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

==== 4.8.1.1 4.8.1.1 Agronomic and soil management measures ====

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Rebuilding soil carbon is an important goal of SLM, particularly in the context of climate change (Rumpel et al. 2018 <sup>[[#fn:r951|951]]</sup> ). The two most important reasons why agricultural soils have lost 20–60% of the soil carbon they contained under natural ecosystem conditions are the frequent disturbance through tillage and harvesting, and the change from deep- rooted perennial plants to shallow-rooted annual plants (Crews and Rumsey 2017 <sup>[[#fn:r952|952]]</sup> ). Practices that build soil carbon are those that increase organic matter input to soil, or reduce decomposition of SOM.

Agronomic practices can alter the carbon balance significantly, by increasing organic inputs from litter and roots into the soil. Practices include retention of residues, use of locally adapted varieties, inter-cropping, crop rotations, and green manure crops that replace the bare field fallow during winter and are eventually ploughed before sowing the next main crop (Henry et al. 2018 <sup>[[#fn:r953|953]]</sup> ). Cover crops (green manure crops and catch crops that are grown between the main cropping seasons) can increase soil carbon stock by between 0.22 and 0.4 t C ha <sup>–1</sup> yr <sup>–1</sup> (Poeplau and Don 2015 <sup>[[#fn:r954|954]]</sup> ; Kaye and Quemada 2017 <sup>[[#fn:r955|955]]</sup> ).

Reduced tillage (or no-tillage) is an important strategy for reducing soil erosion and nutrient loss by wind and water (Van Pelt et al. 2017 <sup>[[#fn:r956|956]]</sup> ; Panagos et al. 2015 <sup>[[#fn:r957|957]]</sup> ; Borrelli et al. 2016 <sup>[[#fn:r958|958]]</sup> ). But the evidence that no-till agriculture also sequesters carbon is not compelling (VandenBygaart 2016 <sup>[[#fn:r959|959]]</sup> ). Soil sampling of only the upper 30 cm can give biased results, suggesting that soils under no-till practices have higher carbon content than soils under conventional tillage (Baker et al. 2007 <sup>[[#fn:r960|960]]</sup> ; Ogle et al. 2012 <sup>[[#fn:r961|961]]</sup> ; Fargione et al. 2018 <sup>[[#fn:r962|962]]</sup> ; VandenBygaart 2016 <sup>[[#fn:r963|963]]</sup> ).

Changing from annual to perennial crops can increase soil carbon content (Culman et al. 2013 <sup>[[#fn:r964|964]]</sup> ; Sainju et al. 2017 <sup>[[#fn:r965|965]]</sup> ). A perennial grain crop (intermediate wheatgrass) was, on average, over four years a net carbon sink of about 13.5 tCO <sub>2</sub> ha <sup>–1</sup> yr <sup>–1</sup> (de Oliveira et al. 2018 <sup>[[#fn:r966|966]]</sup> ). Sprunger et al. (2018) <sup>[[#fn:r967|967]]</sup> compared an annual winter wheat crop with a perennial grain crop (intermediate wheatgrass) and found that the perennial grain root biomass was 15 times larger than winter wheat, however, there was no significant difference in soil carbon pools after the four-year experiment. Exactly how much, and over what time period, carbon can be sequestered through changing from annual to perennial crops depends on the degree of soil carbon depletion and other local biophysical factors (Section 4.9.2).

Integrated soil fertility management is a sustainable approach to nutrient management that uses a combination of chemical and organic amendments (manure, compost, biosolids, biochar), rhizobial nitrogen fixation, and liming materials to address soil chemical constraints (Henry et al. 2018 <sup>[[#fn:r968|968]]</sup> ). In pasture systems, management of grazing pressure, fertilisation, diverse species including legumes and perennial grasses can reduce erosion and enhance soil carbon (Conant et al. 2017 <sup>[[#fn:r969|969]]</sup> ).

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