Published online by Cambridge University Press: 11 May 2010
The world's soils contain approximately 1500 Pg (1 Pg = 1 Gt = 1015 g) of organic carbon (Batjes, 1996), roughly three times the amount of carbon in vegetation and twice the amount in the atmosphere (IPCC, 2001; Denman et al., 2007). The annual fluxes of CO2 from atmosphere to land (global net primary productivity, NPP) and land to atmosphere (respiration and fire) are of the order of 60 Pg C y−1 (IPCC, 2000b). During the 1990s, fossil fuel combustion and cement production emitted 6.4 ± 1.3 Pg C y−1 to the atmosphere, while land-use change emitted 1.6 ± 0.8 Pg C y−1. Atmospheric carbon increased at a rate of 3.2 ± 0.1 Pg C y−1, the oceans absorbed 2.3 ± 0.8 Pg C y−1 and there was an estimated terrestrial sink of 2.6 ± 1.3 Pg C y−1 (Schimel et al., 2001; Denman et al., 2007). The amount of carbon stored in soils globally is therefore large compared to gross and net annual fluxes of carbon to and from the terrestrial biosphere, and the pools of carbon in the atmosphere and vegetation. Because of this, increasing the size of the global soil carbon pool by even a small proportion has the potential to sequester large amounts of carbon, and thus soils have an important role to play in mitigating climate change.
Human intervention, via cultivation and disturbance, has decreased and still is decreasing the soil carbon pools relative to the store typically achieved under native vegetation.