Abstract
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.
Original language | English GB |
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Title of host publication | American Geophysical Union, Fall Meeting 2017 |
Volume | 24 |
State | Published - 1 Dec 2017 |
Externally published | Yes |
Keywords
- 0414 Biogeochemical cycles
- processes
- and modeling
- BIOGEOSCIENCES
- 0426 Biosphere/atmosphere interactions
- 0428 Carbon cycling
- 0452 Instruments and techniques