248 | Nature | Vol 586 | 8 October 2020
Article
A comprehensive quantification of global
nitrous oxide sources and sinks
Hanqin Tian
1
✉
, Rongting Xu
1
, Josep G. Canadell
2
, Rona L. Thompson
3
, Wilfried Winiwarter
4,5
,
Parvadha Suntharalingam
6
, Eric A. Davidson
7
, Philippe Ciais
8
, Robert B. Jackson
9,10,11
,
Greet Janssens-Maenhout
12,13
, Michael J. Prather
14
, Pierre Regnier
15
, Naiqing Pan
1,16
,
Shufen Pan
1
, Glen P. Peters
17
, Hao Shi
1
, Francesco N. Tubiello
18
, Sönke Zaehle
19
, Feng Zhou
20
,
Almut Arneth
21
, Gianna Battaglia
22
, Sarah Berthet
23
, Laurent Bopp
24
, Alexander F. Bouwman
25,26,27
,
Erik T. Buitenhuis
6,28
, Jinfeng Chang
8,29
, Martyn P. Chipperield
30,31
, Shree R. S. Dangal
32
,
Edward Dlugokencky
33
, James W. Elkins
33
, Bradley D. Eyre
34
, Bojie Fu
16,35
, Bradley Hall
33
,
Akihiko Ito
36
, Fortunat Joos
22
, Paul B. Krummel
37
, Angela Landoli
38,39
, Goulven G. Laruelle
15
,
Ronny Lauerwald
8,15,40
, Wei Li
8,41
, Sebastian Lienert
22
, Taylor Maavara
42
, Michael MacLeod
43
,
Dylan B. Millet
44
, Stefan Olin
45
, Prabir K. Patra
46,47
, Ronald G. Prinn
48
, Peter A. Raymond
42
,
Daniel J. Ruiz
14
, Guido R. van der Werf
49
, Nicolas Vuichard
8
, Junjie Wang
27
, Ray F. Weiss
50
,
Kelley C. Wells
44
, Chris Wilson
30,31
, Jia Yang
51
& Yuanzhi Yao
1
Nitrous oxide (N
2
O), like carbon dioxide, is a long-lived greenhouse gas that accumulates
in the atmosphere. Over the past 150 years, increasing atmospheric N
2
O concentrations
have contributed to stratospheric ozone depletion
1
and climate change
2
, with the
current rate of increase estimated at 2 per cent per decade. Existing national inventories
do not provide a full picture of N
2
O emissions, owing to their omission of natural
sources and limitations in methodology for attributing anthropogenic sources. Here we
present a global N
2
O inventory that incorporates both natural and anthropogenic sources
and accounts for the interaction between nitrogen additions and the biochemical
processes that control N
2
O emissions. We use bottom-up (inventory, statistical
extrapolation of ux measurements, process-based land and ocean modelling) and
top-down (atmospheric inversion) approaches to provide a comprehensive
quantication of global N
2
O sources and sinks resulting from 21 natural and human
sectors between 1980 and 2016. Global N
2
O emissions were 17.0 (minimum–maximum
estimates: 12.2–23.5) teragrams of nitrogen per year (bottom-up) and 16.9 (15.9–17.7)
teragrams of nitrogen per year (top-down) between 2007 and 2016. Global human-induced
emissions, which are dominated by nitrogen additions to croplands, increased by 30%
over the past four decades to 7.3 (4.2–11.4) teragrams of nitrogen per year. This
increase was mainly responsible for the growth in the atmospheric burden. Our
ndings point to growing N
2
O emissions in emerging economies—particularly Brazil,
China and India. Analysis of process-based model estimates reveals an emerging
N
2
O–climate feedback resulting from interactions between nitrogen additions and
climate change. The recent growth in N
2
O emissions exceeds some of the highest
projected emission scenarios
3,4
, underscoring the urgency to mitigate N
2
O emissions.
