Nitrogen fertilization and harvest management of switchgrass: Impacts on biomass yield and nitrogen removal

Authors: CASLER, MICHAEL - Retired ARS Employee

Submitted to: BioEnergy Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/17/2022
Publication Date: 3/23/2022
Citation: Casler, M.D. 2022. Nitrogen fertilization and harvest management of switchgrass: Impacts on biomass yield and nitrogen removal. BioEnergy Research. 15(1). https://doi.org/10.1007/s12155-022-10435-z.
DOI: https://doi.org/10.1007/s12155-022-10435-z

Interpretive Summary: 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 and nitrogen fertilizer is the most expensive input for perennial energy grasses. 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. Early harvests had the highest biomass yield, but also removed significant amounts of nitrogen, preventing it from being moved into roots for next year's growth and production. Harvesting after the winter was the most sustainable practice if the crop is fertilized with nitrogen. As observed in the literature, nitrogen fertilizer had widely variable impacts on biomass yield, often with no effect. These results add significant information to the literature, helping to clarify when and how best to use nitrogen fertilizer for researchers and practitioners.

Technical Abstract: 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 five 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.