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Carbon Sequestration, Terrestrial☆
Carbon Sequestration, Terrestrial☆ R Lal, Carbon Management and Sequestration Center, The Ohio State University, Columbus, OH, USA ã 2013 Elsevier Inc. All rights reserved.
Introduction Carbon Sequestration Processes of Terrestrial Carbon Sequestration Enhancing Terrestrial Carbon Sequestration Ancillary Benefits Restoration of Degraded Ecosystems and Terrestrial Carbon Sequestration Potential and Challenges of Soil Carbon Sequestration Climate Change and Terrestrial Carbon Sequestration Nutrient Requirements for Terrestrial Carbon Sequestration The Permanence of Carbon Sequestered in Terrestrial Ecosystems Terrestrial Carbon Sequestration and Global Food Security Conclusions
Glossary ancillary benefits Additional benefits of a process. anthropogenic Human-induced changes. biomass/biosolids Material of organic origin. carbon sequestration Transfer of carbon from atmosphere into the long-lived carbon pools such as vegetation and soil. humus The decomposed organic matter in which remains of plants and animals are not identifiable.
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mean residence time The average time an atom of carbon spends within a pool; calculated by dividing the pool by flux. soil organic matter Sum of all organic substances in the soil. soil quality Ability of a soil to perform functions of interest to humans such as biomass production, water filtration, biodegradation of pollutants, and soil carbon sequestration. terrestrial carbon Carbon contained in the vegetation and soil.
Introduction Specific options of mitigating climate change can be grouped broadly into two categories: adaptive responses and mitigative responses (Figure 1). Adaptive responses are aimed at managing the risks and identifying beneficial opportunities. Mitigative responses are aimed at reducing overall emissions of greenhouse gases (GHGs). Adaptive responses involve identification of options for sustainable management of natural resources (e.g., terrestrial ecosystems, water resources and wetlands, agricultural ecosystems, deserts) under changed climate with projected alterations in precipitation, frequency of extreme events, growing season duration, and incidence of pests and pathogens affecting health of human and other biota. Mitigative responses involve controlling atmospheric concentration of carbon dioxide (CO2) either by removing it from the atmosphere or by avoiding its emission into the atmosphere. Important among mitigative strategies are reducing and sequestering emissions. Emissions reduction may involve improving energy use/conversion efficiency, developing alternatives to fossil fuel or developing carbon (C)-neutral fuel sources, using space reflectors to reduce the amount of radiation reaching the earth, absorbing CO2 from the atmosphere using special extractants, and sequestering C in long-lived pools. Both adaptive and mitigative strategies are important to reducing the rate of enrichment of atmospheric concentration of CO2. This article describes the principles, opportunities, and challenges of C sequestration in terrestrial pools composed of soils and vegetation.
Carbon Sequestration Carbon sequestration implies capture and secure storage of C that would otherwise be emitted to or remain in the atmosphere. The objective is to transfer C from the atmosphere into long-lived pools (e.g., biota, soil, geologic strata, ocean). It is the net removal of CO2 from the atmosphere or prevention of CO2 emissions from natural and managed ecosystems. Strategies of C sequestration can be grouped under two broad categories: biotic and abiotic (Figure 2). Biotic strategies are based on natural process of photosynthesis and
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Change History: May 2013. R Lal updated all parts of the text.
Reference Module in Earth Systems and Environmental Sciences
http://dx.doi.org/10.1016/B978-0-12-409548-9.01211-2
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Figure 1 Carbon management strategies to mitigate the greenhouse effect.
Figure 2 Carbon sequestration strategies.
transfer of CO2 from atmosphere into vegetative, pedologic, and aquatic pools through mediation via green plants. Abiotic strategies involve separation, capture, compression, transport, and injection of CO2 from power plant flue gases and effluent of industrial processes deep into ocean and geologic strata. The strategy is to use engineering techniques for keeping industrial emissions from reaching the atmosphere.
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Terrestrial C sequestration is a natural process. It involves transfer of atmospheric CO2 through photosynthesis into vegetative and pedologic/soil C pools. The terrestrial pool is the third largest among the five global C pools (Figure 3). It includes 2300 petagrams (Pg, where 1 Pg ¼ 1015 g ¼ 1 billion metric tons) of C in soil to 1-m depth and 620 Pg in the vegetative pool, including the detritus material. The soil C pool to 3-m depth, including the revised estimates of the C pool in Cryosols and permafrost, may be as much as 4000Pg. Thus, the terrestrial C pool is approximately four times the size of the atmospheric pool and 60% the size of the fossil C pool. It is also a very active pool and is continuously interacting with atmospheric and oceanic pools. The natural processes of terrestrial C sequestration are already absorbing a large fraction of the anthropogenic emissions. For example, oceanic uptake is about 2.2 Pg C/year, and unknown terrestrial sinks absorbed a total of 2.7 Pg C/year during the 1990s, and 3.2 Pg during 2000s. Thus 57% of the 9.5 Pg C emitted was absorbed by natural sinks. Therefore, it is prudent to identify and enhance the capacity of natural processes. Carbon sequestration, both biotic and abiotic, is an important strategy for mitigating risks of global warming. It can influence the global C cycle over a short period and reduce the equilibrium level of atmospheric CO2 until the alternatives to fossil fuel take effect. A hypothetical scenario in Figure 4 shows that atmospheric concentration of CO2 by 2100may be 700 parts per million in terms of volume (ppmv) with business as usual and only 550ppmv by adoption of several C sequestration strategies. Feasibility of a C sequestration strategy depends on numerous factors: (1) sink capacity of the pool (e.g., soil, vegetation, geologic strata), (2) fate of the C sequestered and its residence time, (3) environmental impact on terrestrial or aquatic ecosystems, and (4) costeffectiveness of the strategy. The distribution of C pool in different biomes shows the high amount of terrestrial C in forest and savanna ecoregions and in northern peat lands (Table 1). The terrestrial C pool in managed ecosystems (e.g., cropland, permanent crops) differs from others in terms of the biomass C pool that is nearly negligible because it is harvested for human and animal consumption. The data in Table 2 show the potential of terrestrial C sequestration at 5.7 to 10.1 Pg C/year, of which the potential of agricultural lands at 0.8–0.9 Pg C/year includes only soil C. Similar estimates for croplands and world soils have been made by others, yet there are numerous uncertainties in these estimates.
Processes of Terrestrial Carbon Sequestration The terrestrial sequestration involves biological transformation of atmospheric CO2 into biomass and soil humus. Currently, the process leads to sequestration of approximately 2 Pg C/year. With improved management and natural CO2 fertilization effect under the enhanced CO2 levels, the potential of terrestrial C sequestration may be much larger (Table 2). Principal among management options include the following:
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Conversion of marginal croplands to biomass energy crop production
Figure 3 Global C pools and fluxes between them. All numbers are in petagrams (Pg ¼ 1015 g ¼ 1 billion metric tons). MRT, mean residence time.
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Figure 4 A hypothetical scenario about the impact of C sequestration by biotic and abiotic strategies on atmospheric concentration of CO2.
Table 1
Distribution of Terrestrial C Pool in Various Ecosystems
Ecosystem Forest Tropical Temperate Boreal Savanna Temperate woodlands Chaparral Tropical Temperate grassland Tundra, arctic & alpine Deserts Lake and streams Wetlands Peat lands and Cryosols Cultivated and permanent crops Perpetual ice Urban Total
Area (Mha)
NPP (Pg C/year)
Biomass C (Pg)
Soil C to 1 m (Pg)
1480 750 900
13.7 5.0 3.2
244.2 92.0 22.0
123 90 135
200 250 2250 1250 950 3000 200 280 340 1480 1550 200 15 080
1.4 0.9 17.8 4.4 1.0 1.5 0.4 3.3 0.0 6.3 0.0 0.2 59.1
16.0 8.0 65.9 9.0 6.0 7.2 0.0 12.0 0.0 3.0 0.0 1.0 486.4
24 30 263 295 121 191 0 202 496 117 0 10 2097
Source: Adopted from Amthor, J. S. and Huston, M. A. (1998). Terrestrial ecosystem responses to global change: A research strategy (ORNL/TM-1998/27). Nashville, TN: Oak Ridge National Laboratory, Intergovernment Panel on Climate Change (2000). Land use, land use change, and forestry: Special report. Cambridge, UK: Cambridge University Press and U.S. Department of Energy (1999). Carbon sequestration: Research and development. U.S. DOE, Springfield, VA: National Technical Information Service, Jungkunst, H. F., Kru¨ger, J. P., Heitkamp, F., Ersami, S., Glatzel, S., Fiedler, S. and Lal, R. (2012). Accounting more precisely for peat and other soil carbon sources. In: Lal, R., Lorenze, K., Hu¨ttl, R. F., Schneider, B. W., Von Braun J. (eds.) Recarbonization of the Biosphere, pp 127–157. Donrecht: Springer.
