response of the terrestrial biosphere to global climate change and human perturbation

Vegetatio 104/105: 295-305, 1993. J. Rozema, H. Lambers, S.C. van de Geijn and M.L. Cambridge (eds). CO 2 and Biosphere © 1993 Kluwer Academic Publishers. Printed in Belgium.

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Response of the terrestrial biosphere to global climate change and human perturbation

William H. Schlesinger Departments of Botany and Geology, Duke University, Durham, NC 27706, USA

Keywords: Biogeochemistry, Biomass, Carbon cycle, Climate change, Fertilization, Global warming, Soil organic matter, Vegetation distribution

Abstract

Despite 20 years of intensive effort to understand the global carbon cycle, the budget for carbon dioxide in the atmosphere is unbalanced. To explain why atmospheric CO2 is not increasing as rapidly as it should be, various workers have suggested that land vegetation acts as a sink for carbon dioxide. Here, I examine various possibilities and find that the evidence for a sink of sufficient magnitude on land is poor. Moreover, it is unlikely that the land vegetation will act as a sink in the postulated warmer global climates of the future. In response to rapid human population growth, destruction of natural ecosystems in the tropics remains a large net source of CO2 for the atmosphere, which is only partially compensated by the potential for carbon storage in temperate and boreal regions. Direct and inadvertent human effects on land vegetation might increase the magnitude of regional CO2 storage on land, but they are unlikely to play a significant role in moderating the potential rate of greenhouse warming in the future.

Introduction

We are entering an era of unprecedented change in our environment. The concentrations of several biogenic gases in the atmosphere are increasing by 0.3 to 1.0 ~o per year. Destruction of natural vegetation, largely in the tropics, proceeds at a rate of about 17 x 106 ha/yr. These documented changes are likely to affect the climate of the planet through "greenhouse warming" and through indirect feedbacks to the global cycles of water and carbon dioxide (Houghton et al. 1990). Direct and indirect human perturbations of the global system are likely to reduce the habitability of the planet for other species, and most importantly, for ourselves. Already, declining populations of some wide-ranging species offer a somber and

provocative index of the declining health of the biosphere (Robbins et al. 1989). The onset of climatic change will be most difficult to prove in an unequivocal fashion, but other global changes wrought by a single species, the human, are already established without question.

It is, of course, possible to believe that the global system will respond in ways that will overcome the anticipated changes in climate as a result of "greenhouse warming." After all, during the last several million years, our planet has shown dramatic cycles in climate, associated with con- tinental glaciations and with the historical occur- rences of famine and drought. But there is little evidence of long-term, unidirectional change that has permanently disrupted the biosphere. Evi- dently, interactions among the biogeochemical

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cycles and the Earth's physical system yield feedbacks that have moderated past changes in climate and maintain near steady-state conditions on the planet.

One potential global feedback derives from the response of plants to elevated CO2 in the atmosphere. A wide variety of species show increases in growth rate and water-use efficiency in response to high CO2 (Strain & Cure 1985; Allen 1990; Bazzaz 1990). If land plants are growing more rapidly, greater amounts of organic matter could be stored in vegetation and soils, moderating the increase in atmospheric CO2 and in the potential for global warming. At one of the first conferences on carbon dioxide and the biosphere, held at Brookhaven National Laboratory in 1972, Ba- castow and Keeling suggested that net carbon storage in the terrestrial biosphere was necessary to balance the carbon dioxide budget of the atmosphere. In addition to the direct response of plants to high CO2, changes in the distribution of land vegetation in response to climate could lead to increases in terrestrial productivity and carbon storage, moderating the rate of greenhouse warming (e.g., Prentice & Fung 1990).

During the last 20 years, various attempts to balance the carbon dioxide budget of the atmosphere have failed, to a greater or lesser degree, unless they included a substantial increase in car-

bon storage on land (Table 1). A recent provocative paper by Tans et al. (1990), which presents substantially lower estimates of carbon dioxide uptake by the oceans, shows that we are no closer to understanding the atmospheric CO2 budget than we were in the early 1970s, despite an enor- mous scientific effort directed toward this prob- lem. Tans et al. (1990) focus our attention on the possibility that land vegetation could be serving as a CO2 sink, especially in the mid-latitude northern hemisphere. At the same time, there is little doubt that destruction of vegetation in the tropics leads to a loss of carbon from the land ecosystems of the world (Houghton et al. 1987).

