Editing IPCC:AR6/SRCCL/Chapter-2 (section)

==== 2.3.3.2 Land use effects ====

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Agriculture is responsible for approximately two-thirds of N <sub>2</sub> O emissions ( ''robust evidence, high agreement'' ) (Janssens-Maenhout et al. 2017b <sup>[[#fn:r678|678]]</sup> ). Total emissions from this sector are the sum of direct and indirect emissions. Direct emissions from soils are the result of mineral fertiliser and manure application, manure management, deposition of crop residues, cultivation of organic soils and inorganic nitrogen inputs through biological nitrogen fixation. Indirect emissions come from increased warming, enrichment of downstream water bodies from runoff, and downwind nitrogen deposition on soils. The main driver of N <sub>2</sub> O emissions in croplands is a lack of synchronisation between crop nitrogen demand and soil nitrogen supply, with approximately 50% of nitrogen applied to agricultural land not taken up by the crop (Zhang et al. 2017 <sup>[[#fn:r679|679]]</sup> ). Cropland soils emit over 3 TgN <sub>2</sub> O-N yr <sup>–1</sup> ( ''medium evidence, high agreement'' ) (Janssens-Maenhout et al. 2017b <sup>[[#fn:r680|680]]</sup> ; Saikawa et al. 2014 <sup>[[#fn:r681|681]]</sup> ). Regional inverse modelling studies show larger tropical emissions than the inventory approaches and they show increases in N <sub>2</sub> O emissions from the agricultural sector in South Asia, Central America, and South America (Saikawa et al. 2014 <sup>[[#fn:r682|682]]</sup> ; Wells et al. 2018 <sup>[[#fn:r683|683]]</sup> ).

Emissions of N <sub>2</sub> O from pasturelands and rangelands have increased by as much as 80% since 1960 due to increased manure production and deposition ( ''robust evidence, high agreement'' ) (de Klein et al. 2014 <sup>[[#fn:r684|684]]</sup> ; Tian et al. 2018 <sup>[[#fn:r685|685]]</sup> ; Chadwick et al. 2018 <sup>[[#fn:r686|686]]</sup> ; Dangal et al. 2019 <sup>[[#fn:r687|687]]</sup> ; Cardenas et al. 2019 <sup>[[#fn:r689|689]]</sup> ). Studies consistently report that pasturelands and rangelands are responsible for around half of the total agricultural N <sub>2</sub> O emissions (Davidson 2009 <sup>[[#fn:r690|690]]</sup> ; Oenema et al. 2014 <sup>[[#fn:r691|691]]</sup> ; Dangal et al. 2019 <sup>[[#fn:r692|692]]</sup> ). An analysis by Dangal et al. (2019) shows that, while managed pastures make up around one-quarter of the global grazing lands, they contribute 86% of the net global N <sub>2</sub> O emissions from grasslands and that more than half of these emissions are related to direct deposition of livestock excreta on soils.

Many studies calculate N <sub>2</sub> O emissions from a linear relationship between nitrogen application rates and N <sub>2</sub> O emissions. New studies are increasingly finding nonlinear relationships, which means that N <sub>2</sub> O emissions per hectare are lower than the Tier 1 EFs (IPCC 2003 <sup>[[#fn:r693|693]]</sup> ) at low nitrogen application rates, and higher at high nitrogen application rates ( ''robust evidence, high agreement'' ) (Shcherbak et al. 2014 <sup>[[#fn:r694|694]]</sup> ; van Lent et al. 2015 <sup>[[#fn:r695|695]]</sup> ; Satria 2017 <sup>[[#fn:r696|696]]</sup> ). This not only has implications for how agricultural N <sub>2</sub> O emissions are estimated in national and regional inventories, which now often use a linear relationship between nitrogen applied and N <sub>2</sub> O emissions, it also means that in regions of the world where low nitrogen application rates dominate, increases in nitrogen fertiliser use would generate relatively small increases in agricultural N <sub>2</sub> O emissions. Decreases in application rates in regions where application rates are high and exceed crop demand for parts of the growing season are likely to have very large effects on emissions reductions ( ''medium evidence, high agreement'' ).