Nitrous oxide (N
2
O) is a long-lived stratospheric ozone-depleting
substance and greenhouse gas with a current atmospheric lifetime
of 116±9 years
1
. The concentration of atmospheric N
2
O has increased
by more than 20% from 270 parts per billion (ppb) in 1750 to 331 ppb
in 2018 (Extended Data Fig.1), with the fastest growth observed in
the past five decades
5,6
. Two key biochemical processes—nitrification
and denitrification—control N
2
O production in both terrestrial and
aquatic ecosystems and are regulated by multiple environmental and
biological factors including temperature, water and oxygen levels,
acidity, substrate availability
7
(which is linked to nitrogen fertilizer
use and livestock manure management) and recycling
8–10
. In the com-
ing decades, N
2
O emissions are expected to continue to increase as a
result of the growing demand for food, feed, fibre and energy, and an
increase in sources from waste generation and industrial processes
4,11,12
.
Since 1990, anthropogenic N
2
O emissions have been reported annually
by Annex I Parties to the United Nations Framework Convention on
Climate Change (UNFCCC). More recently, over 190 national signatories
to the Paris Agreement have been required to report biannually their
https://doi.org/10.1038/s41586-020-2780-0
Received: 28 December 2019
Accepted: 14 August 2020
Published online: 7 October 2020
Check for updates
A list of afiliations appears at the end of the paper.
Nature | Vol 586 | 8 October 2020 | 249
national greenhouse-gas inventory with sufficient detail and transpar-
ency to track progress towards their nationally determined contribu-
tions. However, these inventories do not provide a full picture of N
2
O
emissions owing to their omission of natural sources, the limitations
in methodology for attributing anthropogenic sources, and missing
data for a number of key regions (for example, South America and
Africa)
2,9,13
. Moreover, a complete account of all human activities that
accelerate the global nitrogen cycle and that interact with the bio-
chemical processes controlling the fluxes of N
2
O in both terrestrial and
aquatic ecosystems is required
2,8
. Here we present a comprehensive,
consistent analysis and synthesis of the global N
2
O budget across all
sectors, including natural and anthropogenic sources and sinks, using
both bottom-up and top-down methods and their cross-constraints.
Our assessment enhances understanding of the global nitrogen cycle
and will inform policy development for N
2
O mitigation, which could
help to curb warming to levels consistent with the long-term goal of
the Paris Agreement.
A reconciling framework (described in Extended Data Fig.2) was
used to take full advantage of bottom-up and top-down approaches
for estimating and constraining sources and sinks of N
2
O. Bottom-up
approaches include emission inventories, spatial extrapolation of field
flux measurements, nutrient budget modelling and process-based
modelling for land and ocean fluxes. The top-down approaches com-
bine measurements of N
2
O mole fractions with atmospheric transport
models in statistical optimization frameworks (inversions) to constrain
the sources. Here we constructed a total of 43 flux estimates, includ-
ing 30 using bottom-up approaches, 5 using top-down approaches,
and 8 other estimates using observation and modelling approaches
(Methods, Extended Data Fig.2).
With this extensive data and bottom-up/top-down framework, we
established comprehensive global and regional N
2
O budgets that
include 18 sources and various different chemical sinks. These sources
and sinks are further grouped into six categories (Fig.1, Table1): (1) natu-
ral sources (no anthropogenic effects) including a very small biogenic
surface sink; (2) perturbed fluxes from ecosystems induced by changes
in climate, carbon dioxide (CO
2
) and land cover; (3) direct emissions
from nitrogen additions in the agricultural sector (agriculture); (4)
other direct anthropogenic sources—including fossil fuel and industry,
waste and waste water, and biomass burning; (5) indirect emissions
from ecosystems that are either downwind or downstream from the
initial release of reactive nitrogen into the environment—including N
2
O
release after transport and deposition of anthropogenic nitrogen via
the atmosphere or water bodies as defined by the Intergovernmental
Panel on Climate Change (IPCC)
14
; and (6) the atmospheric chemical
sink, for which one value is derived from observations and the other
is derived from the inversion models. To quantify and attribute the
regional N
2
O budget, we further partition the Earth’s ice-free land into
ten regions (Fig.2, Supplementary Fig.1). With the construction of these
budgets, we explore the relative temporal and spatial importance of
multiple sources and sinks that drive the atmospheric burden of N
2
O,
their uncertainties, and interactions between anthropogenic forcing
and natural fluxes of N
2
O as an emerging climate feedback.
