Nitrogen Fertilization and Harvest Management of Switchgrass: Impacts on Biomass Yield and Nitrogen Removal

Perennial energy grasses, such as switchgrass, represent a potentially important component of sustainable bioenergy production. However, both economic and environmental sustainability will depend on reducing fossil fuel inputs for production to their bare minimum levels. The purpose of this study was to identify the impacts of nitrogen fertilization and harvest management on biomass production and nitrogen removal of two switchgrass cultivars. Five rates of nitrogen addition were evaluated across four developmental growth stages at two field sites for up to 5 years. The sites differed in biomass yield and nitrogen removal rates, with one site responding strongly to nitrogen addition and the other site showing weak and inconsistent responses. Biomass yield was generally reduced by delaying harvest until after senescence, but the yield reduction was small compared to the relatively large reductions in nitrogen removal rates. Following senescence, nitrogen removal rates never exceeded 50 kg N ha−1 and were generally below 30 kg N ha−1, averaged over years. Because these values are below estimates of potential associative nitrogen fixation for switchgrass on silt loam soils, this research suggests that nitrogen fertilization may not be a long-term sustainable practice for soils such as those utilized in this study.