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Restoration of degraded soils and ecosystems Agricultural intensification on cropland and pastures Afforestation and forest soil management Restoration of wetlands
If the rate of C sequestration by adopting these options can be increased by 0.2% per year over 25 years, it can transfer 100 Pg C from the atmosphere into the terrestrial ecosystem through natural processes of biological transformation.
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Enhancing Terrestrial Carbon Sequestration The natural processes governing terrestrial C sequestration are outlined in Figure 5. The vegetative C pool can be enhanced by improving biomass production, increasing the lignin content that reduces the rate of decomposition, and diverting biomass into a deep and prolific root system. Afforestation, conversion of agriculturally marginal soils into restorative land uses, choice of fastgrowing species with deep root systems, and adoption of recommended management practices (RMPs) for stand, soil fertility, and pest management. In addition to the return of biomass to soil, there are several processes that moderate the soil C pool. The humification efficiency, the proportion of biomass converted to the stable humic fraction, depends on soil water and temperature regimes and the availability of essential elements (e.g., N, P, S, Zn, Cu, Mo, Fe). The amount and nature of clay, essential to formation and stabilization of aggregates, strongly influence the sequestration and residence time of C in soil. Effectiveness of chemical, biochemical, and physical processes of soil C sequestration depends on formation of stable microaggregates or organo– mineral complexes. Soil biodiversity, activity and species diversity of soil fauna such as earthworms and termites, plays an important role in bioturbation, C cycle, and humification of biosolids. Transfer of humus into the subsoil, precipitation of
Table 2
Global Potential of Terrestrial Carbon Sequestration
Biome
Potential of C sequestration (Pg C/year)
Agricultural lands Biomass croplands Grass lands Forest lands Urban lands Deserts and degraded lands Terrestrial sediments Boreal peat lands and other wetlands Total
0.4–1.2 0.50–0.80 0.3–0.5 1.6–1.9 0.15–0.3 0.2–0.7 0.4–0.8 0.1–0.7 3.65–6.9
Source: U.S. Department of Energy (1999). Carbon sequestration: Research and development. U.S. DOE, Springfield, VA: National Technical Information Service, Lal, R. (2010a). Carbon sequestration in saline soils. Journal of Soil Salinity and Water Quality 1(1&2), 30–40, Lal, R. and Augustin, B. (eds.) (2011). Carbon sequestration in urban ecosystems. Dordrecht, Netherlands: Springer, 383 pp.
Figure 5 Processes of enhancing terrestrial carbon sequestration.
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dissolved organic C (DOC), leaching and precipitation of carbonates, and formation of secondary carbonates are important processes that enhance soil C pool and increase its residence time. Conversion of natural ecosystems to agricultural ones (e.g., cropland, grazing land) depletes soil organic C (SOC) pool by as much as 50–80%. The magnitude and rate of depletion are generally greater in warm than in cool climates, in coarse-textured soils than in heavy-textured soils, and in soils with high antecedent SOC than in low antecedent SOC pool. Therefore, most agricultural soils now contain lower SOC pool than their potential capacity for the specific ecoregion. Conversion to a restorative land use and adoption of RMPs can enhance SOC pool. Feasible RMPs of agricultural intensification include using no-till farming with frequent incorporation of cover crops (meadows) in the rotation cycle, using manure and other biosolids as a component of integrated nutrient management strategy, and improving pasture growth by planting appropriate species and controlling stocking rate and the like. Restoring eroded/degraded soils and ecosystems is extremely important to enhancing the terrestrial C pool.
Ancillary Benefits There are numerous ancillary benefits of terrestrial C sequestration. Enhancing vegetative/ground cover and soil C pool reduces the risks of accelerated runoff, erosion, and sedimentation. The attendant decline in dissolved and suspended loads reduces risks of eutrophication of surface waters. Similarly, an increase in soil C pool decreases the transport of agricultural chemicals and other pollutants into the ground water. There are notable benefits of increased activity and species diversity of soil fauna and flora with an increase in SOC pool. In addition to enhancing soil structure, in terms of aggregation and its beneficial impacts on water retention and transmission, an increase in soil biodiversity also strengthens elemental/nutrient cycling. Soil is a living membrane between bedrock and the atmosphere, and its effectiveness is enhanced by an increase in soil C pool and its quality. An increase in terrestrial C sequestration in general, and in soil C sequestration in particular, improves soil physical, chemical, and biological quality (Figure 6). Improvements in soil physical quality lead to decreased risks of accelerated runoff and erosion, reduce crusting and compaction, and improve plant-available water capacity. Improvements in soil chemical quality lead to increased cation/anion exchange capacity, elemental balance, and favorable soil reaction. Improvements in soil biological quality lead to increased soil biodiversity and bioturbation, elemental/nutrient cycling, and biodegradation and bioremediation of pollutants. An integrative effect of improvement in soil quality is to increase biomass/agronomic productivity. On a global scale, soil C sequestration can have an important impact on food security.
Figure 6 Favorable effects of soil carbon sequestration on soil quality and agronomic/biomass productivity.
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Restoration of Degraded Ecosystems and Terrestrial Carbon Sequestration Accelerated soil erosion, soil degradation by other degradative processes (e.g., salinization, nutrient depletion, elemental imbalance, acidification), and desertification are severe global issues. Accelerated erosion by water and wind is the most widespread type of soil degradation. The land area affected by water erosion is estimated at 1094 million hectares (Mha), of which 751 Mha is severely affected. The land area affected by wind erosion is estimated at 549 Mha, of which 296 Mha is severely affected. In addition, 239 Mha is likely affected by chemical degradation and 83 Mha by physical degradation. Thus, the total land area affected by soil degradation may be 1965 Mha or 15% of the earth’s land area. With a sediment delivery ratio of 13–20%, the suspended sediment load is estimated at 20 109 metric tons (Mg)/year. Soil erosion affects SOC dynamics by slaking and breakdown of aggregates, preferential removal of C in surface runoff or wind, redistribution of C over the landscape, and mineralization of displaced/redistributed C. The redistributed SOC is generally light or labile fraction composed of particulate organic carbon (POC) and is easily mineralized. As much as 4–6 Pg C/year is transported globally by water erosion. Of this, 2.8–4.2 Pg C/year is redistributed over the landscape and transferred to depressional sites, 0.4–0.6 Pg C/year is transported into the ocean, and 0.8–1.2 Pg C/year is emitted into the atmosphere. The historic loss of SOC pool from soil degradation and other anthropogenic processes is 66–90 Pg C (78 12 Pg C), compared with total terrestrial C loss of 136 55 Pg C. Of the total SOC loss, that due to erosion by water and wind is estimated at 19–32 Pg C (26 9 Pg C) or 33% of the total loss. There is also a strong link between desertification and emission of C from soil and vegetation of the dryland ecosystems. Desertification is defined as the diminution or destruction of the biological potential of land that ultimately can lead to desert-like conditions. Estimates of the extent of desertification are wide-ranging and are often unreliable. The land area affected by desertification is estimated at 3.5–4.0 billion ha. The available data on the rate of desertification is also highly speculative and is estimated by some to be 5.8 Mha/year. Similar to other degradative processes, desertification leads to depletion of the terrestrial C pool. The historic loss of C due to desertification is estimated at 8–12 Pg from the soil C pool and 10–16 Pg from the vegetation C pool. Thus, the total historic C loss due to desertification may be 18–28 Pg C. Therefore, desertification control and restoration of degraded soils and ecosystems can reverse the degradative trends and sequester a large fraction of the historic C loss. The potential of desertification control for C sequestration is estimated at 0.6–1.4 Pg C/year. Management of drylands through desertification control has an overall C sequestration potential of 1.0 Pg C/year. These estimates of high C sequestration potential through restoration of degraded/desertified soils and ecosystems are in contrast to the low overall potential of world soils reported by some.