The global N
2
O budget (2007–2016)
The bottom-up and top-down approaches give consistent estimates
of global total N
2
O emissions in the decade between 2007 and 2016
to well within their respective uncertainties, with values of 17.0 (mini-
mum–maximum estimates: 12.2–23.5) Tg N yr
−1
and 16.9 (15.9–17.7) Tg N
yr
−1
for bottom-up and top-down approaches, respectively. The global
calculated atmospheric chemical sink (that is, N
2
O losses via photolysis
and reaction with electronically excited atomic oxygen (O(
1
D)) in the
troposphere and stratosphere) is 13.5 (12.4–14.6) Tg N yr
−1
. The imbal-
ance of sources and sinks of N
2
O derived from the averaged bottom-up
and top-down estimates is 4.1 Tg N yr
−1
. This imbalance agrees well
with the observed increase in atmospheric abundance of N
2
O between
2007 and 2016 of 3.8–4.8 Tg N yr
−1
(seeMethods). Natural sources from
soils and oceans contributed 57% of total emissions (mean: 9.7; min–
max: 8.0–12.0 Tg N yr
−1
) during this time, according to our bottom-up
estimate. We further estimate the natural soil flux at 5.6 (4.9–6.5) Tg N
yr
−1
and the ocean flux at 3.4 (2.5–4.3) Tg N yr
−1
(seeMethods).
Anthropogenic sources contributed, on average, 43% to the total N
2
O
emission (mean: 7.3; min–max: 4.2–11.4 Tg N yr
−1
), of which direct and
indirect emissions from nitrogen additions in agriculture and other
sectors contributed around 52% and around 18%, respectively. Of the
remaining anthropogenic emissions, about 27% were from other direct
anthropogenic sources including fossil fuel and industry (around 13%),
with about 3% from perturbed fluxes caused by changes in climate,
CO
2
or land cover.
Four decades of the global N
2
O budget
The atmospheric N
2
O burden increased from 1,462 Tg N in the 1980s to
1,555 Tg N in 2007–2016, with a possible uncertaintyof ±20 Tg N. Our
results (Table1) show a substantial increase in global N
2
O emissions
that is primarily driven by anthropogenic sources, as natural sources
remained relatively steady throughout the study period. Global N
2
O
emissions obtained from our bottom-up and top-down approaches
are comparable in magnitude during 1998–2016, but top-down results
imply a larger inter-annual variability (1.0 Tg N yr
−1
; Extended Data
Fig.3a). Bottom-up and top-down approaches diverge when estimat-
ing the magnitude of land emissions compared with ocean emissions,
although they are consistent with respect to trends. Specifically, the
bottom-up land estimate during 1998–2016 was on average 1.8 Tg N yr
−1
higher than the top-down estimate, but showed a slightly slower rate
of increase of 0.8±0.2 Tg N yr
−1
per decade (95% confidence interval;
P<0.05) compared with 1.1±0.6 Tg N yr
−1
per decade (P<0.05) from
the top-down approach (Extended Data Fig.3b). Since 2005, the differ-
ence in the magnitude of emissions between the two approaches has
become smaller owing to a large increase in emission—particularly in
South America, Africa and East Asia—that is inferred by the top-down
approach (Extended Data Fig.3d, f, i). Oceanic N
2
O emissions from the
bottom-up approach (3.6 (2.7–4.5) Tg N yr
−1
) indicate a slight decline
at a rate of 0.06 Tg N yr
−1
per decade (P<0.05), whereas the top-down
approach gives a higher but stable value of 5.1 (3.4–7.1) Tg N yr
−1
during
1998–2016 (Table1).
On the basis of bottom-up approaches, anthropogenic N
2
O emissions
increased from 5.6 (3.6–8.7) Tg N yr
−1
in the 1980s to 7.3 (4.2–11.4) Tg N
yr
−1
in 2007–2016, at a rate of 0.6±0.2 Tg N yr
−1
per decade (P<0.05).
Up to 87% of this increase results from direct emission from agriculture
(71%) and indirect emission from anthropogenic nitrogen additions
into soils (16%). Direct soil emission from fertilizer application is the
major source of increases in emission from agriculture, followed by
a small but notable increase in emissions from livestock manure and
aquaculture. Model-based estimates of direct soil emissions
15–17
show a
faster increase than in the three inventories used in our study (seeMeth-
ods; Extended Data Fig.4a); this is largely attributed to the interac-
tive effects between climate change and nitrogen additions, as well as
spatio-temporal variability in environmental factors such as rainfall
and temperature, that modulate the N
2
O yield from nitrification and
denitrification. This result is in line with the increased emission factor
deduced from the top-down estimates, in which the inversion-based
soil emissions increased at a faster rate than suggested by the IPCC
Tier 1 emission factor
14
(which assumes a linear response), especially
after 2009 (ref.
