Drought minimized nitrogen fertilization effects on bioenergy feedstock quality

Switchgrass (Panicum virgatum) is one of the leading candidates for sustainable lignocellulosic biofuel production in North America. Most current management recommendations for switchgrass include applications of synthetic nitrogen (N) fertilizers to increase production, particularly when grown on marginal lands. However, this management can be costly to growers and have negative impacts on ecosystem functioning. Also, N-fertilization does not always result in higher yield in switchgrass and may have unintended effects on plant biomass quality, including cell wall composition, that can affect the efficiency of fermentation processes for biofuel production. Drought stress may reduce biomass responses to N-fertilization, further reducing the value of fertilizer application. To examine whether N-fertilization and reduced precipitation affected switchgrass productivity and cell wall composition, we conducted a two-year field experiment in mature stands of two switchgrass cultivars grown for bioenergy at the W.K. Kellogg Biological Station Long Term Ecological Research Site in Michigan, USA. Nitrogen was added at a rate of 56 kg N ha(-1) (urea and ammonium nitrate), and precipitation was reduced using rainout shelters. Overall, we did not observe any effect of N-fertilization on biomass production. However, under ambient rainfall conditions, N-fertilization altered switchgrass biomass quality by reducing hemicellulose. Reduced precipitation minimized the effects of N-fertilization on switchgrass cell wall composition. Switchgrass is a relatively drought-tolerant species, and our results indicate that this crop will be a viable bioenergy feedstock even in a changing climate. However, in this study, N-fertilization had no effect on biomass quality or quantity under drought conditions.