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

=== 4.9.2 Perennial grains and soil organic carbon (SOC) ===

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The severe ecological perturbation that is inherent in the conversion of native perennial vegetation to annual crops, and the subsequent high frequency of perturbation required to maintain annual crops, results in at least four forms of soil degradation that will be exacerbated by the effects of climate change (Crews et al. 2016 <sup>[[#fn:r1259|1259]]</sup> ). First, soil erosion is a very serious consequence of annual cropping, with median losses exceeding rates of formation by one to two orders of magnitude in conventionally plowed agroecosystems, and while erosion is reduced with conservation tillage, median losses still exceed formation by several fold (Montgomery 2007 <sup>[[#fn:r1260|1260]]</sup> ). More severe storm intensity associated with climate change is expected to cause even greater losses to wind and water erosion (Nearing et al. 2004 <sup>[[#fn:r1261|1261]]</sup> ). Second, the periods of time in which live roots are reduced or altogether absent from soils in annual cropping systems allow for substantial losses of nitrogen from fertilised croplands, averaging 50% globally (Ladha et al. 2005 <sup>[[#fn:r1262|1262]]</sup> ). This low retention of nitrogen is also expected to worsen with more intense weather events (Bowles et al. 2018 <sup>[[#fn:r1263|1263]]</sup> ). A third impact of annual cropping is the degradation of soil structure caused by tillage, which can reduce infiltration of precipitation, and increase surface runoff. It is predicted that the percentage of precipitation that infiltrates into agricultural soils will decrease further under climate-change scenarios (Basche and DeLonge 2017 <sup>[[#fn:r1264|1264]]</sup> ; Wuest et al. 2006 <sup>[[#fn:r1265|1265]]</sup> ). The fourth form of soil degradation that results from annual cropping is the reduction of soil organic matter (SOM), a topic of particular relevance to climate change mitigation and adaptation.

Undegraded cropland soils can theoretically hold far more SOM (which is about 58% carbon) than they currently do (Soussana et al. 2006 <sup>[[#fn:r1266|1266]]</sup> ). We know this deficiency because, with few exceptions, comparisons between cropland soils and those of proximate mature native ecosystems commonly show a 40–75% decline in soil carbon attributable to agricultural practices. What happens when native ecosystems are converted to agriculture that induces such significant losses of SOM? Wind and water erosion commonly results in preferential removal of light organic matter fractions that can accumulate on or near the soil surface (Lal 2003 <sup>[[#fn:r1267|1267]]</sup> ). In addition to the effects of erosion, the fundamental practices of growing annual food and fibre crops alters both inputs and outputs of organic matter from most agroecosystems, resulting in net reductions in soil carbon equilibria (Soussana et al. 2006 <sup>[[#fn:r1268|1268]]</sup> ; McLauchlan 2006 <sup>[[#fn:r1269|1269]]</sup> ; Crews et al. 2016 <sup>[[#fn:r1270|1270]]</sup> ). Native vegetation of almost all terrestrial ecosystems is dominated by perennial plants, and the below-ground carbon allocation of these perennials is a key variable in determining formation rates of stable soil organic carbon (SOC) (Jastrow et al. 2007 <sup>[[#fn:r1271|1271]]</sup> ; Schmidt et al. 2011 <sup>[[#fn:r1272|1272]]</sup> ). When perennial vegetation is replaced by annual crops, inputs of root-associated carbon (roots, exudates, mycorrhizae) decline substantially. For example, perennial grassland species allocate around 67% of productivity to roots, whereas annual crops allocate between 13–30% (Saugier 2001 <sup>[[#fn:r1273|1273]]</sup> ; Johnson et al. 2006 <sup>[[#fn:r1274|1274]]</sup> ).

At the same time, inputs of SOC are reduced in annual cropping systems, and losses are increased because of tillage, compared to native perennial vegetation. Tillage breaks apart soil aggregates which, among other functions, are thought to inhibit soil bacteria, fungi and other microbes from consuming and decomposing SOM (Grandy and Neff 2008 <sup>[[#fn:r1275|1275]]</sup> ). Aggregates reduce microbial access to organic matter by restricting physical access to mineral-stabilised organic compounds as well as reducing oxygen availability (Cotrufo et al. 2015 <sup>[[#fn:r1276|1276]]</sup> ; Lehmann and Kleber 2015 <sup>[[#fn:r1277|1277]]</sup> ). When soil aggregates are broken open with tillage in the conversion of native ecosystems to agriculture, microbial consumption of SOC and subsequent respiration of CO <sub>2</sub> increase dramatically, reducing soil carbon stocks (Grandy and Robertson 2006 <sup>[[#fn:r1278|1278]]</sup> ; Grandy and Neff 2008 <sup>[[#fn:r1279|1279]]</sup> ).

