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

==== 2.6.1.3 Land management of soils ====

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The global mitigation potential for increasing soil organic matter stocks in mineral soils is estimated to be in the range of 0.4–8.64 GtCO <sub>2</sub> yr <sup>–1</sup> ( ''high confidence'' ), though the full literature range is wider with high uncertainty related to some practices (Fuss et al. 2018 <sup>[[#fn:r1634|1634]]</sup> ; Sommer and Bossio 2014 <sup>[[#fn:r1635|1635]]</sup> ; Lal 2010 <sup>[[#fn:r1636|1636]]</sup> ; Lal et al. 2004 <sup>[[#fn:r1637|1637]]</sup> ; Conant et al. 2017 <sup>[[#fn:r1638|1638]]</sup> ; Dickie et al. 2014 <sup>[[#fn:r1639|1639]]</sup> ; Frank et al. 2017a <sup>[[#fn:r1640|1640]]</sup> ; Griscom et al. 2017 <sup>[[#fn:r1641|1641]]</sup> ; Herrero et al. 2015 <sup>[[#fn:r1642|1642]]</sup> , 2016 <sup>[[#fn:r1643|1643]]</sup> ; McLaren 2012 <sup>[[#fn:r1644|1644]]</sup> ; Paustian et al. 2016 <sup>[[#fn:r1645|1645]]</sup> ; Poeplau and Don 2015 <sup>[[#fn:r1646|1646]]</sup> ; Powlson et al. 2014 <sup>[[#fn:r1647|1647]]</sup> ; Smith et al. 2016c <sup>[[#fn:r1648|1648]]</sup> ; Zomer et al. 2017 <sup>[[#fn:r1649|1649]]</sup> ). Some studies have separate potentials for soil carbon sequestration in croplands (0.25–6.78 GtCO <sub>2</sub> yr <sup>–1</sup> ) (Griscom et al. 2017 <sup>[[#fn:r1650|1650]]</sup> ; Hawken 2017 <sup>[[#fn:r1651|1651]]</sup> ; Frank et al. 2017a <sup>[[#fn:r1652|1652]]</sup> ; Paustian et al. 2016 <sup>[[#fn:r1653|1653]]</sup> ; Herrero et al. 2016 <sup>[[#fn:r1654|1654]]</sup> ; Henderson et al. 2015 <sup>[[#fn:r1655|1655]]</sup> ; Dickie et al. 2014 <sup>[[#fn:r1656|1656]]</sup> ; Conant et al. 2017 <sup>[[#fn:r1657|1657]]</sup> ; Lal 2010 <sup>[[#fn:r1658|1658]]</sup> ) and soil carbon sequestration in grazing lands (0.13–2.56 GtCO <sub>2</sub> yr <sup>–1</sup> ) (Griscom et al. 2017 <sup>[[#fn:r1659|1659]]</sup> ; Hawken 2017 <sup>[[#fn:r1660|1660]]</sup> ; Frank et al. 2017a <sup>[[#fn:r1661|1661]]</sup> ; Paustian et al. 2016 <sup>[[#fn:r1662|1662]]</sup> ; Powlson et al. 2014 <sup>[[#fn:r1663|1663]]</sup> ; McLaren 2012 <sup>[[#fn:r1664|1664]]</sup> ; Zomer et al. 2017 <sup>[[#fn:r1665|1665]]</sup> ; Smith et al. 2015 <sup>[[#fn:r1666|1666]]</sup> ; Sommer and Bossio 2014 <sup>[[#fn:r1667|1667]]</sup> ; Lal 2010 <sup>[[#fn:r1668|1668]]</sup> ). The potential for soil carbon sequestration and storage varies considerably depending on prior and current land management approaches, soil type, resource availability, environmental conditions, microbial composition and nutrient availability among other factors (Hassink and Whitmore 1997 <sup>[[#fn:r1669|1669]]</sup> ; Smith and Dukes 2013 <sup>[[#fn:r1670|1670]]</sup> ; Palm et al. 2014 <sup>[[#fn:r1671|1671]]</sup> ; Lal 2013 <sup>[[#fn:r1672|1672]]</sup> ; Six et al. 2002 <sup>[[#fn:r1673|1673]]</sup> ; Feng et al. 2013 <sup>[[#fn:r1674|1674]]</sup> ). Soils are a finite carbon sink and sequestration rates may decline to negligible levels over as little as a couple of decades as soils reach carbon saturation (West et al. 2004 <sup>[[#fn:r1675|1675]]</sup> ; Smith and Dukes 2013 <sup>[[#fn:r1676|1676]]</sup> ). The sink is at risk of reversibility, in particular due to increased soil respiration under higher temperatures (Section 2.3).

