Nitrous Oxide Emissions Increase Exponentially When Optimum Nitrogen Fertilizer Rates Are Exceeded in the North China Plain

The IPCC assume a linear relationship between nitrogen (N) application rate and nitrous oxide (N<sub>2</sub>O) emissions in inventory reporting, however, a growing number of studies show a nonlinear relationship under specific soil-climatic conditions. In the North China plain, a global hotspot of N<sub>2</sub>O emissions, covering a land as large as Germany, the correlation between N rate and N<sub>2</sub>O emissions remains unclear. We have therefore specifically investigated the N<sub>2</sub>O response to N applications by conducting field experiments with five N rates, and high-frequency measurements of N<sub>2</sub>O emissions across contrasting climatic years. Our results showed that cumulative and yield-scaled N<sub>2</sub>O emissions both increased exponentially as N applications were raised above the optimum rate in maize (<i>Zea mays</i> L.). In wheat (<i>Triticum aestivum</i> L.) there was a corresponding quadratic increase in N<sub>2</sub>O emissions with the magnitude of the response in 2012–2013 distinctly larger than that in 2013–2014 owing to the effects of extreme snowfall. Existing empirical models (including the IPCC approach) of the N<sub>2</sub>O response to N rate have overestimated N<sub>2</sub>O emissions in the North China plain, even at high N rates. Our study therefore provides a new and robust analysis of the effects of fertilizer rate and climatic conditions on N<sub>2</sub>O emissions.