18
). The remaining causes of the increase are attributed
to other direct anthropogenic sources (6%) and perturbed fluxes from
changes in climate, CO
2
or land cover (8%). The contribution from fossil
fuel and industry emissions decreased rapidly between 1980 and 2000,
largely due to the installation of emissions-abatement equipment in
industrial facilities that produce nitric and adipic acid. However, after
250 | Nature | Vol 586 | 8 October 2020
Article
2000, such emissions began to increase slowly, owing to increasing
fossil fuel combustion (Extended Data Fig.5a, b).
Our analysis of process-based model estimates indicates that soil N
2
O
emissions have accelerated substantially as a result of climate change
since the early 1980s, and this has offset the reduction due to feedback
withincreased CO
2
concentrationand climate (Extended Data Fig.6a).
Increased CO
2
concentrations enhance plant growth and thus increase
nitrogen uptake, which in turn decreases soil N
2
O emissions
16,19
. Con-
version of land from tropical mature forests, which have higher N
2
O
emissions, to pastures and other unfertilized agricultural lands has
considerably reduced global natural N
2
O emissions
11,20,21
. This decrease,
however, has been partly offset by an increase in soil N
2
O emissions
attributed to the temporary increase in emissions after deforestation
(the post-deforestation pulse effect) and to background emissions from
converted croplands or pastures
21
(seeMethods; Extended Data Fig.7).
From the ensemble of process-based land model emissions
15,16
, we esti-
mate a global agricultural soil emission factor of 1.8% (1.3%–2.3%), which
is considerably larger than the IPCC Tier 1 default for direct emission of
1%. This higher emission factor, derived from process-based models,
suggests a strong interactive effect between nitrogen additions and other
global environmental changes (Table1, ‘Perturbed fluxes from climate,
atmospheric CO
2
and land cover change’). Previous field experiments
reported a better fit to local observations of soil N
2
O emissions when
assuming a nonlinear response to fertilizer nitrogen inputs under varied
climate and soil conditions
17,22
. The nonlinear response is also likely to
be associated with long-term nitrogen accumulation in agricultural
soils from nitrogen fertilizer use and in aquatic systems from nitrogen
loads (the legacy effect)
18,23
, which provides more substrate for microbial
processes
18,24
. The increasing N
2
O emissions estimated by process-based
models
16
also suggest that recent climate change—particularly warm-
ing—could have boosted soil nitrification and denitrification processes,
contributing to the growing trend in N
2
O emissions together with increas-
ing nitrogen additions to agricultural soils
16,25–27
(Extended Data Fig.8).
Regional N
2
O budgets (2007–2016)
Bottom-up approaches give estimates of N
2
O emissions in each of the
five source categories, whereas top-down approaches provide only
total emissions (Fig.2). Bottom-up and top-down approaches indicate
that Africa was the largest source of N
2
O in the last decade, followed
by South America (Fig.2). Bottom-up and top-down approaches agree
well regarding the magnitudes and trends of N
2
O emissions from South
Asia and Oceania (Extended Data Fig.3j, l). For the remaining regions,
bottom-up and top-down estimates are comparable in terms of trends
but diverge when estimating the strengths of the sources. Clearly,
much more work on regional N
2
O budgets is needed, particularly for
South America and Africa where there are larger differences between
bottom-up and top-down estimates and larger uncertainties in each
Atmospheric chemical sink
13.5
(12.4–14.6)
Change in atmospheric
abundance
7.3
(4.2–11.4)
Anthropogenic sources
9.7
(8.0–12.0)
Natural sources
4.3
(3.8–4.8)
Natural soils
5.6
(4.9–6.5)
Surface sink
0.01
(0.0–0.3)
Oceans
3.4
(2.5–4.3)
Lightning and
atmospheric production
0.4
(0.2–1.2)
Inland and coastal waters
Anthropogenic
0.5
(0.2–0.7)
Natural
0.3
(0.3–0.4)
Waste and
waste water
0.3
(0.2–0.5)
Fossil fuel
and industry
1.0
(0.8–1.1)
Agriculture
3.8
(2.5–5.8)
Atmospheric N
deposition on ocean
0.1
(0.1–0.2)
Atmospheric N
deposition on land
0.8
(0.4–1.4)
Deforestation
reduction
1.1
(1.0–1.1)
Post deforestation
pulse effect
0.8
(0.7–0.8)
Climate and
CO
2
effects
0.5
(–0.3–1.4)
Biomass
burning
0.6
(0.5–0.8)
Natural emissions
Anthropogenic emissions
Fig. 1 | Global N
2
O budget for 2007–2016. The coloured arrows represent N
2
O
fluxes (in Tg N yr
−1
for 2007–2016) as follows: red, direct emissions from
nitrogen additions in the agricultural sector (agriculture); orange, emissions
from other direct anthropogenic sources; maroon, indirect emissions from
anthropogenic nitrogen additions; brown, perturbed fluxes from changes in
climate, CO
2
or land cover; green, emissions from natural sources. The
anthropogenic and natural N
2
O sources are derived from bottom-up estimates.
