Editing IPCC:AR6/SRCCL/Chapter-2 (section)

=== 2.7.5 Soil carbon responses to warming and changes in soil moisture ===

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Annually, 119 GtC is estimated to be emitted from the terrestrial ecosystem to the atmosphere, of which about 50% is attributed to soil microbial respiration (Auffret et al. 2016 <sup>[[#fn:r2063|2063]]</sup> ; Shao et al. 2013 <sup>[[#fn:r2064|2064]]</sup> ). It is yet not possible to make mechanistic and quantitative projections about how multiple environmental factors influence soil microbial respiration (Davidson et al. 2006a <sup>[[#fn:r2065|2065]]</sup> ; Dungait et al. 2012 <sup>[[#fn:r2066|2066]]</sup> ). Soil warming experiments show significant variability in temperature and moisture responses across biomes and climates; Crowther et al. (2016) <sup>[[#fn:r2067|2067]]</sup> found that warming-induced SOC loss is greater in regions with high initial carbon stocks, while an analysis of an expanded version of the same dataset did not support this conclusion (Gestel et al. 2018 <sup>[[#fn:r2068|2068]]</sup> ). Studies of SOC responses to warming over time have also shown complex responses. In a multi-decadal warming experiment, Melillo et al. (2017) <sup>[[#fn:r2069|2069]]</sup> found that soil respiration response to warming went through multiple phases of increasing and decreasing strength, which were related to changes in microbial communities and available substrates over time. Conant et al. (2011) <sup>[[#fn:r2070|2070]]</sup> and Knorr et al. (2005) <sup>[[#fn:r2071|2071]]</sup> suggested that transient decomposition responses to warming could be explained by depletion of labile substrates, but that long-term SOC losses could be amplified by high temperature sensitivity of slowly decomposing SOC components. Overall, long-term SOC responses to warming remain uncertain (Davidson et al. 2006a <sup>[[#fn:r2072|2072]]</sup> ; Dungait et al. 2012 <sup>[[#fn:r2073|2073]]</sup> ; Nishina et al. 2014 <sup>[[#fn:r2074|2074]]</sup> ; Tian et al. 2015 <sup>[[#fn:r2075|2075]]</sup> ).

It is widely known that soil moisture plays an important role in SOM decomposition by influencing microbial processes (e.g., Monard et al. (2012) <sup>[[#fn:r2076|2076]]</sup> , Moyano et al. (2013) <sup>[[#fn:r2077|2077]]</sup> , Yan et al. (2018) <sup>[[#fn:r2078|2078]]</sup> ), as confirmed by a recent global meta-analysis ( ''high confidence'' ) (Hawkes et al. 2017 <sup>[[#fn:r2079|2079]]</sup> ). A likely mechanism is that increased soil moisture lowers carbon mineralisation rates under anaerobic conditions, resulting in enhanced carbon stocks, but experimental analyses have shown that this effect may last for only 3–4 weeks after which iron reduction can actually accelerate the loss of previously protected OC by facilitating microbial access (Huang and Hall 2017 <sup>[[#fn:r2080|2080]]</sup> ).

Experimental studies of responses of microbial respiration to warming have found variable results (Luo et al. 2001 <sup>[[#fn:r2081|2081]]</sup> ; Bradford et al. 2008 <sup>[[#fn:r2082|2082]]</sup> ; Zhou et al. 2011 <sup>[[#fn:r2083|2083]]</sup> ; Carey et al. 2016 <sup>[[#fn:r2084|2084]]</sup> ; Teramoto et al. 2016 <sup>[[#fn:r2085|2085]]</sup> ). No acclimation was observed in carbon-rich calcareous temperate forest soils (Schindlbacher et al. 2015 <sup>[[#fn:r2086|2086]]</sup> ) and arctic soils (Hartley et al. 2008 <sup>[[#fn:r2087|2087]]</sup> ), and a variety of ecosystems from the Arctic to the Amazon indicated that microbes appear to enhance the temperature sensitivity of soil respiration in Arctic and boreal soils, thereby releasing even more carbon than currently projected (Karhu et al. 2014 <sup>[[#fn:r2088|2088]]</sup> ). In tropical forests, phosphorus limitation of microbial processes is a key factor influencing soil respiration (Camenzind et al. 2018 <sup>[[#fn:r2089|2089]]</sup> ). Temperature responses of symbiotic mycorrhizae differ widely among host plant species, without a clear pattern that may allow generalisation across plant species and vegetation types (Fahey et al. 2016 <sup>[[#fn:r2090|2090]]</sup> ).

Some new insights have been obtained since AR5 from investigations of improved mechanistic understanding of factors that regulate temperature responses of soil microbial respiration. Carbon use efficiency and soil nitrogen dynamics have large influence on SOC responses to warming ( ''high confidence'' ) (Allison et al. 2010 <sup>[[#fn:r2091|2091]]</sup> ; Frey et al. 2013 <sup>[[#fn:r2092|2092]]</sup> ; Wieder, William R., Bonan, Gordon B., Allison 2013 <sup>[[#fn:r2093|2093]]</sup> ; García-Palacios et al. 2015 <sup>[[#fn:r2094|2094]]</sup> ). More complex community interactions including competitive and trophic interactions could drive unexpected responses to SOC cycling to changes in temperature, moisture and carbon inputs (Crowther et al. 2015 <sup>[[#fn:r2095|2095]]</sup> ; Buchkowski et al. 2017 <sup>[[#fn:r2096|2096]]</sup> ).

Competition for nitrogen among bacteria and fungi could also suppress decomposition (Averill et al. 2014 <sup>[[#fn:r2097|2097]]</sup> ). Overall, the roles of soil microbial community and trophic dynamics in global SOC cycling remain very uncertain.

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