Editing IPCC:AR6/WGI/Chapter-5 (section)

==== 5.2.3.4 Emissions and Sinks in Non-agricultural Land ====

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Soils are the largest natural source of N <sub>2</sub> O, arising primarily from nitrogen processing associated with microbial nitrification and denitrification (Table 5.3; [[#Butterbach-Bahl--2013|Butterbach-Bahl et al., 2013]] ; [[#Snider--2015|Snider et al., 2015]] ). Under some conditions, soils can also act as a net sink of N <sub>2</sub> O, but this effect is small compared to the overall source ( [[#Schlesinger--2013|Schlesinger, 2013]] ). Since AR5 (WGI, Section 6.4.3), improved global process-based models ( [[#Tian--2019|Tian et al., 2019]] ) suggest a present-day source of 6.7 (5.3–8.1) TgN yr <sup>–1</sup> (2007–2016 average), which is consistent with the estimate in AR5. Process-based models and inventory-based methods show that increased N deposition has enhanced terrestrial N <sub>2</sub> O emissions by 0.8 (0.4–1.4 TgN yr <sup>–1</sup> ) relative to approximately pre-industrial times, and by 0.2 (0.1–0.2) TgN yr <sup>–1</sup> between the 1980s and 2007–2016 ( ''limited evidence'' , ''medium agreement'' ) (Figure 5.16; [[#Tian--2019|Tian et al., 2019]] ). This estimate is at the high end of the range reported in AR5 (WGI, Section 6.4.3). Model projections further show that global warming has led to increased soil N <sub>2</sub> O emissions of 0.8 (0.3–1.3) TgN yr <sup>–1</sup> since approximately pre-industrial times, of which about half occurred since the 1980s ( ''limited evidence'' , ''high agreement'' ) ( [[#Tian--2019|Tian et al., 2019]] , 2020).

The SRCCL assessed that deforestation and other forms of land-use change significantly alter terrestrial N <sub>2</sub> O emissions through emission pulses following conversions, generally resulting in long-term reduced emissions in unfertilized ecosystems ( ''medium evidence, high agreement'' ). This conclusion is supported by a recent study demonstrating that the deforestation-pulse effect is offset by the effect of reduced area of mature tropical forests ( [[#Tian--2020|Tian et al., 2020]] ).

Uncertainties remain in process-based models with respect to their ability to capture the complicated responses of terrestrial N <sub>2</sub> O emissions to rain pulses, freeze–thaw cycles and the net consequences of elevated levels of CO <sub>2</sub> accurately ( [[#Tian--2019|Tian et al., 2019]] ). Emerging literature suggests that permafrost thaw may contribute significantly to arctic N <sub>2</sub> O emissions ( [[#Voigt--2020|Voigt et al., 2020]] ), but these processes are not yet adequately represented in models and upscaling to large-scale remains a significant challenge.

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