Cross-Chapter Box 3: Fire and Climate Change
2.3 Greenhouse gas fluxes between land and atmosphere 20
2.3.3 Nitrous Oxide 22
evidence, high agreement). Peatland management and restoration, alters the exchange of CH4 with the 3
atmosphere (medium evidence, high agreement). Management of peat soils typically converts them from CH4
4
sources to sinks (Augustin et al. 2011; Strack and Waddington 2008; Abdalla et al. 2016) (robust evidence, 5
high agreement). While restoration decreases CO2 emissions (see Section 4.9.4), CH4 emissions often 6
increase relative to the drained conditions (robust evidence, high agreement) (Osterloh et al. 2018; Christen 7
et al. 2016; Koskinen et al. 2016; Tuittila et al. 2000; Vanselow-Algan et al. 2015; Abdalla et al. 2016).
8
Drained peatlands are usually considered to be negligible methane sources, but they emit CH4 under wet 9
weather conditions and from drainage ditches (Drösler et al. 2013; Sirin et al. 2012). While ditches cover 10
only a small percentage of the drained area, emissions can be sufficiently high that drained peatlands emit 11
comparable CH4 as undrained ones (medium evidence, medium agreement) (Sirin et al. 2012; Wilson et al.
12
2016).
13 14
Because of the large uncertainty in the tropical peatland area, estimates of the global flux are highly 15
uncertain. A meta-analysis of the effect of conversion of primary forest to rice production showed that 16
emissions increased by a factor of 4 (limited evidence, high agreement) (Hergoualc’h and Verchot, 2012).
17
For land uses that required drainage, emissions decreased by a factor of 3 (limited evidence, high 18
agreement). There are no representative measurements of emissions from drainage ditches in tropical 19
peatlands.
20 21
2.3.3 Nitrous Oxide 22
23
2.3.3.1 Atmospheric trends 24
The atmospheric abundance of N2O has increased since 1750, from a pre-industrial concentration of 270 25
ppbv to 330 ppbv in 2017 (U.S. National Oceanographic and Atmospheric Agency, Earth Systems Research 26
Laboratory; Figure 2.10) (high agreement, robust evidence). The rate of increase has also increased, from 27
approximately 0.15 ppbv yr-1 100 years ago, to 0.85 ppbv yr-1 over the period 2001 to 2015 (Wells et al.
28
2018). Atmospheric N2O isotopic composition (14/15N) was relatively constant during the pre-industrial 29
period (Prokopiou et al. 2018) and shows a decrease in the δ15N as the N2O mixing ratio in the atmosphere 30
has increased between 1940 and 2005. This recent decrease indicates as that terrestrial sources are the 31
primary driver of increasing trends and marine sources contribute around 25% (Snider et al. 2015).
32
Microbial denitrification and nitrification processes are responsible for more than 80% of total global N2O 33
emissions, which includes natural soils, agriculture, and oceans, with the remainder coming from non-34
biological sources such as biomass burning and fossil-fuel combustion (Fowler et al. 2015). The isotopic 35
trend also indicates a shift from denitrification to nitrification as the primary source of N2O as a result of the 36
use of synthetic nitrogen (N) fertiliser (high evidence, high agreement) (Park et al. 2012; Toyoda et al. 2013;
37
Snider et al. 2015; Prokopiou et al. 2018).
38
39
Figure 2.10 Globally averaged atmospheric N2O mixing ratios since 1984. Data sources: NOAA/ESRL 40
Year
1985 1990 1995 2000 2005 2010 2015 2020
N
2O ( pp bv )
300 305 310 315 320 325 330 335
1 2
The three independent sources of N2O emissions estimates from agriculture at global, regional, and national 3
levels are: U.S.E.P.A., EDGAR and FAOSTAT (USEPA 2013; Tubiello et al. 2015; Janssens-Maenhout et 4
al. 2017). EDGAR and FAOSTAT have temporal resolution beyond 2005 and we these databases compare 5
well with national inventory data (Figure 2.10). USEPA has historical estimates through 2005 and 6
projections thereafter. The independent data use IPCC methods, with Tier 1 emission factors and national 7
reporting of activity data. Tier 2 approaches are also available based on top-down and bottom-up 8
approaches. Recent estimates using inversion modelling and process models estimate total annual global 9
N2O emissions of 16.1-18.7 (bottom-up) and 15.9-17.7 Tg N (top-down), demonstrating relatively close 10
agreement (Thompson et al. 2014). Agriculture is the largest source and has increased with the 11
extensification and intensification. Recent modelling estimates of terrestrial sources show a higher emissions 12
range that is slightly more constrained than what was reported in AR5: approximately 9 (7–11) Tg N2O-N yr -13
1 (Saikawa et al. 2014; Tian et al. 2016)compared to 6.6 (3.3–9.0) Tg N2O-N yr-1 (Ciais et al. 2013a).