Many management approaches are being evaluated to reduce soil degradation in general, especially by increasing mineral-protected forms of SOC in the world’s croplands (Paustian et al. 2016 <sup>[[#fn:r1280|1280]]</sup> ). The menu of approaches being investigated focuses either on increasing below-ground carbon inputs, usually through increases in total crop productivity, or by decreasing microbial activity, usually through reduced soil disturbance (Crews and Rumsey 2017 <sup>[[#fn:r1281|1281]]</sup> ). However, the basic biogeochemistry of terrestrial ecosystems managed for production of annual crops presents serious challenges to achieving the standing stocks of SOC accumulated by native ecosystems that preceded agriculture. A novel new approach that is just starting to receive significant attention is the development of perennial cereal, legume and oilseed crops (Glover et al. 2010 <sup>[[#fn:r1282|1282]]</sup> ; Baker 2017 <sup>[[#fn:r1283|1283]]</sup> ).

There are two basic strategies that plant breeders and geneticists are using to develop new perennial grain crop species. The first involves making wide hybrid crosses between existing elite lines of annual crops, such as wheat, sorghum and rice, with related wild perennial species in order to introgress perennialism into the genome of the annual (Cox et al. 2018 <sup>[[#fn:r1284|1284]]</sup> ; Huang et al. 2018 <sup>[[#fn:r1285|1285]]</sup> ; Hayes et al. 2018 <sup>[[#fn:r1286|1286]]</sup> ). The other approach is de ''novo'' domestication of wild perennial species that have crop-like traits of interest (DeHaan et al. 2016 <sup>[[#fn:r1287|1287]]</sup> ; DeHaan and Van Tassel 2014 <sup>[[#fn:r1288|1288]]</sup> ). New perennial crop species undergoing de ''novo'' domestication include intermediate wheatgrass, a relative of wheat that produces grain known as Kernza (DeHaan et al. 2018 <sup>[[#fn:r1289|1289]]</sup> ; Cattani and Asselin 2018 <sup>[[#fn:r1290|1290]]</sup> ) and ''Silphium integrifolium'' , an oilseed crop in the sunflower family (Van Tassel et al. 2017 <sup>[[#fn:r1291|1291]]</sup> ). Other grain crops receiving attention for perennialisation include pigeon pea, barley, buckwheat and maize (Batello et al. 2014 <sup>[[#fn:r1292|1292]]</sup> ; Chen et al. 2018c <sup>[[#fn:r1293|1293]]</sup> ) and a number of legume species (Schlautman et al. 2018 <sup>[[#fn:r1294|1294]]</sup> ). In most cases, the seed yields of perennial grain crops under development are well below those of elite modern grain varieties. During the period that it will take for intensive breeding efforts to close the yield and other trait gaps between annual and perennial grains, perennial proto-crops may be used for purposes other than grain, including forage production (Ryan et al. 2018 <sup>[[#fn:r1295|1295]]</sup> ). Perennial rice stands out as a high-yielding exception, as its yields matched those of elite local varieties in the Yunnan Province for six growing seasons over three years (Huang et al. 2018 <sup>[[#fn:r1296|1296]]</sup> ).

