Native perennial grasses have been proposed as sources of feedstocks for producing sustainable lignocellulosic biofuels in North America, especially on marginal lands. Switchgrass (Panicum virgatum L.) is a particularly promising species, given its extensive geographic distribution, high biomass yields, low requirements for fertilizer and tolerance for a wide range of environmental conditions. Using species adapted to varied environments as bioenergy feedstocks may reduce productivity losses in a changing climate and increase the potential to provide other ecosystem services. Droughts are expected to increase in frequency and duration with climate change and how species respond to drought will impact their value for bioenergy. Severe droughts reduce biomass and can affect cell wall composition important for energy production. Less is known about the impact of more moderate droughts and how responses could interact with fertilizer use. In this study, we examined how reductions in precipitation, manipulated by rain-out shelters, affected the biomass production and cell wall composition of two switchgrass varieties. To determine if this response interacted with soil fertility, we established the rainout shelters in two fertilizer treatments (with/without added N). We compared cell wall/tissue composition from samples taken in July (peak biomass) and October (post senescence) to determine how harvest time affected potential digestibility and ethanol yield.
Samples from July showed significant differences between the two varieties in hemicellulose and lignin content. Both of these varied in response to drought, but hemicellulose was not affected by fertilizer. Lignin content increased with fertilizer, particularly for Southlow plants in the drought treatment. Cellulose content was responsive to fertilizer, and the magnitude and direction of effect differed among the two varieties; increasing in Cave-in-Rock and decreasing in Southlow. Drought did not affect cellulose content. Analysis of the cell wall composition from samples taken in October is underway; this analysis will also allow calculation of digestibility and potential ethanol yields in response to these treatments. Our results to-date show that cell wall and tissue composition of switchgrass varieties differ and change in response to even moderate reductions in water availability. The optimal variety of Panicum to grow for bioenergy and the management regime that will maximize both biomass and fuel production will likely differ depending on environmental conditions. Increasing variability in climate, particularly rainfall, will make this selection challenging.