Cover crop effects on soil carbon and nitrogen under bioenergy sorghum crops

Cover crops can increase soil carbon (C) and nitrogen (N) storage and reduce the potential for N leaching under agronomic crops, but little is known about their effect under bioenergy crops whose aboveground biomass is removed either for biofuel production or for livestock feed. The objective of the study was to evaluate the effect of cover crops on soil organic C (SOC), total N (STN), ammonium nitrogen (NH4-N), and nitrate-N (NO3-N) contents at the 0 to 30 cm (0 to 12 in) depth under bioenergy sorghum (Sorghum bicolor L. Moench) crops from 2010 to 2013 in the southeastern United States. Treatments were two sorghum species (forage sorghum [Sorghum vulgare Pers.] and sweet sorghum) as the main plot and four cover crops as the split-plot treatments arranged in a randomized complete block with three replications. Cover crop treatments were legume (hairy vetch [Vicia villosa Roth]), nonlegume (rye [Secale cereal L.]), biculture of legume and nonlegume (hairy vetch/rye), and no cover crop (control). At 15 to 30 cm (6 to 12 in), SOC was greater with hairy vetch/rye than the control under forage sorghum. At 0 to 5 and 5 to 15 cm (0 to 2 and 2 to 6 in), STN was greater with hairy vetch and hairy vetch/rye than rye under forage sorghum and greater with hairy vetch/rye than the control under sweet sorghum. At 5 to 15 cm, NO3-N was greater with hairy vetch/rye than rye in 2011, but was greater with rye and hairy vetch than hairy vetch/rye in 2012. At all depths, NH4-N was greater under forage than sweet sorghum in 2012. Regardless of treatments, SOC and STN increased, but NH4-N and NO3-N varied from 2010 to 2013. Although soil available N varied with cover crops, sorghum types, and climatic conditions from year to year, hairy vetch/rye can conserve and/or increase soil C and N storage compared with vetch or rye alone, or the control under bioenergy sorghum in the southeastern United States. The results can be used to claim C credit, increase N cycling, and improve soil and environmental quality under bioenergy sorghum in the regions with similar soil and climatic conditions.