Editing IPCC:AR6/WGI/Chapter-6 (section)

==== 6.2.2.2 NO <sub>x</sub> emissions from Soils ====

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Soil NO <sub>x</sub> (SNO <sub>x</sub> ) emissions occur in connection with complex biogenic/microbial nitrification and denitrification processes ( [[#Ciais--2013|Ciais et al., 2013]] ), which in turn are sensitive – in a non-linear manner – to temperature, precipitation, soil moisture, carbon and nutrient content, and the biome itself (e.g., [[#Hudman--2012|Hudman et al., 2012]] ). Global SNO <sub>x</sub> estimates, based on observationally constrained chemistry-transport model and vegetation model studies, show a broad range between 4.7–16.8 TgN yr <sup>–1</sup> ( [[#Young--2018|Young et al., 2018]] ). This estimate is generally larger than the current source strength used in CMIP6 simulations, which is prescribed using an early empirical estimate, typically scaled to about 5 TgN yr <sup>–1</sup> ( [[#Yienger--1995|Yienger and Levy, 1995]] ).

By the end of the 21st century, the overall nitrogen fixation in non-agricultural ecosystems could be 40% larger than in 2000, due to increased enzyme activity with growing temperatures, but the emission rates of NO (and N <sub>2</sub> O) could be dominated by changes in precipitation patterns and evapotranspiration fluxes ( [[#Fowler--2015|Fowler et al., 2015]] ). Current Earth system models (ESMs) incorporate biophysical and biogeochemical processes only to a limited extent ( [[#Jia--2019|Jia et al., 2019]] ), precluding adequate climate sensitivity studies for SNO <sub>x</sub> . Hence, while the current strength source of soil NO <sub>x</sub> has been better constrained over the last decade, adequate representations of SNO <sub>x</sub> and how it escapes from the canopy, which could provide quantitative estimates of climate-driven changes in SNO <sub>x</sub> , are still missing in ESMs.

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