Impact of Nitrogen Application Rate on Switchgrass Yield, Production Costs, and Nitrous Oxide Emissions

Switchgrass ( L.) has been promoted as a potential feedstock for cellulosic biofuel in the United States. Switchgrass is known to respond to N fertilizer, but optimal rates remain unclear. Given the potential nonlinear response of nitrous oxide (NO) emissions to N inputs, N additions to switchgrass above optimal levels could have large impacts on the greenhouse gas balance of switchgrass-based biofuel. Additionally, N additions are likely to have large impacts on switchgrass production costs. Yield, N removal, and net returns were measured in switchgrass receiving 0 to 200 kg N ha in Manhattan, KS, from 2012 to 2014. Emissions of NO were measured in the 0- to 150-kg N ha treatments. Total emissions of NO increased from 0.2 to 3.0 kg NO-N ha as N inputs increased from 0 to 150 kg N ha. The 3-yr averages of fertilizer-induced emission factors were 0.7, 2.1, and 2.6% at 50, 100, and 150 kg N ha, respectively. Removal of N at harvest increased linearly with increasing N rate. Switchgrass yields increased with N inputs up to 100 to 150 kg N ha, but the critical N level for maximum yields decreased each year, suggesting that N was being applied in excess at higher N rates. Net returns were maximized at 100 kg N ha at both a high and low urea cost (US$394.71 and $945.91 ha, respectively). These results demonstrate that N inputs were necessary to increase switchgrass productivity, but rates exceeding optimal levels resulted in excessive NO emissions and increased costs for producers.