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

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Leilei Ruan
G. Philp Robertson

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.

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Background Photo by: Nicole Hansen - Jornada (JRN) LTER