Potential and Challenges of Soil Carbon Sequestration Of the two components of terrestrial C sequestration—soil and vegetation—the importance of soil C sequestration is not widely recognized. The Kyoto Protocol has not yet accepted the soil C sink as offset for the fossil fuel emission. Yet soil C sequestration has vast potential and is a truly win–win situation. Assuming that 66 Pg C has been depleted from the SOC pool, perhaps 66% of it can be resequestered. The global potential of soil C sequestration in the world’s croplands is estimated at 0.6–1.2 Pg C/year. This potential can be realized over 50 years or until the sink capacity is filled. Some have questioned the practicability or feasibility of realizing the potential of soil C sequestration. The caution against optimism is based on the hidden C costs of fertilizers, irrigation, and other input that may produce no net gains by soil C sequestration. However, it is important to realize that SOC sequestration is a by-product of adopting RMPs and agricultural intensification, which are needed to enhance agronomic production for achieving global food security. The global population of 7.2 billion is increasing at a rate of 73million persons (or 1.3%) per year and is projected to reach 7.5 billion by 2020 and 9.2 billion by 2050. Consequently, the future food demand will increase drastically, and this will necessitate adoption of restorative measures to enhance soil quality and use of off-farm input (e.g., fertilizers, pesticides, irrigation, improved varieties) to increase productivity. Thus, fertilizers and other inputs are being used not for soil C sequestration but rather for increasing agricultural production to meet the needs of the rapidly increasing population. Soil C sequestration is an ancillary benefit of this inevitability and global necessity. There is also a question of the finite nature of the terrestrial C sink in general and that of the soil C sink in particular. The soil C sink capacity is approximately 50–60 Pg C out of the total capacity of the terrestrial C sink of 136 Pg. In contrast, the capacity of geologic and terrestrial sinks is much larger. However, soil C sequestration is the most cost-effective option in the near future, and there are no adverse ecological impacts. Engineering techniques for abiotic (e.g., deep injection of CO2 into geologic strata and ocean) strategies remain a work in progress, and are expensive. The question of permanence also needs to be addressed. How long can C sequestered in soil and biota remain in these pools? Does an occasional plowing of a no-till field negate all of the gains made so far in soil C sequestration? These are relevant questions, and the answers may vary for different soils and ecoregions. Total C stored in terrestrial ecosystems must be assessed by transparent, credible, precise, and cost-effective methods. Questions have often been raised about the assessment of soil C pool and fluxes on soilscape, farm, regional, and national scales. It is
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important to realize that standard methods are available to quantitatively assess SOC pool and fluxes on scales ranging from the pedon or soilscape level and then extrapolated to the ecosystem, regional, and national levels. Soil scientists and agronomists have studied changes in soil organic matter in relation to soil fertility and agronomic productivity since 1900 or earlier. They have adapted and improved the analytical procedures for assessing soil C sequestration for mitigating climate change since the 1980s. New procedures are being developed to assess soil C rapidly, precisely, and economically. Promising among new analytical procedures are accelerator mass spectrometry (AMS), pyrolysis molecular beam mass spectrometry (Py-MBMS), laser-induced breakdown spectroscopy (LIBS), and inelastic neutron scattering (INS). The LIBS technique presumably has good detection limits, precision, and accuracy.
Climate Change and Terrestrial Carbon Sequestration The increase in global temperature during the 20th century has been estimated at 0.6 C and is projected to be 1.4–5.8 C by 2100 relative to the global temperature in 1990. The projected increase in global temperature is attributed to enrichment of the atmospheric concentration of CO2 (which has increased from 280 to 400 ppm and is increasing at the rate of >2.0 ppm/year), CH4 (which has increased from 0.70–1.81 ppm and is increasing at the rate of 0.276% or 05 ppb/year), and N2O (which also has increased from 270 to 324 ppb and is increasing at the rate of 0.25% or 0.8 ppb/year). The implications of rapidly changing atmospheric chemistry are complex and not very well understood. The projected climate change may have a strong impact on terrestrial biomes, biomass productivity, and soil quality. It is estimated that for every 1 C increase in global temperature, the biome or vegetational zones (e.g., boreal forests, temperate forests, wooded/grass savannas) may shift poleward by 200–300 km.There may be a change in species composition of the climax vegetation within each biome. In northern latitudes where the temperature change is likely to be most drastic, every 1 C increase in temperature may prolong the growing season duration by 10 days. With predicted climate change, the rate of mineralization of soil organic matter may increase with adverse impacts on soil quality. There is a general consensus that CO2 enrichment will have a fertilization effect and will enhance C storage in the vegetation (both above and below ground) for 50–100 years. Numerous free-air CO2 enrichment (FACE) experiments have predicted increases in biomass production through increased net primary productivity (NPP), particularly in high latitudes. It was predicted that with an increase in temperature, tundra and boreal biomass will emit increasingly more C to the atmosphere while the humid tropical forests will continue to be a net sink. Soils of the tundra biome contain 393 Pg C as SOC and 17 Pg C as soil inorganic C (SIC) (410 Pg total), which is 16.4% of the world soil C pool. Soils of the boreal biome contain 382 Pg of SOC and 258 Pg of SIC (640 Pg total), which is 25.6% of the world soil C pool. Together, soils of the tundra and boreal (taiga) biomes contain 772 Pg or 42% of the world soil C pool. Recent estimates show that permafrost and peat lands may contain as much as 1700 Pg C to 3-m depth. Some soils of these regions, especially histosols and cryosols, have been major C sinks in the historic past. The projected climate change and some anthropogenic activities may disrupt the terrestrial C cycle of these biomes and render them a major source of CO2, CH4, and N2O. There exists a real danger that an increase in global temperature may result in a long-term loss of the soil C pool. There are numerous knowledge gaps and uncertainties regarding the impact of climate change on terrestrial C pool and fluxes. Principal knowledge gaps and uncertainties exist regarding fine root biology and longevity, nutrient (especially N and P) availability, interaction among cycles of various elements (e.g., C, N, P, H2O), and below-ground response.
Nutrient Requirements for Terrestrial Carbon Sequestration Carbon is only one of the building blocks of the terrestrial C pool. Other important components of the biomass and soil are essential macroelements (e.g., N, P, K, Ca, Mg) and micronutrients (e.g., Cu, Zn, B, Mo, Fe, Mn). A total of 1 Mg (or 1000 kg) of biomass produced by photosynthesis contains approximately 350–450 kg of C, 5 to 12 kg of N, 1 to 2 kg of P, 12 to 16 kg of K, 2 to 15 kg of Ca, and 2 to 5 kg of Mg. Therefore, biomass production of 10 Mg/ha/year requires the availability of 50–120 kg of N, 10–20 kg of P, 120–160 kg of K, 20–150 kg of Ca, and 20–50 kg of Mg. Additional nutrients are required for grain production. Nutrient requirements are lesser for wood and forages production than for grain and tuberous crops. Biomass (e.g., crop residues, leaf litter, detritus material, roots) is eventually converted into SOC pool through humification mediated by microbial processes. The humification efficiency is between 5% and 15%, depending on the composition of the biomass, soil properties, and climate. Soil C sequestration implies an increase in the SOC pool through conversion of biomass into humus. Similar to biomass production through photosynthesis, humification of biomass into SOC involves availability of nutrients. Additional nutrients required are N, P, S, and other minor elements. Assuming an elemental ratio of 12:1 for C:N, 50:1 for C:P, and 70:1 for C:S, sequestering 10 000 kg of C into humus will require 25 000 kg of residues (40% C), 833 kg of N, 200 kg of P, and 143 kg of S. Additional nutrients may be available in crop residue or soil or may be supplied through fertilizers and amendments. Nitrogen may also be made available through biological nitrogen fixation (BNF) and atmospheric deposition. It is argued that sequestration of soil C can be achieved only by the availability of these nutrients. Nutrient recycling, from those contained in the biomass and in the subsoil layers, is crucial to terrestrial C sequestration in general and to soil C sequestration in particular. For soil C sequestration of 500 kg C/ha/year, as is the normal rate for conversion from plow till to no till in Ohio, sequestration of 500 kg of C into humus (58% C) will require about 14 Mg of biomass (40% C and humification efficiency of 15%). This amount of residue from cereals will contain 65–200 kg of N, 8–40 kg of P, 70–340 kg of K,
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22–260 kg of Ca, and 10–130 kg of Mg. In comparison, the nutrients required for sequestration of 500 kg of C into humus are 42 kg of N, 10 kg of P, and 7 kg of S. Indeed, most of the nutrients required are contained in the biomass. Therefore, soil C sequestration is limited mostly by the availability of biomass. However, the production of biomass is limited by the availability of nutrients, especially N and P.
