The 2007 U.S. Energy Independence and Security Act mandates production of 16 billion gallons of cellulosic ethanol by 2022, and switchgrass (Panicum virgatum), a perennial C4 grass native to North America, is a leading candidate cellulosic feedstock crop. Switchgrass fields may provide more ecosystem services than annual crops and could provide a sustainable agricultural crop on marginal lands. However, management considerations, such as what switchgrass variety to plant and whether to fertilize, may alter ecosystem services provided by these grasslands, especially in a changing climate. Our objective in this study was to evaluate the effects of fertilizer and reduced precipitation on the physiology, growth, and belowground ecology of two varieties of switchgrass. We conducted our research in the Cellulosic Biofuels Diversity Experiment located at the W.K. Kellogg Biological Station Long Term Ecological Research (KBS LTER) site in Michigan. In May 2015, we established a precipitation manipulation treatment using rainout shelters in this ongoing experiment initiated in 2008 that factorially manipulated switchgrass variety (Cave-in-Rock or Southlow) and fertilizer addition (0 or 56 kg N/ha). The rain-out shelters removed 70% of ambient rainfall, causing significant decreases in soil moisture during the growing season. We quantified leaf water potential and belowground mycorrhizal activity for the two switchgrass varieties in all treatments at three times during the growing season and harvested plots to estimate aboveground biomass production at the end of the growing season.
Cave-in-Rock had higher biomass production in dry compared to ambient conditions, whereas Southlow biomass was reduced in dry conditions. During the growing season, dry conditions decreased Cave-in-Rock midday leaf water potential, but only in unfertilized plots. Southlow plants had more negative leaf water potential in dry conditions independent of soil fertility. Mycorrhizae associated with Cave-in-Rock were more active in fertilized soils, while those associated with Southlow were more active in unfertilized soils. These results suggest that the Cave-in-Rock variety may be more drought tolerant and a better candidate for bioenergy crop production in a future climate involving reduced precipitation. This may be due to Cave-in-Rock plants taking advantage of beneficial mycorrhizal associations to alleviate water stress. However, these benefits were mostly limited to fertilized soils, indicating that farmers may want to incorporate low levels of fertilizer if growing Cave-in-Rock on marginal lands.