photosynthesizing plants.

Gross primary production is about twice the net primary production for the biosphere as a whole. The fraction of the gross productivity respired by plants can be as high as 5075% in many forests, especially in the Tropics. In many other ecosystems, especially in the sea, respiration amounts to 2040% of the gross production (Whittaker and Likens, 1975).

Lieth (1975) has estimated net primary production in terrestrial ecosystems at 100.2 x 10¹⁵g dw. In Baes et al. (1976) a lower value is given for terrestrial net primary production,about56 x 10¹⁵g/year. Bazilevich et al. (1971) have estimated the total potential phytomass of land at 2.4 x 10¹⁸ g dw. The bulk of this organic mass is in the tropical zone (56%), followed by the boreal (18%), subtropical (14%), sub-boreal (12%), and polar (1%) zones. The majority of phytomass is concentrated in forests (82%). For example, in moist tropical forests, according to Rodin and Bazilevich (1964), the accumulation is approximately 5.0 x 10¹² g/m² or more. The values for subtropical forests and broadleaved forests of the temperate zone are about 0.370.41 x 10¹² g/m². The biomass in northern spruceforests in the taiga regions amounts to about 1.0 x 10¹² g/m². In tropical forests the assimilating parts of the plants account for a large amount, about 0.4 x 10⁸ g/ha or 8% of the biomass. In subtropical forests, the figure is 1.2 x 10⁷ g/ha or 3% of the biomass. The broadleaved forests of the temperate zone have about 0.40.5 x 10⁷ g/ha, about 1% in the assimilating parts, while the amounts for the temperate coniferous forests are about 0.81.7 x 10⁷ g/ha or 58% of the biomass (Rodin and Bazilevich, 1964).

Table 6.1 Primary production and biomass estimates for the biosphere (after Whittaker and Likens, 1975)

The total potential primary production of land is estimated to be 172 x 10¹⁵ g/year. The tropical belt produces 60% of this total, subtropical 20%, sub-boreal 10%, boreal 9%, and polar 0.8%. Forests produce 49% of primary production (Bazilevich et al., 1971). Total phytomass in the world's oceans amounts to 0.17 x 10⁵ g, which is about 15 000 times lower than that of the land, although the productivity is much higher. The total primary production of the oceans is estimated at 4772 x 10¹⁵g/year (Steeman Nielsen et al., 1957; Koblentz-Mishke et al., 1968; Bogorov, 1969). Liebig (1862) made an estimation of what world production would be if the world's surface were covered by a moderately productive meadow (500 g dry matter/m² per year). The data on which the other early estimations were based was too limited for them to be relevant (Whittaker and Likens, 1975). For more detailed discussion on global primary productivity see Ajtay et al. (Chapter 5, this volume).

6.2.2 Fauna

Only very small fractions of all the terrestrial carbon pools are represented by animals. Whittaker and Likens (1975) estimate the total biomass of animals in the world at 906 x 10¹² g C. About 457 x 10¹² g C is bound in continental animals. The annual animal production is about 372 x 10¹² g C for continental fauna and about 138 x 10¹² g C for marine animals. The total herbivore consumption in the world is about 124 x 10¹² g C/year. The continental animals consume about 32.6 x 10¹² g C/year (Whittaker and Likens, 1975). Data on efficiencies of food-use by invertebrate animals is limited and much data on vertebrates is of doubtful applicability to populations under field conditions. Estimations give the somewhat surprising result that the secondary production is less than 1% of the net primary production on land, and 56% in the sea. In most terrestrial ecosystems, animal biomass is concentrated in invertebrates rather than in vertebrates. Vertebrate biomass may, however, exceed invertebrate biomass in grasslands, due to populations of large grazing mammals (Whittaker and Likens, 1975).

6.2.3 Microorganisms

In order to estimate the amount of carbon bound in microorganisms, a mean value of 100 g/m² (dry weight) for microbial biomass was used in most soils (Rosswall, 1976). Microorganisms have a carbon content of about 50% (Rosswall, personal communication). If we approximate the total microbiologically active area of the world at 135 x 10¹² m², we can then estimate the total amount of carbon bound in microbial biomass at 6.8 x 10¹⁵ g C.

6.2.4 Soil Organic Matter

The decay of soil organic matter is one of the largest CO₂ inputs to the atmosphere. The mass of organic carbon in the world's soils, which is often quoted, is based on the carbon contents of nine U.S.A soils (Twenhofel, 1926). An extrapolation of these values for the whole world's soils give 710 x 10¹⁵ g C (Rubey, 1951). More recent and more extensive data, such as the FAOUNESCO world soil map, should give a more accurate estimation of the organic carbon in the world's soils. These FAOUNESCO values indicate that the amount of soil organic carbon in the world is about 3000 x 10¹⁵ g (Bohn, 1976). Bohn has calculated organic carbon content based on a soil depth of 1 m. This will, in some instances, give an overestimation, but Bohn argues that many soils are much deeper than 1 m, and this is a realistic mean value. The largest quantity of litter and surface humus is found in shrubby tundras (0.84 x 10¹² g/m²) followed by spruce forests in central, southern, and northern taiga (0.45 x 10¹², 0.45 x 10¹² , and 0.30 x 10¹² g/m² respectively). Broadleaved forests have about 0.15 x 10¹² g/ha, closely followed by subtropical forests with 0.10 x 10¹² g/ha. The steppes often have about 0.062 x 10¹² g/m². In the tropical rain forests, the amount of litter and surface humus is very small, only about 0.020 x 10¹² g/m². In savannas it is even smaller, about 0.013 x 10¹² g/m² (Rodin and Brazilevich, 1964). Bohn (1976) has estimated the total organic carbon content in soils in South America at 301 x 10¹⁵ g C, in North America at 665 x 10¹⁵ g C, and in the rest of the world at 1980 x 10¹⁵ g C.

6.3 CHANGES INDUCED BY MAN

6.3.1 Forests

A. The Effects of Forestry on the Carbon Cycle

In most countries in the world, the total wood production has increased due, for example, to the use of fertilizers, draining of swamps, and rationalization. The estimates of the total global area of forests are, however, somewhat uncertain. Only 46% of the world's forests are confirmed by reliable values. The estimates are insufficient for 33% of the world's forest areas (Persson,1974).

The world's total area of forests has been estimated at 2.8 x l0⁷ km². This corresponds to 22% of the global land area. Coniferous forests have been estimated at about 40% of the total forest area (Table 6.2) (Persson, 1974; Skogsstyrelsen, 1976). There are about 300 x 10⁹ m³ of wood in the world's forests with a diameter (1 m above ground) of more than 2030 cm for deciduous trees and 510 cm for coniferous trees. This would correspond to about 37.5 x 10¹⁵ g C. However, it is to be noted that the calculated amount of carbon in the tree-stems only constitutes a minor part of the total carbon pool in the forest ecosystem. The part of the carbon pool which is removed from the ecosystem when cutting the trees, will probably sooner or later be mineralized or burnt.

