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

==== 7.4.3.1 Soil Carbon Management in Croplands and Grasslands ====

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'''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.

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