One difficulty in reconciling past attempts to understand the global carbon cycle stems from differences in the breadth of environmental perturbations that are considered. Some efforts have only considered changes in current vegetation in response to higher atmospheric CO2. Other efforts have considered changes in the distribution of terrestrial vegetation that may result from climatic change. The most realistic attempts are those that recognize that by the end of this century nearly all of the Earth's land surface will be managed by humans, as we strive to provide hab- itat and food for our exponentially increasing population. To use general circulation models (GCMs) to predict the future distribution of veg-

Table I. A history of attempts to balance the atmospheric CO 2 budget. All data are given in 1015 gC/yr.

Inputs Fates Unknown Reference sinks

Fossil Net Increase in Oceanic fuel biomass atmospheric uptake combustion destruction pool

3.6

5.2 3.3 5.0 1.3

5.4 1.6

5.3 1.8

1.8

2.5 2.9

3.4

3.0

0.5-0.8 1.0-1.3

2.0 4.0 2.4 1.0

2.0 1.6

1.0-1.6 2.5-3.1

Reiners (1973) The Brookhaven Symposium Woodwell et aL (1983) Trabalka (1985) U.S. Department of Energy State-of-the-Art Report Houghton et aL (1990) Intergovernmental Panel on Climate Change Tans etaL (1990)

etation as if humans were not present is an esoteric exercise; we must remember that most natural vegetation will be destroyed.

It is my aim in this paper to consider probable interactions between the terrestrial biosphere and atmospheric COa during the next century. The critical question is whether the terrestrial biosphere will provide a sink for atmospheric CO2 that moderates the rate of global climate change or an additional source of CO: that exacerbates the anticipated greenhouse warming.

Changes in the distribution of vegetation

A variety of general circulation models predict the global distribution of temperature change on Earth in response to a doubling of atmospheric CO2. There is widespread agreement among these models that we should expect a mean warming on the order of 3 ° C, with the greatest change near the poles (Houghton et al. 1990). In response to global warming, most models also predict an increase in cloudiness and in global precipitation (Raval & Ramanathan 1989). The effect of increased cloudiness is essentially unknown, and investigations of cloud feedbacks are the highest priority in the US Global Change Research Pro- gram (CES 1991).

Although they differ in detail, all models of the future distribution of vegetation indicate a substantial decline in the area of boreal forest and tundra with climatic warming. Emanuel et al.

(1985) suggest that the area of boreal forest will decline from 23 ~o of forested lands to <1 ~o under the warming expected with a doubling of atmospheric CO2. Tundra essentially disappears from the surface of the Earth. Prentice & Fung (1990) indicate a decline of 66 ~o and 63 ~o from the present area of boreal forest and tundra, re- spectively, under similar conditions of climatic change.

Most models indicate that greater global rainfall will accompany greenhouse warming, but the predicted changes in precipitation are not uni- form across the Earth's surface, nor is there much agreement about future rainfall patterns. Substan-

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tial portions of the interior of continents may ex- perience increasing drought during the summer growing season (Manabe & Wetherald 1987), and the area of irrigated agriculture may increase dra- matically (Adams et al. 1990a). Emanuel et al.

(1985) predict an increase in the world's desert area of 6 to 17 ~o under the climatic warming associated with a doubling of CO2. On the other hand, Prentice & Fung (1990) suggest a decrease of 62 ~o in the current area of desert land.

Changes in the terrestrial carbon pool

Shifts in the global distribution of vegetation are accompanied by changes in the relative storage of organic carbon on land. For example, the changes in vegetation predicted by Prentice & Fung (1990) yield a net increase in the carbon storage in the terrestrial biosphere of 235 × 10 ~5 gC, with nearly all of the change occurring in vegetation. Their model suggests that many areas of low-statured vegetation will be replaced by tropical forests, which store larger amounts of carbon. Assuming that atmospheric CO2 doubles within the next 50 years, and that the anticipated global warming occurs over roughly the same interval, their model would suggest an annual net uptake of 4.7 x 1015 gC by the terrestrial biosphere. This value is slightly higher than the annual uptake currently postulated by Tans et al. (1990).

