"Soils are critically important in determining global carbon cycle dynamics because they serve as the link between the atmosphere, vegetation, and oceans. Globally, the soil carbon pool (also referred to as the pedologic pool) is estimated at 2,500 Gt up to a 2-m depth. Out of this, the soil organic carbon pool comprises 1,550 Gt, while the soil inorganic carbon and elemental pools make up the remaining 950 Gt (Batjes 1996). The soil carbon pool is more than 3 times the size of the atmospheric pool (760 Gt) and about 4.5 times the size of the biotic pool (560 Gt)" (page 6).
"The soil organic carbon pool represents a dynamic balance between gains and losses. The amount changes over time depending on photosynthetic C added and the rate of its decay. Under undisturbed natural conditions, inputs of carbon from litter fall and root biomass are cycled by output through erosion, organic matter decomposition, and leaching. The potential carbon sequestration is controlled primarily by pedological factors that set the physico-chemical maximum limit to storage of carbon in the soil. Such factors include soil texture and clay mineralogy, depth, bulk density, aeration, and proportion of coarse fragments" (page XVIII).
"Attainable carbon sequestration is determined by factors that limit the input of carbon to the soil system. Net primary productivity (NPP)—the rate of photosynthesis minus autotrophic respiration—is the major factor influencing attainable sequestration and is modified by above-ground versus below-ground allocation. Land management practices that increase carbon input through increasing NPP tend to increase the attainable carbon sequestration to nearer to the potential level. Climate has both direct and indirect effects on attainable sequestration. Decomposition rate increases with temperature but decreases with increasingly anaerobic conditions. Actual carbon sequestration is determined by land management factors that reduce carbon storage such as erosion, tillage, residue removal, and drainage. Theoretically, the potential soil carbon sequestration capacity is equivalent to the cumulative historical carbon loss. However, only 50 to 66 percent of this capacity is attainable through the adoption of sustainable land management practices" (page XVIII).
"The current rate of carbon loss due to land-use change (deforestation) and related land-change processes (erosion, tillage operations, biomass burning, excessive fertilizers, residue removal, and drainage of peat lands) is between 0.7 and 2.1 Gt carbon per year. Soil erosion is the major land degradation process that emits soil carbon. Because soil organic matter is concentrated on the soil surface, accelerated soil erosion leads to progressive depletion of soil carbon. The annual rate of soil loss ranges from 7.6 Gt for Oceania to 74.0 Gt for Asia (table E2). This corresponds to carbon emissions ranging from 0.02 to 0.04 Gt per year for Oceania to 0.30 to 0.44 Gt per year for Asia. Globally, 201 Gt of soil is lost to erosion, corresponding to 0.8 to 1.2 Gt of emitted carbon per year. Africa, Asia, and South America emit between 0.60 and 0.92 Gt of carbon per year through soil erosion. Agricultural soils must be prevented from being washed into streams and rivers where the relatively stable soil carbon pools are rapidly oxidized to carbon dioxide (XVIII).
Estimates of Erosion-Induced Carbon Emission Across World Regions
SOIL CARBON DISPLACED BY
EROSION (2 TO 3 PERCENT OF SEDIMENT; Gt C/YEAR)
EMISSION (20 PERCENT OF DISPLACED SOIL CARBON;Gt C/YEAR)
Africa 38.9 0.8–1.2 0.16–0.24
Asia 74.0 1.5–2.2 0.30–0.44
South America 39.4 0.8–1.2 0.16–0.24
North America 28.1 0.6–0.8 0.12–0.16
Europe 13.1 0.2–0.4 0.04–0.08
Oceania 7.6 0.1–0.2 0.02–0.04
Total 201.1 4.0–6.0 0.8–1.2
Source:Lal, R. (2003).
CARBON SEQUESTRATION IN AGRICULTURAL SOILS
ECONOMIC AND SECTOR WORK
"Soil respiration, the flux of microbially and plant-respired carbon dioxide, estimated at 75 to 100 Gt carbon per year, is the next largest terrestrial carbon flux following photosynthesis. Soil respiration is a potentially important mechanism of positive feedback to climate change. A small change in soil respiration can significantly alter the balance of atmospheric carbon dioxide concentration compared to soil carbon stores. Conventional tillage leads to the destruction of soil aggregates, excessive respiration, and soil organic matter decomposition, leading to reduced crop production and decreased resilience of the soil ecosystem. When other factors are at optimum, conservation tillage, use of cover crops (green manure), crop rotations, use of deep-rooted crops, application of manure, and water management can optimize soil respiration in addition to improving soil carbon leading to the triple win of enhanced agricultural productivity, adaptation, and mitigation" (page XIX).
World Bank, “Carbon Sequestration in Agricultural Soils,” 67395-GLB (Washington DC, May 2012), accessed May 31, 2014, http://www.vieskanmetalli.fi/Carbon_Sept2013.pdf.