The blue arrows represent the surface sink and the observed atmospheric
chemical sink, of which about 1% occurs in the troposphere. The total budget
(sources + sinks) does not exactly match the observed atmospheric
accumulation, because each of the terms has been derived independently and
we do not force top-down agreement by rescaling the terms. This imbalance
readily falls within the overall uncertainty in closing the N
2
O budget, as
reflected in each of the terms. The N
2
O sources and sinks are given in Tg N yr
−1
.
Copyright the Global Carbon Project.
Nature | Vol 586 | 8 October 2020 | 251
approach. Advancing the understanding and model representation
of key processes responsible for N
2
O emissions from land and ocean
are priorities to reduce uncertainties in bottom-up estimates. Atmos-
pheric observations in underrepresented regions of the world and
better atmospheric transport models are essential to reduce uncer-
tainty in top-down estimates, whereas more accurate activity data and
robust emission factors are critical for greenhouse-gas inventories (see
Methods for additional discussion on uncertainty).
According to estimates from the Global N
2
O Model Intercompari-
son Project
16
, natural soil emissions dominate (to different extents) in
tropical and sub-tropical regions. Soil N
2
O emissions in the tropics
(0.1±0.04 g N m
−2
yr
−1
) are about 50% higher than the global average,
because many lowland, highly weathered tropical soils have excess
nitrogen relative to phosphorus
20
. Total anthropogenic emissions in
the 10 terrestrial regions (Fig.2) were highest in East Asia (1.5 (0.8–2.6)
Tg N yr
−1
), followed by North America, Africa and Europe. High direct
Table 1 | The global N
2
O budget in the 1980s, 1990s, 2000s and 2007–2016
1980s 1990s 2000s 2007–2016
Anthropogenic sources Mean Min. Max. Mean Min. Max. Mean Min. Max. Mean Min. Max.
Direct emissions from
nitrogen additions in
the agricultural sector
(Agriculture)
Direct soil emissions 1.5 0.9 2.6 1.7 1.1 3.1 2.0 1.3 3.4 2.3 1.4 3.8
Manure left on pasture 0.9 0.7 1.0 1.0 0.7 1.1 1.1 0.8 1.2 1.2 0.9 1.3
Manure management 0.3 0.2 0.4 0.3 0.2 0.4 0.3 0.2 0.5 0.3 0.2 0.5
Aquaculture 0.01 0.00 0.03 0.03 0.01 0.1 0.1 0.02 0.2 0.1 0.02 0.2
Subtotal 2.6 1.8 4.1 3.0 2.1 4.8 3.4 2.3 5.2 3.8 2.5 5.8
Other direct
anthropogenic sources
Fossil fuels and
industry
0.9 0.8 1.1 0.9 0.9 1.0 0.9 0.8 1.0 1.0 0.8 1.1
Waste and waste water 0.2 0.1 0.3 0.3 0.2 0.4 0.3 0.2 0.4 0.3 0.2 0.5
Biomass burning 0.7 0.7 0.7 0.7 0.6 0.8 0.6 0.6 0.6 0.6 0.5 0.8
Subtotal 1.8 1.6 2.1 1.9 1.7 2.1 1.8 1.6 2.1 1.9 1.6 2.3
Indirect emissions
from anthropogenic
nitrogen additions
Inland waters,
estuaries, coastal
zones
0.4 0.2 0.5 0.4 0.2 0.5 0.4 0.2 0.6 0.5 0.2 0.7
Atmospheric nitrogen
deposition on land