Examinations of past changes in the distribution of land plants and of the size of the terrestrial biosphere give some indication of whether a response of this magnitude is reasonable. Adams et al. (1990b) use a wide variety of paleoecological literature to reconstruct a best-estimate of the vegetation at the last glacial maximum, roughly 18,000 years ago. Their data suggest that the size of the terrestrial biosphere increased by 1300 x 1015 g as the Earth warmed to its present- day climate. Although the climate change associated with deglaciation occurred rather rapidly, it is likely that changes in vegetation and soils were slower. Most soils require at least 3000 years to achieve a steady-state in soil organic matter with respect to new conditions of vegetation or climate

298

(Birkeland 1984; Almendinger 1990). Assuming that the size of the terrestrial biosphere increased uniformly during the last 10,000 years, the net accumulation on land was 0.13 x 1015 gC/yr during the Holocene. Thus, Adams et al. (1990b) show that land ecosystems have been a sink for carbon in warmer climates, but the rate of carbon accumulation observed during past climatic warming is nearly an order of magnitude lower than the response anticipated by Prentice & Fung (1990).

Soils are important carbon sinks, for they contain resistant humic compounds with relatively long turnover times. Humic compounds and peatlands are the only significant long-term sinks for organic carbon on land. Recently, I compiled values for the long-term accumulation of organic matter accompanying soil development during the Holocene (Table 2). Deglaciated soils appear to

have accumulated about 0.04-0.10 x 1015 gC/yr during the last 10,000 years (Schlesinger 1990; Gorham 1991), and the total storage in undisturbed soils is presently not likely to be greater than 0.4 x 1015 gC/yr. This value is very close to the estimated riverine transport of dissolved and particulate organic carbon (Schlesinger & Melack 1981), so it is possible that the formation ofhumic substances on land is roughly balanced by their net transport to the sea, where they may ulti- mately be added to ocean sediments (Berner 1982; Lugo & Brown 1986). In any case, this compila- tion suggests that soils in the undisturbed terrestrial biosphere could be only a small potential sink for atmospheric CO2, and much less than the losses of soil organic matter from agricultural soils.

The alternative vegetation changes postulated by Emanuel et al. (1985) indicate a reduction in

Table 2. Long-term rates of accumulation of organic carbon in Holocene-age soils.

Ecosystem Vegetation Soil origin type in terminal state

Accumulation interval (yr)

Long-term rate of accumulation (gC m- 2 yr- 1)

Tundra Polar desert Glacial retreat Polar desert Glacial retreat Polar desert Glacial retreat Sedge moss Glacial retreat Sedge moss Glacial retreat Sedge moss Glacial retreat

Boreal forest Spruce Glacial retreat Spruce-fir Glacial retreat Spruce-fir Glacial retreat

Temperate forest Broadleaf evergreen Volcanic ash Coniferous Volcanic mudflow Deciduous Alluvium Deciduous Dunes Podocarpus Dunes Angophora Dunes Eucalyptus Dunes Eucalyptus Dunes Low forest Glacial deposits

Tropical forest Metrosideros Volcanic ash Rain forest Volcanic ash

Temperate grassland Chionochloa Glacial deposits Temperate desert Grassland Alluvium

8,000 9,000 2,600 1,000 9,000 8,700 3,500 5,435 2,740 1,277 1,200 1,955

10,000 10,000 4,200 6,500 5,500 9,000 3,500 8,620 9,000 3,040

0.2 0.2 2.4 2.4 1.1 0.7*

11.7 0.8 2.2

12.0 10.0 5.1 0.7 2.1 1.7 1.4 2.1 2.5 2.5 2.3 2.2 0.8

From Schlesinger (1990); citations to original literature are given therein. * Corrected from value given in original publication.