In a perennial agroecosystem, the biogeochemical controls on SOC accumulation shift dramatically, and begin to resemble the controls that govern native ecosystems (Crews et al. 2016 <sup>[[#fn:r1297|1297]]</sup> ). When erosion is reduced or halted, and crop allocation to roots increases by 100–200%, and when soil aggregates are not disturbed thus reducing microbial respiration, SOC levels are expected to increase (Crews and Rumsey 2017 <sup>[[#fn:r1298|1298]]</sup> ). Deep roots growing year round are also effective at increasing nitrogen retention (Culman et al. 2013 <sup>[[#fn:r1299|1299]]</sup> ; Jungers et al. 2019 <sup>[[#fn:r1300|1300]]</sup> ). Substantial increases in SOC have been measured where croplands that had historically been planted to annual grains were converted to perennial grasses, such as in the US Conservation Reserve Program or in plantings of second-generation perennial biofuel crops. Two studies have assessed carbon accumulation in soils when croplands were converted to the perennial grain Kernza. In one, researchers found no differences in soil labile (permanganate-oxidisable) carbon after four years of cropping to perennial Kernza versus annual wheat in a sandy textured soil. Given that coarse textured soils do not offer the same physicochemical protection against microbial attack as many finer textured soils, these results are not surprising, but these results do underscore how variable the rates of carbon accumulation can be (Jastrow et al. 2007 <sup>[[#fn:r1301|1301]]</sup> ). In the second study, researchers assessed the carbon balance of a Kernza field in Kansas, USA over 4.5 years using eddy covariance observations (de Oliveira et al. 2018). They found that the net carbon accumulation rate of about 1500 gC m <sup>–2</sup> yr <sup>–1</sup> in the first year of the study corresponding to the biomass of Kernza, increasing to about 300 gC m <sup>–2</sup> yr <sup>–1</sup> in the final year, where CO <sub>2</sub> respiration losses from the decomposition of roots and SOM approached new carbon inputs from photosynthesis. Based on measurements of soil carbon accumulation in restored grasslands in this part of the USA, the net carbon accumulation in stable organic matter under a perennial grain crop might be expected to sequester 30–50 gC m <sup>–2</sup> yr <sup>–1</sup> (Post and Kwon 2000 <sup>[[#fn:r1302|1302]]</sup> ) until a new equilibrium is reached. Sugar cane, a highly productive perennial, has been shown to accumulate a mean of 187 gC m–2 yr <sup>–1</sup> in Brazil (La Scala Júnior et al. 2012 <sup>[[#fn:r1303|1303]]</sup> ).

Reduced soil erosion, increased nitrogen retention, greater water uptake efficiency and enhanced carbon sequestration represent improved ecosystem functions, made possible in part by deep and extensive root systems of perennial crops (Figure 4.8).

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'''Figure-4.8'''

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'''Comparison of root systems between the newly domesticated intermediate wheatgrass (left) and annual wheat (right). Photo: Copyright © Jim Richardson.'''
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Comparison of root systems between the newly domesticated intermediate wheatgrass (left) and annual wheat (right). Photo: Copyright © Jim Richardson.

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When compared to annual grains like wheat, single species stands of deep-rooted perennial grains such as Kernza are expected to reduce soil erosion, increase nitrogen retention, achieve greater water uptake efficiency and enhance carbon sequestration (Crews et al. 2018 <sup>[[#fn:r1304|1304]]</sup> ) (Figure 4.8). An even higher degree of ecosystem services can, at least theoretically, be achieved by strategically combining different functional groups of crops such as a cereal and a nitrogen-fixing legume (Soussana and Lemaire 2014 <sup>[[#fn:r1305|1305]]</sup> ). Not only is there evidence from plant-diversity experiments that communities with higher species richness sustain higher concentrations of SOC (Hungate et al. 2017 <sup>[[#fn:r1306|1306]]</sup> ; Sprunger and Robertson 2018 <sup>[[#fn:r1307|1307]]</sup> ; Chen, S. 2018 <sup>[[#fn:r1308|1308]]</sup> ; Yang et al. 2019 <sup>[[#fn:r1309|1309]]</sup> ), but other valuable ecosystem services such as pest suppression, lower GHG emissions, and greater nutrient retention may be enhanced (Schnitzer et al. 2011 <sup>[[#fn:r1310|1310]]</sup> ; Culman et al. 2013 <sup>[[#fn:r1311|1311]]</sup> ).

Similar to perennial forage crops such as alfalfa, perennial grain crops are expected to have a definite productive lifespan, probably in the range of three to 10 years. A key area of research on perennial grains cropping systems is to minimise losses of SOC during conversion of one stand of perennial grains to another. Recent work demonstrates that no-till conversion of a mature perennial grassland to another perennial crop will experience several years of high net CO <sub>2</sub> emissions as decomposition of copious crop residues exceed ecosystem uptake of carbon by the new crop (Abraha et al. 2018 <sup>[[#fn:r1312|1312]]</sup> ). Most, if not all, of this lost carbon will be recaptured in the replacement crop. It is not known whether mineral-stabilised carbon that is protected in soil aggregates is vulnerable to loss in perennial crop succession.

Perennial grains hold promises of agricultural practices, which can significantly reduce soil erosion and nutrient leakage while sequestering carbon. When cultivated in mixes with N-fixing species (legumes) such polycultures also reduce the need for external inputs of nitrogen – a large source of GHG from conventional agriculture.

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