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

=== 7.4.3 Agriculture ===

<div id="h2-15-siblings" class="h2-siblings"></div>

<div id="7.4.3.1" class="h3-container"></div>

<span id="soil-carbon-management-in-croplands-and-grasslands"></span>
==== 7.4.3.1 Soil Carbon Management in Croplands and Grasslands ====

<div id="h3-26-siblings" class="h3-siblings"></div>

'''Activities, co-benefits, risks and implementation opportunities and barriers.''' Increasing soil organic matter in croplands are agricultural management practices that include (i) crop management: for example, high input carbon practices such as improved crop varieties, crop rotation, use of cover crops, perennial cropping systems (including agroforestry; see [[#7.4.3.3|Section 7.4.3.3]] ), integrated production systems, crop diversification, agricultural biotechnology; (ii) nutrient management including fertilisation with organic amendments/green manures ( [[#7.4.3.6|Section 7.4.3.6]] ); (iii) reduced tillage intensity and residue retention, (iv) improved water management: including drainage of waterlogged mineral soils and irrigation of crops in arid/semi-arid conditions, (v) improved rice management ( [[#7.4.3.5|Section 7.4.3.5]] ) and (vi) biochar application (P. [[#Smith--2019|Smith et al. 2019]] a) ( [[#7.4.3.2|Section 7.4.3.2]] ). For increased soil organic matter in grasslands, practices include (i) ''management of vegetation'' : including improved grass varieties/sward composition, deep rooting grasses, increased productivity, and nutrient management, (ii) ''livestock management'' : including appropriate stocking densities fit to carrying capacity, fodder banks, and fodder diversification, and (iii) ''fire management'' : improved use of fire for sustainable grassland management, including fire prevention and improved prescribed burning (Smith et al. 2014, 2019b). All these measures are recognised as Sustainable Soil Management Practices by FAO ( [[#Baritz--2018|Baritz et al. 2018]] ). While there are co-benefits for livelihoods, biodiversity, water provision and food security (P. [[#Smith--2019|Smith et al. 2019]] a), and impacts on leakage, indirect land-use change and foregone sequestration do not apply (since production in not displaced), the climate benefits of soil carbon sequestration in croplands can be negated if achieved through additional fertiliser inputs (potentially causing increased N 2 O emissions; ( [[#Guenet--2021|Guenet et al. 2021]] ), and both saturation and permanence are relevant concerns. When considering implementation barriers, soil carbon management in croplands and grasslands is a low-cost option at a high level of technology readiness (it is already widely deployed globally) with low socio-cultural and institutional barriers, but with difficulty in monitoring and verification proving a barrier to implementation ( [[#Smith--2020a|Smith et al. 2020a]] ).

'''Conclusions from AR5 and IPCC Special Reports (SR1.5, SROCC and SRCCL); mitigation potential, costs, and pathways.''' Building on AR5, the SRCCL reported the global mitigation potential for soil carbon management in croplands to be 1.4–2.3 GtCO 2 -eq yr –1 (Smith et al. 2014), though the full literature range was 0.3–6.8 GtCO 2 -eq yr –1 ( [[#Sommer--2014|Sommer and Bossio 2014]] ; [[#Powlson--2014|Powlson et al. 2014]] ; [[#Dickie--2014b|Dickie et al. 2014b]] ; [[#Henderson--2015|Henderson et al. 2015]] ; [[#Herrero--2016|Herrero et al. 2016]] ; [[#Paustian--2016|Paustian et al. 2016]] ; [[#Zomer--2016|Zomer et al. 2016]] ; [[#Frank--2017|Frank et al. 2017]] ; [[#Conant--2017|Conant et al. 2017]] ; [[#Griscom--2017|Griscom et al. 2017]] ; [[#Hawken--2017|Hawken 2017]] ; [[#Sanderman--2017|Sanderman et al. 2017]] ; [[#Fuss--2018|Fuss et al. 2018]] ; [[#Roe--2019|Roe et al. 2019]] ). The global mitigation potential for soil organic carbon management in grasslands was assessed to be 1.4–1.8 GtCO 2 -eq yr –1 , with the full literature range being 0.1–2.6 GtCO 2 -eq yr –1 ( [[#Herrero--2013|Herrero et al. 2013]] ; 2016; [[#Conant--2017|Conant et al. 2017]] ; [[#Roe--2019|Roe et al. 2019]] ). Lower values in the range represented economic potentials, while higher values represented technical potentials – and uncertainty was expressed by reporting the whole range of estimates. The SR1.5 outlined associated costs reported in literature to range from USD –45 to 100 tCO 2 –1 , describing enhanced soil carbon sequestration as a cost-effective measure ( [[#IPCC--2018|IPCC 2018]] ). Despite significant mitigation potential, there is limited inclusion of soil carbon sequestration as a response option within IAM mitigation pathways ( [[#Rogelj--2018a|Rogelj et al. 2018a]] ).

