Soil Nitrogen Responses to Cover Crop and Nitrogen Fertilization Under Bioenergy Sorghum.

Soil N storage and availability may be reduced by removing aboveground biomass for bioenergy/feedstock in bioenergy cropping systems. Cover crop and N fertilization may provide additional N inputs that sustain soil N storage and availability. We evaluated the effect of four winter cover crops (control or no cover crop, cereal rye, hairy vetch, and hairy vetch/cereal rye mixture) and two N fertilization rates (0 and 90 kg N ha^-1) on soil total N (STN), NH₄-N, and NO₃-N contents at 0-5, 5-15, and 15-30 cm depths under forage and sweet sorghums from 2010 to 2013 in Fort Valley, GA. Cover crop biomass N was greater with vetch/rye than vetch, rye, and the control under forage sorghum and greater with vetch/rye than the control under sweet sorghum in 2013. Soil total N at 0-5 and 15-30 cm was greater with vetch/rye than rye and the control under forage sorghum in 2011 and 2013. Under sweet sorghum, STN at 0-5 cm was greater with rye and vetch/rye than the control. Soil NH₄-N content at all depths was greater with rye or vetch than other cover crops in 2012 and 2013. Soil NO₃-N content at all depths was greater with vetch and vetch/rye than rye or the control in all years. Soil NO₃-N content was also greater in vetch with 90 kg N ha^-1 than vetch with 0 kg N ha^-1 and rye with 0 kg N ha^-1. The STN increased from 2010 to 2013, but NH₄-N and NO₃-N contents varied with years, regardless of treatments. Because of increased biomass N returned to the soil, hairy vetch/rye mixture increased soil N storage and availability compared with rye and the control and vetch with N fertilization increased N availability compared with rye with no N fertilization.