The Permanence of Carbon Sequestered in Terrestrial Ecosystems The mean residence time (MRT) of C sequestered in soil and biota depends on numerous factors, including land use, management system, soil type, vegetation, and climate. The MRT is computed as a ratio of the pool (Mg ha1) to the flux (Mg/ha/year). On this basis, the global MRT of C is 5 years in the atmosphere, 10 years in the vegetation, 35 years in the soils, and 100 years in the ocean (Figure 3). However, there is a wide range of MRT depending on the site-specific conditions and characteristics of the organic matter. For example, the MRT of soil organic matter is less than 1 year for the labile fraction, 1–100 years for the intermediate fraction, and several millennia for the passive fraction. The MRT also depends on management. Conversion from plow till to no till sequesters C in soil, and the MRT depends on continuous use of no till. Even occasional plowing can lead to emission of CO2. Biofuel production is another important strategy. The C sequestered in biomass is released when the biosolids are burned. However, C released from biofuel is recycled again and is in fact a fossil fuel offset. There is a major difference between the two. The fossil fuel combustion releases new CO2 into the atmosphere, and burning of biofuels merely recycles the CO2.
Terrestrial Carbon Sequestration and Global Food Security An important ancillary benefit of terrestrial C sequestration in general, and of soil C sequestration in particular, is the potential for increasing world food security. The projected global warming, with a likely increase in mean global temperature of 4 2 C, may adversely affect soil temperature and water regimes and the NPP. The increase in soil temperature may reduce soil organic matter pool with an attendant decrease in soil quality, increase in erosion and soil degradation, and decrease in NPP and agronomic yields. In contrast, the degradative trends and downward spiral may be reversed by an increase in soil C sequestration leading to improvement in soil quality and an increase in agronomic/biomass productivity. There already exists a large intercountry/ interregional variation in average yields, the so-called “yield gap”, of wheat (Triticum aestivum), corn (Zea mays), and other crops. Yields of wheat range from a high of 6.0–7.8 Mg ha1 in European Union countries (e.g., United Kingdom, Denmark, Germany, France), to a middle range of 2.4 to 2.7 Mg ha1 (e.g., United States, India, Romania, Ukraine, Argentina, Canada), to a very low range of 1.0 to 2.2 Mg ha1 (e.g., Pakistan, Turkey, Australia, Iran, Russia, Kazakhstan). Similarly, average yields of corn range from a high of 8–10 Mg ha1 (e.g., Italy, France, Spain, United States), to a middle range of 4 to 7 Mg ha1 (e.g., Canada, Egypt, Hungary, Argentina), to a low range of 1 to 2 Mg ha1 (e.g., Nigeria, Philippines, India). A concern regarding projected global warming is whether the agroecological yield may be adversely affected by the change in soil temperature and moisture regimes and the threat of increase in food deficit in Sub-Saharan Africa and elsewhere in developing countries. The adverse impact of decline in SOC pool on global food security is also evident from the required increase in crop yield in developing countries to meet the demands of increased population. With a little or no possibility of bringing new land area under cultivation in most developing countries, the yields of cereals in developing countries will have to increase from 2.64 Mg ha1 in 1997–1998 to 4.4 Mg ha1 by 2025 and to 6.0 Mg ha1 by 2050. Can this drastic increase in yields of cereals in developing countries be attained by agricultural intensification in the face of projected global warming? An important factor that can help to attain this challenging goal is improvement in soil quality through terrestrial and soil C sequestration. Improvements in soil C pool can be achieved by increasing below-ground C directly through input of root biomass. The importance of irrigation, fertilizer and nutrient acquisition, erosion control, and use of mulch farming and conservation tillage cannot be overemphasized.
Conclusions Terrestrial C sequestration is a truly win–win scenario. In addition to improving the much needed agronomic/biomass production to meet the needs of a rapidly increasing world population, terrestrial C sequestration improves soil quality, reduces sediment transport in rivers and reservoirs, enhances biodiversity, increases biodegradation of pollutants, and mitigates the risks of climate change. Besides, it is a natural process with numerous ancillary benefits. Although it has a finite capacity to absorb atmospheric CO2, it is the most desirable option available at the onset of the 21st century. The strategy of terrestrial C sequestration buys us time until C-neutral fuel options take effect.
Further Reading Amthor JS and Huston MA (1998) Terrestrial ecosystem responses oak ridge national laboratory to global change: A research strategy (ORNL/TM-1998/27). Nashville, TN: Oak Ridge National Laboratory.
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Eswaran H, Vanden Berg E, Reich P, and Kimble JM (1995) Global soil carbon resources. In: Lal R, Kimble JM, Levine E, and Stewart BA (eds.) Soils and global change, pp. 27–43. Boca Raton, FL: CRC Press. Halmann MM and Steinberg M (1999) Greenhouse gas carbon dioxide mitigation: Science and technology. Boca Raton, FL: Lewis Publishers. Hao Y, Lal R, Owens L, Izaurralde RC, Post M, and Hothem D (2002) Effect of cropland management and slope position on soil organic carbon pools at the North Appalachian Experimental Watersheds. Soil Tillage Res. 68: 133–142. Himes FL (1998) Nitrogen, sulfur, and phosphorus and the sequestering of carbon. In: Lal R, Kimble JM, Follett RF, and Stewart BA (eds.) Soil processes and the carbon cycle, pp. 315–319. Boca Raton, FL: CRC Press. Intergovernment Panel on Climate Change (1996) Climate change 1995: Impacts, adaptations, and mitigation of climatic change—Scientific–technical analyses. Cambridge, UK: Cambridge University Press. Intergovernment Panel on Climate Change (2000) Land use, land use change, and forestry: Special report. Cambridge, UK: Cambridge University Press. Intergovernment Panel on Climate Change (2001) Climate change: The scientific basis. Cambridge, UK: Cambridge University Press. Jungkunst HF, Kru¨ger JP, Heitkamp F, Ersami S, Glatzel S, Fiedler S, and Lal R (2012) Accounting more precisely for peat and other soil carbon sources. In: Lal R, Lorenze K, Hu¨ttl RF, Schneider BW, and Von Braun J (eds.) Recarbonization of the Biosphere, pp. 127–157. Donrecht: Springer. Lal R (1995) Global soil erosion by water and carbon dynamics. In: Lal R, Kimble JM, Levine E, and Stewart BA (eds.) Soils and global change, pp. 131–141. Boca Raton, FL: CRC/ Lewis Publishers. Lal R (1999) Soil management and restoration for C sequestration to mitigate the greenhouse effect. Progress in Environmental Science 1: 307–326. Lal R (2001a) Potential of desertification control to sequester carbon and mitigate the greenhouse effect. Climate Change 15: 35–72. Lal R (2001b) World cropland soils as a source or sink for atmospheric carbon. Advances in Agronomy 71: 145–191. Lal R (2003a) Global potential of soil C sequestration to mitigate the greenhouse effect. Critical Reviews in Plant Sciences 22: 151–184. Lal R (2003b) Soil erosion and the global carbon budget. Environment International 29: 437–450. Lal R (2004) Soil carbon sequestration impacts on global climate change and food security. Science 304: 1623–1627, (www.sciencemag.org/cgi/content/full/305/5690/1567DCI). Lal R (2010a) Carbon sequestration in saline soils. Journal of Soil Salinity and Water Quality 1(1&2): 30–40. Lal R (2010b) Managing soils and ecosystems for mitigating anthropogenic carbon emissions and advancing global food security. Bioscience 60(9): 708–721. Lal R and Augustin B (eds.) (2011) Carbon Sequestration in Urban Ecosystems. Dordrecht, Netherlands: Springer, 383 pp. Mainguet M (1991) Desertification: Natural background and human mismanagement. Berlin: Springer-Verlag. Oberthu¨r S and Ott HE (2001) The Kyoto protocol: International climate policy for the 21st century. Berlin: Springer. Oldeman LR (1994) The global extent of soil degradation. In: Greenland DJ and Szabolcs I (eds.) Soil resilience and sustainable land use, pp. 99–118. Wallingford, UK: CAB International. Oldeman LR and Van Lynden GWJ (1998) Revisiting the GLASOD methodology. In: Lal R, Blum WH, Valentine C, and Stewart BA (eds.) Methods for assessment of soil degradation, pp. 423–440. Boca Raton, FL: CRC Press. Schlesinger WH (1999) Carbon sequestration in soils. Science 286: 2095. U.S. Department of Energy (1999) Carbon sequestration: Research and development. Springfield, VA: National Technical Information Service. WMO (2012). Greenhouse Gas Bulletin, 4pp, Geneva, Switzerland, #8. Nov.
