Introduction
Livestock production has increased substantially in response to growing meat
consumption across the globe in the past century (Bouwman et al., 2013;
Dangal et al., 2017). Agriculture occupies 37 % of earth's ice-free land
surface for use as cropland and “permanent meadows and pastures”
(Tubiello, 2018). Land used by livestock for permanent meadows and pastures is the
largest component, using 25 % of the total land earth surface (FAOSTAT,
2018) to generate 33 %–50 % of world total agricultural GDP (Herrero
et al., 2013). While livestock is a major source of income for more than 1.3
billion people, it is also a major user of crop and freshwater resources
(Dangal et al., 2017; Herrero et al., 2013). Overall, livestock production
plays a major role as a driver of global change in land use and nutrient
cycles (Havlík et al., 2014; Herrero et al., 2013; Zhang et al., 2017).
There is a growing recognition that livestock production is linked to
increasing global greenhouse gas (GHG) and ammonia emissions (Tian et al.,
2016; Tubiello et al., 2018; Xu et al., 2018, 2019).
Unsustainable practices, especially in intensive systems, may lead to severe
pollution of aquatic systems and soil degradation locally, regional and
globally, in particular through nitrate leaching to water bodies (Dangal et
al., 2017; Davis et al., 2015; Fowler et al., 2013; Yang et al., 2016).
Growing global demand for livestock products has increased grain production
for feed in many regions, and has become a global driver of fertilizers
trends, through an increase in manure availability and synthetic fertilizer N
use (FAOSTAT, 2018).
Livestock production systems therefore play an important role in global
nutrient cycles. For example, nitrogen excretion from livestock increased
from 21 Tg N yr-1 in 1860 to 123 Tg N yr-1 in 2016 (FAOSTAT, 2018;
Zhang et al., 2017). Livestock contribute roughly two-thirds of non-CO2
GHG emissions from agriculture (Smith et al., 2014), with roughly an equal
share of CH4 and N2O emissions (Dangal et al., 2017; Tubiello et
al., 2013). Importantly, about 45 % of total anthropogenic N2O
emissions are linked to manure deposited through grazing and manure applied
to croplands or left on pasture (Davidson, 2009; FAOSTAT, 2018). Globally,
emissions from manure N applied to soils or left on pastures increased
from 0.44 to 0.88 Gt CO2eq. yr-1 during 1961–2010 (FAOSTAT, 2018).
Increased meat and dairy product consumption worldwide was a major driver
behind the documented increase in cattle herds globally (FAOSTAT, 2018), and
thus a major cause in the observed atmospheric increase of N2O and
CH4 over the past several decades (Bai et al., 2018; Bouwman et al.,
2013; Dangal et al., 2017; Tubiello, 2018).
While the availability of national-level statistics is a fundamental
component of our knowledge base, environmental problems related to nitrogen
pollution or emissions are best tackled at the local scale and often require
finer, geospatial information, for example to assess proximity to water
bodies and thus pollution risks. In particular, a number of studies have
focused on downscaling existing national information to develop geospatially
explicit regional and global datasets of nitrogen fertilizer and livestock
manure production and use, to better understand their feedback on the
climate system. Several datasets of N fertilizer use were used in this
study, in particular the FAOSTAT annual, country-specific statistics on
mineral and chemical fertilizers and livestock manure over the period
1961–2016 (FAOSTAT, 2018), as well as specific geospatially downscaled
products (e.g., Bouwman et al., 2005; Lu and Tian, 2017; Mueller et al.,
2012; Nishina et al., 2017; Potter et al., 2010; Sheldrick et al., 2002).
Further, global manure production datasets were developed in different
studies to achieve various research goals (Bouwman et al., 2009, 2013;
Holland et al., 2005; Potter et al., 2010; Zhang et al., 2017).
Although datasets of manure application in croplands are increasingly
becoming available, there is considerable uncertainty in the estimation of total
manure application and their spatial distribution across different studies
(Gerber et al., 2016; Herrero et al., 2013; Liu et al., 2010; Zhang et al., 2017).
