Carbon Sequestration and Greenhouse Gas Fluxes from Cropland Soils – Climate Opportunities and Threats

Abstract

Globally, soils contain approximately 1500 Pg (1 Pg = 1 Gt = 10 15 g) of organic carbon (C) (Batjes 1996), roughly three times the amount of carbon in vegeta-tion and twice the amount in the atmosphere (IPCC 2001). The annual fluxes of carbon dioxide (CO 2) from atmosphere to land (global Net Primary Productivity [NPP]) and land to atmosphere (respiration and fire) are of the order of 60 Pg C yr −1 (IPCC 2001). during 1990s, fossil fuel combustion and cement production emitted 6.3 ± 1.3 Pg C yr −1 to the atmosphere, while land-use change accounted for 1.6 ± 0.8 Pg C yr −1 (Schimel et al. 2001; IPCC 2001). Atmospheric C increased at a rate of 3.2 ± 0.1 Pg C yr −1 , the oceans absorbed 2.3 ± 0.8 Pg C yr −1 and there was an esti-mated terrestrial sink of 2.3 ± 1.3 Pg C yr −1 (Schimel et al. 2001; IPCC 2001). The amount of carbon stored in soils globally is, therefore, very 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. Human intervention, via cultivation and disturbance, has also decreased the soil carbon pools relative to the store typically achieved under native vegetation. Historically, these processes have caused a loss of soil C between 40 and 90 Pg C globally (Paustian et al. 1998; Houghton et al. 1999; Lal 1999). Hence, 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 help mitigate climate change. The trace gases, like methane (CH 4) and nitrous oxide (N 2 O), are also potent greenhouse gases and emitted from, and absorbed by soils. For this reason, soils also have a second role to play – reducing trace gas emissions to the atmosphere – in combating climate change. Nitrous oxide is formed primarily from nitrification P. Falloon (B) Met Office Hadley Centre for Climate Change,