the storage of carbon on land by 38 x 1015 g as a result of climatic warming. Land vegetation gains 7 x 1015 gC, but 45 x 1015 gC are lost from soils. Losses from soil organic matter occur as organic- rich boreal forest and tundra soils are replaced by the northward development of temperate soils on those lands. Overall the loss from the terrestrial biosphere is consistent with the observation that decomposition proceeds most rapidly in warm, wet conditions. Global warming is likely to increase soil respiration worldwide (Schleser 1982; Jenkinson et al. 1991), and losses of organic matter are seen in field experiments that impose soil warming (Van Cleve et al. 1990). Assuming a uni- form global warming of 0.03 °C/yr, Jenkinson et al. (1991) estimate that there will be a net release of 61 x 10 is gC from the soil carbon pool to the atmosphere during the next 60 years. These data all suggest that the terrestrial biosphere is likely to be an additional source of atmospheric CO2 during global warming.

Land-use changes

There is little doubt that humans are making a dramatic impact on the use of land worldwide. Current estimates suggest that native tropical forests are cleared at a rate of 17 x 10 6 ha/yr (Houghton 1992). Cropland, pasture, and sec- ond-growth forest contain lower amounts of vegetation than primary forest, and the net effect of such land use change is a large release of carbon dioxide to the atmosphere. Organic matter is also lost from soils, as the rate of organic inputs is reduced, while decomposition rates are stable or increased (Schlesinger 1986). Houghton et al.

(1987) calculate that 1.8 × 1015 gC/yr are lost from vegetation and soils of the world as a result of land-use change. Other global estimates are similar to or slightly smaller than this value (Molof- sky et al. 1984; Detwiler & Hall 1988; Hall & Uhling 1991). The loss from the Amazon basin alone appears to exceed 0.67110 ~5 gC/yr (Houghton et al. 1991), and estimated losses from tropical Asia range from 0.16 to 0.31 x 1015 gC/yr (Houghton 1991).

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Nearly all of the carbon lost from the terrestrial biosphere is derived from tropical latitudes; temperate and boreal zones show little change. This pattem reflects the distribution of human population growth around the world. A potential sink for carbon in the temperate zone stems from im- provements in agricultural productivity that have allowed many nations to reduce the acreage de- voted to agriculture, allowing native vegetation to return on some areas. It is possible that the regrowth of vegetation and the redevelopment of soil profiles has resulted in a sink for atmospheric CO2 in these areas. Delcourt & Harris (1980) calculate a storage of 0.07 x 1015 gC/yr as a result of reforestation in the southeastern United States, and forests now cover substantial areas of New England that were cleared for agriculture a century ago. Worldwide, temperate-zone reforestation may serve as a sink for as much as 1.9 x 1015 gC/yr (Johnson & Sharpe 1983), although Hough- ton et al. (1987) suggest much less. The large global sink for carbon on abandoned lands stems from the greater rates of carbon accumulation seen when native vegetation replaces cropland than when climatic change causes one native vegetation to replace another.

The net uptake of carbon by vegetation is short-lived, because forest regrowth is nearly complete in a few decades (Schiffman & Johnson 1989; Vitousek 1991). However, soils abandoned from agriculture or naturally-occurring youthful soils often show accretion of 30 to 50 gC m - 2 yr-~ in humic matter for several centuries (Table 3). Globally, the strength of this sink is limited; if all agricultural lands of the world (14 x 1012 m 2) were abandoned tomorrow and their soils accumulated carbon at 30 g m - 2 yr- 1, the global sink would sequester only 0.42 x 1015 gC/yr.

To arrive at an accurate estimate of the net change in the terrestrial biosphere, the carbon storage on abandoned agricultural lands of the temperate zone must be balanced against the net clearing of land for agricultural use in the tropics (Houghton et al. 1987). Thus, as long as the human population is increasing exponentially, the terrestrial biosphere would seem to be a most

300

Table 3. Accumulation of soil organic matter in abandoned agricultural soils and in other disturbed sites, which are allowed to return to native vegetation.

