Effects of Biosolids Rates and Harvest Frequency on Switchgrass Yield and Biomass Feedstock Quality.

Biosolids can be safely recycled and applied as an alternative fertilizer to sustainably maintain productive soils and stimulate plant growth. However, no study has tested the response of switchgrass to biosolids as part of a nutrient management program for energy cropping systems.  We conducted a study to evaluate the effects of biosolids rates and harvest frequencies on switchgrass yield and forage quality. The study was conducted at Orange, VA, on Davidson clay soils (Fine, kaolinitic, thermic Rhodic Kandiudults) with 0 to 10% slopes.  A long-term switchgrass stand (‘Cave in Rock’) was burned in late April 2010.  In May, 2.4-×4.8-m plots (0.6-m borders) were laid out. Biosolids (0, 154, 308, 462 kg N ha^-1) were surface applied, and two harvest frequencies (July + December vs. December) were imposed.   The study was an RCBD with a split plot treatment arrangement with three replications; harvest frequency was the main plot and biosolids was the subplot.

Switchgrass harvested twice had greater (P=0.004) dry matter(DM) than once-cut biomass (847 vs. 719 g kg^-1).  Biosolids did not affect DM concentrations (mean = 773 g kg^-1).   Cutting twice tended (P=0.132) to reduce yield (4.9 vs. 5.6 Mg ha^-1).  Biosolids applications increased (P=0.014) switchgrass yields over controls (mean=5.56 vs. 3.92 Mg ha^-1).   Cutting switchgrass twice increased (P<0.0001) biomass N concentrations in November-harvested biomass (8.7 vs. 5.7 g kg^-1).  Biomass N concentration differences were not detectable among biosolids treatments but were greater (P=0.003) than in control biomass (8.0 vs. 5.3 g kg^-1). Greater (P=0.016) N removal occurred with biosolids applications (mean=45.3 vs. 21.8 kg N ha^-1).  However, average N use efficiency for biosolids was 16.7 %. Biosolids applications can increase switchgrass production in bioenergy systems but may face nutrient loading limitations.