The Role of Agricultural Soil in Influencing the GHG Budget and Their Potential for C Mitigation

According to Houghton and Hackler (2001), the net flux of carbon between the terrestrial biosphere and the atmosphere between 1850 and 1990 was 124 Pg C from deliberate changes in land cover and land use. DeFries et al. (1999) estimated that total C loss from human-induced land cover changes were almost 200 Pg C for the same period. Lal et al. (1998) estimated that agricultural soils globally have lost 40 to 50 Pg C during the last two centuries, and about 80 to 117 Pg C have been released from biomass due to change in land use for agriculture. Until the 1970's more CO2 had been released into the atmosphere from agricultural activities than from fossil fuel burning. The annual net release of C from agriculture has recently been estimated at 0.8 Pg C, that is, about 14% of current fossil fuel emissions (Schlesinger, 1995). Considering all these substantial losses in soil C, it is not surprising that land management strategies are considered to have considerable potential for carbon mitigation.

Cole et al. (1996) estimated that globally, between 0.4 and 0.8 Pg /yr of C could be sequestered in agricultural soils for 50-100 yr through good soil management. Paustian et al. (1997) calculated that the capacity for C sequestration in agricultural soils on a global scale is about 20-30 Pg C over the next 50-100 yr. Lal and Bruce (1999) estimated that the total soil C sequestration potential of the world cropland is about 0.75-1.0 Pg/yr or about 50% of the annual emissions by deforestation and other agricultural activities. This finite soil C sink could be filled over a 20 to 50-yr period. The International Panel for Climate Change (IPCC, 2000) recently reported that by 2010 the potential net C storage from additional activities under Article 3.4 of the Kyoto Protocol could be 0.29 Pg C/yr for developed countries and 0.72 Pg C/yr for developing countries. By 2040, the potential net C storage is expected to be 0.85 Pg C/yr for developed countries and 1.32 Pg C/yr for developing countries. These values represent the contribution from both agriculture and forest management.

There are still large uncertainties regarding the total amount of C that can be sequestered by agricultural soils. These uncertainties are primarily due to the difficulty of measuring a small change in soil carbon (e.g., 3 Mg C/ha) above a large background (e.g., 60 Mg C/ha) with high variability (e.g., ±15 Mg C/ha). Garten and Wullschleger (1999) estimated that the smallest difference in SOC inventories that can be detected, and this only with exceedingly large sample size (n > 100), is approximately 2-3% or 1 Mg C/ha.

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