Carbon Debt of CRP Lands Converted to Annual and Perennial Bioenergy Crops

The net greenhouse gas fluxes of an ecosystem are directly influenced by land use conversions. In the USA, 5 Mha of grassland in the Conservation Reserve Program (CRP) have been converted to agricultural production in response to higher demand for corn grain biofuel. The global warming impact (GWI) of these biofuel crops can remain positive for many years following the conversions until the "carbon debt" incurred upon conversion is repaid. Model estimates suggest that 340-351 ×106 Mt of carbon dioxide equivalents (CO2eq) would be released to the atmosphere after the conversions. These estimates, while highly uncertain, appear to have payback times of decades or even centuries. In a field experiment conducted from 2009-16, we converted CRP grassland and conventionally-tilled agricultural (AGR) land to grain (corn) and cellulosic (switchgrass and restored prairie) biofuel feedstocks. We conducted life cycle analysis (LCA) on all converted lands by accounting for greenhouse gas fluxes related to farming operations, agronomic inputs, and soil-atmosphere greenhouse gas exchanges. We found that cumulative carbon debt for the conversion on former CRP grasslands over the 8 years is -295, 652 and 7661 gCO2eq m-2 for switchgrass, restored prairie and corn, respectively, where a positive debt indicates net emissions to the atmosphere. These indicate that the switchgrass field repaid its carbon debt in the 8th year following conversion; and the restored prairie field will likely repay its carbon debt in the next year. The corn field, however, is projected to pay its carbon debt in another 250 years. The same biofuel crops established on former AGR lands became net CO2eq sinks within two years following the conversion. Our findings indicate that the GWI estimates and the time needed to repay CO2eq debt due to conversion of grasslands to bioenergy crops is underestimated by current models.