Non-linear nitrous oxide (N2O) response to N fertilizer during switchgrass establishment

N₂O is known for its high global warming potential. Various studies have shown that soil N₂O emissions are significantly correlated with the amount of N fertilizer applied. However, the form of the relationship between the amount of N applied and soil N₂O emissions is still unclear. If nonlinear, as for many annual crops, the contribution of soil N₂O from fertilizer will be significantly higher than that estimated by IPCC default methods, as will the mitigation benefit of a lower N fertilizer rate.

We conducted a three-year study of N₂O emissions from newly established switchgrass in southwest Michigan to determine the response in soil N₂O emissions to the input of N fertilizer. Nitrogen was applied to replicate field plots as urea in May-June each year at 8 rates of 0, 28, 56, 84, 112, 140, 168 and 196 kg N ha^-1. Plots were established in spring, 2009, in a randomized complete block design with four replications. From 2009 to 2011, mean daily N₂O emissions ranged between 1.86 and 20.0 g N ha^-1d^-1 for the low fertilizer (0 kg N ha^-1) and high fertilizer (196 kg N ha^-1) N rates, respectively, with a maximum daily N₂O flux of 312 g N ha^-1d^-1 in the highest N rate  plots and a minimum daily flux of 0 in  plots that received no fertilizer N. A linear relationship between N input and cumulative N₂O emissions occurred in the establishment year 2009 but a non-linear relationship occurred in 2010 and 2011. N₂O fluxes were strongly correlated with precipitation. Most of the N fertilizer-associated N₂O increase occurred within 40 days following fertilization. Switchgrass yields were responsive to N fertilizer in 2009 (to 140 kg N ha^-1) and 2010 (to 56 kg N ha^-1) but there were no significant yield differences among treatments in 2011.