Carbon debt of CRP lands converted to annual and perennial bioenergy crops

Abraha, Michael, Ilya Gelfand, Stephen K. Hamilton, Jiquan Chen, and G. Philip Robertson
Michigan State University

Presented at the All Scientist Meeting and Investigators Field Tour (2017-10-06 to 2017-10-07 )

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 Mt to 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 to 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 -297, 651 and 8491 gCO2eq m-2 for switchgrass, restored prairie and corn, respectively, where a positive debt indicates net emissions to the atmosphere. The carbon debt is repaid in the 8th year for switchgrass; and will be repaid in the 9th year for restored prairie. The corn field, however, will require additional 380 years to pay its C debt. 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.

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