Deforestation and other forms of land-use change alter soil N <sub>2</sub> O emissions. Typically, N <sub>2</sub> O emissions increase following conversion of native forests and grasslands to pastures or croplands (McDaniel et al. 2019 <sup>[[#fn:r697|697]]</sup> ; van Lent et al. 2015 <sup>[[#fn:r698|698]]</sup> ). This increase lasts from a few years to a decade or more, but there is a trend toward decreased N <sub>2</sub> O emissions with time following land use change and, ultimately, lower N <sub>2</sub> O emissions than had been occurring under native vegetation, in the absence of fertilisation ( ''medium evidence, high agreement'' ) (Meurer et al. 2016 <sup>[[#fn:r2132|2132]]</sup> ; van Lent et al. 2015 <sup>[[#fn:r699|699]]</sup> ) (Figure 2.12). Conversion of native vegetation to fertilised systems typically leads to increased N <sub>2</sub> O emissions over time, with the rate of emission often being a function of nitrogen fertilisation rates, however, this response can be moderated by soil characteristics and water availability ( ''medium evidence, high agreement'' ) (van Lent et al. 2015 <sup>[[#fn:r700|700]]</sup> ; Meurer et al. 2016 <sup>[[#fn:r701|701]]</sup> ). Restoration of agroecosystems to natural vegetation, over the period of one to two decades does not lead to recovery of N <sub>2</sub> O emissions to the levels of the original vegetation (McDaniel et al. 2019 <sup>[[#fn:r702|702]]</sup> ). To conclude, findings since AR5 increasingly highlight the limits of linear N <sub>2</sub> O emission factors, particularly from field to regional scales, with emissions rising nonlinearly at high nitrogen application rates ( ''high confidence'' ). Emissions from unfertilised systems often increase and then decline over time with typically lower emissions than was the case under native vegetation ( ''high confidence'' ).

While soil emissions are the predominant source of N <sub>2</sub> O in agriculture, other sources are important (or their importance is only just emerging). Biomass burning is responsible for approximately 0.7 TgN <sub>2</sub> O-N yr <sup>–1</sup> (0.5–1.7 TgN <sub>2</sub> O-N yr <sup>–1</sup> ) or 11% of total gross anthropogenic emissions due to the release of N <sub>2</sub> O from the oxidation of organic nitrogen in biomass (UNEP 2013 <sup>[[#fn:r703|703]]</sup> ). This source includes crop residue burning, forest fires, household cook stoves and prescribed savannah, pasture and cropland burning. Aquaculture is currently not accounted for in most assessments or compilations. While it is currently responsible for less than 0.1 TgN <sub>2</sub> O-N yr <sup>–1</sup> , it is one of the fastest growing sources of anthropogenic N <sub>2</sub> O emissions (Williams and Crutzen 2010 <sup>[[#fn:r704|704]]</sup> ; Bouwman et al. 2013 <sup>[[#fn:r705|705]]</sup> ) ( ''limited evidence, high agreement'' ). Finally, increased nitrogen deposition from terrestrial sources is leading to greater indirect N <sub>2</sub> O emissions, particularly since 1980 ( ''moderate evidence, high agreement'' ) (Tian et al. 2018 <sup>[[#fn:r706|706]]</sup> , 2016 <sup>[[#fn:r2133|2133]]</sup> ). In marine systems, deposition is estimated to have increased the oceanic N <sub>2</sub> O source by 0.2 TgN <sub>2</sub> O-N yr <sup>–1</sup> or 3% of total gross anthropogenic emissions (Suntharalingam et al. 2012 <sup>[[#fn:r707|707]]</sup> ).

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'''Figure 2.12'''

<span id="effect-of-time-since-conversion-on-n2o-fluxes-in-unfertilised-orange-circles-and-fertilised-blue-circles-tropical-croplands-left-frame-and-in-unfertilised-tropical-pastures-right-frame.-average-n2o-flux-and-95-confidence-intervals-are-given-for-upland-forests-orange-inverted-triangle-and-low-canopy-forests-blue-inverted-triangle-for-comparison.-the-solid-lines-represent"></span>
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'''Effect of time since conversion on N2O fluxes in unfertilised (orange circles) and fertilised (blue circles) tropical croplands (left frame) and in unfertilised tropical pastures (right frame). Average N2O flux and 95% confidence intervals are given for upland forests (orange inverted triangle) and low canopy forests (blue inverted triangle), for comparison. The solid lines represent […]'''
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[[File:3fe437700a08981212c063d3ec0e286f Figure-2.12-1024x440.jpg]]

Effect of time since conversion on N <sub>2</sub> O fluxes in unfertilised (orange circles) and fertilised (blue circles) tropical croplands (left frame) and in unfertilised tropical pastures (right frame). Average N <sub>2</sub> O flux and 95% confidence intervals are given for upland forests (orange inverted triangle) and low canopy forests (blue inverted triangle), for comparison. The solid lines represent the trends for unfertilised and fertilised cases. Data source: van Lent et al. (2015) <sup>[[#fn:r708|708]]</sup> .

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