0.6 0.3 1.2 0.7 0.4 1.4 0.7 0.4 1.3 0.8 0.4 1.4
Atmospheric nitrogen
deposition on ocean
0.1 0.1 0.2 0.1 0.1 0.2 0.1 0.1 0.2 0.1 0.1 0.2
Subtotal 1.1 0.6 1.9 1.2 0.7 2.1 1.2 0.6 2.1 1.3 0.7 2.2
Perturbed luxes from
climate/CO
2
/land
cover change
CO
2
effect −0.2 −0.3 0.0 −0.2 −0.4 0.0 −0.3 −0.5 0.1 −0.3 −0.6 0.1
Climate effect 0.4 0.0 0.8 0.5 0.1 0.9 0.7 0.3 1.2 0.8 0.3 1.3
Post-deforestation
pulse effect
0.7 0.6 0.8 0.7 0.6 0.8 0.7 0.7 0.8 0.8 0.7 0.8
Long-term effect of
reduced mature forest
area
−0.8 −0.8 −0.9 −0.9 −0.8 −1.0 −1.0 −0.9 −1.1 −1.1 −1.0 −1.1
Subtotal 0.1 −0.4 0.7 0.1 −0.5 0.7 0.2 −0.4 0.9 0.2 −0.6 1.1
Anthropogenic total 5.6 3.6 8.7 6.2 3.9 9.7 6.7 4.1 10.3 7.3 4.2 11.4
Natural fluxes
Natural soils baseline 5.6 4.9 6.6 5.6 4.9 6.5 5.6 5.0 6.5 5.6 4.9 6.5
Ocean baseline 3.6 3.0 4.4 3.5 2.8 4.4 3.5 2.7 4.3 3.4 2.5 4.3
Natural (inland waters, estuaries, coastal zones) 0.3 0.3 0.4 0.3 0.3 0.4 0.3 0.3 0.4 0.3 0.3 0.4
Lightning and atmospheric production 0.4 0.2 1.2 0.4 0.2 1.2 0.4 0.2 1.2 0.4 0.2 1.2
Surface sink −0.01 0.00 −0.3 −0.01 0.00 −0.3 −0.01 0.00 −0.3 −0.01 0.00 −0.3
Natural total 9.9 8.5 12.2 9.8 8.3 12.1 9.8 8.2 12.0 9.7 8.0 12.0
Bottom-up total source 15.5 12.1 20.9 15.9 12.2 21.7 16.4 12.3 22.4 17.0 12.2 23.5
Top-down ocean 5.1 3.1 7.2 5.1 3.4 7.1
Top-down land 10.8 9.3 12.5 11.8 10.6 13.8
Top-down total source 15.9 15.1 16.9 16.9 15.9
17.7
Top-down stratospheric sink 12.1 11.4 13.1 12.4 11.7 13.3
Observed atmospheric chemical sink
a
13.3 12.2 14.4 13.5 12.4 14.6
Change in atmospheric abundance
b
3.7 3.2 4.2 4.3 3.8 4.8
Atmospheric burden 1,462 1,442 1,482 1,493 1,472 1,514 1,531 1,510 1,552 1,555 1,533 1,577
Bottom-up estimates include four categories of anthropogenic source and one category for natural sources and sinks. The sources and sinks of N
2
O are given in Tg N yr
−1
. The atmospheric
burden is given in Tg N.
a
Calculated from satellite observations with a photolysis model (about 1% of this sink occurs in the troposphere).
b
Calculated from the combined NOAA and AGAGE record of surface N
2
O, and adopting the uncertainty of the IPCC Assessment Report 5 (Chapter 6)
2
. Detailed information on calculating each
sub-category is shown in Supplementary Tables1–13.
252 | Nature | Vol 586 | 8 October 2020
Article
agricultural N
2
O emissions can be attributed to the large-scale applica-
tion of synthetic nitrogen fertilizers in East Asia, Europe, South Asia and
North America, which together consume over 80% of the world’s syn-
thetic nitrogen fertilizers
28
. By contrast, direct agricultural emissions
from Africa and South America mainly arise from livestock manure
that is deposited in pastures and rangelands
28,29
. East Asia contributed
71%–79% of global aquaculture N
2
O emissions; South Asia and South-
east Asia together contributed 10%–20% (refs.
