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

==== 7.4.3.6 Crop Nutrient Management ====

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'''Activities, co-benefits, risks and implementation opportunities and barriers.''' Improved crop nutrient management can reduce N 2 O emissions from cropland soils. Practices include optimising fertiliser application delivery, rates and timing, utilising different fertiliser types (i.e., organic manures, composts and synthetic forms), and using slow or controlled-released fertilisers or nitrification inhibitors (Smith et al. 2014; [[#Griscom--2017|Griscom et al. 2017]] ; P. [[#Smith--2019|Smith et al. 2019]] a). In addition to individual practices, integrated nutrient management that combines crop rotations including intercropping, nitrogen biological fixation, reduced tillage, use of cover crops, manure and bio-fertiliser application, soil testing and comprehensive nitrogen management plans, is suggested as central for optimising fertiliser use, enhancing nutrient uptake and potentially reducing N 2 O emissions ( [[#Bationo--2012|Bationo et al. 2012]] ; [[#Lal--2018|Lal et al. 2018]] ; [[#Bolinder--2020|Bolinder et al. 2020]] ; [[#Jensen--2020|Jensen et al. 2020]] ; [[#Namatsheve--2020|Namatsheve et al. 2020]] ). Such practices may generate additional mitigation by indirectly reducing synthetic fertiliser manufacturing requirements and associated emissions, though such mitigation is accounted for in the Industry Sector and not considered in this chapter. Tailored nutrient management approaches, such as 4R nutrient stewardship, are implemented in contrasting farming systems and contexts and supported by best management practices to balance and match nutrient supply with crop requirements, provide greater stability in fertiliser performance and to minimise N 2 O emissions and nutrient losses from fields and farms ( [[#Fixen--2020|Fixen 2020]] ; [[#Maaz--2021|Maaz et al. 2021]] ). Co-benefits of improved nutrient management can include enhanced soil quality (notably when manure, crop residues or compost is utilised), carbon sequestration in soils and biomass, soil water holding capacity, adaptation capacity, crop yields, farm incomes, water quality (from reduced nitrate leaching and eutrophication), air quality (from reduced ammonia emissions) and in certain cases, it may facilitate land sparing ( [[#Sapkota--2014|Sapkota et al. 2014]] ; [[#Johnston--2014|Johnston and Bruulsema 2014]] ; [[#Zhang--2017|Zhang et al. 2017]] ; P. [[#Smith--2019|Smith et al. 2019]] a; [[#Mbow--2019|Mbow et al. 2019]] ).

A potential risk under certain circumstances, is yield reduction, while implementation of practices should consider current soil nutrient status. There are significant regional imbalances, with some regions experiencing nutrient surpluses from over fertilisation and others, nutrient shortages and chronic deficiencies ( [[#FAO--2021e|FAO 2021e]] ). Additionally, depending on context, practices may be inaccessible, expensive or require expertise to implement ( [[#Hedley--2015|Hedley 2015]] ; [[#Benson--2018|Benson and Mogues 2018]] ) while impacts of climate change may influence nutrient use efficiency ( [[#Amouzou--2019|Amouzou et al. 2019]] ) and therefore, mitigation potential.

'''Conclusions from AR5 and IPCC Special Reports (SR1.5, SROCC and SRCCL); mitigation potential, costs, and pathways.''' The SRCCL broadly identified the same practices as outlined in AR5 and estimated that improved cropland nutrient management could mitigate between 0.03 and 0.71 GtCO 2 -eq yr –1 between 2020 and 2050 (SRCCL Chapter 2) ( [[#Dickie--2014a|Dickie et al. 2014a]] ; [[#Beach--2015|Beach et al. 2015]] ; [[#Paustian--2016|Paustian et al. 2016]] ; [[#Griscom--2017|Griscom et al. 2017]] ; [[#Hawken--2017|Hawken 2017]] ).

'''Developments since AR5 and IPCC Special Reports (SR1.5, SROCC and SRCCL).''' Research since the SRCCL highlights the mitigation potential and co-benefits of adopting improved nutrient management strategies, notably precision fertiliser application methods and nutrient expert systems, and applicability in both large-scale mechanised and small-scale systems ( [[#USEPA--2019|USEPA 2019]] ; [[#Hijbeek--2019|Hijbeek et al. 2019]] ; [[#Griscom--2020|Griscom et al. 2020]] ; [[#Tian--2020|Tian et al. 2020]] ; [[#Aryal--2020|Aryal et al. 2020]] ; [[#Sapkota--2021|Sapkota et al. 2021]] ). Improved crop nutrient management is feasible in all regions, but effectiveness is context dependent. Sub-Saharan Africa has one of the lowest global fertiliser consumption rates, with increased fertiliser use suggested as necessary to meet projected future food requirements ( [[#Mueller--2012|Mueller et al. 2012]] ; [[#ten%20Berge--2019|ten Berge et al. 2019]] ; [[#Adam--2020|Adam et al. 2020]] ; [[#Falconnier--2020|Falconnier et al. 2020]] ). Fertiliser use in Developed Countries is already high (Figure 7.10) with increased nutrient use efficiency among the most promising mitigation measures ( [[#Roe--2019|Roe et al. 2019]] ; [[#Hijbeek--2019|Hijbeek et al. 2019]] ). Considering that Asia and Pacific, and Developed Countries accounted for the greatest share of global nitrogen fertiliser use, it is not surprising that these regions are estimated to have greatest economic mitigation potential (up to USD100 tCO 2 -eq –1 ) between 2020 and 2050, at 161.8 and 37.1 MtCO 2 -eq yr –1 respectively (using the IPCC AR4 GWP100 value for N 2 O) ( [[#Roe--2021|Roe et al. 2021]] ).

'''Critical assessment and conclusion.''' There is ''medium confidence'' that crop nutrient management has a technical potential of 0.3 (0.06–0.7) GtCO 2 -eq yr –1 of which 0.2 (0.05–0.6) GtCO 2 -eq yr –1 is available up to USD100 tCO 2 -eq –1 . This value is based on GWP100 using a mixture of IPCC values for N 2 O and may slightly differ if calculated using AR6 values. The development of national roadmaps for sustainable fertiliser (nutrient) management can help in scaling-up related practices and in realising this potential. Crop nutrient management measures can contribute not only to mitigation, but food and nutrition security and wider environmental sustainability goals.

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