Switchgrass and Mixed Perennial Biomass Production on Two Marginally Productive Soils As Affected By Nitrogen Fertility and Harvest Management.

Biofuel production using native perennials can reduce U.S. dependence on foreign oil and curtail greenhouse gas emissions. Moreover, native perennials can be grown on fragile or low-productivity soils, providing valuable ecosystem services such as topsoil retention and nutrient loss reduction. In this research, we quantified native perennial biomass production and nitrogen (N) removal as a function of harvest regime and N application rate (0, 56, and 112 kg ha^-1) on two marginal sites in Minnesota over three years. We examined three switchgrass (Panicum virgatum L.) monocultures, including ‘Liberty’, a new F1 hybrid bioenergy variety, and three polycultures of grass-only, grass-legume, and grass-legume-forb mixes. ‘Shawnee’ and ‘Sunburst’ produced an average biomass yield of 2.7 and 9.6 Mg ha^-1 at Becker and Lamberton, respectively, compared to only 1.7 and 7.2 Mg ha^-1 for ‘Liberty’ at the same sites. We also found that biomass production in the low-diversity grass mix was, on average, similar to ‘Shawnee’ switchgrass at both sites. However, in the post-frost harvest of the second treatment year at Lamberton, the low-diversity grass mix produced 38% more biomass than ‘Shawnee’. Average N removal for the near-anthesis and post-frost harvests, respectively, was 21.2 and 12.5 kg ha^-1 at Becker 82.8 and 44.2 kg ha^-1 at Lamberton. While there was little difference in yield between harvest regimes in ‘Shawnee’, ‘Sunburst’, or the low-diversity grass mix, producers will remove less N in a post-frost regime, which may promote stand longevity with fewer inputs over time. Best management practices for native perennial biomass production on marginal soils in Minnesota include planting either a well-adapted switchgrass variety or a low-diversity grass mix fertilized with 56 kg N ha^-1 annually, post-establishment. Producers may have flexibility in harvest timing (near-anthesis or post-frost) in the first few years following establishment: little-to-no stand loss in the best-adapted feedstocks was observed with near-anthesis harvest, although this could change over time.