Sensitivity of Nitrous Oxide Emissions to Climate

Production of synthetic nitrogen (N) fertilizer is forecast to exceed 200 Tg N in 2018— a 25% increase since 2008. The current trajectory of global N fertilizer production is following one of the most aggressive scenarios considered by the Food and Agricultural Organization of the United Nations. Regional and global analyses demonstrate that about 4.5% of the anthropogenic N (fertilizer, deposition, manure) in agricultural systems is returned to the atmosphere as nitrous oxide (N₂O)—a potent greenhouse gas and ozone-depleting substance. While growing population and demands for food, fiber, and fuel are accelerating the use of synthetic N, it remains unclear how sensitive N₂O emissions are to climate variations. In this study, six years of hourly N₂O concentration measurements from a very tall tower located within the US Corn Belt were used within a Bayesian inverse modeling framework to estimate the inter-annual variability in regional N₂O emissions. The variability in emissions was large, ranging from 315 to 555 Gg N₂O-N y^-1 (mean = 386 +/- 105 Gg N₂O-N y^-1) with a coefficient of variation of 27%. The sensitivity of emissions to climate variability was assessed by considering: 1) the response of direct and indirect emissions to climate drivers; and 2) by examining the seasonal and episodic fluctuations in tall tower concentrations to climate drivers using wavelet coherence analyses. These analyses were used to compute climate sensitivity factors for N₂O emissions. They indicate that precipitation (timing, frequency, and magnitude) and surface runoff are key regional drivers that cause large fluctuations in the indirect emissions. State-of-the-art land surface schemes did not accurately capture the seasonal and inter-annual variability in N₂O emissions, suggesting that the sensitivity of key model processes to climate variations is poorly constrained. The implications of increasing production of synthetic nitrogen and current climate trends on future N₂O emissions will be discussed.