Nitrogen and harvest management effects on switchgrass and mixed perennial biomass production

Biofuel production using native perennials on marginal soils can reduce US dependence on foreign oil and curtail greenhouse gas emissions with minimal impact on food crop production. We quantified post-establishment biomass yield and N removal as a function of harvest regime (anthesis and post-frost) and N application rate (0, 56, and 112 kg N ha^–1) at two marginal sites in Minnesota. We examined three switchgrass (Panicum virgatum L.) monocultures: ‘Shawnee’, ‘Sunburst’, and ‘Liberty’, a new bioenergy variety, and three polycultures: grass-only, grass-legume, and grass-legume-forb. On an excessively-drained loamy sand soil, maximum productivity occurred in ‘Sunburst’ and ‘Shawnee’ switchgrass fertilized at 112 kg N ha^–1 yr^–1 in the post-frost harvest regime, with 3-yr total yields of 11.0 Mg ha^–1, while ‘Liberty’ produced 7.0 Mg ha^–1. On an eroded loam soil, maximum post-frost productivity in ‘Shawnee’, ‘Sunburst’, and ‘Liberty’ was similar at 56 and 112 kg N ha^–1 yr^–1, with total yields of 32.6, 29.8, and 23.5 Mg ha^–1, respectively. Yields of the low-diversity (LD) grass mix were similar to ‘Shawnee’ switchgrass at both locations, indicating that an LD grass mix or well-adapted switchgrass monoculture are likely the most productive feedstocks for this region. Most feedstocks produced similar or greater yields and removed less N in the post-frost relative to the anthesis harvest. While producers may have flexibility in harvest timing for some feedstocks in the first few years following establishment, a post-frost harvest regime may promote stand longevity with less N fertilizer use over time.