Optimized fertilizer N rates for bioenergy feedstock production and water quality in drylands

Bioenergy feedstocks offset demand for conventional petroleum-based energy resources. Switchgrass (Panicum virgatum L.) is a warm-season perennial grass that has been utilized for lingo-cellulosic ethanol production and direct energy via combustion. However, little is known about its potential as a feedstock in the semi-arid northern Great Plains USA, including effects of N fertilizer application on system productivity and environmental quality. A field study initiated in 2009 seeded ‘Sunburst’ switchgrass into plots with fertilizer N broadcast each spring at 0, 28, 56, and 84 kg N per ha as urea, with four treatment replicates. Each fall beginning in 2011, aboveground biomass was harvested, weighed, and dried. Soil cores to a depth of 1.2 m were taken in fall 2018, air-dried, and analyzed for soil nitrate. Switchgrass biomass ranged from 1.8 to 12.3 Mg per ha. In most years, N application increased switchgrass biomass, but response to N rates above 28 kg per ha was inconsistent. Biomass from fertilized switchgrass averaged 6.5 Mg per ha compared to 4.4 Mg per ha for the unfertilized control.  Soil nitrate levels indicated the potential of over-fertilization of switchgrass feedstocks to negatively impact water resources in drylands.