Ecosystem type Previous Period of Rate of Reference land use abandonment accumulation

(yr) (gC m - 2 yr- 1)

Subtropical forest Temperate deciduous forest Temperate coniferous forest Tempeate coniferous forest Temperate deciduous forest Temperate grassland

Cultivation 40 30-50 Cultivation 100 45 Cultivation 50 21-26 Diked soils 100 26 Mine spoils 50 55 Mine spoils 28-40 28

Lugo et al. (1986) Jenkinson (1990) Schiffman & Johnson (1989) Beke (1990) Leisman (1957) Anderson (1977)

unlikely sink for much atmospheric CO2 (Vi- tousek et al. 1986).

Fertilization of the terrestrial biosphere

Inadvertently, humans are fertilizing most of the terrestrial biosphere. All land plants of the world are now bathed by an atmosphere containing over 350 umol mol- 1 of CO2, nearly 10 ~o higher than a few decades ago. Many land areas that are downwind of human population centers receive a substantial excess deposition of NO3- as a com- ponent of acid rain. The carbon dioxide budget of the atmosphere might be balanced if these fertil- izations increase net primary production (ca. 60 x 1015 gC/yr) by about 5 ~o over the rate of decomposition. At the same time, a change of this magnitude would be nearly impossible to detect with our best methods for studying the terrestrial biosphere.

Dramatic increases in plant growth when native and agricultural species have been exposed to high CO2 in greenhouse experiments have led many workers to believe that similar effects are already occurring in nature. However, plants in natural ecosystems do not grow in the well- watered and fertilized conditions of the greenhouse. The experimental evidence for a CO2 fertilization of the natural terrestrial biosphere is weak (Graumlich 1991). Moreover, few studies have examined changes in soil processes; a CO2 fertilization of the terrestrial biosphere will provide a sink for atmospheric CO2 only if decomposition rates do not increase as well.

I know of only two ecosystems where this question has been examined in much detail--the wet tundra of Alaska and an estuarine salt marsh along the eastern coast of Maryland. Tissue & Oechel (1987) conducted a field experiment in the Alaskan tundra, where Eriophorum vaginatum was exposed to 680 ppm CO2 for 10 weeks. Within three weeks, plants exposed to high CO2 showed downward physiological adjustments, so that their photosynthetic rate was similar to those grown at ambient CO2. Plants grown at high CO2 produced a greater number of tillers, but the net carbon storage per unit of ground surface was scarcely increased (Grulke et al. 1990). Tissue & Oechel (1987) suggest that the lack of response of arctic tundra to high CO2 was due to nutrient deficiencies in the soil. In contrast, Curtis et al. (1989) found an increase in plant growth during a 3-year study in which salt marsh plants were exposed to high CO2 in an estuary, which is flushed by nutrient-rich tidal waters at regular intervals.

Tans et al. (1990) suggest that the global terrestrial sink must be 2.5 to 3.1 x 101SgC/yr to balance the budget of CO2 in the atmosphere (Table 1). They further suggest that the sink must reside in the northern latitudes, being especially likely in boreal forest regions. Recent estimates of aboveground biomass in boreal forests of North America (Botkin & Simpson 1990), applied to the area of boreal forest vegetation of the world, indicate a total biomass of 22.8 x 1015 gC. A sink for CO 2 of the magnitude suggested by Tans et al. (1990) would have doubled the size of boreal forest biomass during the 1980s. Even if the larger

biomass values of Whittaker & Likens (1973) are correct, the sink would have doubled the size of the boreal forest in the last 30 years. Surely this kind of change in the land vegetation should be easy to document, yet I know of no evidence for changes that are anywhere close to this magnitude.

Clearly, we desperately need more studies of the CO2 response of terrestrial ecosystems, especially forests, but my suspicion is that the widespread deficiency of soil nutrients will limit the ability of most native plants to respond. The salt marsh experiment could well be the exception, rather than the rule, owing to its unusual hydro- logic and nutrient regime.