Introduction Carbon Sequestration Processes of Terrestrial Carbon Sequestration Enhancing Terrestrial Carbon Sequestration Ancillary Benefits Restoration of Degraded Ecosystems and Terrestrial Carbon Sequestration Potential and Challenges of Soil Carbon Sequestration Climate Change and Terrestrial Carbon Sequestration Nutrient Requirements for Terrestrial Carbon Sequestration The Permanence of Carbon Sequestered in Terrestrial Ecosystems Terrestrial Carbon Sequestration and Global Food Security Conclusions
Glossary ancillary benefits Additional benefits of a process. anthropogenic Human-induced changes. biomass/biosolids Material of organic origin. carbon sequestration Transfer of carbon from atmosphere into the long-lived carbon pools such as vegetation and soil. humus The decomposed organic matter in which remains of plants and animals are not identifiable.
1 1 3 5 6 7 7 8 8 9 9 9
mean residence time The average time an atom of carbon spends within a pool; calculated by dividing the pool by flux. soil organic matter Sum of all organic substances in the soil. soil quality Ability of a soil to perform functions of interest to humans such as biomass production, water filtration, biodegradation of pollutants, and soil carbon sequestration. terrestrial carbon Carbon contained in the vegetation and soil.
Introduction Specific options of mitigating climate change can be grouped broadly into two categories: adaptive responses and mitigative responses (Figure 1). Adaptive responses are aimed at managing the risks and identifying beneficial opportunities. Mitigative responses are aimed at reducing overall emissions of greenhouse gases (GHGs). Adaptive responses involve identification of options for sustainable management of natural resources (e.g., terrestrial ecosystems, water resources and wetlands, agricultural ecosystems, deserts) under changed climate with projected alterations in precipitation, frequency of extreme events, growing season duration, and incidence of pests and pathogens affecting health of human and other biota. Mitigative responses involve controlling atmospheric concentration of carbon dioxide (CO2) either by removing it from the atmosphere or by avoiding its emission into the atmosphere. Important among mitigative strategies are reducing and sequestering emissions. Emissions reduction may involve improving energy use/conversion efficiency, developing alternatives to fossil fuel or developing carbon (C)-neutral fuel sources, using space reflectors to reduce the amount of radiation reaching the earth, absorbing CO2 from the atmosphere using special extractants, and sequestering C in long-lived pools. Both adaptive and mitigative strategies are important to reducing the rate of enrichment of atmospheric concentration of CO2. This article describes the principles, opportunities, and challenges of C sequestration in terrestrial pools composed of soils and vegetation.
Carbon Sequestration Carbon sequestration implies capture and secure storage of C that would otherwise be emitted to or remain in the atmosphere. The objective is to transfer C from the atmosphere into long-lived pools (e.g., biota, soil, geologic strata, ocean). It is the net removal of CO2 from the atmosphere or prevention of CO2 emissions from natural and managed ecosystems. Strategies of C sequestration can be grouped under two broad categories: biotic and abiotic (Figure 2). Biotic strategies are based on natural process of photosynthesis and
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Change History: May 2013. R Lal updated all parts of the text.
Reference Module in Earth Systems and Environmental Sciences
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Figure 1 Carbon management strategies to mitigate the greenhouse effect.
Figure 2 Carbon sequestration strategies.
transfer of CO2 from atmosphere into vegetative, pedologic, and aquatic pools through mediation via green plants. Abiotic strategies involve separation, capture, compression, transport, and injection of CO2 from power plant flue gases and effluent of industrial processes deep into ocean and geologic strata. The strategy is to use engineering techniques for keeping industrial emissions from reaching the atmosphere.
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Terrestrial C sequestration is a natural process. It involves transfer of atmospheric CO2 through photosynthesis into vegetative and pedologic/soil C pools. The terrestrial pool is the third largest among the five global C pools (Figure 3). It includes 2300 petagrams (Pg, where 1 Pg ¼ 1015 g ¼ 1 billion metric tons) of C in soil to 1-m depth and 620 Pg in the vegetative pool, including the detritus material. The soil C pool to 3-m depth, including the revised estimates of the C pool in Cryosols and permafrost, may be as much as 4000Pg. Thus, the terrestrial C pool is approximately four times the size of the atmospheric pool and 60% the size of the fossil C pool. It is also a very active pool and is continuously interacting with atmospheric and oceanic pools. The natural processes of terrestrial C sequestration are already absorbing a large fraction of the anthropogenic emissions. For example, oceanic uptake is about 2.2 Pg C/year, and unknown terrestrial sinks absorbed a total of 2.7 Pg C/year during the 1990s, and 3.2 Pg during 2000s. Thus 57% of the 9.5 Pg C emitted was absorbed by natural sinks. Therefore, it is prudent to identify and enhance the capacity of natural processes. Carbon sequestration, both biotic and abiotic, is an important strategy for mitigating risks of global warming. It can influence the global C cycle over a short period and reduce the equilibrium level of atmospheric CO2 until the alternatives to fossil fuel take effect. A hypothetical scenario in Figure 4 shows that atmospheric concentration of CO2 by 2100may be 700 parts per million in terms of volume (ppmv) with business as usual and only 550ppmv by adoption of several C sequestration strategies. Feasibility of a C sequestration strategy depends on numerous factors: (1) sink capacity of the pool (e.g., soil, vegetation, geologic strata), (2) fate of the C sequestered and its residence time, (3) environmental impact on terrestrial or aquatic ecosystems, and (4) costeffectiveness of the strategy. The distribution of C pool in different biomes shows the high amount of terrestrial C in forest and savanna ecoregions and in northern peat lands (Table 1). The terrestrial C pool in managed ecosystems (e.g., cropland, permanent crops) differs from others in terms of the biomass C pool that is nearly negligible because it is harvested for human and animal consumption. The data in Table 2 show the potential of terrestrial C sequestration at 5.7 to 10.1 Pg C/year, of which the potential of agricultural lands at 0.8–0.9 Pg C/year includes only soil C. Similar estimates for croplands and world soils have been made by others, yet there are numerous uncertainties in these estimates.
Processes of Terrestrial Carbon Sequestration The terrestrial sequestration involves biological transformation of atmospheric CO2 into biomass and soil humus. Currently, the process leads to sequestration of approximately 2 Pg C/year. With improved management and natural CO2 fertilization effect under the enhanced CO2 levels, the potential of terrestrial C sequestration may be much larger (Table 2). Principal among management options include the following:
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Conversion of marginal croplands to biomass energy crop production
Figure 3 Global C pools and fluxes between them. All numbers are in petagrams (Pg ¼ 1015 g ¼ 1 billion metric tons). MRT, mean residence time.
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Figure 4 A hypothetical scenario about the impact of C sequestration by biotic and abiotic strategies on atmospheric concentration of CO2.
Table 1
Distribution of Terrestrial C Pool in Various Ecosystems
Ecosystem Forest Tropical Temperate Boreal Savanna Temperate woodlands Chaparral Tropical Temperate grassland Tundra, arctic & alpine Deserts Lake and streams Wetlands Peat lands and Cryosols Cultivated and permanent crops Perpetual ice Urban Total
Area (Mha)
NPP (Pg C/year)
Biomass C (Pg)
Soil C to 1 m (Pg)
1480 750 900
13.7 5.0 3.2
244.2 92.0 22.0
123 90 135
200 250 2250 1250 950 3000 200 280 340 1480 1550 200 15 080
1.4 0.9 17.8 4.4 1.0 1.5 0.4 3.3 0.0 6.3 0.0 0.2 59.1
16.0 8.0 65.9 9.0 6.0 7.2 0.0 12.0 0.0 3.0 0.0 1.0 486.4
24 30 263 295 121 191 0 202 496 117 0 10 2097
Source: Adopted from Amthor, J. S. and Huston, M. A. (1998). Terrestrial ecosystem responses to global change: A research strategy (ORNL/TM-1998/27). Nashville, TN: Oak Ridge National Laboratory, Intergovernment Panel on Climate Change (2000). Land use, land use change, and forestry: Special report. Cambridge, UK: Cambridge University Press and U.S. Department of Energy (1999). Carbon sequestration: Research and development. U.S. DOE, Springfield, VA: National Technical Information Service, Jungkunst, H. F., Kru¨ger, J. P., Heitkamp, F., Ersami, S., Glatzel, S., Fiedler, S. and Lal, R. (2012). Accounting more precisely for peat and other soil carbon sources. In: Lal, R., Lorenze, K., Hu¨ttl, R. F., Schneider, B. W., Von Braun J. (eds.) Recarbonization of the Biosphere, pp 127–157. Donrecht: Springer.