Table 6.2 The amount of stem wood in the forests of the world and forestry cutting (from Skogsstyrelsen, 1976)

Figure 6.1 gives the changes in volume of cutting between 1950 and 1973 (roundwood). During this period cutting increased by about 80%. The increase varies in different parts of the world and is most marked in Asia at little over 200%, but is only about 2025% in Europe and North America. The above increase in cutting can mainly be explained by the fact that roundwood cutting in several countries in the Far East is now much better reported.

The annual cuttings of coniferous and deciduous forests are shown in Table 6.3. According to Table 6.2 the share of industrial wood in North America was approximately 96%, while in Africa and Asia (except U.S.S.R) it was only 14% and 29%, respectively. Most of the wood in Africa and Asia is used for fuel and will thus rapidly be converted in CO₂.

The cutting of fuel wood has decreased in industrialized countries, while it remains almost unchanged in Africa and increased considerable in Asia (except U.S.S.R.) (Figure 6.1(b)). This is an average which is not relevant for Japan, since it is a highly industrialized country (Skogsstyrelsen, 1976). The increased cutting of fuel wood would result in a global increase in fuel wood consumption from 1025 x 10⁶ m3 in 1961 to about 1120 x 10⁶ m³ in 1970. This means an increase for the type of wood which is rapidly converted to CO₂. At the same time, the amounts of wood used for pulp and paper production have rapidly increased (Figure 6.1(c)). Thus, an increasing amount of the organic matter in the world's forests is converted to CO₂ rather rapidly. However, the timber and pulp production fluctuates over the years due to the state of the market. Thus, the production of pulp and paper in the major producing countries fell by 13% in 1975 (Anon., 1976).

Figure 6.1 The global cutting (a), cutting of fuel wood (b), and cutting of pulp wood (c), all figures in 10⁶ m³ solid volume under bark (after Skogsstyrelsen, 1976. Reproduced by permission of the National Board of Forestry, Sweden.)

Table 6.3 Removals in certain countries with extensive logging in 1973. (Skogsstyrelsen, 1976)

	Coniferous			Broadleaved
	industrial			industrial			Total	Total
	wood	fuel wood	total	wood	fuel wood	total	1973	1972
Country	million m3 solid volume under bark
Argentina *	0.6		0.6	2.5	8.0	10.5	11.5	12.7
Australia	2.7		2.7	8.7	0.7	9.4	12.1	14.1
Austria	9.6	0.29	9.8	1.4	0.7	2.1	11.9	12.5
Brazil*	11.1	15.0	26.1	12.7	125.0	137.7	163.8	163.8
Canada*	111.4	1.2	112.6	9.4	2.2	11.6	124.2	119.7
China*	28.5	54.0	82.5	18.5	82.2	100.7	183.2	179.0
Columbia*	0.0		0.0	4.9	20.0	24.9	24.9	26.8
Czechoslovakia	10.6	0.7	11.3	3.0	0.8	3.8	15.1	14.7
East Germany	7.2	0.8	8.0	1.4		1.4	9.4	7.9
Ethiopia*	0.2	2.8	3.0	1.1	20.2	21.3	24.3	24.2
Finland*	29.9	1.4	31.3	5.7	5.9	11.6	42.9	42.9
France*	13.7	1.0	14.7	4.8	4.8	19.2	33.9	33.9
Great Britain	1.6	0.2	1.8	0.8	0.1*	0.9	2.7	2.5
India*	1.4	3.2	4.6	9.2	102.8	112.0	116.6	116.6
Indonesia*	0.1		0.1	27.1	104.0	131.1	131.2	120.0
Italy	1.1*	0.3*	1.4*	5.0	5.3	10.3	11.7	13.1
Japan*	25.6	0.0	25.6	17.5	1.5	19.0	44.6	46.2
Mexico	5.0	2.6	7.6	0.5	6.0	6.5	14.1	14.2
New Zealand*	8.0	0.5	8.5	0.2	0.1	0.3	8.8	8.8
Nigeria*				3.0	56.8	59.8	59.8	59.8
Norway	7.5	0.2	7.7	0.4	0.5*	0.9	8.6	8.3
The Philippines*	0.0		0.0	13.8	21.1	34.9	34.9	33.1
Poland	15.6	0.9	16.5	4.5	0.9	5.4	21.9	18.8
Portugal	4.9	0.3	5.2	2.0	0.2	2.2	7.4	7.0
Romania	6.5	0.4	6.9	9.6	4.9	14.5	21.4	21.2
South Africa	4.4	0.1*	4.5	5.1	0.9*	6.0	10.5	9.6
Spain	5.6	2.1	7.7	3.2	5.9*	9.1	16.8	16.0
Sweden	50.5	1.2	51.7	4.4	1.9	6.3	58.0	58.0
Tanzania*	0.1	0.0	0.1	1.1	31.5	32.6	32.7	32.7
U.S.A.*	269.7	2.5	272.2	72.5	10.9	83.4	355.6	355.7
U.S.S.R.*	263.5	55.5	319.0	34.1	29.9	64.0	383.0	383.0
Venezuela*				0.5	6.9	7.4	7.4	7.4
West Germany	22.6	0.8	23.4	6.1	1.1	7.2	30.6	23.8
Yugoslavia	4.1	0.0	4.1	5.3	3.8	9.1	13.2	13.2
Zaire*				1.9	12.8	14.7	14.7	14.7
WORLD 1973	949.7	174.1	1123.8	401.5	972.2	1373.7	2497.5
1972	935.3	172.9	1108.2	378.0	967.4	1345.4		2453.6
*Estimations made by the FAO secretariat.

Bolin (1977) estimates the rate of deforestation and fuel wood production to about 1.1 x 10¹⁵ g/year. However, there is an increase in biomass due to reforestation of about 0.3 x 10¹⁵ g/year, which will, to some extent, counteract the increased CO₂ output to the atmosphere. Due, for example, to fertilization, the wood production has increased during the last few years. In Finland, for example, forest fertilization is at present (1976) carried out on well over 200 000 ha annually. The effects of the synthetic fertilizers on accumulation of carbon in the world's forests can, however, be discussed. The increased growth of the trees will lead to an earlier cutting. Thus, the real accumulation will not increase to any larger extent. Nitrogenous fertilizers also result in an increased mineralization of the organic matter in the soil, which decreases the carbon pool in the soil. However, if organic fertilizers (e.g. compost or sewage sludge) are used, this will make up for the loss of organic material by supplying humus. This will, for example, increase the water- and nutrient-holding capacity (Bramryd, 1976). Also, in subarctic forests the soil conditions and nutrient status are made more favourable. This can probably lead to more rapid productivity in these forests (Bramryd in manuscript).

