Abstract
Over 5 million ha of US Conservation Reserve Program (CRP) grasslands have been converted to annual crops since 2000, driven mainly by demand for corn grain ethanol. Much of the soil carbon sequestered under CRP is lost upon conversion, creating a ‘carbon debt’ that is presumed to be repaid by future greenhouse gas (GHG) savings from ethanol’s substitution for petroleum. Model simulations, extrapolations, and national statistics rather than direct measurements have been used thus far to estimate the long-term global warming impact (GWI) of such conversions. Here we report measured GWIs for three 22-year-old CRP grassland fields and three conventionally tilled agricultural (AGR) fields (11–17 ha) converted to either annual no-till corn or perennial cellulosic (switchgrass or restored prairie) bioenergy crops. We assessed GWIs for each field over eight years using whole-system life cycle analysis (LCA) by measuring: (a) GHG fluxes via eddy covariance and static chamber methodologies, (b) farming operations and agronomic inputs, and (c) the fossil fuel offset by ethanol use. Payback times were much longer than those estimated by prior modeling efforts. After 8 years, cumulative GWIs of switchgrass, restored prairie, and corn at the CRP grasslands were, respectively, −2.6 ± 4.0, 6.9 ± 3.6 and 85.2 ± 5.1 Mg CO2-equivalent ha−1. The switchgrass system had repaid its carbon debt by year eight and the restored prairie will have likely repaid by year ten; however, the no-till corn system appears likely to require >300 years. The same bioenergy crops grown on former agricultural lands, with no sequestered carbon lost on conversion, repaid their carbon debts within two years. Results indicate that GWI estimates and carbon debt payback times due to conversion of CRP lands to annual bioenergy crops have been substantially underestimated by current models.
Highlights
Bioenergy-derived fuel will be key to avoiding fossil fuel related greenhouse gas (GHG) emissions until transportation systems are sufficiently electrified [1], and as well bioenergy with carbon (C) capture and storage (BECCS) is a crucial part of almost all mitigation scenarios capable of constraining the global average temperature rise to 1.5 °C by 2100 [2]
Much of the soil carbon sequestered under Conservation Reserve Program (CRP) is lost upon conversion, creating a ‘carbon debt’ that is presumed to be repaid by future greenhouse gas (GHG) savings from ethanol’s substitution for petroleum
Global warming impact (GWI) The annual global warming impact (GWI) of corn at the CRP field showed high CO2-eq emission to the atmosphere in all study years, while that of the corn at the AGR field fluctuated around neutrality
Summary
Bioenergy-derived fuel will be key to avoiding fossil fuel related greenhouse gas (GHG) emissions until transportation systems are sufficiently electrified [1], and as well bioenergy with carbon (C) capture and storage (BECCS) is a crucial part of almost all mitigation scenarios capable of constraining the global average temperature rise to 1.5 °C by 2100 [2]. Growing bioenergy crops will require extensive land use changes that will have direct and indirect implications for slowing warming trends and mitigating the impacts of climate change [3–5]. Estimates suggest that ∼360 Mt of CO2-equivalents (CO2-eq) could be potentially released to the atmosphere due to conversion of CRP grasslands to corn bioenergy [3, 7]. These estimates, while highly uncertain, appear to have payback times of decades. Diversion of existing croplands to bioenergy crops, not incurring direct C debt, may cause land conversion to new croplands elsewhere to meet food demands displaced by bioenergy production, thereby creating indirect C debt [4, 14]
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