'''Developments since AR5 and IPCC Special Reports (SR1.5, SROCC and SRCCL).''' No recent literature has been published which conflict with the mitigation potentials reported in the SRCCL. Relevant papers include [[#Lal--2018|Lal et al. (2018)]] which estimated soil carbon sequestration potential to be 0.7–4.1 GtCO 2 -eq yr –1 for croplands and 1.1–2.9 GtCO 2 -eq yr –1 for grasslands. [[#Bossio--2020|Bossio et al. (2020)]] assessed the contribution of soil carbon sequestration to natural climate solutions and found the potential to be 5.5 GtCO 2 yr –1 across all ecosystems, with only small portions of this (0.41 GtCO 2 -eq yr –1 for cover cropping in croplands; 0.23, 0.15, 0.15 GtCO 2 -eq yr –1 for avoided grassland conversion, optimal grazing intensity and legumes in pastures, respectively) arising from croplands and grasslands. Regionally, soil carbon management in croplands is feasible anywhere, but effectiveness can be limited in very dry regions ( [[#Sanderman--2017|Sanderman et al. 2017]] ). For soil carbon management in grasslands the feasibility is greatest in areas where grasslands have been degraded (e.g., by overgrazing) and soil organic carbon is depleted. For well managed grasslands, soil carbon stocks are already high and the potential for additional carbon storage is low. [[#Roe--2021|Roe et al. (2021)]] estimate the greatest economic (up to USD100 tCO 2 –1 ) potential between 2020 and 2050 for croplands to be in Asia and the Pacific (339.7 MtCO 2 yr –1 ) and for grasslands, in Developed Countries (253.6 MtCO 2 yr –1 ).

'''Critical assessment and conclusion.''' In conclusion, there is ''medium confidence'' that enhanced soil carbon management in croplands has a global technical mitigation potential of 1.9 (0.4–6.8) GtCO 2 yr –1 , and in grasslands of 1.0 (0.2–2.6) GtCO 2 yr –1 , of which, 0.6 (04–0.9) and 0.9 (0.3–1.6) GtCO 2 yr –1 is estimated to be available at up to USD100 tCO 2 –1 respectively. Regionally, soil carbon management in croplands and grasslands is feasible anywhere, but effectiveness can be limited in very dry regions, and for grasslands it is greatest in areas where degradation has occurred (e.g., by overgrazing) and soil organic carbon is depleted. Barriers to implementation include regional capacity for monitoring and verification (especially in developing countries), and more widely through concerns over saturation and permanence.

<div id="7.4.3.2" class="h3-container"></div>

<span id="biochar"></span>
==== 7.4.3.2 Biochar ====

<div id="h3-27-siblings" class="h3-siblings"></div>

'''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.

