Carbon balance of converting conservation reserve program (CRP) grasslands to agriculture.

To analyze the net carbon balance of land-use conversion from grasslands to cropping agriculture, we conducted a full carbon accounting of biodiesel and grain production from three CRP grassland ecosystems converted to no-till soybean management, with reference to an unmanaged CRP grassland. The analysis included measured fluxes of greenhouse gases (CO₂, N₂O, CH₄), agricultural chemicals and fuel use, and net ecosystem productivity (NEP) measured by eddy-covariance technique during the year of conversion, all expressed as CO₂ equivalents (CO₂e). Conversion of CRP grasslands to no-till cropping systems for soybean production caused net emission of 518 ± 30 g CO₂ m^-2 during the first year. This initial debt is augmented by other sources of CO₂e emissions: increased N₂O fluxes (210 ± 58 g CO₂e m^-2) and soil organic carbon oxidation (up to 15 × 10³ g CO₂e m^-2, depending on management). Conversion of grasslands to permanent no-till would help attenuate the conversion carbon debt (68 vs. 222 Mg CO₂e ha^-1). Converting CRP grasslands to agriculture may help reduce conflict between bioenergy and food production, while the large carbon debt due to conversion would have negative climatic impacts. Carbon debt could be repaid with production of grain ethanol/biodiesel on converted lands, whereas food production will effectively keep all emitted CO₂ in the atmosphere. The fossil fuel offsets due to production of renewable bioenergy in this case would allow the carbon debt to be paid back between 30 and 123 years. Alternatively, production of cellulosic feedstocks on the unconverted grasslands could provide CO₂ sequestration benefits of 311 ± 126 g CO₂ m^-2 yr^-1 for several decades, together with renewable energy production of ~26 GJ ha^-1 yr^-1.