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 (NO) 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 NO emissions, covering a land as large as Germany, the correlation between N rate and NO emissions remains unclear. We have therefore specifically investigated the NO response to N applications by conducting field experiments with five N rates, and high-frequency measurements of NO emissions across contrasting climatic years. Our results showed that cumulative and yield-scaled NO emissions both increased exponentially as N applications were raised above the optimum rate in maize ( Zea mays L.). In wheat ( Triticum aestivum L.) there was a corresponding quadratic increase in NO 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 NO response to N rate have overestimated NO 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 NO emissions.