Climate Policy Watcher

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

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

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.