Although previous studies have provided spatially explicit datasets of N
inputs in the form of mineral or chemical and manure N in cropland systems,
the spatially explicit datasets on N inputs in grassland systems are still
missing (Chang et al., 2016; Lassaletta et al., 2014; Stehfest and Bouwman,
2006). By grassland systems we mean the FAO livestock land use definition,
i.e., land used as permanent meadows and pastures (FAOSTAT, 2018). The same
may also be referred to in the literature as “pastures and rangelands”. We
note that “grassland” is in fact a land cover definition. In order to avoid
the confusion often made in the literature between land cover and land use
terminology, we will adopt FAO land use terminology of permanent meadows
and pastures, to which the various national regional and global land use
statistics cited in this work refer. Furthermore, using results from the
HYDE 3.2 dataset (Klein Goldewijk et al., 2017), we may split the FAO land use
category into “pastures” and “rangelands”, to highlight differences between
managed intensive and unmanaged extensive systems, as needed. To enhance our
understanding of the role of livestock in the global GHG balance and
nutrient budgets (e.g., ammonia emissions, nitrate leaching), global
biogeochemistry models require spatially explicit estimates of N inputs. In
this study, we developed datasets for major sources of N inputs in
agriculture (i.e., manure and fertilizer application and manure deposition
on permanent meadows and pastures), using the recently published FAOSTAT
statistics on manure N use in agriculture (FAOSTAT, 2018). The latter are
estimates based on IPCC Tier 1 methodology; i.e., they rely on default
coefficients prescribing, among other variables, N excretion rates by animal
type and region, as well as regional compositions of manure management
systems (FAOSTAT, 2018).
Through combining the land use dataset HYDE 3.2, FAOSTAT fertilizer N
statistics and gridded manure production data in Zhang et al. (2017), we
developed three annual global datasets at a spatial resolution of
0.5∘ × 0.5∘, as follows: (1) manure N
application rates to pastures (1860–2016); (2) synthetic N fertilizer
application rates to pastures (1961–2016); and (3) manure deposition rates
by grazing livestock to rangelands and pastures (1860–2016). We
quantified regional variations in N inputs, identified hotspots of N inputs
from different N sources from livestock and discussed their uncertainty.
These datasets are developed for global model simulation studies in model
intercomparison projects (e.g., NMIP; Tian et al., 2018, 2019), and
will be updated annually based on regular annual updates of FAO
fertilizers and land use statistics and other sources of data such as global
land use data products.
Methods
Land use categories
The concepts of grassland, pastures and meadows span several international
land cover and land use statistical definitions, specifically those used by
FAO (FAOSTAT, 2018). In this paper, we follow the relevant FAO land use
definition of permanent meadows and pastures, considering our focus on
livestock production. Importantly, complete country, regional and global
statistics available from FAO refer to this land use category. This land use
definition is roughly equivalent to the one adopted by the academic
community engaged in global biogeochemical modeling, for which “grassland
systems” are thought of as land cover/land use areas dominated by herbaceous
and shrub vegetation, including savannas (Africa, South America and India),
steppes (Eurasia), prairies (North America), shrub-dominated areas (Africa),
meadows and pastures (United Kingdom and Ireland) and tundra (Breymeyer,
1990; White et al., 2000).
For mineral and chemical fertilizers, we further split the FAO definition
using HYDE 3.2, into pastures and rangelands, the former representing
land use areas managed to support high stocking densities of grass
production for hay and silage, whereas the latter represents unmanaged and
grazed at low stocking densities. Although FAOSTAT land use statics cover in
principle these two subcategories of land use, data coverage needed is
insufficient for the consistent global mapping needed herein. The spatial
distribution map of pastures and rangelands provided by HYDE are nonetheless
based on and normalized to FAOSTAT land use statistics, complemented by
additional information (Klein Goldewijk et al., 2017). To investigate N inputs from
livestock at a regional level, the global landmass was disaggregated into
seven regions: North America, South America, Africa, Europe, southern Asia
(i.e., west, south, east, central and southeast Asia), northern Asia and
Oceania (Fig. S1 in the Supplement).
Global synthetic fertilizer N application on pastures
We obtained national-level datasets of “Agricultural use of mineral or
chemical fertilizers” from the FAOSTAT (2018) “Fertilizers by Nutrient”
domain, over the time series 1961–2016. The FAOSTAT statistics of
agricultural use include use for both agriculture and forestry, as well as
use in aquaculture. Furthermore, agricultural use includes both cropland and
permanent meadows and pastures. We assumed that the use of fertilizers for forestry
and aquaculture was zero, as well as fertilizer applications on rangelands.
