Nitrogen Management of Switchgrass and Miscanthus on Marginal Soils.

Miscanthus × giganteus and switchgrass yield and fertilizer N requirements have been well studied in Europe and parts of the United States, but few reports have investigated their production on eroded claypan soils economically marginal for grain crops. This study was conducted to evaluate yield potential and fertilizer N strategies for young and mature M. × giganteus and switchgrass on eroded soils. Fertilizer N treatments were applied to both crops in separate trials during 2009 to 2015. Additionally, the impact of topsoil thickness or depth to claypan (DTC) on switchgrass N requirements was evaluated. Miscanthus × giganteus yield ranged from 13.3 to 23.8 Mg ha^-1 and the response to N was infrequent, but could be detected using relative chlorophyll content in Mid-June. The most efficient N fertilization strategy for Miscanthus × giganteus at responsive sites was 67 kg N ha^-1 yr^-1. Switchgrass yield ranged from 6.3 to 14.9 Mg ha^-1 across years and DTC. Fertilization with 101 kg N ha^-1 on exposed, shallow, or moderate DTC and 67 kg N ha^-1 on deep DTC obtained the highest biomass yield, but it also increased nutrient removal. Strikingly, partial profit across years was greatest with no fertilizer on all DTC classes. Therefore, yield levels and N requirements of these two bioenergy crops on marginal soils were similar to more productive soils. The scarce and sometimes unprofitable response to N indicates that more site-specific management is needed. On claypan soils, N management could be refined by using chlorophyll meters and topsoil thickness.