In many parts of the world, the forest areas have increased due to drainage of mires. In Finland alone, almost 300 000 ha of mires are drained every year. For example, between 1967 and 1973 the forest area in Finland increased by 0.5 x 10⁶ ha, owing to swamp drainage and reforestation of farming lands. Forest land in Finland now covers over 19.2 x 10⁶ ha or 63% of the total Finnish land area (Kuusela, 1976). Drainage of mires usually increases the decomposition of peat and thus, to some extent conteracts carbon accumulation in the trees (Holmen, personal communication). However, this means that the input of organic matter from the new forest would make up for this decomposition, The new litter layer could also prevent the peat layer from a further rapid degradation.

The forest plantations planned for energy extraction will increase the flow of carbon into the forests in a short-term view, but their total effects on carbon accumulation are hard to predict. At least the liberation of carbon from fossil fuels may decrease. One million tons of treedry substance could equal about 0.5 million tons of oil in thermal value (Kuusela, 1976). The wood is then, however, burned and the CO₂ returns to the atmosphere. The plans for whole-tree utilization including stumps, bark and other logging residues as alternative to fossil fuel, would markedly decrease the amount of organic material available for humus production. An increased use of fertilizers would be needed to compensate for nutrients, which are often bound up in needles and bark. Thus, this would also result in an increased energy demand for production of fertilizers.

B. Case Studies

Sweden: The total land area of Sweden is about 41 x 10⁴ km². Of this area, about 57% or 23.5 x 10⁴ km² are covered by productive forests. The forest covered area has increased by about 1015 x 10³ km² during the last few decades, mainly at the expense of agricultural area. The total wood supply in Swedish forests in 1970 was about 2360 x 10⁶ m³. If we estimate the density of wood at 0.5 g/cm³, the water content at about 50% and the content of carbon in dry wood at 50% (Nihlgård, 1972), we find that, in 1970, the pool of carbon in above-ground parts of standing trees in Swedish forests was about 590 x 10¹² g C. Since 192329, the supply of wood has increased by almost 700 x 10⁶ m³ (175 x 10¹² g C). About 67% of the increase derives from the southern parts of Sweden. Due to fertilization and modern forestry, the wood supply has increased from 7200 m³/km² in 192329, to 10 000 m³ /km² in 196872. The annual total growth has increased from 57 x 106 m3, to 77 x 10⁶ m³ during the same period (Jordbruksdepartementet, 1974; Skogsstyrelsen, 1976) (see Table 6.4).

Table 6.4 The increase of wood supply in Sweden (Skogsstatistisk årsbok, 1974, from Skogsstyrelsen, 1976)

		Annual growth 106	Annual growth
Part of Sweden	Year	m3 solid volume	m3 per ha
Norrland	19381952	24.5	1.8
	19531962	32.6	2.6
	19641968	33.0	2.5
	19681972	30.2	2.3
Svealand	19381952	19.3	3.7
	19531962	21.6	4.1
	19641968	20.2	3.7
	19681972	18.1	3.4
Götaland	19381952	19.1	4.3
	19531962	23.5	5.2
	19641968	24.0	5.0
	19681972	21.6	4.5
Total Sweden	19381952	62.9	2.7
	19531962	77.7	3.5
	19641968	77.2	3.3
	19681972	69.9	3.0

The volume of fellings in 1972/73 was 73.6 x 10⁶ m³. This was an increase of about 1.4 x 10⁶ m³ compared with previous years. The annual flow of carbon from the Swedish forests due to wood and pulp production is, therefore, about 18.4 x 10¹² g carbon (Table 6.4) (Skogsstyrelsen, 1976). Most of this wood was used for paper production (about 52%) and other products, which are used rather quickly and then turned into garbage (Table 6.5, Figure 6.2). Hence the organic matter in most timber is decomposed and turned into carbon dioxide within a few years. Only a small percentage is stored for centuries in furniture, buildings, and other construction works.

Prognoses from the Swedish State Department of Agriculture estimate that the draining and fertilization of mires and moist woodland growing on peat could increase the annual wood production by about1015 x 10⁵ m³ (Virkesbalansutredningen, 1968). This would correspond to an amount of about 2.53.8 x 10¹² g C/year.

Table 6.5 Consumed and exported quantities of roundwood in 1974 in Sweden (Skogsstyrelsen, 1976)

The total clearcut area in Sweden has not increased during the twentieth century. By clear cut area we mean all forest ground which is not sufficiently covered by trees or is only covered by seed plants. Forest fertilization means a speeding up of the production of organic matter in forests, but this is mostly compensated by more rapid felling and, hence, more rapid mineralization of the assimilated carbon. Fertilization also tends to stimulate mineralization of litter and organic matter in soil.

U.S.S.R.: The total forest area in the U.S.S.R. is about 7.65 x 10⁶ km² (5.53 x 10⁶ km² coniferous and 1.75 x 10⁶ km² broadleaved trees) (Persson, 1974).

The total supply of wood has been calculated at about 73.3 x 10⁹ m³ (Skogsstyrelsen, 1976). The pool of carbon in above-ground parts of standing trees in the U.S.S.R. is about 9.16 x 10¹⁵ g. The annual clearing in the U.S.S.R. in 1973 was about 0.38 x 10⁹ m³. This corresponds to an annual output of carbon from the Soviet forests of about 47.5 x 10¹² g. Of this, about 0.021 x 10⁹ m³ is pulp (2.6 x 10¹² g C) (Skogsstyrelsen,1976).

The production of roundwood in the U.S.S.R. has increased from about 0.37 x 10⁹ m³ in 1963 to 0.38 x 10⁹ m³ in 1973, and pulpwood from 0.019 x 10⁹ m³ to 0.035 x 10⁹ m³ during the same period. This is probably, in part, due to the increased use of fertilizers in U.S.S.R. Fuel wood and charcoal production, however, have decreased from 0.10 x 10⁹ m³ to 0.085 x 10⁹ m³ (FAO, 1975).

U.S.A. and Canada: The total forest area in the U.S.A. and Canada is about 6.30 x 10⁶ km². The total supply of wood has been calculated as 58.5 x 10⁹ m³ (Skogsstyrelsen, 1976). From this we can roughly estimate the total carbon pool in the trees in North America at 7.31 x 10¹⁵ g. The annual clearing in the U.S.A. and Canada was about 0.48 x 10⁹ m³ in 1973. About one-quarter of this is pulpwood (Skogsstyrelsen, 1976). The net loss of forest land for agricultural purposes amounted to 2000 km² per year between 1962 and 1970 (Spurr and Vaux, 1976).

Figure 6.2 Quantities of pulpwood, sawtimber, and fuel wood in Sweden from 1955/56 to 1974/75 (after Skogsstyrelsen, 1976. Reproduced by permission of the National Board of Forestry, Sweden.)

