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

=== 4.9.5 Biochar ===

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Biochar is organic matter that is carbonised by heating in an oxygen-limited environment, and used as a soil amendment. The properties of biochar vary widely, dependent on the feedstock and the conditions of production. Biochar could make a significant contribution to mitigating both land degradation and climate change, simultaneously.

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==== 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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==== 4.9.5.2 Role of biochar in management of land degradation ====

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Biochars generally have high porosity, high surface area and surface-active properties that lead to high absorptive and adsorptive capacity, especially after interaction in soil (Joseph et al. 2010 <sup>[[#fn:r1450|1450]]</sup> ). As a result of these properties, biochar could contribute to avoiding, reducing and reversing land degradation through the following documented benefits:

* Improved nutrient use efficiency due to reduced leaching of nitrate and ammonium (e.g., Haider et al. 2017 <sup>[[#fn:r1451|1451]]</sup> ) and increased availability of phosphorus in soils with high phosphorus fixation capacity (Liu et al. 2018c <sup>[[#fn:r1452|1452]]</sup> ), potentially reducing nitrogen and phosphorus fertiliser requirements.
* Management of heavy metals and organic pollutants: through reduced bioavailability of toxic elements (O’Connor et al. 2018 <sup>[[#fn:r1453|1453]]</sup> ; Peng et al. 2018 <sup>[[#fn:r1454|1454]]</sup> ), by reducing availability, through immobilisation due to increased pH and redox effects (Rizwan et al. 2016 <sup>[[#fn:r1455|1455]]</sup> ) and adsorption on biochar surfaces (Zhang et al. 2013 <sup>[[#fn:r1456|1456]]</sup> ) thus providing a means of remediating contaminated soils, and enabling their utilisation for food production.
* Stimulation of beneficial soil organisms, including earthworms and mycorrhizal fungi (Thies et al. 2015 <sup>[[#fn:r1457|1457]]</sup> ).
* Improved porosity and water-holding capacity (Quin et al. 2014 <sup>[[#fn:r1458|1458]]</sup> ), particularly in sandy soils (Omondi et al. 2016 <sup>[[#fn:r1459|1459]]</sup> ), enhancing microbial function during drought (Paetsch et al. 2018 <sup>[[#fn:r1460|1460]]</sup> ).
* Amelioration of soil acidification, through application of biochars with high pH and acid-neutralising capacity (Chan et al. 2008 <sup>[[#fn:r1461|1461]]</sup> ; Van Zwieten et al. 2010 <sup>[[#fn:r1462|1462]]</sup> ).

Biochar systems can deliver a range of other co-benefits, including destruction of pathogens and weed propagules, avoidance of landfill, improved handling and transport of wastes such as sewage sludge, management of biomass residues such as environmental weeds and urban greenwaste, reduction of odours and management of nutrients from intensive livestock facilities, reduction in environmental nitrogen pollution and protection of waterways. As a compost additive, biochar has been found to reduce leaching and volatilisation of nutrients, increasing nutrient retention through absorption and adsorption processes (Joseph et al. 2018 <sup>[[#fn:r1463|1463]]</sup> ).

