Carbon debt of Conservation Reserve Program (CRP) grasslands converted to bioenergy production

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 108; no. 33; pp. 13864 - 13869
• Main Authors: Gelfand, Ilya, Zenone, Terenzio, Jasrotia, Poonam, Chen, Jiquan, Hamilton, Stephen K, Robertson, G. Philip
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 16.08.2011; National Acad Sciences
• Subjects: Agricultural land; Agricultural production; Agricultural soils; Animal Feed; Biodiesel fuels; biodiversity; Biofuels; Biological Sciences; biomass; Carbon; Carbon - metabolism; Carbon dioxide; carbon sequestration; carbon sinks; Cellulose; climate; Climate change; Climate change mitigation; Conservation; Conservation of Natural Resources - methods; Conservation Reserve Program; conventional tillage; Corn; credit; Crop production; cropland; crops; Crops, Agricultural; Debt; Energy crops; Farm buildings; feedstocks; Fossil fuels; Government Programs; grasses; Grassland soils; Grasslands; Greenhouse Effect - prevention & control; Greenhouse gases; harvesting; Land use; magnesium; methane; methane production; Nitrous oxide; No tillage; soil; Soybeans; Tillage; United States; Vegetables; Water quality; United States
• ISSN: 0027-8424; 1091-6490; 1091-6490
• DOI: 10.1073/pnas.1017277108

Over 13 million ha of former cropland are enrolled in the US Conservation Reserve Program (CRP), providing well-recognized biodiversity, water quality, and carbon (C) sequestration benefits that could be lost on conversion back to agricultural production. Here we provide measurements of the greenhouse gas consequences of converting CRP land to continuous corn, corn–soybean, or perennial grass for biofuel production. No-till soybeans preceded the annual crops and created an initial carbon debt of 10.6 Mg CO2 equivalents (CO2e)·ha–1 that included agronomic inputs, changes in C stocks, altered N2O and CH4 fluxes, and foregone C sequestration less a fossil fuel offset credit. Total debt, which includes future debt created by additional changes in soil C stocks and the loss of substantial future soil C sequestration, can be constrained to 68 Mg CO2e·ha–1 if subsequent crops are under permanent no-till management. If tilled, however, total debt triples to 222 Mg CO2e·ha–1 on account of further soil C loss. Projected C debt repayment periods under no-till management range from 29 to 40 y for corn–soybean and continuous corn, respectively. Under conventional tillage repayment periods are three times longer, from 89 to 123 y, respectively. Alternatively, the direct use of existing CRP grasslands for cellulosic feedstock production would avoid C debt entirely and provide modest climate change mitigation immediately. Incentives for permanent no till and especially permission to harvest CRP biomass for cellulosic biofuel would help to blunt the climate impact of future CRP conversion.