The tropics Most nutrients in the tropical rain forests are bound in the aboveground biomass (De las Salas and Fölster, 1976) (Table 6.6). Therefore, an increased exploitation of these forests will lead to the removal of nutrients, which, ultimately, can lead to impoverishment of the soil and a fundamental disturbance of the ecosystem. The most successful permanent crops in the tropics are those which make relatively small demands on the soil, for example, rubber, teak, and cocoa, because only a limited amount of the nutrients is removed from the ecosystem during harvest (Richards, 1973).

Tropical rain forests serve as large assimilators of CO₂, converting it into organic substances. A disruption of this process, together with the large CO₂ liberation resulting from burning and mineralization of products (wood, paper, etc.), can lead to severe consequences for the ecosystem. For Brazil, Adams et al. (1977) have estimated the annual consumption of wood per capita at about 3 x 10⁶ g C. At least 75% of the cutting is for firewood, while per capita use of other wood products is low. Only about 20% of the cuttings in the Sao Paulo area are replaced. The total reforestation is much lower and is estimated to be approximately 1.0 x 10⁶ g/year per capita (Adams et al., 1977).

Table 6.6 Total carbon and bioelement stores of the primary forest, and the percentage of their distribution between vegetation, organic layer and soil. Data obtained from Middle Magdalena Valley, Columbia, U.S.A. (De las Salas and Fölster, 1976)

In tropical regions, forests are often cut down and the trees are burnt to give new land for agriculture. The land is frequently left fallow after only two or three crops and a new patch of forest is cleared. In many tropical areas, the land is exploited again before fertility has recovered and this can lead to impoverishment of the soil. Tropical forest clearings, if not kept under continuous cultivation, soon become covered with weeds, shrubs, and young trees. Re-establishment of the primary forest might be possible, but this process will probably take centuries (Richards, 1973; Goodland and Irwin, 1974). This, however, is only possible if enough primary forest is left to serve as a refuge for the flora and fauna, which is seldom the case in Brazil, Indonesia, and many other tropical areas.

Among the different types of fallows in warm humid tropics, the early stages of a secondary forest are most capable of restoring the soil productivity potential. Trees have deep roots and therefore they can take up nutrients from deeper soil layers (Van Wambeke, 1974). After about 115 years, the original level of organic matter is reached in a rain forest. At later stages the older regrowths immobilize the nutrients almost exclusively in the woody parts (Laudelout, 1961).

Kellman (1969) has studied the effects of clearings for agricultural purposes in Mindanao in the Philippines. He found that the carbon content of the topsoil had decreased from 8-10% obtained under primary forest, to about 3% after being used for agriculture and then left fallow. In addition to accelerated decomposition of organic matter in soil, a loss of organic matter through erosion also occurs (Table 6.7) (Kellman, 1969).

Gosden (1956) showed that 60 g/m². of soil was lost per cm rainfall on a 10-year-old teak plantation in Trinidad, compared with 47 g/m² per cm lost from secondary evergreen forests (Beard, 1946). Many scientists recommend that burning and cultivation should be controlled or banned completely on high altitudes with steep slopes in order to prevent erosion (Cornforth, 1970). Thus, large amounts of soil organic matter will be transported from rain forests by the rivers to the sea (Walters 1971; Ferri, 1974; Lundgren, 1976). Similar erosion problems are found in tropical savannas exposed to human cultivation (Lundgren, 1976).

Table 6.7 Estimated annual losses of organic matter through erosion from various plots (Kellman, 1969)

Figure 6.3 Environmental disruption caused by deforestation (Goodland and Irwin, 1974. Reproduced by permission of the Elsevier Scientific Publishing Company.)

Figure 6.4 The relationship between deforestation and crop failure (Goodland and Irwin, 1974. Reproduced by permission of the Elsevier Scientific Publishing Company.)

Richards (1973) considers that the role of the tropical rain forest as converter of carbon dioxide to oxygen is overestimated, since the oxygen produced by forests probably does not account for more than that consumed by the organisms decomposing dead organic matter. Deforestation also leads to other ecological disruptions (Figures 6.3 and 6.4) (Goodland and Irwin, 1974). To prevent all tropical rain forests in Brazil from being felled, the principal governmental body responsible for conservation, IBDF (The Brazilian Institute for Forestry Development), has passed a law requiring colonists to preserve 50% of the forest on their lot. Sioli (1973) calculated that the Amazonian forest contains about 600 t of organic matter per ha, representing 30 x 10³ g C/m². Multiplied by 4 million km², this will give 120 x 10¹⁵ g C in the whole Amazonian forest, equivalent to approximately 15% of the atmospheric gaseous carbon (Goodland and Irwin, 1974). The dangers of an increased exploitation and burning of the Amazonian rain forest are thus apparent. 

6.3.2 Agriculture

The organic matter content greatly affects the soil's ability to produce high crop yields. It affects the supply of many essential plant nutrients, especially nitrogen, phosphorus and sulfur (Vitosh et al., 1974). In addition the water-holding capacity and soil structure are affected (Harris et al., 1966; Bramryd, 1976). Soil organic matter levels also regulate the effect of herbicides (Meggitt, 1974).

A field usually contains more organic matter than an adjacent pine plantation. This is, in most cases, due to higher microfloral and meio- and microfaunal populations, plus a more favourable moisture regime for the activity of these organisms in the forest than in the field (Fisher et al., 1975). Soils under cultivation lose about half their original organic matter through microbial oxidation to CO₂. The mixing of soil and the exposure of undecomposed organic material set up a new steady-state carbon concentration,` equal to about half the original state. However, Revelle (1957, 1966) estimated that during the last 100 years, the CO₂ release by cultivation is less than 5% of the CO₂ in the atmosphere and thus should be insignificant compared with the CO₂ coming from combustion of fossil fuels. In contrast, Deevey (1971) has suggested that this estimate is too low, and that the amount of CO₂ released by cultivation of new lands in the last 100 years is 10% or more of the CO₂ in the atmosphere.

The average organic content of the mineral soils of the world is estimated to be 14 000 g/m². This estimation includes noncultivable mountainous and desert soils (Bohn, personal communication). Revelle (1957, 1966) estimated that in 1850 the global area of agricultural cropland was about 9.3 x 10⁶ km². In 1970 this had increased to 14 x 10⁶ km² . However, the world's population during the same time had increased from 1.4 x 10⁹ to 3.5 x 10⁹ and, therefore, the cultivated area per capita had decreased from 8000 to 4000 m².

Revelle also assumes that, during the period 1170 to 1850, the net land area per capita increased linearly with time from 0 to 8000 m². Around the year 1170, humans are assumed to have lived for the first time under shifting cultivation. Because of the relatively small proportion of land under cultivation and the relative ease with which new land could be obtained, the amount of land under permanent cultivation was probably very small. The freedom to move to new soils diminished with time. This meant that more and more land was cultivated annually, or was in crop rotation.