Garrels et al. (1975) were among the first bio- geochemists to note that the worldwide human mobilization of nitrogen and phosphorus approx- imately matched the stoichiometric requirements of these elements for increased plant growth, which might sequester carbon from the atmosphere. Nitrogen is provided for plant growth by direct industrial nitrogen fixation--the Haber process--for fertilizer, and by indirect fixation during the combustion of fossil fuels in high compression engines. Phosphorus is provided by the direct mining and distribution of phosphate rock.

Is it possible that the excess nitrogen deposition received by many ecosystems provides the soil nutrients that might allow a plant response to high CO2? Consider an extreme scenario: assume that all of the human mobilization of fixed nitrogen (ca. 100 × 1012 gN/yr) is deposited in forest ecosystems, where the stimulation of growth al- lows a long-term storage of carbon. The growth all consists of wood, with a C/N ratio of 160 (Vitousek et al. 1988). Under such conditions, added storage of carbon on land would amount to over 16 x 1015 gC/yr--far more than enough to balance the release from fossil fuel and net biomass destruction in the tropics.

A number of aspects of nitrogen biogeochemistry make this case unrealistic. A large portion of plant growth consists of non-woody tissues, so the overall C/N ratio of net primary production is usually closer to 50, reducing our estimate of

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carbon storage by about 2/3. Much of the fertilizer nitrogen is deposited on land surfaces that are not managed for the long-term storage of carbon (e.g., farm fields). In the eastern United States, a substantial portion of the inadvertent atmospheric emission of fixed nitrogen compounds is transported offshore to the North At- lantic Ocean (Galloway & Whelpdale 1987). In many cases, it appears that excessive deposition of nitrogen may actually be reducing the growth of forests (Aber et al. 1989), and the additional nitrogen exacerbates underlying deficiencies of P, which is not widely distributed by air pollution (Mohren et aL 1986). Other areas show reduc- tions in forest growth due to concurrent exposure to ozone and acid rain. Thus, the net redistribu- tion of nitrogen in air pollutants is likely to im- pinge on only a small portion of the land area that might be conducive to greater carbon storage (Peterson & Melillo 1985). A liberal estimate might suggest that only half of the anthropogenic fixed nitrogen is effective in enhancing land plant growth, further reducing our estimate of potential storage. Finally, in most fertilization experiments that have followed the fate of isotopically labelled nitrogen applied to forests, only a small portion, typically <25~o, remains after several years (Keeney 1980). In a Scots pine forest, Melin et al.

(1983) found that only 79 ~o remained after one year. Presumably, the rest was lost to runoff waters and through microbial denitrification.

If we reduce our extreme estimate of enhanced carbon storage (16 × 1015 gC/yr) by these per- centages, it would appear that the maximum response of the terrestrial biosphere is about 0.6 x 1015 gC/yr. This value is less than that calculated by Kohlmaier et al. (1987) using a 4-biome model of the terrestrial biosphere. If portions of the terrestrial biosphere are a sink for 0.6 × 1015 gC/yr, they only partially compensate for the loss of 1.8 × 10 ~5 gC/yr due to land-use conversion in the tropics (Table 1). An analysis of the changes of CO2 concentration and of its iso- topic content in ice cores suggests that the terrestrial biosphere is currently decreasing at a rate of 0 to 0.9 × 1015 gC/yr, suggesting that the growth of vegetation in some areas makes up for some,

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but not all, of the destruction of vegetation in the tropics (Siegenthaler & Oeschger 1987).

Future global climate change may stimulate rates of decomposition in soils, reducing the pool of soil organic matter (Jenkinson et al. 1991). This decomposition will be associated with concurrent mineralization of nitrogen that is held in soil organic matter at a C/N ratio of about 12. If this nitrogen is taken up by land plants and enters long-term storage in wood, with a C/N ratio of about 160, a substantial terrestrial carbon sink might develop on land. Of course, it is possible that this nitrogen will be lost from terrestrial ecosystems by leaching and gaseous emissions. We know very little about the potential magnitude of this sink and whether it is currently being real- ized.

