Estimating the Overall Impact of A Change in Agricultural Practices on Atmospheric CO2

Introduction One option for sequestering carbon in the terrestrial biosphere is to increase the carbon (C) stocks in agricultural soils. There is now an extensive literature on the amount of C that has been lost from soils as a consequence of humans disturbing natural ecosystems, and of the amount of C that might be returned to soils with improved management practices. Improvements in management practices could include efficient use of fertilizers and irrigation water, use of crop rotations, and changing from conventional tillage (CT) to conservation tillage (or, more specifically, to no-till (NT)). The Intergovernmental Panel on Climate Change (IPCC) has estimated that 55 x 10 Mg of soil C have been lost, globally, largely as a result of cultivating former grasslands, forests, and wetlands (Cole et al. 1996). The IPCC estimated further that 2229 x 10 Mg of C could be returned to existing, world, agricultural soils under improved management regimes. Historical losses of soil organic C (SOC) in the U.S., due to cultivation, have been estimated to be 1.3 ± 0.3 x 10 Mg (Kern and Johnson 1993). Kern and Johnson projected that by increasing NT practice in the U.S. from 27% in 1990 to 76%, a total of 0.4 ± 0.1 x 10 Mg C could be sequestered in the soil during the interval 1990-2020. These studies tend to focus on increasing the C stocks in soils rather than on the overall effect that changes in agricultural practice would have on C stocks in the atmosphere. Changing agricultural practice can impact net CO2 emissions to the atmosphere in three fundamental ways: (1) it can lead to an increase in the C held in agricultural soils, (2) it can lead to a change in emissions of CO2 from fossil fuel burning, and (3) it can change agricultural productivity, and hence the amount of cultivated land needed to meet the demand for agricultural products. Changing agricultural practice can also affect the net emissions of other greenhouse gases, such as N2O emissions associated with nitrogen (N) fertilizer application. This study focuses on a comprehensive analysis of the first two factors, including N2O emissions, and inquires into the balance between C sequestered and the change in C equivalent (Ceq) emissions associated with a change in agricultural practices. N2O emissions are converted to C equivalent emissions, based on their time-integrated effect on the global atmospheric energy balance, as suggested by the IPCC (Schimel et al. 1996).