Bohn (personal communication) estimates that the CO₂ released to the atmosphere, due to soil cultivation, from the year 1200 to the present day has meant a CO₂ evolution of about 1.6 x 10¹⁷ g C. Since 1870, about 0.95 x 10¹⁷ g C. have been emitted to the atmosphere due to cultivation of new lands. There was a marked increase in CO₂ evolution since 1930 probably due to technological advances in crop production.

The amount of organic matter in crop lands depends on the types of crops, methods of cultivation, amounts of fertilizers, etc. When virgin land on a sodpodzolic soil is ploughed, there is, at first, a period of intense decomposition of the organic matter, and after a period of 13 years of continuous fallow, the humus and nitrogen content has decreased by 43% (see Table 6.8). The process subsequently slows down probably because microorganisms have used up the most easily available organic matter. During a 32-year period, humus in the fallowed soil has decreased by only 9% (Kononova et al., 1966). In chernozems, however, much of the humus is lost, approximately 4560 t/ha, during a period of 1012 years of continuous fallowing. This constitutes about 1927% of the total amount of humus in the virgin land. Thus, the absolute amount of humus decomposed under continuous fallowing on ordinary chernozem soils considerably exceeded the amount decomposed under similar conditions in sodpodzolic soil (Lazarev, 1936). However, a comparison with the total reserve indicates that the decomposition of humus proceeds more economically in chernozem than in sodpodzolic soil (Kononova et al., 1966).

Table 6.8 Humus and nitrogen content of a sodpodzolic light loamy soil in the longterm experiment at the Timiryazev Academy (Kononova et al., 1966)

			Decrease in humus content
	Humus	Nitrogen	% of	% of
	% of	% of	soil	virgin
	soil	soil	weight	land	Author
Land	weight	weight
Virgin land	2.222.19	0.159
Continuous fallow
13 years
unmanured	1.27	0.091	0.95	43	Drachev (1927)
farmyard manure	2.14	0.125	0.08	9	Drachev (1927)
Continuous fallow
48 years
unmanured	1.05		l.14	52	Lykov(1961)
farmyard manure	1.62		0.57	26	Egorov (1961)
Continuous rye
48 years
unmanured	1.55		0.64	29	Egorov (1961)
farmyard manure	2.50		0.31	14	Egorov (1961)
Rotation
48 years
unmanured	l.57		0.62	28	Egorov (1961)

6.3.3 Fire

In addition to industrial combustion of fuel and waste, fire is often used in forestry and agricultural management. Furthermore, wild fires caused by the sun or by lightning can be of great significance. Fire used in forestry is an easy way of removing logging slash, However, this increases the leaching of nutrients from the soil into the groundwater, ultimately resulting in a less favourable soil structure. Due to modern mechanized forestry, alternative methods, where litter is left to natural mineralization, are more common nowadays. In addition to destruction of standing timber, fire often damages the forest soil which has been built up over hundreds of years. This can sometimes have severe effects on the productivity of the forest.

For uncontrolled forest fires, average figures must be used, since the amount of fires can vary from year to year, depending largely on weather conditions. In 1954, in Canada about 2950 fires occurred, but in 1961 there were as many as 8458 fires. During these years, the total area burned also varied greatly 2 064 000 acres in 1954 and 8, 460 000 acres (1 acre = 4047 m²) in 1961 (Canadian Forestry Service, 1974). It is likely that similar trends can be found in most boreal forests of the world. In highly productive parts of the world, straw is often burned on the fields. Burning has, however, negative effects on nutrients and the equilibrium in the soil. More and more farmers, nowadays, have begun to plough straw down into the soil. Fire is widespread in all tropical countries, usually connected with primitive agriculture and grazing (Bartlett, 1955, 1956). In most tropical countries, fire is now used in pasture management, clearing of forest vegetation, and in agriculture. Surface fires constitute the great majority of these fires, although they may vary considerably in intensity and frequency. Fire is most predominant in grassland areas, and usually spreads from there into the surrounding forests.

Wild fires are considered a very important problem in many tropical American countries, especially Brazil, Venezuela, Colombia, and Honduras (Budowski, 1966). Most savannas and steppe lands of the tropics are subject to grass fires during the annual dry season. In Africa, the major area of grassland fires extends from central Nigeria, east to the Ethiopian highlands. This area ranges from 800 to 1400 km from north to south (Deshler, personal communication). Repeated annually, these fires alter the composition of the vegetation by increasing the grass cover at the expense of the trees. This is a major factor in the conditioning of the vegetation, which is in itself an important component of the resource base for the farmers and herdsmen in these lands (Deshler, personal communication). Furthermore, uncontrolled fires break out periodically in some deserts and desert grasslands, if not regulated by man. In most dry desert areas the growth is probably too sparse to allow fires to run (Humphrey, 1963).

Controlled burning, as a modern land-management practice, is increasing in pasture and forest exploitation in parts of the tropics. Although fires due to controlled burning are significant in South Africa, Australia, and India, they are far less significant than uncontrolled fires set by swidden cultivators and graziers (Batchelder, 1967). The values available are too scattered to give a reliable global estimate of carbon lost from the ecosystem as CO₂ due to fire.

About 20%, on average, of the biomass of trees become logging slash when the trees are cut (Nihlgård and Lindgren, 1977). When burned, this will be oxidized into CO₂. Nowadays, however, the use of fire in forestry seems to be decreasing. This is not the case in tropical areas, however, where large forest areas are clear-cut and burned. To decrease the CO₂ flow into the atmosphere, the use of fire to obtain quick oxidation of litter and other organic material ought to be restricted whenever possible.

6.3.4. Mires

Peat accumulation is a mechanism of CO₂ removal rarely discussed. It is one of the few forms of soil organic matter accumulation which can be easily measured (Bohn,, personal communication). Twenhofel (1926), and Moore and Bellamy (1973) estimate the rate of peat accumulation at about 1 mm/year as global average. This corresponds to a carbon accumulation of about 300 x 10¹² g C/year or about one-third of that released by cultivation of new lands. The data for this estimate of carbon accumulation by peat is: 3.3 x 10⁶ km² wetlands of the world (Deevey, 1971), 0.2 g/cm³ as density of peat (Boelter and Blake, 1964), and 50% carbon content of peat on a dry weight basis (Bohn, personal communication). Moore and Bellamy (1973) give an estimation that the world's peat resource is about 230 x 10⁶ ha covered by 330 x 10¹⁵ g of organic matter. This would correspond to about 165 x 10¹⁵ g C. The world's peat resources are given in Figure 6.5.

The estimate of the annual peat accumulation rate as 1 mm/year is probably an overestimation. A more relevant figure would be a global average of 0.5 mm/year (Sjörs, personal communication). Most estimates of 1 mm/year are probably based on research on mires in central parts of Europe and the United States. Mires in cold regions usually have a much lower annual accumulation. This is due not only to the climate, but also to the large water-pool systems on these mires. The average for Sweden is 0.5 mm/year. The mires have a very high productivity in the southern parts of this country, but perhaps only an average increase of about 0.2 mm/year in the northern parts. It should be the same situation in the large boreal areas in North America and the U.S.S.R (Figure 6.5).

