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

=== 2.7.4 Vertical distribution of soil organic carbon ===

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It has long been recognised that dynamics of soil organic carbon (SOC) represent a large source of uncertainties on biogeochemical interactions of land with atmosphere and climate as detailed below. Since AR5, there have been new understandings on SOC size, as well as on the microbial processes that influence SOM dynamics under climate change and LULCC. Three existing databases (SoilGrids, the Harmonized World Soil Data Base and Northern Circumpolar Soil Database) substantially differ in the estimated size of global SOC stock down to 1 m depth, varying between 2500 Pg to 3400 Pg with differences among databases largely attributable to carbon stored in permafrost (Joosten 2015 <sup>[[#fn:r2049|2049]]</sup> ; Köchy et al. 2015 <sup>[[#fn:r2050|2050]]</sup> ; Tifafi et al. 2018 <sup>[[#fn:r2051|2051]]</sup> ). These values are four to eight times larger than the carbon stock associated with the terrestrial vegetation (Bond-Lamberty et al. 2018 <sup>[[#fn:r2052|2052]]</sup> ). New estimates since AR5 show that much larger areas in the Amazon and Congo basins are peatlands (Gumbricht et al. 2017 <sup>[[#fn:r2053|2053]]</sup> ; Dargie et al. 2019 <sup>[[#fn:r2054|2054]]</sup> ).

Deep soil layers can contain much more carbon than previously assumed ( ''limited evidence, medium agreement'' ) (e.g., González- Jaramillo et al. (2016) <sup>[[#fn:r2055|2055]]</sup> ). Based on radiocarbon measurements, deep SOC can be very old, with residence times up to several thousand years (Rumpel and Kögel-Knabner 2011 <sup>[[#fn:r2056|2056]]</sup> ) or even several tens of thousands of years (Okuno and Nakamura 2003 <sup>[[#fn:r2057|2057]]</sup> ). Dynamics associated with such deeply buried carbon remain poorly studied and ignored by the models, and are not addressed in most of the studies assessed in this subsection. Deep soil carbon is thought to be stabilised by mineral interactions, but recent experiments suggest that CO <sub>2</sub> release from deep soils can also be increased by warming, with a 4 ̊C warming enhancing annual soil respiration by 34–37% (Hicks Pries et al. 2017 <sup>[[#fn:r2058|2058]]</sup> ), or with the addition of fresh carbon (Fontaine et al. 2007 <sup>[[#fn:r2059|2059]]</sup> ). While erosion is not typically modelled as a carbon flux in ESMs, erosion and burial of carbon-containing sediments is likely a significant carbon transfer from land to ocean ( ''medium confidence'' ) (Berhe et al. 2007 <sup>[[#fn:r2060|2060]]</sup> ; Asefaw et al. 2008 <sup>[[#fn:r2061|2061]]</sup> ; Wang et al. 2017e <sup>[[#fn:r2062|2062]]</sup> ).

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