<div id="7.4.3.3" class="h3-container"></div>

<span id="agroforestry"></span>
==== 7.4.3.3 Agroforestry ====

<div id="h3-28-siblings" class="h3-siblings"></div>

'''Activities, co-benefits, risks and implementation opportunities and barriers.''' Agroforestry is a set of diverse land management systems that integrate trees and shrubs with crops and/or livestock in space and/or time. Agroforestry accumulates carbon in woody vegetation and soil ( [[#Ramachandran%20Nair--2010|Ramachandran Nair et al. 2010]] ) and offers multiple co-benefits such as increased land productivity, diversified livelihoods, reduced soil erosion, improved water quality, and more hospitable regional climates ( [[#Ellison--2017|Ellison et al. 2017]] ; [[#Kuyah--2019|Kuyah et al. 2019]] ; [[#Mbow--2020|Mbow et al. 2020]] ; [[#Zhu--2020|Zhu et al. 2020]] ). Incorporation of trees and shrubs in agricultural systems, however, can affect food production, biodiversity, local hydrology and contribute to social inequality ( [[#Amadu--2020|Amadu et al. 2020]] ; [[#Fleischman--2020|Fleischman et al. 2020]] ; [[#Holl--2020|Holl and Brancalion 2020]] ). To minimise risks and maximise co-benefits, agroforestry should be implemented as part of support systems that deliver tools, and information to increase farmers’ agency. This may include reforming policies, strengthening extension systems and creating market opportunities that enable adoption ( [[#Jamnadass--2020|Jamnadass et al. 2020]] ; [[#Sendzimir--2011|Sendzimir et al. 2011]] ; P. [[#Smith--2019|Smith et al. 2019]] a). Consideration of carbon sequestration in the context of food and fuel production, as well as environmental co-benefits at the farm, local, and regional scales can further help support decisions to plant, regenerate and maintain agroforestry systems ( [[#Kumar--2011|Kumar and Nair 2011]] ; [[#Miller--2020|Miller et al. 2020]] ). In spite of the advantages, biophysical and socio-economic factors can limit the adoption ( [[#Pattanayak--2003|Pattanayak et al. 2003]] ). Contextual factors may include, but are not limited to; water availability, soil fertility, seed and germplasm access, land policies and tenure systems affecting farmer agency, access to credit, and to information regarding the optimum species for a given location.

'''Conclusions from AR5 and IPCC Special Reports (SR1.5, SROCC and SRCCL); mitigation potential, costs, and pathways.''' The SRCCL estimated the global technical mitigation potential of agroforestry, with medium confidence, to be between 0.08 and 5.6 GtCO 2 -eq yr –1 by 2050 ( [[#Griscom--2017|Griscom et al. 2017]] ; [[#Dickie--2014a|Dickie et al. 2014a]] ; [[#Zomer--2016|Zomer et al. 2016]] ; [[#Hawken--2017|Hawken 2017]] ). Estimates are derived from syntheses of potential area available for various agroforestry systems, for example, windbreaks, farmer managed natural regeneration, and alley cropping and average annual rates of carbon accumulation. The cost-effective economic potential, also with medium confidence, is more limited at 0.3–2.4 GtCO 2 -eq yr –1 ( [[#Zomer--2016|Zomer et al. 2016]] ; [[#Griscom--2017|Griscom et al. 2017]] ; [[#Roe--2019|Roe et al. 2019]] ). Despite this potential, agroforestry is currently not considered in integrated assessment models used for mitigation pathways ( [[#7.5|Section 7.5]] ).

