Soil Carbon and Nitrogen in Response to Perennial Bioenergy Grass, Cover Crop and Nitrogen Fertilization

Cover crop and nitrogen (N) fertilization may maintain soil organic matter under bioenergy perennial grass where removal of aboveground biomass for feedstock to produce cellulosic ethanol can reduce soil quality. We evaluated the effects of cover crops and N fertilization rates on soil organic carbon (C) (SOC), total N (STN), ammonium N (NH4-N), and nitrate N (NO3-N) contents at the 0-5, 5-15, and 15-30 cm depths under perennial bioenergy grass from 2010 to 2014 in the southeastern USA. Treatments included unbalanced combinations of perennial bioenergy grass, energy cane (Saccharum spontaneum L.) or elephant grass (Pennisetum purpureum Schumach.), cover crop, crimson clover (Trifolium incarnatum L.), and N fertilization rates (0, 100, and 200 kg N ha−1). Cover crop biomass and C and N contents were greater in the treatment of energy cane with cover crop and 100 kg N ha−1 than in the treatment of energy cane and elephant grass. The SOC and STN contents at 0-5 and 5-15 cm were 9%-20% greater in the treatments of elephant grass with cover crop and with or without 100 kg N ha−1 than in most of the other treatments. The soil NO3-N content at 0-5 cm was 31%-45% greater in the treatment of energy cane with cover crop and 100 kg N ha−1 than in most of the other treatments. The SOC sequestration increased from 0.1 to 1.0 Mg C ha−1 year−1 and the STN sequestration from 0.03 to 0.11 Mg N ha−1 year−1 from 2010 to 2014 for various treatments and depths. In contrast, the soil NH4-N and NO3-N contents varied among treatments, depths, and years. Soil C and N storages can be enriched and residual NO3-N content can be reduced by using elephant grass with cover crop and with or without N fertilization at a moderate rate.