Soil Carbon Dynamics

Several important carbon management strategies exist, including the adaptive and mitigative options found in Figure 5.1. Adaptive options are based on better management

Agricultural Organization
Figure 5.1 Adaptive and mitigative strategies of carbon management and sequestration to address global climate change.

of terrestrial and aquatic ecosystems, desert lands, and wetlands. Adopting RMPs such as fertilizer use and irrigation on croplands and grazing lands is an important C management option. Mitigative options include enhancement of energy-use efficiency, finding alternatives to fossil fuel, and using geoengineering techniques, such as space reflectors and CO2 extraction from the atmosphere in order to influence the energy budget and the rate of enrichment of atmospheric concentration of CO2. Carbon sequestration is a key mitiga-tive strategy.

Carbon sequestration implies transferring CO2 from a pool that has a short turnover time into a pool with a longer turnover time. Specifically, it involves the removal of CO2 from the atmosphere and its storage in long-lived pools, such as soil, vegetation, wetlands, oceans, and geologic strata. There are two main ways to sequester C as illustrated in Figure 5.2. The biotic strategy involves conversion of CO2 into carbohydrates, lignin, cellulose, and other forms of biomass through

Plant Functional Traits
Figure 5.2 Categories of technological options for carbon sequestration through biotic and abiotic processes.

biotic processes such as photosynthesis. The biotic sequestration of CO2 is relevant to the transfer of CO2 into vegetation, soils, and the oceans. In contrast, the abiotic strategy involves a technical transfer of CO2 from the atmosphere into geologic, oceanic, and other long-lived pools, and the conversion of CO2 into other products such as CaCO3.

Soil C sequestration is a biotic strategy, based on a transfer of atmospheric CO2 into humic substances in soil. The terrestrial C pool is the third largest pool. As shown in Table 5.9, it holds at least 486 Pg (gigatons) C in biota and 2542 Pg C in the soil. Carbon sequestration in terrestrial

Table 5.9 Global Estimates of Land Area and Carbon Stocks in Plant Matter and Soil for Ecosystems

Area

Plant C

Soil

Total

Ecosystem

(1012 m2)

(Pga)

(Pg)

(Pg)

Forest, tropical

14.8

244.2

123

367

Forest, temperate and plantation

7.5

92.0

90

182

Forest, boreal

9.0

22.0

135

157

Woodland, temperate

2.0

16.0

24

40

Chaparral

2.5

8.0

30

38

Savanna, tropical

22.5

65.9

263

329

Grassland, temperate

12.5

9.0

295

304

Tundra, arctic and alpine

9.5

6.0

121

127

Desert and semidesert, scrub

21.0

6.9

168

175

Desert, extreme

9.0

0.3

23

23

Perpetual ice

15.5

0.0

0

0

Lake and stream

2.0

0.0

0

0

Wetland

2.8

12.0

202

214

Peatland, northern

3.4

0.0

455

455

Cultivated and permanent crop

14.8

3.0

117

120

Human area

2.0

1.0

10

11

Total

150.8

486.4

2056

2542

a Pg, 1015 grams, or 1 gigaton.

Source: From Amthor, J.S., M.A. Huston, et al., 1998. Terrestrial ecosystems responses to global change: a research strategy. ORNL/TM-1998/27. Oak Ridge National Laboratory, Oak Ridge, TN; and U.S. Department of Energy. 1999. Carbon Sequestration: Research and Development. National Technical Information Service, Springfield, VA.

a Pg, 1015 grams, or 1 gigaton.

Source: From Amthor, J.S., M.A. Huston, et al., 1998. Terrestrial ecosystems responses to global change: a research strategy. ORNL/TM-1998/27. Oak Ridge National Laboratory, Oak Ridge, TN; and U.S. Department of Energy. 1999. Carbon Sequestration: Research and Development. National Technical Information Service, Springfield, VA.

Table 5.10 Terrestrial Carbon Sequestration Potential

Biome

C Sequestration Potential (Pga C/year)

Agricultural lands Biomass croplands Grasslands Rangelands Forest lands

Deserts and degraded lands Terrestrial sediments Boreal peatlands and wetlands

Total a Pg, 1015 grams, or 1 gigaton.

Source: From U.S. Department of Energy. 1999. Carbon Sequestration: Research and Development. National Technical Information Service, Springfield, VA.

ecosystems can be achieved by enhancing the C pool in living plant matter, roots, and soil. Soil C storage involves both organic and inorganic C pools. Formation of secondary carbonates is one of the mechanisms of soil C sequestration.

There are numerous estimates of the potential for terrestrial C sequestration. The estimate of 5.7 to 10.1 Pg C/year shown in Table 5.10 is quite large (U.S. Department of Energy, 1999). However, the attainable potential of terrestrial C sequestration may only represent 10% to 20% of this amount through the use of adaptive strategies. IPCC (2000) estimated the potential of terrestrial C sequestration in agricultural and forestry ecosystems at 2.5 Pg C/year over the next 40 years. As indicated in Table 5.11, such a level of sequestration will yield a net increase of only 0.5 to 0.7 Pg C/year in atmospheric concentration, assuming the same rate of increase as observed in the 1990s.

Soil-specific research on adaptive and mitigative strategies aimed at soil C sequestration and enhancement of the SOC pool is needed. This would be a "no-regret approach" (Wittwer, 1995). Soil C sequestration would yield benefits with or without future climate change as it improves soil quality

Table 5.11 Carbon Sequestration Potential in Managed Terrestrial Ecosystems S

Average Adoption/Conversion

Average Carbon

Potential

Area

(% of area)

Sequestration

(Tga C year)

Scenario

(million ha)

2010

2040

(Mg C/ha/year)

2010

2040

Improved Management

Cropland

1289

29

59

0.33

125

258

Rice paddies

153

51

81

0.10

8

13

Agroforestry

400

22

40

0.28

26

45

Grazing land

3401

10

20

0.70

237

474

Forest land

4051

10

39

0.41

170

703

Urban land

100

5

15

0.3

2

4

Land Use Change

Agroforestry

630

20

30

3.1

391

586

Restoring degraded soils

277

5

10

0.25

4

8

Grassland

1457

3

7

0.8

38

82

Wetland restoration

230

5

15

0.4

4

14

Forest Products

300

300

Totals

1302

2485

a Tg= teragram, or 1012 grams, or 1 megaton.

Note: Average adoption rate and average carbon sequestration rate are calculated as weighted mean averages. Source: Modified from Inter-Government Panel on Climate Change 2000. Land Use, Land Use Change and Forestry. Cambridge University Press, London, 181-281.

a Tg= teragram, or 1012 grams, or 1 megaton.

Note: Average adoption rate and average carbon sequestration rate are calculated as weighted mean averages. Source: Modified from Inter-Government Panel on Climate Change 2000. Land Use, Land Use Change and Forestry. Cambridge University Press, London, 181-281.

and subsequent productivity. Adaptive strategies involve: (1) the use of transgenic plants that are more resilient to environmental stresses such as drought and heat and with pest resistance in crops, trees, and livestock; (2) conservation-effective measures to improve soil quality and reduce the risks of soil degradation; and (3) improved energy-use efficiency and development of fossil-fuel offsets.

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