Impact of Nitrogen Rate on N2O Emissions, Yield and Production Costs in Switchgrass.

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 non-linear 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 a large impact on the switchgrass production costs.  Yield, N₂O emissions, N removal, and costs of production were measured in switchgrass receiving 0-200 kg N ha^-1 in Manhattan, KS from 2012-2014.  Response of N₂O to N rate varied each year, showing an exponential, linear and sigmoidal response to N₂O emissions in 2012, 2013 and 2014, respectively.  Fertilizer induced emission factor (EF) increased from 0.7% at 50 kg N ha^-1 to 2.6% at 150 kg N ha^-1, demonstrating a non-linear increase in N₂O emissions.  Switchgrass yields increased with N inputs up to 100-150 kg N ha^-1, but the critical N level for maximum yields decreased each year, suggesting N was being applied in excess at higher N rates.  Removal of N at harvest increased linearly with increasing N rate.  Yield-scaled costs of production were minimized at 100 kg N ha^-1 ($70.91 Mg^-1).  Harvest costs, land rent and N fertilizer made up the majority of production costs.  These results demonstrate N inputs are necessary to increase switchgrass productivity, but rates exceeding optimal levels could result in excessive N_­2O emissions and increased costs for producers.