Editing IPCC:AR6/WGIII/Chapter-7 (section)

==== 7.4.3.2 Biochar ====

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'''Activities, co-benefits, risks and implementation opportunities and barriers.''' Biochar is produced by heating organic matter in oxygen-limited environments (pyrolysis and gasification) ( [[#Lehmann--2012|Lehmann and Joseph 2012]] ). Feedstocks include forestry and sawmill residues, straw, manure and biosolids. When applied to soils, biochar is estimated to persist from decades to thousands of years, depending on feedstock and production conditions (J. [[#Wang--2016|Wang et al. 2016]] ; [[#Singh--2015|Singh et al. 2015]] ). Biochar systems producing biochar for soil application plus bioenergy, generally give greater mitigation than bioenergy alone and other uses of biochar, and are recognised as a CDR strategy. Biochar persistence is increased through interaction with clay minerals and soil organic matter ( [[#Fang--2015|Fang et al. 2015]] ). Additional CDR benefits arise through ‘negative priming’ whereby biochar stabilises soil carbon and rhizodeposits ( [[#Weng--2015|Weng et al. 2015]] ; J. [[#Wang--2016|Wang et al. 2016]] ; [[#Archanjo--2017|Archanjo et al. 2017]] ; [[#Hagemann--2017|Hagemann et al. 2017]] ; [[#Han%20Weng--2017|Han Weng et al. 2017]] ; [[#Weng--2018|Weng et al. 2018]] ). Besides CDR, additional mitigation can arise from displacing fossil fuels with pyrolysis gases, lower soil N 2 O emissions ( [[#Cayuela--2014|Cayuela et al. 2014]] , 2015; [[#Song--2016|Song et al. 2016]] ; [[#He--2017|He et al. 2017]] ; [[#Verhoeven--2017|Verhoeven et al. 2017]] ; [[#Borchard--2019|Borchard et al. 2019]] ), reduced nitrogen fertiliser requirements due to reduced nitrogen leaching and volatilisation from soils ( [[#Liu--2019|Liu et al. 2019]] ; [[#Borchard--2019|Borchard et al. 2019]] ), and reduced GHG emissions from compost when biochar is added ( [[#Agyarko-Mintah--2017|Agyarko-Mintah et al. 2017]] ; [[#Wu--2017|Wu et al. 2017]] ). Biochar application to paddy rice has resulted in substantial reductions (20–40% on average) in N 2 O ( [[#Song--2016|Song et al. 2016]] ; [[#Awad--2018|Awad et al. 2018]] ; [[#Liu--2018|Liu et al. 2018]] ) ( [[#7.4.3.5|Section 7.4.3.5]] ) and smaller reduction in CH 4 emissions ( [[#Song--2016|Song et al. 2016]] ; [[#Kammann--2017|Kammann et al. 2017]] ; [[#Kim--2017a|Kim et al. 2017a]] ; [[#He--2017|He et al. 2017]] ; [[#Awad--2018|Awad et al. 2018]] ). Potential co-benefits include yield increases particularly in sandy and acidic soils with low cation exchange capacity ( [[#Woolf--2016|Woolf et al. 2016]] ; [[#Jeffery--2017|Jeffery et al. 2017]] ); increased soil water-holding capacity ( [[#Omondi--2016|Omondi et al. 2016]] ), nitrogen use efficiency ( [[#Liu--2019|Liu et al. 2019]] ; [[#Borchard--2019|Borchard et al. 2019]] ), biological nitrogen fixation ( [[#Van%20Zwieten--2015|Van Zwieten et al. 2015]] ); adsorption of organic pollutants and heavy metals (e.g., [[#Silvani--2019|Silvani et al. 2019]] ); odour reduction from manure handling (e.g., [[#Hwang--2018|Hwang et al. 2018]] ) and managing forest fuel loads ( [[#Puettmann--2020|Puettmann et al. 2020]] ). Due to its dark colour, biochar could decrease soil albedo ( [[#Meyer--2012|Meyer et al. 2012]] ), though this is insignificant under recommended rates and application methods. Biochar could reduce enteric CH 4 emissions when fed to ruminants ( [[#7.4.3.4|Section 7.4.3.4]] ). Barriers to upscaling include insufficient investment, limited large-scale production facilities, high production costs at small scale, lack of agreed approach to monitoring, reporting and verification, and limited knowledge, standardisation and quality control, restricting user confidence ( [[#Gwenzi--2015|Gwenzi et al. 2015]] ).

