Will Energy Crops Release Soil Carbon?

Grasslands in the Conservation Reserve Program (CRP) in the United States may be increasingly converted to growing bioenergy grain crops. Questions abound regarding the fate of carbon sequestered in the soil during the CRP program by perennial grasses if the land is converted to grain crop production and the potential effectiveness of no-till production systems to conserve the sequestered soil organic carbon (SOC).

The effect of no-till conversion of land that had been in smooth bromegrass for 13 years to no-till corn production on soil organic carbon in eastern Nebraska was observed for six years by U.S. Department of Agriculture scientists.

The bromegrass was killed with herbicides in late fall of 1998, and corn was planted directly into the killed sod in the spring of 1999. No plowing or cultivation was conducted during the entire study. Weeds were controlled with herbicides. Soil samples were collected at three different depths at the beginning and several times during the study and were analyzed for soil organic carbon. Carbon isotope ratio analyses made it possible to determine the amount of soil organic carbon that originated from bromegrass or corn.

The results of this study were reported at the October meeting of the Soil Science Society of America in Houston, Texas, and are published in the March-April 2009 issue of Agronomy Journal.

During the study, the origin of the soil carbon in the two upper soil layers (0- to 5-, and 5- to 10-cm depths) changed with the soil carbon from bromegrass gradually replaced by that from corn. Total soil organic carbon, however, did not change significantly at any depth. There was no loss of sequestered soil carbon during continuous no-till corn production.

Ronald Follett, who led this study, states, "If Conservation Reserve Program grasslands are converted to grain crop production, data from this study strongly supports the use of no-till farming practices. The use of no-till was observed to conserve both previously sequestered SOC while also enhancing sequestration of SOC by the bioenergy crops."

Coauthors Gary Varvel and Ken Vogel indicate that no-till conversion of CRP grasslands into grain crops or perennial biomass crops such as switchgrass is significantly less expensive than using extensive tillage including plowing. No-till conversion of grasslands is feasible because of effective herbicides and improved no-till planting equipment that has been developed.

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