The growth in the height of the peat-cover has varied considerably during different climatic periods. At the end of the climatic optimum it was only 0.2 mm/year and even less when at a minimum in the case of raised bogs, while the average is more than 0.5 mm/year for Sphagnum peat. A general feature is the increase in the rate of peat formation during the subatlantic period, i.e. during the past 3000 years, during which time it has averaged at about 1 mm per year (Tolonen, 1973). Pakarinen (1975) estimated the global peat accumulation of temperate and boreal peatlands at about 135270 x 10¹² g dry weight (67.5135 x 10¹² g C), which is only a small percentage (less than 5%) of the current annual CO₂ emission from fossil fuels.

Different rules for the definition of mires can also cause problems. For example, in Finland pineswamps are included in the mire area, but not in Sweden. There are also other definitions of mire; a geological definition states a minimum depth, while a forestry definition settles that the annual increase of wood must be less than 1 m³ /ha on mires. It would be desirable to have a more uniform definition of mires in all parts of the world.

The total area in Sweden covered by mires is probably about 55 x 10³ km² (Holmen et al., 1967). Large areas of the subarctic regions in northern Scandinavia are covered by mires of varying depth. The sloping fens in the western mountain areas are often about 0.51 m deep (Bramryd and Holgersson, in press). In some areas, the peat-layers can be as deep as 810 m.

Figure 6.5 Major peat areas. 1 = Regions with intensive peat accumulation 2 = Regions with minor peat accumulation 3 = Tropical and subtropical areas with little peat accumulation 4 = Mountain areas with little peat accumulation 5 = Polar areas with a low rate of peat accumulation 6 = Regions without peat accumulation (Heikurainen, 1973, Reproduced by permission of P. A. Norstedt and Söners Förlag, Sweden.)

The peat-covered area (peat depth more than 0.5 m) in Europe is about 4% of the total land area. The corresponding values are: for Asia 11%, North America 0.6%, South America 0.13%, Africa 0.18%, and Australia 0.04% (Heikurainen, 1973). The conclusion that can be drawn from this, and from Figure 6.5, is that most peat accumulation is found in boreal parts of the world. There are, however, peatlands in temperate latitudes of both the northern and the southern hemispheres.

New Zealand has quite large deposits of peat, and it is estimated that there are 1.2 x 10⁶ m² of peatlands, about 0.6% of the total area of the islands, mainly on the North Island (Moore and Bellamy, 1973).

Table 6.9 The world's peat resources (Moore and Bellamy, 1973)

Table 6.10 Peat exploitation (Sheridan, 1965)

Tropical and subtropical peat deposits are probably far more extensive than can be realized. South America, Uruguay, and Paraguay have considerable peatland. Pakistan also has local peat deposits, mainly in river deltas. The reserves there are estimated to be about 200 million t of dry peat. Mangrove swamps are important peat-producing communities in Africa and in Florida (Moore and Bellamy, 1973). The world's peat resources are shown in Table 6.9.

Many countries in the world use peat for industrial purposes and drain the peatlands to make new forest and agricultural lands. Table 6.10 gives the percentage of the annual peat harvest in some countries. It can be seen that the Soviet Union strongly dominates the world exploitation of peat. Already at the turn of this century, Russia was harvesting l.65 x 10⁶ g of peat per year. The bulk of the harvested peat is used as an energy source (Moore and Bellamy, 1973).

Canada, on the other hand, exploits little of its peatlands and exports 90% of the produce to the United States for horticulture. Some large peatland areas in the world are drained and fertilized every year to increase wood production. The ditched peatland in Sweden, until 1960, can be estimated at some 6.57.0 x 10⁵ ha. It is not known exactly how much of this area was afforested or had an increased production. It may be roughly estimated at 33.5 x 10⁵ ha, i.e. about half of the drained area, or about 1.5% of the total forest area of the country. Today, forest drainage operations in Sweden cover an area of 0.150.20 x 10⁵ ha annually. An estimation of 30 x 10⁵ ha of utilizable peatland includes all the mires in south and north central Sweden (Holmen et al., 1967). If unrestricted, the drainage for afforestation would totally spoil the many typical kinds of mires in these regions. It is also these mires in south Sweden that would give the best productivity if planted with forests. The same type of problem occurs in all wood-producing countries of the world. Therefore, it is imperative that a conservation programme should be established.

Forest drainage was started in Finland as early as in the nineteenth century, usually on land owned by one of the Finnish wood-producing companies. On state-owned land in Finland, forest drainage activities were started in 1908. On privately owned land forest drainage activities were begun in 1928 when the first Finnish Forest Improvement law was passed. According to this law, public funds could be used to support forest improvement operations on lands owned by private citizens, for example by farmers (Heikurainen, 1973). Mires are being drained in Finland at an ever-increasing rate. In 1950, an area of about 10 000 ha was drained, while in 1970 the rate had increased to 291 000 ha per year. By the beginning of the 1970s, a total area of 3.5 million ha of peatland and land suffering from excess water had been drained. However, pine swamps are also included in this value (Heikurainen, 1973). In Finland, peatlands form the greatest part of the arable land, 3 x 10⁵ ha of which has been drained since 1945. This is particularly true of the northern parts of the country, where in many cases more than half of the arable land of a farm originates from peatland. However, liming of peatland is mostly necessary for crop farming (Pessi, 1971). After about 30 years of forest management on former peat areas, the thickness of the peat layer often decreases by 1-.52 m (Mikkola, personal communication).

For a period of about 50 years after 1880, a rather rapid cultivation of mires for agricultural purposes took place in Sweden. It reached its greatest extent around 1930 (Holmen et al., 1967). Since then, it has gradually decreased and by 1974 had almost completely ceased. In 1975 and 1976, however, efforts were made to gain new agricultural land from mires. Therefore, this could be an increasing threat to the mires in south Sweden in the future. It is estimated that in 1961 about 4 x 10⁵ ha. about 1 per cent of the total land area and slightly more than 10 per cent of the agricultural area of Sweden, were of organic soil origin (Holmen et al., 1967).

In Ireland, for example, about 2 x 10⁵ ha of mires have been drained for agricultural purposes. Most of this area has been abandoned during the last few years, as these lands are often too isolated and difficult to use (Holmen, personal communication).

Peat is also used for fuel and soil improvement. Earlier, the technical use of peat in Sweden consisted mainly in manufacturing fuel or moss litter. The production of peat briquettes has now declined to only 25 00030 000 t per year (peat with 65% dry matter) (Holmen et al., 1967). Of great demand at present is the so-called meal-peat, which is used to a large extent in greenhouses and for soil improvement. Hasselfors Garden in Sweden, one of the two main users of peat for soil improvement, has an annual requirement of 400 000 m³ peat (1976). This industry plans to increase production by 5% per year (Hasselfors Garden, personal communication). Peat exploitation in Finland amounted to 750 000 m³ in 1970 and has, during the last years, increased to well over one million m³ /year. This corresponds to about 107 x 10⁹ g C. Although this was mainly used for soil improvement, 300 000 m³ were used as fuel. Furthermore, peat was used in the prevention of oil pollution and as raw material in the production of peat pots (Suoninen, 1971).

