PNAS paper on carbon debt from conversion of CRP grassland to bioenergy (Gelfand et al.)
Gelfand et al. investigate the emissions impacts of converting US Conservation Reserve Program land to corn, soy or perennial grasses for bioenergy. They observe substantial carbon loss and nitrous oxide emission in the year of conversion, as well as lost sequestration, for a carbon debt of 68 – 222 Mg CO2e per hectare (amortised over 30 years this would be 2.3 – 7.4 Mg CO2e per hectare per year). The paper concludes that converting former cropland enrolled in the US Conservation Reserve Program (CRP) to continuous corn for ethanol would result in a carbon payback period of 40 – 123 years, while a corn soy rotation would take 29 – 89 years to pay back (the shorter payback times require no-till agriculture to be implemented). By these calculations, conversion of CRP grassland to biofuel production would not generate carbon savings against either the 30-year land-use change carbon accounting use in the US or the 20-year carbon accounting used in Europe.
As an alternative to conversion, they find that cultivation and harvesting for cellulosic ethanol production of existing perennial grasses would give lower energy yields per hectare but almost immediate carbon savings. If such harvesting were undertaken on land unsuitable for conversion to food production, they note that indirect land use (iLUC) change could also be avoided.