Spatial changes in nitrogen inputs drive short- and long-term variability in global nitrous oxide&#160;emissions
Authors: Harris Eliza , Yu Longfei , Wang Ying Ping , Mohn Joachim , Bai Edith , Barthel Matti , Six Johan , Bauters Marijn , Boeckx Pascal , Dorich Christopher , Farrell Mark , Henne Stephan , Krummel Paul , Loh Zoe , Steinbacher Martin , Zellweger Christoph , Wells Naomi S. , Bahn Michael , Rayner Peter 
Published: March 2021

&lt;p&gt;Anthropogenic activities, particularly fertilisation, have resulted in significant increases in reactive nitrogen (&lt;em&gt;r&lt;/em&gt;N) in soils globally, leading to eutrophication, acidification, poor air quality, and emissions of the important greenhouse gas N&lt;sub&gt;2&lt;/sub&gt;O. Understanding the partitioning of &lt;em&gt;r&lt;/em&gt;N losses into different environmental compartments is critical to mitigate negative impacts, however, loss pathways are poorly quantified, and potential changes driven by climate warming and societal shifts are highly uncertain. We present a coupled soil-atmosphere isotope model (IsoTONE; &lt;strong&gt;ISO&lt;/strong&gt;topic &lt;strong&gt;T&lt;/strong&gt;racing &lt;strong&gt;O&lt;/strong&gt;f &lt;strong&gt;N&lt;/strong&gt;itrogen in the &lt;strong&gt;E&lt;/strong&gt;nvironment) to partition &lt;em&gt;r&lt;/em&gt;N losses into leaching, harvest, NH&lt;sub&gt;3&lt;/sub&gt; volatilization, and production of NO, N&lt;sub&gt;2&lt;/sub&gt; and N&lt;sub&gt;2&lt;/sub&gt;O based on a global dataset of soil &amp;#948;&lt;sup&gt;15&lt;/sup&gt;N, as well as numerous other geoclimatic and experimental datasets. The model was optimized in a Bayesian framework using a time series of N&lt;sub&gt;2&lt;/sub&gt;O mixing ratios and isotopic compositions since the preindustrial era, as well as a global dataset of N&lt;sub&gt;2&lt;/sub&gt;O emission factors (EF). The posterior model results showed that the total anthropogenic flux in 2020 (7.8 Tg N&lt;sub&gt;2&lt;/sub&gt;O-N a&lt;sup&gt;-1&lt;/sup&gt;) was dominated by indirect emissions resulting from N deposition, while the growth rate and trend in anthropogenic N&lt;sub&gt;2&lt;/sub&gt;O was driven by both direct N fertilisation and deposition inputs. In contrast, inputs from fixation N drive natural N&lt;sub&gt;2&lt;/sub&gt;O emissions, and were responsible for subdecadal interannual variability in total emissions.&lt;/p&gt;&lt;p&gt;Total N gas (N&lt;sub&gt;2&lt;/sub&gt;O + NO + N&lt;sub&gt;2&lt;/sub&gt;) production and N&lt;sub&gt;2&lt;/sub&gt;O losses were strongly dependent on geoclimate and thus spatially variable, therefore the spatial pattern of N inputs strongly impacted resulting EFs and total N&lt;sub&gt;2&lt;/sub&gt;O emissions. The area-weighted global EF for N&lt;sub&gt;2&lt;/sub&gt;O was 1% &amp;#160;of anthropogenic N inputs in 2020, similar to the current IPCC default of 1.4%, however the N input-weighted global EF was 4.3%. Shifts in fertilisation inputs from the temperate Northern hemisphere towards warmer regions with higher EFs such as India and China have led to accelerating N&lt;sub&gt;2&lt;/sub&gt;O emissions (1.02&amp;#177;0.7 Tg N&lt;sub&gt;2&lt;/sub&gt;O-N a&lt;sup&gt;-1&lt;/sup&gt;). In addition, N&lt;sub&gt;2&lt;/sub&gt;O emissions have increased over the past decades due to climate warming (0.76&amp;#177;0.4 Tg N&lt;sub&gt;2&lt;/sub&gt;O-N a&lt;sup&gt;-1&lt;/sup&gt;). Predicted increases in fertilisation in India and Africa in the coming decades could further accelerate N&lt;sub&gt;2&lt;/sub&gt;O-driven climate warming, unless mitigation measures are implemented to increase fertiliser N use efficiency and reduce N&lt;sub&gt;2&lt;/sub&gt;O emission factors.&lt;/p&gt;