Land management practices to increase carbon interact with agricultural and fire management practices (Cross-chapter Box 3 and Chapter 5) and include improved rotations with deeper rooting cultivars, addition of organic materials and agroforestry (Lal 2011 <sup>[[#fn:r1677|1677]]</sup> ; Smith et al. 2008 <sup>[[#fn:r1678|1678]]</sup> ; Lorenz and Pitman 2014 <sup>[[#fn:r1679|1679]]</sup> ; Lal 2013 <sup>[[#fn:r1680|1680]]</sup> ; Vermeulen et al. 2012 <sup>[[#fn:r1681|1681]]</sup> ; de Rouw et al. 2010 <sup>[[#fn:r1682|1682]]</sup> ). Adoption of green manure cover crops, while increasing cropping frequency or diversity, helps sequester SOC (Poeplau and Don 2015 <sup>[[#fn:r1683|1683]]</sup> ; Mazzoncini et al. 2011 <sup>[[#fn:r1684|1684]]</sup> ; Luo et al. 2010 <sup>[[#fn:r1685|1685]]</sup> ). Studies of the long-term SOC sequestration potential of conservation agriculture (i.e., the simultaneous adoption of minimum tillage, (cover) crop residue retention and associated soil surface coverage, and crop rotations) include results that are both positive (Powlson et al. 2016 <sup>[[#fn:r1686|1686]]</sup> ; Zhang et al. 2014 <sup>[[#fn:r1687|1687]]</sup> ) and inconclusive (Cheesman et al. 2016 <sup>[[#fn:r1688|1688]]</sup> ; Palm et al. 2014 <sup>[[#fn:r1689|1689]]</sup> ; Govaerts et al. 2009 <sup>[[#fn:r1690|1690]]</sup> ).

The efficacy of reduced and zero-till practices is highly context-specific; many studies demonstrate increased carbon storage (e.g., Paustian et al. (2000) <sup>[[#fn:r1691|1691]]</sup> , Six et al. (2004) <sup>[[#fn:r1692|1692]]</sup> , van Kessel et al. (2013) <sup>[[#fn:r1693|1693]]</sup> ), while others show the opposite effect (Sisti et al. 2004 <sup>[[#fn:r1694|1694]]</sup> ; Álvaro-Fuentes et al. 2008 <sup>[[#fn:r1695|1695]]</sup> ; Christopher et al. 2009 <sup>[[#fn:r1696|1696]]</sup> ). On the other hand, deep ploughing can contribute to SOC sequestration by burying soil organic matter in the subsoil where it decomposes slowly (Alcántara et al. 2016 <sup>[[#fn:r1697|1697]]</sup> ). Meta- analyses (Haddaway et al. 2017 <sup>[[#fn:r1698|1698]]</sup> ; Luo et al. 2010 <sup>[[#fn:r1699|1699]]</sup> ; Meurer et al. 2018 <sup>[[#fn:r1700|1700]]</sup> ) also show a mix of positive and negative responses, and the lack of robust comparisons of soils on an equivalent mass basis continues to be a problem for credible estimates (Wendt and Hauser 2013 <sup>[[#fn:r1701|1701]]</sup> ; Powlson et al. 2011 <sup>[[#fn:r1702|1702]]</sup> ; Powlson et al. 2014 <sup>[[#fn:r1703|1703]]</sup> ).

Soil carbon management interacts with N <sub>2</sub> O (Paustian et al. 2016 <sup>[[#fn:r1704|1704]]</sup> ). For example, Li et al. (2005) <sup>[[#fn:r1705|1705]]</sup> estimate that the management strategies required to increase carbon sequestration (reduced tillage, crop residue and manure recycling) would increase N <sub>2</sub> O emissions significantly, offsetting 75–310% of the carbon sequestered in terms of CO <sub>2</sub> equivalence, while other practices such as cover crops can reduce N <sub>2</sub> O emissions (Kaye and Quemada 2017 <sup>[[#fn:r1706|1706]]</sup> ).

