Biosolids Amendment and Harvest Frequency Affect Nitrogen Use Dynamics of Switchgrass Grown for Biofuel Production

Nitrogen use efficiency (NUE) is a crucial index for developing sustainable bioenergy cropping systems. The objective of this study was to examine switchgrass (Panicum virgatum L.) NUE by using a low-cost organic amendment under different harvest frequencies. Aerobically digested biosolids were applied at 0, 153, 306, and 459 kg N ha−1 in a small plot study, and lime-stabilized biosolids were applied at 0, 77, and 154 kg N ha−1 in a field-scale study in Virginia, USA. Switchgrass was harvested once or twice per season. Switchgrass N concentration and N removal were measured to estimate switchgrass NUE, annual N recovery (ANR), and partial factor productivity (PFP). Across N rates, biosolid application increased biomass N concentration and removal by 29 % and 84 % and decreased NUE, ANR, and PFP in the plot study, but effects were inconsistent in the field study. Low NUE, ANR, and PFP obtained with a single, end-of-season harvest were likely functions of low feedstock N concentrations. Switchgrass harvested in summer had highest N concentrations. Cutting twice per season removed more N than cutting once; the resulting increase in NUE reflects differences in feedstock N concentrations rather than differences in yield. Our results suggest that biosolids can be applied as an alternative N source to support plant growth, and cutting once per season is preferable in sustainable biofuel production systems.