Nitrogenous Fertilizer and Soil Structure Under Switchgrass in Milan, Tennessee.

Switchgrass (Panicum virgatum L., Poaceae) is a warm-season grass with a potential of producing biofuel feedstock. Nitrogen (N) fertilization is needed to sustain biomass yield as a biofuel feedstock, and for soil quality improvements through soil organic carbon (SOC) sequestration. Many studies have focused on the feasibility of bioenergy crop cultivation with a focus on economic considerations, but changes to soil quality are rarely evaluated. Therefore, the objective of this study is to assess the effects of inorganic N application to switchgrass on soil structural properties. Soil was sampled at 0-5, 5-10, and 10-15 cm depths in April and November 2008 after 4 years of switchgrass cultivation in Milan, TN. Nitrogenous fertilizer was applied at the rates of 0, 67, and 202 kg NH₄NO₃-N ha^-1 every year. Root weight density (RWD), root length density (RLD), and SOC concentration were measured under different N treatments as factors influencing soil structural properties. Bulk density, soil moisture characteristic curves, water stable aggregate (WSA), and tensile strength (TS) of wet-sieved aggregates were analyzed as soil quality parameters. The RWD in April and RLD in November significantly decreased in the 0-5 cm depth with 202 kg N ha^-1 application compared with 0 and 67 kg N ha^-1. The SOC concentration was somewhat more in 67 and 202 kg N ha^-1 than in the unfertilized treatment. The ratio of mean weight diameter (MWD) after:before wet-sieving for 5-10 cm depth was significantly higher in 0 kg N ha^-1 treatment than 202 kg N ha^-1 treatment. The TS of WSA in April were significantly higher in the 0 kg N ha^-1 than in the 202 kg N ha^-1 treatment in 0-5 cm layer. Above all, excessive N application to switchgrass may have negative impacts on soil physical properties by influencing root, a major determinant of soil structure.