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

=== 2.7.6 Soil carbon responses to changes in organic matter inputs by plants ===

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While current ESM structures mean that increasing carbon inputs to soils drive corresponding increases in SOC stocks, long-term carbon addition experiments have found contradictory SOC responses. Some litter addition experiments have observed increased SOC accumulation (Lajtha et al. 2014b <sup>[[#fn:r2098|2098]]</sup> ; Liu et al. 2009 <sup>[[#fn:r2099|2099]]</sup> ), while others suggest insignificant SOC responses (Lajtha et al. 2014a <sup>[[#fn:r2100|2100]]</sup> ; van Groenigen et al. 2014 <sup>[[#fn:r2101|2101]]</sup> ). Microbial dynamics are believed to have an important role in driving complex responses to carbon additions. The addition of fresh organic material can accelerate microbial growth and SOM decomposition via priming effects (Kuzyakov et al. 2014 <sup>[[#fn:r2102|2102]]</sup> ; Cheng et al. 2017 <sup>[[#fn:r2103|2103]]</sup> ). SOM cycling is dominated by ‘hot spots’ including the rhizosphere as well as areas surrounding fresh detritus ( ''medium evidence, high agreement'' ) (Finzi et al. 2015 <sup>[[#fn:r2104|2104]]</sup> ; Kuzyakov and Blagodatskaya 2015 <sup>[[#fn:r2105|2105]]</sup> ). This complicates projections of SOC responses to increasing plant productivity as increasing carbon inputs could promote higher SOC storage, but these fresh carbon inputs could also deplete SOC stocks by promoting faster decomposition (Hopkins et al. 2014 <sup>[[#fn:r2106|2106]]</sup> ; Guenet et al. 2018 <sup>[[#fn:r2107|2107]]</sup> ; Sulman et al. 2014 <sup>[[#fn:r2108|2108]]</sup> ). A meta-analysis by van Groenigen et al. (2014) <sup>[[#fn:r2109|2109]]</sup> suggested that elevated CO <sub>2</sub> accelerated SOC turnover rates across several biomes. These effects could be especially important in high-latitude regions where soils have high organic matter content and plant productivity is increasing (Hartley et al. 2012 <sup>[[#fn:r2110|2110]]</sup> ), but have also been observed in the tropics (Sayer et al. 2011 <sup>[[#fn:r2111|2111]]</sup> ).

Along with biological decomposition, another source of uncertainty in projecting responses of SOC to climate change is stabilisation via interactions with mineral particles ( ''high confidence'' ) (Kögel- Knabner et al. 2008 <sup>[[#fn:r2112|2112]]</sup> ; Kleber et al. 2011 <sup>[[#fn:r2113|2113]]</sup> ; Marschner et al. 2008 <sup>[[#fn:r2114|2114]]</sup> ; Schmidt 2011 <sup>[[#fn:r2115|2115]]</sup> ). Historically, conceptual models of SOC cycling have centred on the role of chemical recalcitrance: the hypothesis that long-lived components of SOC are formed from organic compounds that are inherently resistant to decomposition. Under the emerging new paradigm, stable SOC is primarily formed by the bonding of microbially-processed organic material to mineral particles, which limits the accessibility of organic material to microbial decomposers (Lützow et al. 2006 <sup>[[#fn:r2116|2116]]</sup> ; Keiluweit et al. 2015 <sup>[[#fn:r2117|2117]]</sup> ; Kallenbach et al. 2016 <sup>[[#fn:r2118|2118]]</sup> ; Kleber et al. 2011 <sup>[[#fn:r2119|2119]]</sup> ; Hopkins et al. 2014 <sup>[[#fn:r2120|2120]]</sup> ). SOC in soil aggregates can be protected from microbial decomposition by being trapped in soil pores too small for microbes to access (Blanco-Canqui and Lal 2004 <sup>[[#fn:r2121|2121]]</sup> ; Six et al. 2004 <sup>[[#fn:r2122|2122]]</sup> ) or by oxygen limitation (Keiluweit et al. 2016 <sup>[[#fn:r2123|2123]]</sup> ). Some new models are integrating these mineral protection processes into SOC cycling projections (Wang et al. 2017a <sup>[[#fn:r2124|2124]]</sup> ; Sulman et al. 2014 <sup>[[#fn:r2125|2125]]</sup> ; Riley et al. 2014 <sup>[[#fn:r2126|2126]]</sup> ; Wieder et al. 2015 <sup>[[#fn:r2127|2127]]</sup> ), although the sensitivity of mineral-associated organic matter to changes in temperature, moisture, fire (Box 2.1) and carbon inputs is highly uncertain. Improved quantitative understanding of soil ecosystem processes will be critically important for projection of future land–climate feedback interactions.

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