Nitrogenous fertilizer effects on soil structural properties under switchgrass

Nitrogen (N) fertilization is needed to sustain the biomass yield of switchgrass (Panicum virgatum L., Poaceae) as a biofuel feedstock and, consequently, may influence the potential for soil quality improvements through soil organic carbon (SOC) sequestration. Therefore, the objective of this study was to assess how inorganic N application to switchgrass affects soil structural properties, which may feed back to affect the sustainability of biomass production. Soil was sampled at depths of 0–5, 5–10, and 10–15 cm in April and November 2008 during the fifth year of switchgrass growth in Milan, TN. Nitrogenous fertilizer was applied as NH4NO3 at rates of 0, 67, and 202 kg N ha−1 y−1 beginning in the second year. Root weight density (RWD), root length density (RLD) and SOC concentration were measured under different N treatments as factors potentially influencing soil structural properties. Measured soil structural parameters included soil moisture characteristic curve (SMCC), and aggregate stability through wet-sieving. At 0–5 cm depth, spring RWD (3.8 mg cm−3) was significantly lower with 202 kg N ha−1 application compared to 0 and 67 kg N ha−1 (14.1 and 17.0 mg cm−3, respectively). Although fall RWD did not vary among N treatments, RLD under 202 kg N ha−1 (7.1 cm cm−3) was less than half of that at 0 kg N ha−1 (15.7 cm cm−3). The SOC concentration was greater in both fertilized treatments than in the unfertilized treatment. Although SMCC varied somewhat between seasons, it did not exhibit any consistent trends attributable to N application. The proportion of macroaggregates for 0–10 cm depth were significantly greater in the 0 and 67 kg N ha−1 treatments than in the 202 kg N ha−1 treatment. These data suggest that excessive N application to switchgrass could have negative impacts on soil structural properties by reducing root biomass and length, crucial determinants of soil structure despite an increase in SOC.