While many studies report positive responses, some studies have found negative or zero impacts on soil properties or plant response (e.g., Kuppusamy et al. 2016 <sup>[[#fn:r1464|1464]]</sup> ). The risk that biochar may enhance polycyclic aromatic hydrocarbon (PAH) in soil or sediments has been raised (Quilliam et al. 2013 <sup>[[#fn:r1465|1465]]</sup> ; Ojeda et al. 2016 <sup>[[#fn:r1466|1466]]</sup> ), but bioavailability of PAH in biochar has been shown to be very low (Hilber et al. 2017 <sup>[[#fn:r1467|1467]]</sup> ) Pyrolysis of biomass leads to losses of volatile nutrients, especially nitrogen. While availability of nitrogen and phosphorus in biochar is lower than in fresh biomass, (Xu et al. 2016 <sup>[[#fn:r1468|1468]]</sup> ) the impact of biochar on plant uptake is determined by the interactions between biochar, soil minerals and activity of microorganisms (e.g., Vanek and Lehmann 2015 <sup>[[#fn:r1655|1655]]</sup> ; Nguyen et al. 2017 <sup>[[#fn:r1469|1469]]</sup> ). To avoid negative responses, it is important to select biochar formulations to address known soil constraints, and to apply biochar prior to planting (Nguyen et al. 2017 <sup>[[#fn:r1470|1470]]</sup> ). Nutrient enrichment improves the performance of biochar from low nutrient feedstocks (Joseph et al. 2013 <sup>[[#fn:r1471|1471]]</sup> ). While there are many reports of biochar reducing disease or pest incidence, there are also reports of nil or negative effects (Bonanomi et al. 2015 <sup>[[#fn:r1472|1472]]</sup> ). Biochar may induce systemic disease resistance (e.g., Elad et al. 2011 <sup>[[#fn:r1473|1473]]</sup> ), though Viger et al. (2015) <sup>[[#fn:r1474|1474]]</sup> reported down-regulation of plant defence genes, suggesting increased susceptibility to insect and pathogen attack. Disease suppression where biochar is applied is associated with increased microbial diversity and metabolic potential of the rhizosphere microbiome (Kolton et al. 2017 <sup>[[#fn:r1475|1475]]</sup> ). Differences in properties related to feedstock (Bonanomi et al. 2018 <sup>[[#fn:r1476|1476]]</sup> ) and differential response to biochar dose, with lower rates more effective (Frenkel et al. 2017 <sup>[[#fn:r1477|1477]]</sup> ), contribute to variable disease responses.

The constraints on biochar adoption include: the high cost and limited availability due to limited large-scale production; limited amount of unutilised biomass; and competition for land for growing biomass. While early biochar research tended to use high rates of application (10 t ha <sup>–1</sup> or more) subsequent studies have shown that biochar can be effective at lower rates, especially when combined with chemical or organic fertilisers (Joseph et al. 2013 <sup>[[#fn:r1478|1478]]</sup> ). Biochar can be produced at many scales and levels of engineering sophistication, from simple cone kilns and cookstoves to large industrial-scale units processing several tonnes of biomass per hour (Lehmann and Joseph 2015 <sup>[[#fn:r1479|1479]]</sup> ). Substantial technological development has occurred recently, though large-scale deployment is limited to date.

Governance of biochar is required to manage climate, human health and contamination risks associated with biochar production in poorly designed or operated facilities that release methane or particulates (Downie et al. 2012 <sup>[[#fn:r1480|1480]]</sup> ; Buss et al. 2015 <sup>[[#fn:r1481|1481]]</sup> ), to ensure quality control of biochar products, and to ensure that biomass is sourced sustainably and is uncontaminated. Measures could include labelling standards, sustainability certification schemes and regulation of biochar production and use. Governance mechanisms should be tailored to context, commensurate with risks of adverse outcomes.

In summary, application of biochar to soil can improve soil chemical, physical and biological attributes, enhancing productivity and resilience to climate change, while also delivering climate-change mitigation through carbon sequestration and reduction in GHG emissions ( ''medium agreement, robust evidence'' ). However, responses to biochar depend on the biochar’s properties, which are in turn dependent on feedstock and biochar production conditions, and the soil and crop to which it is applied. Negative or nil results have been recorded.Agronomic and methane-reduction benefits appear greatest in tropical regions, where acidic soils predominate and suboptimal rates of lime and fertiliser are common, while carbon stabilisation is greater in temperate regions. Biochar is most effective when applied in low volumes to the most responsive soils and when properties are matched to the specific soil constraints and plant needs. Biochar is thus a practice that has potential to address land degradation and climate change simultaneously, while also supporting sustainable development. The potential of biochar is limited by the availability of biomass for its production. Biochar production and use requires regulation and standardisation to manage risks ( ''strong agreement'' ).

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