30,31
). Indirect emissions
have a moderate role in the total N
2
O budget, with the highest emission
in East Asia (0.3 (0.1–0.5) Tg N yr
−1
). Other direct anthropogenic sources
together contribute N
2
O emissions of approximately 0.2–0.4 Tg N yr
−1
in each of East Asia, Africa, North America and Europe.
Both bottom-up and top-down estimates of ocean N
2
O emissions
for northern, tropical and southern ocean regions (90° N–30° N, 30°
N–30° S and 30° S–90° S, respectively) reveal that the tropical oceans
contribute over 50% to the global oceanic N
2
O source. In particular,
the upwelling regions of the equatorial Pacific, Indian and tropical
Atlantic (Fig.3) provide considerable sources of N
2
O
32–34
. Bottom-up
estimates suggest that the southern ocean region is the second largest
contributor, with emissions around twice as high as those from the
northern oceans (53% tropical oceans, 31% southern oceans, 17% north-
ern oceans), in line with their respective areas. Top-down estimates,
however, suggest approximately equal contributions from the southern
and northern ocean regions.
Four decades of anthropogenic N
2
O emissions
Trends in anthropogenic emissions were found to vary among regions
(Fig.3). Fluxes from Europe and Russia decreased by a total of 0.6
(0.5–0.7) Tg N yr
−1
over the 37 years from 1980 to 2016. The decrease in
Europe is associated with successful emissions abatement in industry
as well as agricultural policies, whereas the decrease in Russia is asso-
ciated with the collapse of the agricultural cooperative system after
1990. By contrast, fluxes from the remaining eight regions increased
by a total of 2.9 (2.4–3.4) Tg N yr
−1
(Fig.3), of which 34% came from East
Asia, 18% from Africa, 18% from South Asia, 13% from South America
and 6% from North America, with the remaining increase attributed
to the three other regions.
0.3 1.5 2.02.5 3.03.5 4.01.00.50
Global ocean
N
2
O emissions (Tg N yr
–1
)
N
2
O emissions (kg N ha
–1
yr
–1
)
2007–2016
Natural uxes without
ocean
Agriculture
Other direct anthropogenic
sources
Indirect emissions from
anthropogenic N additions
Perturbed uxes from
climate/CO
2
/land cove
r
change
Bottom-up Top-down
–0.5
0.0
0.5
1.0
1.5
2.0
2.5
Africa
–0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
East Asia
–2
0
2
4
6
8
10
1.7 1.3
2.5 2.1 2.6 3.2 1.1 1.1
1.1 0.8 0.4 0.5 0.6 0.2
0.5 0.4
0.8 0.7
1.7 1.5
Global land
–0.2
0.0
0.2
0.4
0.6
0.8
Europe
–0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
North America
–0.2
0.0
0.2
0.4
0.6
0.8
Middle East
BU TD
0
2
4
6
8
–0.2
0.0
0.2
0.4
0.6
0.8
Oceania
–0.5
0.0
0.5
1.0
1.5
2.0
2.5
South America
–0.2
0.0
0.2
0.4
0.6
0.8
Russia
–0.2
0.0
0.2
0.4
0.6
0.8
Southeast Asia
–0.2
0.0
0.2
0.4
0.6
0.8
South Asia
13.5 11.8
Fig. 2 | Regional N
2
O sources in the decade 2007–2016. The Earth’s ice-free
land is partitioned into ten regions: North America, South America, Europe,
Middle East, Africa, Russia, East Asia, South Asia, Southeast Asia and Oceania.
Each subplot shows the emissions from five sub-sectors using bottom-up
approaches, followed by the sum of these five categories using bottom-up
approaches (blue) and the estimates from top-down approaches (yellow).
Bottom-up and top-down estimates of ocean emissions are shown at the
bottom left (from bottom to top, lighter to darker, the contributions from the
30°–90° N, 30° S–30° N and 90°–30° S regions). Error bars indicate the spread
between the minimum and the maximum values. The centre map shows the
spatial distribution of 10-year average N
2
O emissions from land and ocean
based on the land and ocean models. Per capita N
2
O emission (kg N per capita
per year) during 2007–2016 is shown in Supplementary Fig.2. The map was
created using ESRI ArcMap 10.4.1.