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Restoration of degraded soils and ecosystems Agricultural intensification on cropland and pastures Afforestation and forest soil management Restoration of wetlands
If the rate of C sequestration by adopting these options can be increased by 0.2% per year over 25 years, it can transfer 100 Pg C from the atmosphere into the terrestrial ecosystem through natural processes of biological transformation.
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Enhancing Terrestrial Carbon Sequestration The natural processes governing terrestrial C sequestration are outlined in Figure 5. The vegetative C pool can be enhanced by improving biomass production, increasing the lignin content that reduces the rate of decomposition, and diverting biomass into a deep and prolific root system. Afforestation, conversion of agriculturally marginal soils into restorative land uses, choice of fastgrowing species with deep root systems, and adoption of recommended management practices (RMPs) for stand, soil fertility, and pest management. In addition to the return of biomass to soil, there are several processes that moderate the soil C pool. The humification efficiency, the proportion of biomass converted to the stable humic fraction, depends on soil water and temperature regimes and the availability of essential elements (e.g., N, P, S, Zn, Cu, Mo, Fe). The amount and nature of clay, essential to formation and stabilization of aggregates, strongly influence the sequestration and residence time of C in soil. Effectiveness of chemical, biochemical, and physical processes of soil C sequestration depends on formation of stable microaggregates or organo– mineral complexes. Soil biodiversity, activity and species diversity of soil fauna such as earthworms and termites, plays an important role in bioturbation, C cycle, and humification of biosolids. Transfer of humus into the subsoil, precipitation of
Table 2
Global Potential of Terrestrial Carbon Sequestration
Biome
Potential of C sequestration (Pg C/year)
Agricultural lands Biomass croplands Grass lands Forest lands Urban lands Deserts and degraded lands Terrestrial sediments Boreal peat lands and other wetlands Total
0.4–1.2 0.50–0.80 0.3–0.5 1.6–1.9 0.15–0.3 0.2–0.7 0.4–0.8 0.1–0.7 3.65–6.9
Source: U.S. Department of Energy (1999). Carbon sequestration: Research and development. U.S. DOE, Springfield, VA: National Technical Information Service, Lal, R. (2010a). Carbon sequestration in saline soils. Journal of Soil Salinity and Water Quality 1(1&2), 30–40, Lal, R. and Augustin, B. (eds.) (2011). Carbon sequestration in urban ecosystems. Dordrecht, Netherlands: Springer, 383 pp.
Figure 5 Processes of enhancing terrestrial carbon sequestration.
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dissolved organic C (DOC), leaching and precipitation of carbonates, and formation of secondary carbonates are important processes that enhance soil C pool and increase its residence time. Conversion of natural ecosystems to agricultural ones (e.g., cropland, grazing land) depletes soil organic C (SOC) pool by as much as 50–80%. The magnitude and rate of depletion are generally greater in warm than in cool climates, in coarse-textured soils than in heavy-textured soils, and in soils with high antecedent SOC than in low antecedent SOC pool. Therefore, most agricultural soils now contain lower SOC pool than their potential capacity for the specific ecoregion. Conversion to a restorative land use and adoption of RMPs can enhance SOC pool. Feasible RMPs of agricultural intensification include using no-till farming with frequent incorporation of cover crops (meadows) in the rotation cycle, using manure and other biosolids as a component of integrated nutrient management strategy, and improving pasture growth by planting appropriate species and controlling stocking rate and the like. Restoring eroded/degraded soils and ecosystems is extremely important to enhancing the terrestrial C pool.
Ancillary Benefits There are numerous ancillary benefits of terrestrial C sequestration. Enhancing vegetative/ground cover and soil C pool reduces the risks of accelerated runoff, erosion, and sedimentation. The attendant decline in dissolved and suspended loads reduces risks of eutrophication of surface waters. Similarly, an increase in soil C pool decreases the transport of agricultural chemicals and other pollutants into the ground water. There are notable benefits of increased activity and species diversity of soil fauna and flora with an increase in SOC pool. In addition to enhancing soil structure, in terms of aggregation and its beneficial impacts on water retention and transmission, an increase in soil biodiversity also strengthens elemental/nutrient cycling. Soil is a living membrane between bedrock and the atmosphere, and its effectiveness is enhanced by an increase in soil C pool and its quality. An increase in terrestrial C sequestration in general, and in soil C sequestration in particular, improves soil physical, chemical, and biological quality (Figure 6). Improvements in soil physical quality lead to decreased risks of accelerated runoff and erosion, reduce crusting and compaction, and improve plant-available water capacity. Improvements in soil chemical quality lead to increased cation/anion exchange capacity, elemental balance, and favorable soil reaction. Improvements in soil biological quality lead to increased soil biodiversity and bioturbation, elemental/nutrient cycling, and biodegradation and bioremediation of pollutants. An integrative effect of improvement in soil quality is to increase biomass/agronomic productivity. On a global scale, soil C sequestration can have an important impact on food security.
Figure 6 Favorable effects of soil carbon sequestration on soil quality and agronomic/biomass productivity.
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Restoration of Degraded Ecosystems and Terrestrial Carbon Sequestration Accelerated soil erosion, soil degradation by other degradative processes (e.g., salinization, nutrient depletion, elemental imbalance, acidification), and desertification are severe global issues. Accelerated erosion by water and wind is the most widespread type of soil degradation. The land area affected by water erosion is estimated at 1094 million hectares (Mha), of which 751 Mha is severely affected. The land area affected by wind erosion is estimated at 549 Mha, of which 296 Mha is severely affected. In addition, 239 Mha is likely affected by chemical degradation and 83 Mha by physical degradation. Thus, the total land area affected by soil degradation may be 1965 Mha or 15% of the earth’s land area. With a sediment delivery ratio of 13–20%, the suspended sediment load is estimated at 20 109 metric tons (Mg)/year. Soil erosion affects SOC dynamics by slaking and breakdown of aggregates, preferential removal of C in surface runoff or wind, redistribution of C over the landscape, and mineralization of displaced/redistributed C. The redistributed SOC is generally light or labile fraction composed of particulate organic carbon (POC) and is easily mineralized. As much as 4–6 Pg C/year is transported globally by water erosion. Of this, 2.8–4.2 Pg C/year is redistributed over the landscape and transferred to depressional sites, 0.4–0.6 Pg C/year is transported into the ocean, and 0.8–1.2 Pg C/year is emitted into the atmosphere. The historic loss of SOC pool from soil degradation and other anthropogenic processes is 66–90 Pg C (78 12 Pg C), compared with total terrestrial C loss of 136 55 Pg C. Of the total SOC loss, that due to erosion by water and wind is estimated at 19–32 Pg C (26 9 Pg C) or 33% of the total loss. There is also a strong link between desertification and emission of C from soil and vegetation of the dryland ecosystems. Desertification is defined as the diminution or destruction of the biological potential of land that ultimately can lead to desert-like conditions. Estimates of the extent of desertification are wide-ranging and are often unreliable. The land area affected by desertification is estimated at 3.5–4.0 billion ha. The available data on the rate of desertification is also highly speculative and is estimated by some to be 5.8 Mha/year. Similar to other degradative processes, desertification leads to depletion of the terrestrial C pool. The historic loss of C due to desertification is estimated at 8–12 Pg from the soil C pool and 10–16 Pg from the vegetation C pool. Thus, the total historic C loss due to desertification may be 18–28 Pg C. Therefore, desertification control and restoration of degraded soils and ecosystems can reverse the degradative trends and sequester a large fraction of the historic C loss. The potential of desertification control for C sequestration is estimated at 0.6–1.4 Pg C/year. Management of drylands through desertification control has an overall C sequestration potential of 1.0 Pg C/year. These estimates of high C sequestration potential through restoration of degraded/desertified soils and ecosystems are in contrast to the low overall potential of world soils reported by some.