14
Estimates of marine N2O emissions are between 2.5 and 4.6 Tg N2O-N yr-1; (Buitenhuis et al., 2017;
15
Saikawa et al., 2014).
16 17
To conclude, N2O is continuing to accumulate in the atmosphere at an increasingly higher rate (very high 18
confidence), driven primarily by increases in manure production and synthetic N fertiliser use from the mid-19
20th century onwards (high confidence). Findings since AR5 have constrained regional and global estimates 20
of annual N2O emissions and improved our understanding of the spatio-temporal dynamics of N2O 21
emissions, with soil rewetting and freeze-thaw cycles, which important determinants of total annual emission 22
fluxes in some regions (medium confidence).
23
24
Figure 2.11 Average agricultural N2O emissions estimates from 1990. Sub-sectorial agricultural emissions 25
are based on the Emissions Database for Global Atmospheric Research (EDGAR v4.3.2; Janssens-26
Maenhout et al. 2017a); FAOSTAT (Tubiello et al. 2013); and National GHGI data (Grassi et al. 2018).
27
GHGI data are aggregate values for the sector.
28
* Note that EDGAR data are complete only through 2012; the EDGAR data in the right-hand panel 29
represent the three years 2010-2012 and are presented for comparison.
30 31
2.3.3.2 Land use effects 32
Agriculture is responsible for approximately two-thirds of N2O emissions (robust evidence, high agreement) 33
(Janssens-Maenhout et al. 2017). Total emissions from this sector are the sum of direct and indirect 1
emissions. Direct emissions from soils are the result of mineral fertiliser and manure application, manure 2
management, deposition of crop residues, cultivation of organic soils and inorganic N inputs through 3
biological nitrogen fixation. Indirect emissions come from increased warming, enrichment of downstream 4
water bodies from runoff, and downwind N deposition on soils. The main driver of N2O emissions in 5
croplands is a lack of synchronisation between crop N demand and soil N supply, with approximately 50%
6
of N applied to agricultural land not taken up by the crop (Zhang et al. 2017). Cropland soils emit over 3 Tg 7
N2O-N yr-1 (medium evidence, high agreement) (Janssens-Maenhout et al. 2017; Saikawa et al. 2014).
8
Regional inverse modelling studies show larger tropical emissions than the inventory approaches and they 9
show increases in N2O emissions from the agricultural sector in South Asia, Central America, and South 10
America (Saikawa et al. 2014; Wells et al. 2018).
11 12
Emissions of N2O from pasturelands and rangelands have increased by as much as 80% since 1960 due to 13
increased manure production and deposition (robust evidence, high agreement) (de Klein et al. 2014; Tian et 14
while managed pastures make up around one-quarter of the global grazing lands, they contribute 86% of the 18
net global N2O emissions from grasslands and that more than half of these emissions are related to direct 19
deposition of livestock excreta on soils.
20 21
Many studies calculate N2O emissions from a linear relationship between nitrogen application rates and N2O 22
emissions. New studies are increasingly finding nonlinear relationships, which means that N2O emissions per 23
hectare are lower than the Tier 1 EFs (IPCC 2003) at low nitrogen application rates, and higher at high 24
nitrogen application rates (robust evidence, high agreement) (Shcherbak et al. 2014; van Lent et al. 2015;
25
Satria 2017). This not only has implications for how agricultural N2O emissions are estimated in national and 26
regional inventories, which now often use a linear relationship between nitrogen applied and N2O emissions, 27
it also means that in regions of the world where low nitrogen application rates dominate, increases in 28
nitrogen fertiliser use would generate relatively small increases in agricultural N2O emissions. Decreases in 29
application rates in regions where application rates are high and exceed crop demand for parts of the growing 30
season are likely to have very large effects on emissions reductions (medium evidence, high agreement).
31
toward decreased N2O emissions with time following land use change and ultimately lower N2O emissions 36
than had been occurring under native vegetation, in the absence of fertilisation (Figure 2.12) (Meurer et al.
37
2016; van Lent et al. 2015) (medium evidence, high agreement). Conversion of native vegetation to fertilised 38
systems typically leads to increased N2O emissions over time, with the rate of emission often being a 39
function of nitrogen fertilisation rates, but this response can be moderated by soil characteristics and water 40
availability (medium evidence, high agreement) (van Lent et al. 2015; Meurer et al. 2016). Restoration of 41
agroecosystems to natural vegetation, over the period of one to two decades does not lead to recovery of N2O 42
emissions to the levels of the original vegetation (McDaniel et al. 2019). To conclude, findings since AR5 43
increasingly highlight the limits of linear N2O emission factors, particularly from field to regional scales, 44
with emissions rising nonlinearly at high nitrogen application rates (high confidence). Emissions from 45
unfertilised systems often increase and then decline over time with typically lower emissions than was the 46
case under native vegetation (high confidence).
47