'''Developments since AR5 and IPCC Special Reports (SR1.5, SROCC and SRCCL).''' Updated estimates of agroforestry’s technical mitigation potential and synthesised estimates of carbon sequestration across agroforestry systems have since been published. The most recent global analysis estimates technical potential of 9.4 GtCO 2 -eq yr –1 ( [[#Chapman--2020|Chapman et al. 2020]] ) of agroforestry on 1.87 and 1.89 billion ha of crop and pasture lands below median carbon content, respectively. This estimate is at least 68% greater than the largest estimate reported in the SRCCL ( [[#Hawken--2017|Hawken 2017]] ) and represents a new conservative upper bound as [[#Chapman--2020|Chapman et al. (2020)]] only accounted for above-ground carbon. Considering both above- and below-ground carbon of windbreaks, alley cropping and silvopastoral systems at a more limited areal extent ( [[#Griscom--2020|Griscom et al. 2020]] ), the economic potential of agroforestry was estimated to be only about 0.8 GtCO 2 -eq yr –1 . Variation in estimates primarily result from assumptions on the agroforestry systems including, extent of implementation and estimated carbon sequestration potential when converting to agroforestry.

Regional estimates of mitigation potential are scant with agroforestry options differing significantly by geography ( [[#Feliciano--2018|Feliciano et al. 2018]] ). For example, multi-strata shaded coffee and cacao are successful in the humid tropics ( [[#Somarriba--2013|Somarriba et al. 2013]] ; [[#Blaser--2018|Blaser et al. 2018]] ), silvopastoral systems are prevalent in Latin American ( [[#Peters--2013|Peters et al. 2013]] ; [[#Landholm--2019|Landholm et al. 2019]] ) while agrosilvopastoral systems, shelterbelts, hedgerows, and windbreaks are common in Europe ( [[#Joffre--1988|Joffre et al. 1988]] ; Rigueiro-Rodriguez 2009). At the field scale, agroforestry accumulates between 0.59 and 6.24 t ha –1 yr –1 of carbon above-ground. Below-ground carbon often constitutes 25% or more of the potential carbon gains in agroforestry systems (De Stefano and Jacobson 2018; [[#Cardinael--2018|Cardinael et al. 2018]] ). [[#Roe--2021|Roe et al. (2021)]] estimate greatest regional economic (up to USD100 tCO 2 –1 ) mitigation potential for the period 2020–2050 to be in Asia and the Pacific (368.4 MtCO 2 -eq yr –1 ) and Developed Countries (264.7 MtCO 2 -eq yr –1 ).

Recent research has also highlighted co-benefits and more precisely identified implementation barriers. In addition to aforementioned co-benefits, evidence now shows that agroforestry can improve soil health, regarding infiltration and structural stability ( [[#Muchane--2020|Muchane et al. 2020]] ); reduces ambient temperatures and crop heat stress ( [[#Arenas-Corraliza--2018|Arenas-Corraliza et al. 2018]] ; [[#Sida--2018|Sida et al. 2018]] ); increases groundwater recharge in drylands when managed at moderate density ( [[#Ilstedt--2016|Ilstedt et al. 2016]] ; Bargués-Tobella et al. 2020); positively influences human health ( [[#Rosenstock--2019|Rosenstock et al. 2019]] ); and can improve dietary diversity ( [[#McMullin--2019|McMullin et al. 2019]] ). Along with previously mentioned barriers, low social capital, assets, and labour availability have been identified as pertinent to adoption. Practically all barriers are interdependent and subject to the context of implementation.

'''Critical assessment and conclusion.''' There is medium confidence that agroforestry has a technical potential of 4.1 (0.3–9.4) GtCO 2 -eq yr –1 for the period 2020–2050, of which 0.8 (0.4–1.1) GtCO 2 -eq yr –1 is available at USD100 tCO 2 –1 . Despite uncertainty around global estimates due to regional preferences for management systems, suitable land availability, and growing conditions, there is high confidence in agroforestry’s mitigation potential at the field scale. With countless options for farmers and land managers to implement agroforestry, there is medium confidence in the feasibility of achieving estimated regional mitigation potential. Appropriately matching agroforestry options, to local biophysical and social contexts is important in maximising mitigation and co-benefits, while avoiding risks ( [[#Sinclair--2019|Sinclair and Coe 2019]] ).

<div id="box-7.3" class="h2-container box-container"></div>

<span id="box-7.3-case-study-agroforestry-in-br-azil-canopies"></span>