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

=== 4.8.1 4.8.1 Actions on the ground to address land degradation ===

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Concrete actions on the ground to address land degradation are primarily focused on soil and water conservation. In the context of adaptation to climate change, actions relevant for addressing land degradation are sometimes framed as ecosystem-based adaptation (Scarano 2017 <sup>[[#fn:r935|935]]</sup> ) or Nature-Based Solutions (Nesshöver et al. 2017 <sup>[[#fn:r936|936]]</sup> ), and in an agricultural context, agroecology (see Glossary) provides an important frame. The site-specific biophysical and social conditions, including local and indigenous knowledge, are important for successful implementation of concrete actions.

Responses to land degradation generally take the form of agronomic measures (methods related to managing the vegetation cover), soil management (methods related to tillage, nutrient supply), and mechanical methods (methods resulting in durable changes to the landscape) (Morgan 2005a <sup>[[#fn:r937|937]]</sup> ). Measures may be combined to reinforce benefits to land quality, as well as improving carbon sequestration that supports climate change mitigation. Some measures offer adaptation options and other co-benefits, such as agroforestry, involving planting fruit trees that can support food security in the face of climate change impacts (Reed and Stringer 2016 <sup>[[#fn:r938|938]]</sup> ), or application of compost or biochar that enhances soil water-holding capacity, so increases resilience to drought.

There are important differences in terms of labour and capital requirements for different technologies, and also implications for land tenure arrangements. Agronomic measures and soil management require generally little extra capital input and comprise activities repeated annually, so have no particular implication for land tenure arrangements. Mechanical methods require substantial upfront investments in terms of capital and labour, resulting in long-lasting structural change, requiring more secure land tenure arrangements (Mekuriaw et al. 2018 <sup>[[#fn:r939|939]]</sup> ). Agroforestry is a particularly important strategy for SLM in the context of climate change because of the large potential to sequester carbon in plants and soil and enhance resilience of agricultural systems (Zomer et al. 2016 <sup>[[#fn:r940|940]]</sup> ).

Implementation of SLM practices has been shown to increase the productivity of land (Branca et al. 2013 <sup>[[#fn:r941|941]]</sup> ) and to provide good economic returns on investment in many different settings around the world (Mirzabaev et al. 2015 <sup>[[#fn:r942|942]]</sup> ). Giger et al. (2018) <sup>[[#fn:r943|943]]</sup> showed, in a meta study of 363 SLM projects over the period 1990 to 2012, that 73% of the projects were perceived to have a positive or at least neutral cost-benefit ratio in the short term, and 97% were perceived to have a positive or very positive cost-benefit ratio in the long term ( ''robust evidence, high agreement'' ). Despite the positive effects, uptake is far from universal. Local factors, both biophysical conditions (e.g., soils,

drainage, and topography) and socio-economic conditions (e.g., land tenure, economic status, and land fragmentation) play decisive roles in the interest in, capacity to undertake, and successful implementation of SLM practices (Teshome et al. 2016 <sup>[[#fn:r944|944]]</sup> ; Vogl et al. 2017 <sup>[[#fn:r945|945]]</sup> ; Tesfaye et al. 2016 <sup>[[#fn:r946|946]]</sup> ; Cerdà et al. 2018 <sup>[[#fn:r947|947]]</sup> ; Adimassu et al. 2016 <sup>[[#fn:r948|948]]</sup> ). From a landscape perspective, SLM can generate benefits, including adaptation to and mitigation of climate change, for entire watersheds, but challenges remain regarding coordinated and consistent implementation ( ''medium evidence, medium agreement'' ) (Kerr et al. 2016 <sup>[[#fn:r949|949]]</sup> ; Wang et al. 2016a <sup>[[#fn:r950|950]]</sup> ).

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==== 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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==== 4.8.1.2 Mechanical soil and water conservation ====

