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

==== 4.9.5.1 Role of biochar in climate change mitigation ====

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Biochar is relatively resistant to decomposition compared with fresh organic matter or compost, so represents a long-term carbon store ( ''very high confidence'' ). Biochars produced at higher temperature (&gt;450°C) and from woody material have greater stability than those produced at lower temperature (300–450°C), and from manures ( ''very high confidence'' ) (Singh et al. 2012 <sup>[[#fn:r1414|1414]]</sup> ; Wang et al. 2016b <sup>[[#fn:r1415|1415]]</sup> ). Biochar stability is influenced by soil properties: biochar carbon can be further stabilised by interaction with clay minerals and native SOM ( ''medium evidence'' ) (Fang et al. 2015 <sup>[[#fn:r1416|1416]]</sup> ). Biochar stability is estimated to range from decades to thousands of years, for different biochars in different applications (Singh et al. 2015 <sup>[[#fn:r1417|1417]]</sup> ; Wang et al. 2016 <sup>[[#fn:r1418|1418]]</sup> ). Biochar stability decreases as ambient temperature increases ( ''limited evidence'' ) (Fang et al. 2017 <sup>[[#fn:r1419|1419]]</sup> ).

Biochar can enhance soil carbon stocks through ‘negative priming’, in which rhizodeposits are stabilised through sorption of labile carbon on biochar, and formation of biochar-organo-mineral complexes (Weng et al. 2015 <sup>[[#fn:r1420|1420]]</sup> , 2017 <sup>[[#fn:r1421|1421]]</sup> , 2018 <sup>[[#fn:r1422|1422]]</sup> ; Wang et al. 2016b). Conversely, some studies show increased turnover of native soil carbon (‘positive priming’) due to enhanced soil microbial activity induced by biochar. In clayey soils, positive priming is minor and short-lived compared to negative priming effects, which dominate in the medium to long term (Singh and Cowie 2014 <sup>[[#fn:r1421|1421]]</sup> ; Wang et al. 2016b <sup>[[#fn:r1422|1422]]</sup> ). Negative priming has been observed particularly in loamy grassland soil (Ventura et al. 2015 <sup>[[#fn:r1423|1423]]</sup> ) and clay-dominated soils, whereas positive priming is reported in sandy soils (Wang et al. 2016b <sup>[[#fn:r1424|1424]]</sup> ) and those with low carbon content (Ding et al. 2018 <sup>[[#fn:r1425|1425]]</sup> ).

Biochar can provide additional climate-change mitigation by decreasing nitrous oxide (N <sub>2</sub> O) emissions from soil, due in part to decreased substrate availability for denitrifying organisms, related to the molar H/C ratio of the biochar (Cayuela et al. 2015 <sup>[[#fn:r1426|1426]]</sup> ). However, this impact varies widely: meta-analyses found an average decrease in N <sub>2</sub> O emissions from soil of 30–54%, (Cayuela et al. 2015 <sup>[[#fn:r1427|1427]]</sup> ; Borchard et al. 2019 <sup>[[#fn:r1428|1428]]</sup> ; Moore 2002 <sup>[[#fn:r1429|1429]]</sup> ), although another study found no significant reduction in field conditions when weighted by the inverse of the number of observations per site (Verhoeven et al. 2017 <sup>[[#fn:r1430|1430]]</sup> ). Biochar has been observed to reduce methane emissions from flooded soils, such as rice paddies, though, as for N <sub>2</sub> O, results vary between studies and increases have also been observed (He et al. 2017 <sup>[[#fn:r1431|1431]]</sup> ; Kammann et al. 2017 <sup>[[#fn:r1432|1432]]</sup> ). Biochar has also been found to reduce methane uptake by dryland soils, though the effect is small in absolute terms (Jeffery et al. 2016 <sup>[[#fn:r1433|1433]]</sup> ).

Additional climate benefits of biochar can arise through: reduced nitrogen fertiliser requirements, due to reduced losses of nitrogen through leaching and/or volatilisation (Singh et al. 2010 <sup>[[#fn:r1434|1434]]</sup> ) and enhanced biological nitrogen fixation (Van Zwieten et al. 2015 <sup>[[#fn:r1435|1435]]</sup> ); increased yields of crop, forage, vegetable and tree species (Biederman and Harpole 2013 <sup>[[#fn:r1436|1436]]</sup> ), particularly in sandy soils and acidic tropical soils (Simon et al. 2017 <sup>[[#fn:r1437|1437]]</sup> ); avoided GHG emissions from manure that would otherwise be stockpiled, crop residues that would be burned or processing residues that would be landfilled; and reduced GHG emissions from compost when biochar is added (Agyarko-Mintah et al. 2017 <sup>[[#fn:r1438|1438]]</sup> ; Wu et al. 2017a <sup>[[#fn:r1439|1439]]</sup> ).

Climate benefits of biochar could be substantially reduced through reduction in albedo if biochar is surface-applied at high rates to light-coloured soils (Genesio et al. 2012 <sup>[[#fn:r1440|1440]]</sup> ; Bozzi et al. 2015 <sup>[[#fn:r1441|1441]]</sup> ; Woolf et al. 2010 <sup>[[#fn:r1442|1442]]</sup> ), or if black carbon dust is released (Genesio et al. 2016 <sup>[[#fn:r1443|1443]]</sup> ). Pelletising or granulating biochar, and applying below the soil surface or incorporating into the soil, minimises the release of black carbon dust and reduces the effect on albedo (Woolf et al. 2010 <sup>[[#fn:r1444|1444]]</sup> ).

Biochar is a potential ‘negative emissions’ technology: the thermochemical conversion of biomass to biochar slows mineralisation of the biomass, delivering long-term carbon storage; gases released during pyrolysis can be combusted for heat or power, displacing fossil energy sources, and could be captured and sequestered if linked with infrastructure for CCS (Smith 2016 <sup>[[#fn:r1445|1445]]</sup> ). Studies of the lifecycle climate change impacts of biochar systems generally show emissions reduction in the range 0.4 –1.2 tCO <sub>2</sub> e t <sup>–1</sup> (dry) feedstock (Cowie et al. 2015 <sup>[[#fn:r1446|1446]]</sup> ). Use of biomass for biochar can deliver greater benefits than use for bioenergy, if applied in a context where it delivers agronomic benefits and/or reduces non-CO <sub>2</sub> GHG emissions (Ji et al. 2018 <sup>[[#fn:r1447|1447]]</sup> ; Woolf et al. 2010 <sup>[[#fn:r1448|1448]]</sup> , 2018; Xuetal.2019).A global analysis of technical potential, 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> e yr <sup>–1</sup> (including 2.6–4.6 GtCO <sub>2</sub> e yr <sup>–1</sup> carbon stabilisation), with theoretical potential to reduce total emissions over the course of a century by 240–475 GtCO <sub>2</sub> e (Woolf et al. 2010). Fuss et al. (2018) propose a range of 0.5–2 GtCO <sub>2</sub> e per year as the sustainable potential for negative emissions through biochar. Mitigation potential of biochar is reviewed in Chapter 2.

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