Potential and Challenges of Soil Carbon Sequestration Of the two components of terrestrial C sequestration—soil and vegetation—the importance of soil C sequestration is not widely recognized. The Kyoto Protocol has not yet accepted the soil C sink as offset for the fossil fuel emission. Yet soil C sequestration has vast potential and is a truly win–win situation. Assuming that 66 Pg C has been depleted from the SOC pool, perhaps 66% of it can be resequestered. The global potential of soil C sequestration in the world’s croplands is estimated at 0.6–1.2 Pg C/year. This potential can be realized over 50 years or until the sink capacity is filled. Some have questioned the practicability or feasibility of realizing the potential of soil C sequestration. The caution against optimism is based on the hidden C costs of fertilizers, irrigation, and other input that may produce no net gains by soil C sequestration. However, it is important to realize that SOC sequestration is a by-product of adopting RMPs and agricultural intensification, which are needed to enhance agronomic production for achieving global food security. The global population of 7.2 billion is increasing at a rate of 73million persons (or 1.3%) per year and is projected to reach 7.5 billion by 2020 and 9.2 billion by 2050. Consequently, the future food demand will increase drastically, and this will necessitate adoption of restorative measures to enhance soil quality and use of off-farm input (e.g., fertilizers, pesticides, irrigation, improved varieties) to increase productivity. Thus, fertilizers and other inputs are being used not for soil C sequestration but rather for increasing agricultural production to meet the needs of the rapidly increasing population. Soil C sequestration is an ancillary benefit of this inevitability and global necessity. There is also a question of the finite nature of the terrestrial C sink in general and that of the soil C sink in particular. The soil C sink capacity is approximately 50–60 Pg C out of the total capacity of the terrestrial C sink of 136 Pg. In contrast, the capacity of geologic and terrestrial sinks is much larger. However, soil C sequestration is the most cost-effective option in the near future, and there are no adverse ecological impacts. Engineering techniques for abiotic (e.g., deep injection of CO2 into geologic strata and ocean) strategies remain a work in progress, and are expensive. The question of permanence also needs to be addressed. How long can C sequestered in soil and biota remain in these pools? Does an occasional plowing of a no-till field negate all of the gains made so far in soil C sequestration? These are relevant questions, and the answers may vary for different soils and ecoregions. Total C stored in terrestrial ecosystems must be assessed by transparent, credible, precise, and cost-effective methods. Questions have often been raised about the assessment of soil C pool and fluxes on soilscape, farm, regional, and national scales. It is
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important to realize that standard methods are available to quantitatively assess SOC pool and fluxes on scales ranging from the pedon or soilscape level and then extrapolated to the ecosystem, regional, and national levels. Soil scientists and agronomists have studied changes in soil organic matter in relation to soil fertility and agronomic productivity since 1900 or earlier. They have adapted and improved the analytical procedures for assessing soil C sequestration for mitigating climate change since the 1980s. New procedures are being developed to assess soil C rapidly, precisely, and economically. Promising among new analytical procedures are accelerator mass spectrometry (AMS), pyrolysis molecular beam mass spectrometry (Py-MBMS), laser-induced breakdown spectroscopy (LIBS), and inelastic neutron scattering (INS). The LIBS technique presumably has good detection limits, precision, and accuracy.
Climate Change and Terrestrial Carbon Sequestration The increase in global temperature during the 20th century has been estimated at 0.6 C and is projected to be 1.4–5.8 C by 2100 relative to the global temperature in 1990. The projected increase in global temperature is attributed to enrichment of the atmospheric concentration of CO2 (which has increased from 280 to 400 ppm and is increasing at the rate of >2.0 ppm/year), CH4 (which has increased from 0.70–1.81 ppm and is increasing at the rate of 0.276% or 05 ppb/year), and N2O (which also has increased from 270 to 324 ppb and is increasing at the rate of 0.25% or 0.8 ppb/year). The implications of rapidly changing atmospheric chemistry are complex and not very well understood. The projected climate change may have a strong impact on terrestrial biomes, biomass productivity, and soil quality. It is estimated that for every 1 C increase in global temperature, the biome or vegetational zones (e.g., boreal forests, temperate forests, wooded/grass savannas) may shift poleward by 200–300 km.There may be a change in species composition of the climax vegetation within each biome. In northern latitudes where the temperature change is likely to be most drastic, every 1 C increase in temperature may prolong the growing season duration by 10 days. With predicted climate change, the rate of mineralization of soil organic matter may increase with adverse impacts on soil quality. There is a general consensus that CO2 enrichment will have a fertilization effect and will enhance C storage in the vegetation (both above and below ground) for 50–100 years. Numerous free-air CO2 enrichment (FACE) experiments have predicted increases in biomass production through increased net primary productivity (NPP), particularly in high latitudes. It was predicted that with an increase in temperature, tundra and boreal biomass will emit increasingly more C to the atmosphere while the humid tropical forests will continue to be a net sink. Soils of the tundra biome contain 393 Pg C as SOC and 17 Pg C as soil inorganic C (SIC) (410 Pg total), which is 16.4% of the world soil C pool. Soils of the boreal biome contain 382 Pg of SOC and 258 Pg of SIC (640 Pg total), which is 25.6% of the world soil C pool. Together, soils of the tundra and boreal (taiga) biomes contain 772 Pg or 42% of the world soil C pool. Recent estimates show that permafrost and peat lands may contain as much as 1700 Pg C to 3-m depth. Some soils of these regions, especially histosols and cryosols, have been major C sinks in the historic past. The projected climate change and some anthropogenic activities may disrupt the terrestrial C cycle of these biomes and render them a major source of CO2, CH4, and N2O. There exists a real danger that an increase in global temperature may result in a long-term loss of the soil C pool. There are numerous knowledge gaps and uncertainties regarding the impact of climate change on terrestrial C pool and fluxes. Principal knowledge gaps and uncertainties exist regarding fine root biology and longevity, nutrient (especially N and P) availability, interaction among cycles of various elements (e.g., C, N, P, H2O), and below-ground response.
Nutrient Requirements for Terrestrial Carbon Sequestration Carbon is only one of the building blocks of the terrestrial C pool. Other important components of the biomass and soil are essential macroelements (e.g., N, P, K, Ca, Mg) and micronutrients (e.g., Cu, Zn, B, Mo, Fe, Mn). A total of 1 Mg (or 1000 kg) of biomass produced by photosynthesis contains approximately 350–450 kg of C, 5 to 12 kg of N, 1 to 2 kg of P, 12 to 16 kg of K, 2 to 15 kg of Ca, and 2 to 5 kg of Mg. Therefore, biomass production of 10 Mg/ha/year requires the availability of 50–120 kg of N, 10–20 kg of P, 120–160 kg of K, 20–150 kg of Ca, and 20–50 kg of Mg. Additional nutrients are required for grain production. Nutrient requirements are lesser for wood and forages production than for grain and tuberous crops. Biomass (e.g., crop residues, leaf litter, detritus material, roots) is eventually converted into SOC pool through humification mediated by microbial processes. The humification efficiency is between 5% and 15%, depending on the composition of the biomass, soil properties, and climate. Soil C sequestration implies an increase in the SOC pool through conversion of biomass into humus. Similar to biomass production through photosynthesis, humification of biomass into SOC involves availability of nutrients. Additional nutrients required are N, P, S, and other minor elements. Assuming an elemental ratio of 12:1 for C:N, 50:1 for C:P, and 70:1 for C:S, sequestering 10 000 kg of C into humus will require 25 000 kg of residues (40% C), 833 kg of N, 200 kg of P, and 143 kg of S. Additional nutrients may be available in crop residue or soil or may be supplied through fertilizers and amendments. Nitrogen may also be made available through biological nitrogen fixation (BNF) and atmospheric deposition. It is argued that sequestration of soil C can be achieved only by the availability of these nutrients. Nutrient recycling, from those contained in the biomass and in the subsoil layers, is crucial to terrestrial C sequestration in general and to soil C sequestration in particular. For soil C sequestration of 500 kg C/ha/year, as is the normal rate for conversion from plow till to no till in Ohio, sequestration of 500 kg of C into humus (58% C) will require about 14 Mg of biomass (40% C and humification efficiency of 15%). This amount of residue from cereals will contain 65–200 kg of N, 8–40 kg of P, 70–340 kg of K,
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22–260 kg of Ca, and 10–130 kg of Mg. In comparison, the nutrients required for sequestration of 500 kg of C into humus are 42 kg of N, 10 kg of P, and 7 kg of S. Indeed, most of the nutrients required are contained in the biomass. Therefore, soil C sequestration is limited mostly by the availability of biomass. However, the production of biomass is limited by the availability of nutrients, especially N and P.
