Nitrogen fertilization of switchgrass increases biomass yield and improves net greenhouse gas balance in northern Michigan, U.S.A

Nitrogen (N) fertilization can increase bioenergy crop production; however, fertilizer production and application can contribute to greenhouse gas (GHG) emissions, potentially undermining the GHG benefits of bioenergy crops. The objective of this study was to evaluate the effects of N fertilization on GHG emissions and biomass production of switchgrass bioenergy crop, in northern Michigan. Nitrogen fertilization treatments included 0 kg ha−1 (control), 56 kg ha−1 (low) and 112 kg ha−1 (high) of N applied as urea. Soil fluxes of CO2, N2O and CH4 were measured every two weeks using static chambers. Indirect GHG emissions associated with field activities, manufacturing and transport of fertilizer and pesticides were derived from the literature. Switchgrass aboveground biomass yield was evaluated at the end of the growing season. Nitrogen fertilization contributed little to soil GHG emissions; relative to the control, there were additional global warming potential of 0.7 Mg ha−1 y−1 and 1.5 Mg ha−1 y−1 as CO2 equivalents (CO2eq), calculated using the IPCC values, in the low and high N fertilization treatments, respectively. However, N fertilization greatly stimulated CO2 uptake by switchgrass, resulting in 1.5- and 2.5-fold increases in biomass yield in the low and high N fertilization treatments, respectively. Nitrogen amendments improved the net GHG benefits by 2.6 Mg ha−1 y−1 and 9.4 Mg ha−1 y−1 as CO2eq relative to the control. Results suggest that N fertilization of switchgrass in this region could reduce (15–50%) the land base needed for bioenergy production and decrease pressure on land for food and forage crop production.