Management of the terrestrial biosphere

When natural land is converted to agriculture using "no-tillage" techniques, there are often smaller losses of soil carbon compared to those seen when traditional cultivation is practiced (Ta- ble 4). Small net gains of soil carbon might be seen if existing cultivated fields were converted to no-till agriculture (Dick 1983; Wood et al. 1991). Soils in no-till agriculture typically show higher soil organic matter in the surface horizons, but

little or no difference in the lower soil layers (Dalai 1989; Havlin et al. 1990).

In many instances, the application of fertilizer increases the storage of soil organic matter in agricultural soils, or at least reduces the rate of loss compared to unfertilized conditions (e.g., Blevins et aL 1977). Because humus typically con- tains a C/N ratio of about 12, the application of 1 mole of N in fertilizer could potentially remove 12 moles of CO2 from the atmosphere. A net storage of carbon is possible if the CO2 released during the manufacture of N fertilizer is not greater than the net increase in soil organic carbon. Assuming 100 ~o industrial efficiency, sim- ple stoichiometry would suggest that the ratio is favorable. In the Haber process,

3 C H 4 + 3 H20 ~ 3 CO2 + 3 H2

N 2 + 3 H 2 , 2 N H 3 ,

so 3 moles of CO2 are released during the production of 2 moles of N H 3.

When a pasture soil containing 5.2 kgC/m 2 was converted to a heavily fertilized (336 kgN ha-1 yr-1), no-till cornfield, soil organic matter increased to 5.5 kgC/m 2 in the 0-30 cm layer over 5 years (Blevins et al. 1977; Table 4). The increase amounts to 25 moles of C stored for each square meter of land. The release of CO2 during

Table 4. Comparisons of soil organic carbon in agricultural soils under conventional cultivation and under no-t• management practice and in undisturbed (virgin) soils from which they were derived.

Location Soil type Depth of Period of Fertilizer (reference) sampling study application

(cm) (yr) (kg N ha- 1 yr- 1)

Carbon content (kg C/m 2)

Cultivated No-till Virgin

Kentucky, USA Paleudalf 30 5 0 3.7 4.4 5.2 (Blevins et al. 1977) 30 5 336 4.4 5.5 5.2

Australia Pellustert 20 13 37 3.5 3.6 (Dalai 1989) 120 13 37 17.0 17.4

Ohio, USA Ochraqualf 22.5 18 150 6.3 7.3 7.3 (Dick 1983) Fragiudalf 22.5 19 150 3.3 3.9 4.4

Kansas, USA Haplustoll 30 12 65 4.7 4,9 (Havlin et al. 1990) ArgiudoU 30 13 115 7.9 7.9

Hapludoll 30 9 0 3.1 Hapludoll 30 9 252 3.1

N.B. If values for bulk density were not given, calculated carbon contents are based on an assumed bulk density of 1.4 g/cm 3 in all horizons of each profile.

the production of the total fertilizer application was 18 moles/m 2, indicating a net storage of carbon in this field. The net storage is equivalent to 16.8 gC m -2 y r - 1, which is significantly higher than storage of carbon that might be expected in a well-developed soil under native vegetation (Ta- ble 2). Small increases in soil organic matter have also been found when cultivated fields are converted to no-till agriculture (cf.Table 4). For example, Wood et al. (1991) found that intensive no-till management resulted in accumulations of 7 to 16 g C m - 2 y r -1 in the 0-10 cm depth of prairie soils that were previously subjected to conventional cultivation. Nevertheless, the potential net soil carbon storage through conservation tillage appears to be small (Kern & Johnson 1991).

Conclusions

Various lines of evidence suggest that land vegetation and soils are not a large net global sink for atmospheric CO2, and they are not likely to act as a large net sink in the forseeable future. A regional sink may exist, as a result of reforestation and improved agricultural practices in the temperate zone, but the maximum storage of carbon in these areas is not likely to exceed the losses of carbon from tropical forest destruction. Direct and inadvertent fertilization of land vegetation, both natural and managed, may slow the rate of CO2 increase in the atmosphere, but it is not likely to reverse it.

Acknowledgements

I thank Pat Megonigal and Lisa Dellwo Schlesinger for critical comments on my manu- script.

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