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In hilly and mountainous terrain, terracing is an ancient but still practised soil conservation method worldwide (Preti and Romano 2014 <sup>[[#fn:r970|970]]</sup> ) in climatic zones from arid to humid tropics (Balbo 2017 <sup>[[#fn:r981|981]]</sup> ). By reducing the slope gradient of hillsides, terraces provide flat surfaces. Deep, loose soils that increase infiltration, reduce erosion and thus sediment transport. They also decrease the hydrological connectivity and thus reduce hillside runoff (Preti et al. 2018 <sup>[[#fn:r972|972]]</sup> ; Wei et al. 2016 <sup>[[#fn:r973|973]]</sup> ; Arnáez et al. 2015 <sup>[[#fn:r974|974]]</sup> ; Chen et al. 2017 <sup>[[#fn:r975|975]]</sup> ). In terms of climate change, terraces are a form of adaptation that helps in cases where rainfall is increasing or intensifying (by reducing slope gradient and the hydrological connectivity), and where rainfall is decreasing (by increasing infiltration and reducing runoff) ( ''robust evidence, high agreement'' ). There are several challenges, however, to continued maintenance and construction of new terraces, such as the high costs in terms of labour and/or capital (Arnáez et al. 2015 <sup>[[#fn:r976|976]]</sup> ) and disappearing local knowledge for maintaining and constructing new terraces (Chen et al. 2017 <sup>[[#fn:r977|977]]</sup> ). The propensity of farmers to invest in mechanical soil conservation methods varies with land tenure; farmers with secure tenure arrangements are more willing to invest in durable practices such as terraces (Lovo 2016 <sup>[[#fn:r978|978]]</sup> ; Sklenicka et al. 2015 <sup>[[#fn:r979|979]]</sup> ; Haregeweyn et al. 2015 <sup>[[#fn:r980|980]]</sup> ). Where the slope is less severe, erosion can be controlled by contour banks, and the keyline approach (Duncan 2016 <sup>[[#fn:r1652|1652]]</sup> ; Stevens et al. 2015 <sup>[[#fn:r982|982]]</sup> ) to soil and water conservation.

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==== 4.8.1.3 Agroforestry ====

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Agroforestry is defined as a collective name for land-use systems in which woody perennials (trees, shrubs, etc.) are grown in association with herbaceous plants (crops, pastures) and/or livestock in a spatial arrangement, a rotation, or both, and in which there are both ecological and economic interactions between the tree and non-tree components of the system (Young, 1995, p. 11 <sup>[[#fn:r983|983]]</sup> ). At least since the 1980s, agroforestry has been widely touted as an ideal land management practice in areas vulnerable to climate variations and subject to soil erosion. Agroforestry holds the promise of improving soil and climatic conditions, while generating income from wood energy, timber and non-timber products – sometimes presented as a synergy of adaptation and mitigation of climate change (Mbow et al. 2014 <sup>[[#fn:r984|984]]</sup> ).

There is strong scientific consensus that a combination of forestry with agricultural crops and/or livestock, agroforestry systems can provide additional ecosystem services when compared with monoculture crop systems (Waldron et al. 2017 <sup>[[#fn:r985|985]]</sup> ; Sonwa et al. 2011 <sup>[[#fn:r986|986]]</sup> , 2014 <sup>[[#fn:r987|987]]</sup> , 2017 <sup>[[#fn:r988|988]]</sup> ; Charles et al. 2013 <sup>[[#fn:r989|989]]</sup> ). Agroforestry can enable sustainable intensification by allowing continuous production on the same unit of land with higher productivity without the need to use shifting agriculture systems to maintain crop yields (Nath et al. 2016 <sup>[[#fn:r990|990]]</sup> ). This is especially relevant where there is a regional requirement to find a balance between the demand for increased agricultural production and the protection of adjacent natural ecosystems such as primary and secondary forests (Mbow et al. 2014 <sup>[[#fn:r991|991]]</sup> ). For example, the use of agroforestry for perennial crops such as coffee and cocoa is increasingly promoted as offering a route to sustainable farming, with important climate change adaptation and mitigation co-benefits (Sonwa et al. 2001 <sup>[[#fn:r992|992]]</sup> ; Kroeger et al. 2017 <sup>[[#fn:r993|993]]</sup> ). Reported co-benefits of agroforestry in cocoa production include increased carbon sequestration in soils and biomass, improved water and nutrient use efficiency and the creation of a favourable micro-climate for crop production (Sonwa et al. 2017 <sup>[[#fn:r994|994]]</sup> ; Chia et al. 2016 <sup>[[#fn:r995|995]]</sup> ). Importantly, the maintenance of soil fertility using agroforestry has the potential to reduce the practice of shifting agriculture (of cocoa) which results in deforestation (Gockowski and Sonwa 2011 <sup>[[#fn:r996|996]]</sup> ). However, positive interactions within these systems can be ecosystem and/or species specific, but co-benefits such as increased resilience to extreme climate events, or improved soil fertility are not always observed (Blaser et al. 2017 <sup>[[#fn:r997|997]]</sup> ; Abdulai et al. 2018 <sup>[[#fn:r998|998]]</sup> ). These contrasting outcomes indicate the importance of field-scale research programmes to inform agroforestry system design, species selection and management practices (Sonwa et al. 2014 <sup>[[#fn:r999|999]]</sup> ).