Fuel prices have increased rapidly during the last few years, and this favours the production of fuel peat. At present 10002000 ha of peatland are brought into peat production every year in Finland. It has been estimated that by 1980, annual production will be 10 x 10⁶ m³ (3.3 x 10¹² g of peat or 1.07 x 10¹² g C) (Suoninen, 1971). Compared with the amounts of fuel peat used before the Second World War (20 000 t annually in 1940), this means an enormous increase in CO₂ liberation from the mires in Finland (Heikurainen, 1973).

In some other countries, there is an even worse exploitation of mires and peatland. In Ireland, the annual production of fuel peat is about 5.0 x 10¹² grams (Heikurainen, 1973). The U.S.S.R., which has about 60% of the peatland area of the world (Gjaerevoll, 1973), has an annual fuel peat production of about 50 x 10¹² g. About 49% of this quantity is used in power stations (Heikurainen, 1973). The Finnish mires, which make up about 32% of the total land area, contain about 10 x.10¹² g dry weight of peat (Heikurainen, 1973). This corresponds to about 5 x 10¹² g C. If this could be used as fuel and turned into the atmosphere as CO₂ it could have extensive effects. Also when oil, coal, or other fossil fuels are used as energy resources, the increase of CO₂ liberation into the atmosphere is considerable. Therefore, all countries should endeavour to find other solutions to the energy problems and should try to find alternate sources of energy (solar energy, wind energy, etc.) than those from organically fixed energy. The rapidly increasing exploitation of the world's mires due to the energy crisis constitutes a serious threat to the environment.

6.3.5 Deserts

The definition and the process of desertification were explained by Le Houérou as early as in 1959. His conclusions were confirmed at a seminar held in Gabes, in Tunisia, in 1972:

Desertification is a man-induced phenomenon; there is no evidence of increased climatic aridity during the period of instrumental record. Desertification is further a result of high demographic pressure which results in

a) generalized overgrazing

b) clearing of natural pastures for cereal production and over cultivation of sandy soils

c) destruction of woody species for fuel 

d) extension of mechanized ploughing. 

These cumulative causes result in accelerated soil erosion which in many cases leads to new deserts'. (Le Houérou, 1976).

Desertification is an acute environmental and social problem on both the northern and southern sides of the Sahara, as well as in many other dry parts of the world. Rapp (1976) states that desertification is mainly man-made and results from overgrazing, overcultivation, woodcutting and, in some areas, also from burning. These activities destroy the plant cover, which is needed for protection against wind and water erosion (Rapp, 1976). Figure 6.6 shows the arid lands of Africa, Each year, desertification affects several tens of thousands of hectares in southern Tunisia. However, it is a discontinuous process, which mainly takes place after many dry years (Le Houérou, 1976). Firewood collection alone destroys 18 000 ha of steppe every year in the country of Gabes (Floret and Le Floch,1972). Estimations have been made based on test plots, that desertized areas in Tunisia will increase from 35% to 65% of the land surface. Production will decrease by 35% (Floret and Le Floch, 1973).

Figure 6.6 Arid regions of Africa (redrawn after McGinnies et al., 1968)

Table 6.11  Average production in kg dry matter per hectare per year in Tunisia. Production decreases by about 50% from each bioclimatic subdivision to the next, and more arid, subdivision (Le Houérou, 1969)

6.3.6 Waste Disposal

Most of the products from forests and agriculture are transported into cities and other societies, and sooner or later become garbage. This solid waste is mostly deposited on sanitary landfills or is burned in incinerators. When burning, the carbon is rapidly oxidized to CO₂. On the landfills, carbon is bound up in the ground and is slowly released as a result of microbiological activity.

In some countries, there is an increasing number of composting plants that deal with the garbage and sewage sludge and make soil improvements out of the waste. In this case, the organic waste is converted to humus that will gradually be oxidized by the soil fauna and the decomposing microorganisms. In the case of both sanitary landfill and composting, carbon is bound up for an extended period of time. The two methods landfill and incineration account for about 98% of the disposed waste in the United States, while the remaining 2% includes composting and other methods (Figure 6.7) (Baum and Parker, 1974).

Estimates from 1971, in the United States, of total production of garbage show that nearly 3.8 x 10⁶ t of plastics and 40 x 10⁶ t of paper were turned into garbage. By 1976, the dry weight of plastic solid wastes was predicted at a little over 5 x 10⁶ t (Baum and Parker, 1973). Solid waste is generated by society in the following approximate proportions: 44% from private families or households; 30% from construction activities and industry; and the remaining 26% from commercial establishments, such as stores and warehouses. Packaging contributes about 25% of the garbage. Packing materials in 1966 were estimated at about 52 x 10⁶ t, and were expected to rise to about 74 x 10⁶ t in 1976 (Baum and Parker, 1974). Table 6.12 shows estimated annual averages of refuse categories.

Figure 6.7 Type of solid waste disposal used by municipalities in the United States with over 25 000 inhabitants in 1966. There are 250 incinerator plants in the United States. (Reproduced by permission from Solid Waste Disposal, Volume 1, Incineration and Landfill, B. Baum and C. Parker. Ann Arbor Science Publishers, Inc., Ann Arbor, Michigan, 1973)

The largest landfills in the United States are located in the Meadowlands in Hackensack, New Jersey, and in Los Angeles. The landfill in New Jersey is the largest in the world, handling about 15 x 10⁶ t/year (Baum and Parker, 1973). In several European countries, incineration is a common means of waste destruction, as, for example, in Paris, Berlin, Munich, Copenhagen, Oslo, and others. In Sweden, the three largest cities, Stockholm, Gothenburg, and Malmö, also burn most of their garbage. Large amounts of organic matter are oxidized to CO₂ . In most countries, the authorities have become aware of the severe risk that air pollutants from incinerators constitute. In London, for example, and in many other cities, incinerators are closed and modern landfills, with compacting trucks and a continuous cover for the garbage, are used. In Holland, and in some cities in Germany, compost plants have been built that can handle large quantities of garbage. In Tokyo, large landfills are used, but also incinerators. Many other cities in Japan have large incinerators. In developing countries, uncontrolled open garbage dumps are common, sometimes with open burning.