The Permanence of Carbon Sequestered in Terrestrial Ecosystems The mean residence time (MRT) of C sequestered in soil and biota depends on numerous factors, including land use, management system, soil type, vegetation, and climate. The MRT is computed as a ratio of the pool (Mg ha1) to the flux (Mg/ha/year). On this basis, the global MRT of C is 5 years in the atmosphere, 10 years in the vegetation, 35 years in the soils, and 100 years in the ocean (Figure 3). However, there is a wide range of MRT depending on the site-specific conditions and characteristics of the organic matter. For example, the MRT of soil organic matter is less than 1 year for the labile fraction, 1–100 years for the intermediate fraction, and several millennia for the passive fraction. The MRT also depends on management. Conversion from plow till to no till sequesters C in soil, and the MRT depends on continuous use of no till. Even occasional plowing can lead to emission of CO2. Biofuel production is another important strategy. The C sequestered in biomass is released when the biosolids are burned. However, C released from biofuel is recycled again and is in fact a fossil fuel offset. There is a major difference between the two. The fossil fuel combustion releases new CO2 into the atmosphere, and burning of biofuels merely recycles the CO2.
Terrestrial Carbon Sequestration and Global Food Security An important ancillary benefit of terrestrial C sequestration in general, and of soil C sequestration in particular, is the potential for increasing world food security. The projected global warming, with a likely increase in mean global temperature of 4 2 C, may adversely affect soil temperature and water regimes and the NPP. The increase in soil temperature may reduce soil organic matter pool with an attendant decrease in soil quality, increase in erosion and soil degradation, and decrease in NPP and agronomic yields. In contrast, the degradative trends and downward spiral may be reversed by an increase in soil C sequestration leading to improvement in soil quality and an increase in agronomic/biomass productivity. There already exists a large intercountry/ interregional variation in average yields, the so-called “yield gap”, of wheat (Triticum aestivum), corn (Zea mays), and other crops. Yields of wheat range from a high of 6.0–7.8 Mg ha1 in European Union countries (e.g., United Kingdom, Denmark, Germany, France), to a middle range of 2.4 to 2.7 Mg ha1 (e.g., United States, India, Romania, Ukraine, Argentina, Canada), to a very low range of 1.0 to 2.2 Mg ha1 (e.g., Pakistan, Turkey, Australia, Iran, Russia, Kazakhstan). Similarly, average yields of corn range from a high of 8–10 Mg ha1 (e.g., Italy, France, Spain, United States), to a middle range of 4 to 7 Mg ha1 (e.g., Canada, Egypt, Hungary, Argentina), to a low range of 1 to 2 Mg ha1 (e.g., Nigeria, Philippines, India). A concern regarding projected global warming is whether the agroecological yield may be adversely affected by the change in soil temperature and moisture regimes and the threat of increase in food deficit in Sub-Saharan Africa and elsewhere in developing countries. The adverse impact of decline in SOC pool on global food security is also evident from the required increase in crop yield in developing countries to meet the demands of increased population. With a little or no possibility of bringing new land area under cultivation in most developing countries, the yields of cereals in developing countries will have to increase from 2.64 Mg ha1 in 1997–1998 to 4.4 Mg ha1 by 2025 and to 6.0 Mg ha1 by 2050. Can this drastic increase in yields of cereals in developing countries be attained by agricultural intensification in the face of projected global warming? An important factor that can help to attain this challenging goal is improvement in soil quality through terrestrial and soil C sequestration. Improvements in soil C pool can be achieved by increasing below-ground C directly through input of root biomass. The importance of irrigation, fertilizer and nutrient acquisition, erosion control, and use of mulch farming and conservation tillage cannot be overemphasized.
Conclusions Terrestrial C sequestration is a truly win–win scenario. In addition to improving the much needed agronomic/biomass production to meet the needs of a rapidly increasing world population, terrestrial C sequestration improves soil quality, reduces sediment transport in rivers and reservoirs, enhances biodiversity, increases biodegradation of pollutants, and mitigates the risks of climate change. Besides, it is a natural process with numerous ancillary benefits. Although it has a finite capacity to absorb atmospheric CO2, it is the most desirable option available at the onset of the 21st century. The strategy of terrestrial C sequestration buys us time until C-neutral fuel options take effect.
Further Reading Amthor JS and Huston MA (1998) Terrestrial ecosystem responses oak ridge national laboratory to global change: A research strategy (ORNL/TM-1998/27). Nashville, TN: Oak Ridge National Laboratory.
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Eswaran H, Vanden Berg E, Reich P, and Kimble JM (1995) Global soil carbon resources. In: Lal R, Kimble JM, Levine E, and Stewart BA (eds.) Soils and global change, pp. 27–43. Boca Raton, FL: CRC Press. Halmann MM and Steinberg M (1999) Greenhouse gas carbon dioxide mitigation: Science and technology. Boca Raton, FL: Lewis Publishers. Hao Y, Lal R, Owens L, Izaurralde RC, Post M, and Hothem D (2002) Effect of cropland management and slope position on soil organic carbon pools at the North Appalachian Experimental Watersheds. Soil Tillage Res. 68: 133–142. Himes FL (1998) Nitrogen, sulfur, and phosphorus and the sequestering of carbon. In: Lal R, Kimble JM, Follett RF, and Stewart BA (eds.) Soil processes and the carbon cycle, pp. 315–319. Boca Raton, FL: CRC Press. Intergovernment Panel on Climate Change (1996) Climate change 1995: Impacts, adaptations, and mitigation of climatic change—Scientific–technical analyses. Cambridge, UK: Cambridge University Press. Intergovernment Panel on Climate Change (2000) Land use, land use change, and forestry: Special report. Cambridge, UK: Cambridge University Press. Intergovernment Panel on Climate Change (2001) Climate change: The scientific basis. Cambridge, UK: Cambridge University Press. Jungkunst HF, Kru¨ger JP, Heitkamp F, Ersami S, Glatzel S, Fiedler S, and Lal R (2012) Accounting more precisely for peat and other soil carbon sources. In: Lal R, Lorenze K, Hu¨ttl RF, Schneider BW, and Von Braun J (eds.) Recarbonization of the Biosphere, pp. 127–157. Donrecht: Springer. Lal R (1995) Global soil erosion by water and carbon dynamics. In: Lal R, Kimble JM, Levine E, and Stewart BA (eds.) Soils and global change, pp. 131–141. Boca Raton, FL: CRC/ Lewis Publishers. Lal R (1999) Soil management and restoration for C sequestration to mitigate the greenhouse effect. Progress in Environmental Science 1: 307–326. Lal R (2001a) Potential of desertification control to sequester carbon and mitigate the greenhouse effect. Climate Change 15: 35–72. Lal R (2001b) World cropland soils as a source or sink for atmospheric carbon. Advances in Agronomy 71: 145–191. Lal R (2003a) Global potential of soil C sequestration to mitigate the greenhouse effect. Critical Reviews in Plant Sciences 22: 151–184. Lal R (2003b) Soil erosion and the global carbon budget. Environment International 29: 437–450. Lal R (2004) Soil carbon sequestration impacts on global climate change and food security. Science 304: 1623–1627, (www.sciencemag.org/cgi/content/full/305/5690/1567DCI). Lal R (2010a) Carbon sequestration in saline soils. Journal of Soil Salinity and Water Quality 1(1&2): 30–40. Lal R (2010b) Managing soils and ecosystems for mitigating anthropogenic carbon emissions and advancing global food security. Bioscience 60(9): 708–721. Lal R and Augustin B (eds.) (2011) Carbon Sequestration in Urban Ecosystems. Dordrecht, Netherlands: Springer, 383 pp. Mainguet M (1991) Desertification: Natural background and human mismanagement. Berlin: Springer-Verlag. Oberthu¨r S and Ott HE (2001) The Kyoto protocol: International climate policy for the 21st century. Berlin: Springer. Oldeman LR (1994) The global extent of soil degradation. In: Greenland DJ and Szabolcs I (eds.) Soil resilience and sustainable land use, pp. 99–118. Wallingford, UK: CAB International. Oldeman LR and Van Lynden GWJ (1998) Revisiting the GLASOD methodology. In: Lal R, Blum WH, Valentine C, and Stewart BA (eds.) Methods for assessment of soil degradation, pp. 423–440. Boca Raton, FL: CRC Press. Schlesinger WH (1999) Carbon sequestration in soils. Science 286: 2095. U.S. Department of Energy (1999) Carbon sequestration: Research and development. Springfield, VA: National Technical Information Service. WMO (2012). Greenhouse Gas Bulletin, 4pp, Geneva, Switzerland, #8. Nov.