Editing IPCC:AR6/SR15/Chapter-4 (section)

==== 4.3.7.3 Soil carbon sequestration and biochar ====

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At local scales there is ''robust evidence'' that soil carbon sequestration (SCS, e.g., agroforestry, De Stefano and Jacobson, 2018) <sup>[[#fn:r639|639]]</sup> , restoration of degraded land (Griscom et al., 2017) <sup>[[#fn:r640|640]]</sup> , or conservation agriculture management practices (Aguilera et al., 2013; Poeplau and Don, 2015; Vicente-Vicente et al., 2016) <sup>[[#fn:r641|641]]</sup> have co-benefits in agriculture and that many measures are cost-effective even without supportive climate policy. Evidence at global scale for potentials and especially costs is much lower. The literature spans cost ranges of −45–100 USD tCO <sub>2</sub> <sup>−1</sup> (negative costs relating to the multiple co-benefits of SCS, such as increased productivity and resilience of soils; P. Smith et al., 2014) <sup>[[#fn:r642|642]]</sup> , and 2050 potentials are estimated at between 0.5 and 11 GtCO <sub>2</sub> yr <sup>−</sup> <sup>1</sup> , narrowed down to 2.3–5.3 GtCO <sub>2</sub> yr <sup>−</sup> <sup>1</sup> considering that studies above 5 GtCO <sub>2</sub> yr <sup>−</sup> <sup>1</sup> often do not apply constraints, while estimates lower than 2 GtCO <sub>2</sub> yr <sup>−</sup> <sup>1</sup> mostly focus on single practices (Fuss et al., 2018) <sup>[[#fn:r643|643]]</sup> .

SCS has negligible water and energy requirements (Smith, 2016) <sup>[[#fn:r644|644]]</sup> , affects nutrients and food security favourably ( ''high agreement, robust evidence'' ) and can be applied without changing current land use, thus making it socially more acceptable than CDR options with a high land footprint. However, soil sinks saturate after 10–100 years, depending on the SCS option, soil type and climate zone (Smith, 2016) <sup>[[#fn:r645|645]]</sup> .

Biochar is formed by recalcitrant (i.e., very stable) organic carbon obtained from pyrolysis, which, applied to soil, can increase soil carbon sequestration leading to improved soil fertility properties. <sup>[[#fn:5|5]]</sup> Looking at the full literature range, the global potential in 2050 lies between 1 and 35 Gt CO <sub>2</sub> yr <sup>−</sup> <sup>1</sup> ( ''low agreement, low evidence'' ), but considering limitations in biomass availability and uncertainties due to a lack of large-scale trials of biochar application to agricultural soils under field conditions, Fuss et al. (2018) <sup>[[#fn:r646|646]]</sup> lower the 2050 range to 0.3–2 GtCO <sub>2</sub> yr <sup>−</sup> <sup>1</sup> . This potential is below previous estimates (e.g., Woolf et al., 2010) <sup>[[#fn:r647|647]]</sup> , which additionally consider the displacement of fossil fuels through biochar. Permanence depends on soil type and biochar production temperatures, varying between a few decades and several centuries (Fang et al., 2014) <sup>[[#fn:r648|648]]</sup> . Costs are 30– 120 USD tCO <sub>2</sub> <sup>−1</sup> ( ''medium agreement, medium evidence'' ) (McCarl et al., 2009; McGlashan et al., 2012; McLaren, 2012; Smith, 2016) <sup>[[#fn:r649|649]]</sup> .

Water requirements are low and at full theoretical deployment, up to 65 EJ yr <sup>−1</sup> of energy could be generated as a side product (Smith, 2016) <sup>[[#fn:r650|650]]</sup> . Positive side effects include a favourable effect on nutrients and reduced N <sub>2</sub> O emissions (Cayuela et al., 2014; Kammann et al., 2017) <sup>[[#fn:r651|651]]</sup> . However, 40–260 Mha are needed to grow the biomass for biochar for implementation at 0.3 GtCO <sub>2</sub> -eq yr <sup>−1</sup> (Smith, 2016) <sup>[[#fn:r652|652]]</sup> , even though it is also possible to use residues (e.g., Windeatt et al., 2014) <sup>[[#fn:r653|653]]</sup> . Biochar is further constrained by the maximum safe holding capacity of soils (Lenton, 2010) <sup>[[#fn:r654|654]]</sup> and the labile nature of carbon sequestrated in plants and soil at higher temperatures (Wang et al., 2013) <sup>[[#fn:r655|655]]</sup> .

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