Table 6.12  Estimated annual average (% by weight) of 1968 municipal refuse on ^'as-discarded' basis. (Baum and Parker, 1973)

Due to the energy crisis, the use of garbage incinerators for energy production has tended to increase both in Europe and in the United States. This means that the rapid output of CO₂ to the atmosphere will increase. The problems connected with air pollution are difficult to solve. There are today no methods for cleaning smoke off heavy metals, as these particles are too small to be ionized in electrofilters and most of them are not easily soluble in water. Heavy metals can poison the plants and thus decrease the total assimilation of carbon dioxide. The heavy metals will, however, also decrease the breakdown of organic matter in the soil (Tyler, 1974). This, however, is perhaps of minor significance for the total carbon balance, as these effects on soil only appear near the emitting source.

During the last few years, more and more countries have started programmes aiming at an increased recycling of waste materials, such as paper. This will decrease the amount of garbage and reduce the flow of carbon from the forests, via society, into the air as carbon dioxide.

Sewage-sludge is a big problem in most cities. Most sludge in the world is probably dumped in landfills or in the sea. Some cities burn their sludge, but the recent trend is to use sludge for soil improvement. This will increase the microbiological activity in the soil and the productivity of the plants (Bramryd, 1976). However, some health hazards may be involved where sludges are derived from industries handling dangerous materials such as cadmium or mercury.

6.3.7 Air Pollution, Toxic Substances

Productivity can be negatively affected by a number of pollutants. Sulfur dioxide from combustion of organic materials can poison plants and reduce their CO₂ assimilation. Sulfur dioxide can also cause acidification in the soil and can, therefore, lead to decreased mineralization of forest litter. This will probably later result in a decreased productivity. Further acidification of already acid forest soils by air pollutants can be a slow process, and it probably takes considerable time before growth effects can be established. However, even moderate additions of sulfuric acid or sulfur have obvious effects on soil biological processes, especially on nitrogen turnover (Tamm et al., 1975). It can, however, be assumed that the carbon accumulation, due to the slowdown of litter mineralization rate in the soil, will probably be compensated by the decrease in productivity. Due to the feedback mechanisms it is possible that acidification has no significant influence on the total carbon cycle. However, we do not yet know enough to make sufficiently relevant estimations on this subject (Tamm, personal communication).

The acidification may ultimately have far-reaching consequences on soil fertility, and more research is urgently required in this field, before the acidification goes too far. Heavy metals also have a negative effect on mineralization in the soil (Tyler, 1974). This, combined with direct poisoning, will decrease productivity. Thus it is probable that similar feedback mechanisms to those described above could keep the total carbon flow on the whole unchanged.

Other air pollutants will also effect productivity in a negative way. Investigations by Bennet and Hill (1973) reveal that HF, O₃, C1₂, SO₂, NO₂ and NO, ranked in this order, will decrease photosynthesis by the end of 2 hours of pollutant exposure. Carbon monoxide was also tested but did not measurably reduce CO₂ uptake when applied in concentrations ranging up to 80 ppm.

The use of herbicides will probably decrease photosynthesis, and thus the carbon fixation, on the treated area but effects on soil respiration, for example, have not been clearly revealed (Grossbard, 1971). However, Westing (1971) assumes that herbicides will cause long-term damage to the ecosystem.

6.3.8 Methane, Carbon Monoxide

Methane produced in the anaerobic decay of organic matter has been investigated in different environments. Games and Hayes (1976) have studied CH₄ release from landfills, and other scientists have investigated methane from sewage sludge, for example. Most of the CH₄ released to the atmosphere, about 80%, is of recent biological origin. Probably 20% of the atmospheric CH₄ is released by fossil-fuel sources.

The annual production of CH₄ has been estimated at somewhere between 650x 10¹² and 7300 x 10¹² g/year. The destruction of CH₄ takes place mainly in the troposphere, while only about 10% of the CH₄ is destroyed in the stratosphere. The total destruction rate is estimated at about 3 x 10¹⁵ g/year. Apparently the CH₄ cycle contributes about 1% to the atmospheric carbon cycle. The amount of CH₄ in the atmosphere is probably about 4 x 10¹⁵ g (Ehhalt, 1973).

Humid or marshy areas that provide anaerobic conditions constitute major sources. There, the production of CH₄ can be as high as 210 x 10⁶ g/km² per year. Marsh areas are also of major importance on a global scale; large amounts of CH₄ can also be emitted from landfills (Ehhalt, 1973). In sewage plants, large amounts of CH₄ are generated during the anaerobic digestion of the sludge. In most plants, CH₄ is collected and burned and is sometimes used as an energy source. Thus, the CH₄ quantities released from sewage plants are limited (Ehhalt, 1973).

Carbon monoxide is produced mainly by combustion of organic substances; for example, by a variety of technological processes, by incomplete combustion of fossil fuel in automobiles, industries, or power plants. It is probably also produced by bacterial action on land and in the oceans. Seiler (1974) has estimated the total anthropogenic production of carbon monoxide at 0.64 x 10¹⁵ g/year in 1973. About 70% of this value should derive from automobiles. The mechanisms for the removal of CO from the atmosphere are not well known, but seem to include chemical reactions, both in the troposphere and stratosphere, and bacterial action in soil (Cadle, 1973). Further details on trace gases in the atmosphere can be found in Chapter 4 (this volume).

6.4. SUMMARY

Due to exploitation of terrestrial ecosystems by man, the outflow of carbon to the atmosphere has increased. In tropical rain forests, having the highest net primary production of all ecosystems, clearcutting and burning have destroyed large areas of forest land. In addition to rapid oxidation of the biomass, the photosynthesizing capacity has decreased on a global scale.

Also in temperate and boreal forests, new forestry techniques could probably cause a net outflow of carbon compared with virgin forests. This would in particular be the case if plans for whole-tree utilization are realized. Very little biomass is then left in the forest to form soil organic matter. Clearing of forests to create new agricultural lands has also reduced the pool of carbon in the world's forests. New agricultural methods as, for example, deeper ploughing, have increased the mineralization rate of accumulated organic matter in agricultural lands.

To claim new forest and agricultural areas, there are plans to drain large areas of peatland. When these areas are used for agricultural purposes, the new aerobic conditions and the intensive mixing of the peat, when ploughed, will rapidly increase the mineralization rate, due to the search for new energy sources, there are plans to exploit large peat areas for fuel production. This might have serious effects as the organic matter in peat, being accumulated during thousands of years, becomes rapidly oxidized. The accumulating capacity of carbon is further reduced when peatlands are destroyed.

Most products from agriculture and forestry are sooner or later turned into garbage. The disposal methods for solid waste could therefore, be of some interest for the carbon cycle. Landfills for waste could, to a certain extent, have similar effects on the carbon cycle as peatlands. As the landfills are often well compacted and situated in anaerobic environments, the mineralization of organic matter is strongly reduced. Incineration of solid waste, on the other hand, will rapidly oxidize the organic matter. When making compost from the waste, a great deal of the organic matter will remain as humus. Productivity of the biota can be negatively affected by several air pollutants and other toxic substances. However, a number of feedback mechanisms also will occur. To what extent the carbon cycle is affected is therefore somewhat difficult to estimate.

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