'''Conclusions from AR5 and IPCC Special Reports (SR1.5, SROCC and SRCCL); mitigation potential, costs, and pathways.''' Biochar is discussed as a mitigation option in AR5 and CDR strategy in the SR1.5. Consideration of potential was limited as biochar is not included in IAMs. The SRCCL estimated mitigation potential of 0.03–6.6 GtCO 2 -eq yr –1 by 2050 based on studies with widely varying assumptions, definitions of potential, and scope of mitigation processes included (SRCCL, Chapters 2 and 4: ( [[#Roberts--2010|Roberts et al. 2010]] ; [[#Pratt--2010|Pratt and Moran 2010]] ; [[#Hristov--2013|Hristov et al. 2013]] ; [[#Lee--2013|Lee and Day 2013]] ; [[#Dickie--2014a|Dickie et al. 2014a]] ; [[#Hawken--2017|Hawken 2017]] ; [[#Fuss--2018|Fuss et al. 2018]] ; [[#Powell--2012|Powell and Lenton 2012]] ; [[#Woolf--2010|Woolf et al. 2010]] ).

'''Developments since AR5 and IPCC Special Reports (SR1.5, SROCC and SRCCL).''' Developments include mechanistic understanding of ‘negative priming’ and biochar-soil-microbes-plant interactions ( [[#DeCiucies--2018|DeCiucies et al. 2018]] ; [[#Fang--2019|Fang et al. 2019]] ). Indirect climate benefits are associated with persistent yield response to biochar ( [[#Kätterer--2019|Kätterer et al. 2019]] ; [[#Ye--2020|Ye et al. 2020]] ), improved crop water use efficiency ( [[#Du--2018|Du et al. 2018]] ; [[#Gao--2020|Gao et al. 2020]] ) and reduced GHG and ammonia emissions from compost and manure ( [[#Sanchez-Monedero--2018|Sanchez-Monedero et al. 2018]] ; [[#Bora--2020a|Bora et al. 2020a]] ,b; [[#Zhao--2020|Zhao et al. 2020]] ). A quantification method based on biochar properties is included in the IPCC guidelines for NGHGIs ( [[#Domke--2019|Domke et al. 2019]] ). Studies report a range of biochar responses, from positive to occasionally adverse impacts, including on GHG emissions, and identify risks ( [[#Tisserant--2019|Tisserant and Cherubini 2019]] ). This illustrates the expected variability ( [[#Lehmann--2014|Lehmann and Rillig 2014]] ) of responses, which depend on the biochar type and climatic and edaphic characteristics of the site ( [[#Zygourakis--2017|Zygourakis 2017]] ). Biochar properties vary with feedstock, production conditions and post-production treatments, so mitigation and agronomic benefits are maximised when biochars are chosen to suit the application context ( [[#Mašek--2018|Mašek et al. 2018]] ). A recent assessment finds greatest economic potential (up to USD100 tCO 2 –1 ) between 2020 and 2050 to be in Asia and the Pacific (793 MtCO 2 yr –1 ) followed by Developed Countries (447 MtCO 2 yr –1 ) ( [[#Roe--2021|Roe et al. 2021]] ). Mitigation through biochar will be greatest where biochar is applied to responsive soils (acidic, low fertility), where soil N 2 O emissions are high (intensive horticulture, irrigated crops), and where the syngas co-product displaces fossil fuels. Due to the early stage of commercialisation, mitigation estimates are based pilot-scale facilities, leading to uncertainty. However, the long-term persistence of biochar carbon in soils has been widely studied ( [[#Singh--2012|Singh et al. 2012]] ; [[#Fang--2019|Fang et al. 2019]] ; [[#Zimmerman--2019|Zimmerman and Ouyang 2019]] ). The greatest uncertainty is the availability of sustainably-sourced biomass for biochar production.

'''Critical assessment and conclusion.''' Biochar has significant mitigation potential through CDR and emissions reduction, and can also improve soil properties, enhancing productivity and resilience to climate change ( ''medium agreement'' , ''robust evidence'' ). There is ''medium evidence'' that biochar has a technical potential of 2.6 (0.2–6.6) GtCO 2 -eq yr –1 , of which 1.1 (0.3–1.8) GtCO 2 -eq yr –1 is available up to USD100 tCO 2 –1 . However, mitigation and agronomic co-benefits depend strongly on biochar properties and the soil to which biochar is applied ( ''strong agreement'' , ''robust evidence'' ). While biochar could provide moderate to large mitigation potential, it is not yet included in IAMs, which has restricted comparison and integration with other CDR strategies.

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