The management of soil erosion could avoid a net emissions of 1.36–3.67 GtCO <sub>2</sub> yr <sup>–1</sup> and create a sink of 0.44–3.67 GtCO <sub>2</sub> yr <sup>–1</sup> ( ''low confidence'' ) (Jacinthe and Lal 2001 <sup>[[#fn:r1707|1707]]</sup> ; Lal et al. 2004 <sup>[[#fn:r1708|1708]]</sup> ; Stallard 1998 <sup>[[#fn:r1709|1709]]</sup> ; Smith et al. 2001 <sup>[[#fn:r1710|1710]]</sup> ; Van Oost et al. 2007 <sup>[[#fn:r1711|1711]]</sup> ). The overall impact of erosion control on mitigation is context-specific and uncertain at the global level and the final fate of eroded material is still debated (Hoffmann et al., 2013 <sup>[[#fn:r1712|1712]]</sup> ).

Biochar is produced by thermal decomposition of biomass in the absence of oxygen (pyrolysis) into a stable, long-lived product like charcoal that is relatively resistant to decomposition (Lehmann et al. 2015 <sup>[[#fn:r1713|1713]]</sup> ) and which can stabilise organic matter when added to soil (Weng et al. 2017 <sup>[[#fn:r1714|1714]]</sup> ). Although charcoal has been used traditionally by many cultures as a soil amendment, ‘modern biochar’, produced in facilities that control emissions, is not widely used. The range of global potential of biochar is 0.03–6.6 GtCO <sub>2</sub> -eq yr <sup>–1</sup> by 2050, including energy substitution, with 0.03–4.9 GtCO <sub>2</sub> yr <sup>–1</sup> for CDR only ( ''medium confidence'' ) (Griscom et al. 2017 <sup>[[#fn:r1715|1715]]</sup> ; Hawken 2017 <sup>[[#fn:r1716|1716]]</sup> ; Paustian et al. 2016 <sup>[[#fn:r1717|1717]]</sup> ; Fuss et al. 2018 <sup>[[#fn:r1718|1718]]</sup> ; Lenton 2014 <sup>[[#fn:r1719|1719]]</sup> , 2010 <sup>[[#fn:r1720|1720]]</sup> ; Powell and Lenton 2012 <sup>[[#fn:r1721|1721]]</sup> ; Woolf et al. 2010 <sup>[[#fn:r1722|1722]]</sup> ; Pratt and Moran 2010 <sup>[[#fn:r1723|1723]]</sup> ; Smith 2016 <sup>[[#fn:r1724|1724]]</sup> ; Roberts et al. 2010 <sup>[[#fn:r1725|1725]]</sup> ). An analysis in which biomass supply constraints were applied to protect against food insecurity, loss of habitat and land degradation, estimated technical potential abatement of 3.7–6.6 GtCO <sub>2</sub> -eq yr <sup>–1</sup> (including 2.6–4.6 GtCO <sub>2</sub> yr <sup>–1</sup> carbon stabilisation) (Woolf et al. 2010 <sup>[[#fn:r1726|1726]]</sup> ). Fuss et al. (2018) <sup>[[#fn:r1727|1727]]</sup> propose a range of 0.5–2 GtCO <sub>2</sub> -eq yr <sup>–1</sup> as the sustainable potential for negative emissions through biochar. Griscom et al. (2017) <sup>[[#fn:r1728|1728]]</sup> suggest a potential of 1.0 GtCO <sub>2</sub> yr <sup>–1</sup> based on available residues. Biochar can provide additional climate change mitigation benefits by decreasing N <sub>2</sub> O emissions from soil and reducing nitrogen fertiliser requirements in agricultural soils (Borchard et al. 2019 <sup>[[#fn:r1729|1729]]</sup> ). Application of biochar to cultivated soils can darken the surface and reduce its mitigation potential via decreases in surface albedo, but the magnitude of this effect depends on soil moisture content, biochar application method and type of land use ( ''low confidence'' ) (Verheijen et al. 2013 <sup>[[#fn:r1730|1730]]</sup> ; Bozzi et al. 2015 <sup>[[#fn:r1731|1731]]</sup> ) (Section 4.9.5).

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