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

Switchgrass (Panicum virgatum 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 (N2O) 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−1 in Manhattan, KS, from 2012 to 2014. Emissions of N2O were measured in the 0‐ to 150‐kg N ha−1 treatments. Total emissions of N2O increased from 0.2 to 3.0 kg N2O‐N ha−1 as N inputs increased from 0 to 150 kg N ha−1. 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−1, 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−1, 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−1 at both a high and low urea cost (US$394.71 and $945.91 ha−1, respectively). These results demonstrate that N inputs were necessary to increase switchgrass productivity, but rates exceeding optimal levels resulted in excessive N2O emissions and increased costs for producers. Core Ideas Losses of fertilizer N as N2O increased with increasing N application rates. Net return was maximized at an N rate of 100 kg N ha−1. Nitrogen inputs were necessary to increase switchgrass productivity. Rates exceeding optimal levels resulted in increased N2O emissions and costs.