Switchgrass (Panicum virgatumL.) is a native perennial warm-season grass that is grown in North America primarily for hay and grazing purposes. Given its wide geographic range across North America, switchgrass has extensive trait variation allowing it to adapt to many environments. There is growing interest in using switchgrass for liquid biofuel production due to its resilient nature that allows it to grow well in marginal lands and potential benefits over annual crops. Currently switchgrass is typically grown as monocultures of a single variety. However, there may also be advantages of growing mixtures of different varieties of switchgrass on marginal lands. We conducted an experiment to determine whether planting multiple switchgrass varieties together (increasing genetic diversity) affects total productivity. Our goal was to determine how mixtures and monocultures of different varieties responded to changes in soil texture, specifically whether or not the effects of genetic diversity were stronger or weaker in marginal soils. We planted monocultures and polycultures of switchgrass from four varieties (Alamo, Cave-in-Rock, Dakota and Trailblazer) in pots with two different types of soils: standard potting soil and potting soil mixed with 50% sand by volume. To assess the potential productivity of the different plantings, we measured height, tiller production, leaf chlorophyll (SPAD), and above- and below-ground biomass.
Alamo plants were tallest in both soil types, but produced the fewest number of tillers. Both above- and below-ground biomass for all planted monocultures and polycultures were greater in potting soil than in 50% sand. Polycultures produced the most biomass on average in both standard soil and 50% sand, but it was not significantly different than the biomass produced by high-yielding monocultures of Alamo and Cave-in-Rock. At an individual level, however, biomass of switchgrass was higher when neighboring plants were of different varieties than when they were the same cultivar, potentially indicating that inter-variety competition was stronger than intra-variety competition. Our results suggest that growing switchgrass in mixtures could offer yield benefits, particularly in fertile, well-drained soils. Further studies of biomass production using different switchgrass varieties grown on marginal farmland could shed light on the benefits of planting switchgrass in polycultures as well as the feasibility of using genetically diverse monocultures as a source for liquid biofuel production.