Despite the many proven benefits, adoption of agroforestry has been low and slow (Toth et al. 2017 <sup>[[#fn:r1000|1000]]</sup> ; Pattanayak et al. 2003 <sup>[[#fn:r1001|1001]]</sup> ; Jerneck and Olsson 2014 <sup>[[#fn:r1002|1002]]</sup> ). There are several reasons for the slow uptake, but the perception of risks and the time lag between adoption and realisation of benefits are often important (Pattanayak et al. 2003 <sup>[[#fn:r1003|1003]]</sup> ; Mercer 2004 <sup>[[#fn:r1004|1004]]</sup> ; Jerneck and Olsson 2013 <sup>[[#fn:r1005|1005]]</sup> ).

An important question for agroforestry is whether it supports poverty alleviation, or if it favours comparatively affluent households. Experiences from India suggest that the overall adoption is low, with a differential between rich and poor households. Brockington el al. (2016) <sup>[[#fn:r1006|1006]]</sup> , studied agroforestry adoption over many years in South India and found that, overall, only 18% of the households adopted agroforestry. However, among the relatively rich households who adopted agroforestry, 97% were still practising it after six to eight years, and some had expanded their operations. Similar results were obtained in Western Kenya, where food-secure households were much more willing to adopt agroforestry than food-insecure households (Jerneck and Olsson 2013 <sup>[[#fn:r1007|1007]]</sup> , 2014). Other experiences from Sub-Saharan Africa illustrate the difficulties (such as local institutional support) of having a continued engagement of communities in agroforestry (Noordin et al. 2001 <sup>[[#fn:r1008|1008]]</sup> ; Matata et al. 2013 <sup>[[#fn:r1009|1009]]</sup> ; Meijer et al. 2015 <sup>[[#fn:r1010|1010]]</sup> ).

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==== 4.8.1.4 Crop–livestock interaction as an approach to managing land degradation ====

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The integration of crop and livestock production into ‘mixed farming’ for smallholders in developing countries became an influential model, particularly for Africa, in the early 1990s (Pritchard et al. 1992 <sup>[[#fn:r1011|1011]]</sup> ; McIntire et al. 1992 <sup>[[#fn:r1012|1012]]</sup> ). Crop–livestock integration under this model was seen as founded on three pillars: improved use of manure for crop fertility management; expanded use of animal traction (draught animals); and promotion of cultivated fodder crops. For Asia, emphasis was placed on draught power for land preparation, manure for soil fertility enhancement, and fodder production as an entry point for cultivation of legumes (Devendra and Thomas 2002 <sup>[[#fn:r1013|1013]]</sup> ). Mixed farming was seen as an evolutionary process to expand food production in the face of population increase, promote improvements in income and welfare, and protect the environment. The process could be further facilitated and steered by research, agricultural advisory services and policy (Pritchard et al. 1992 <sup>[[#fn:r1014|1014]]</sup> ; McIntire et al. 1992 <sup>[[#fn:r1015|1015]]</sup> ; Devendra 2002 <sup>[[#fn:r1016|1016]]</sup> ).

Scoones and Wolmer (2002) <sup>[[#fn:r1017|1017]]</sup> place this model in historical context, including concern about population pressure on resources and the view that mobile pastoralism was environmentally damaging. The latter view had already been critiqued by developing understandings of pastoralism, mobility and communal tenure of grazing lands (e.g., Behnke 1994 <sup>[[#fn:r1018|1018]]</sup> ; Ellis 1994 <sup>[[#fn:r1019|1019]]</sup> ). They set out a much more differentiated picture of crop–livestock interactions, which can take place either within a single-farm household, or between crop and livestock producers, in which case they will be mediated by formal and informal institutions governing the allocation of land, labour and capital, with the interactions evolving through multiple place-specific pathways (Ramisch et al. 2002 <sup>[[#fn:r1020|1020]]</sup> ; Scoones and Wolmer 2002 <sup>[[#fn:r1021|1021]]</sup> ). Promoting a diversity of approaches to crop–livestock interactions does not imply that the integrated model necessarily leads to land degradation, but increases the space for institutional support to local innovation (Scoones and Wolmer 2002 <sup>[[#fn:r1022|1022]]</sup> ).

However, specific managerial and technological practices that link crop and livestock production will remain an important part of the repertoire of on-farm adaptation and mitigation. Howden and coauthors (Howden et al. 2007 <sup>[[#fn:r1023|1023]]</sup> ) note the importance of innovation within existing integrated systems, including use of adapted forage crops. Rivera-Ferre et al. (2016) <sup>[[#fn:r1024|1024]]</sup> list as adaptation strategies with high potential for grazing systems, mixed crop–livestock systems or both: crop–livestock integration in general; soil management, including composting; enclosure and corralling of animals; improved storage of feed. Most of these are seen as having significant co-benefits for mitigation, and improved management of manure is seen as a mitigation measure with adaptation co-benefits.

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