Subsequently, we estimated N application rates to pastures by using the
ratio of pasture to cropland N use total published by Lassaletta et al. (2014).
We finally spatialized the pasture N data using HYDE 3.2, obtaining
gridded maps of synthetic fertilizer N application rates on pastures in
each grid cell area, over the period 1961–2016 (Fig. 1). We assumed even
application rates within each country. Although gridded livestock density
maps were available from FAO, these are currently fixed for specific time
periods, mainly 2010, so that we deemed their use not particularly relevant
to improve estimates for the 1961–2016 time series considered herein.
Improved live density map products from FAO will considerably improve our
work and reduce uncertainty, and will be used when available.
Diagram of the workflow for developing the database of global
annual N fertilizer use rate in pasture during the period 1961–2016.
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Global manure N application to pastures
We obtained country-level datasets of “manure applied to soils” from the
FAOSTAT (2018) “Livestock Manure” domain for the period 1961–2016 (FAO,
2018). Following IPCC guidelines, the data in this domain do not consider N
leaching during treatment (FAOSTAT, 2018). Furthermore, the FAOSTAT data do
not separate manure application to cropland and pastures, and data of manure
N application rates to pastures are currently not available. We therefore
assumed that manure N application rates in pastures and croplands were the
same, considering that the overall uncertainty in the input manure N data
would not justify further assumptions at this stage of knowledge. Improved
FAO statistics on both use and application rates will be used when available
to improve this current work. Through combining land use data HYDE 3.2, we
calculated the total cropland and pasture areas within each country where
manure application amount was larger than zero. We then computed mean manure
N application rates on pastures, annually over the period 1961–2016 (Fig. 2).
Diagram of the workflow for developing the database of global
annual manure N use rate in pastures and manure N deposition rate in
pastures and rangelands during the period 1860–2016.
![]()
We calculated the national-level ratio of manure application to production
(Ra2py,j) by combining gridded manure production data in Zhang
et al. (2017) and the grid cell area. To spatialize the national-level
manure N application amounts to gridded maps of application rates in each
grid area, we multiplied the Ra2py,j in grids where pasture
areas were larger than zero with the time-series gridded spatial
distribution maps of manure production rate in Zhang et al. (2017) during
1961–2014 and based on the spatial distributions of global pastures in
land use data HYDE 3.2 (Klein Goldewijk et al., 2017).
The above-mentioned processes are represented by following equations:
Ra2py,j=TMappy,j∑g=1g=nincountryj(RMprody,g×Ag),
where year is from 1961 to 2016, and country number is 165. Ra2py,j
is the ratio (unitless) of manure application to production in the year
y and country j. TMappy,j is the national total manure application
amount (kg N yr-1) derived from the FAO database for each year. Ag
is the area of each grid (km2).
RMappy,g=Ra2py,j×RMprody,g,
where RMappy,g is the gridded manure application rate
(kg N km-2 yr-1) in year y and country j.
As the national-level manure application amount was not available during
1860–1960, we assumed that Ra2py,j is the same as for 1961.
Combining with the gridded spatial maps of manure production rates in Zhang
et al. (2017), we generated the datasets of spatialized manure application
rates to global pastures during 1860–1960.
Finally, we calculated manure application amounts in each country by
combining RMappy,g and grid areas to compare with
national-level deposition amounts from the FAOSTAT database during
1961–2016. As we calculated national-level manure application amounts
during 1860–1960 using Ra2py,j in 1961, these data served as
national total manure N application amounts to adjust RMappy,g
during 1860–1960.
The adjustment procedure is represented in the following equations:
CTMappy,j=∑g=1g=nincountryj(RMappy,g×Ag),
where year is from 1860–2016. CTMappy,j (kg N yr-1) is
the calculated national-level manure application amounts in the year y and
country j. If CTMappy,j is less or more than
TMappy,j, an adjustment is needed to keep calculated national total amounts
consistent with amounts from the FAOSTAT database. In this case,
CTMappy,j is less than TMappy,j using Eq. (3), thus
an adjustment is needed, using the following equations:
Ray,j=TMappy,jCTMappy,j,
where Ray,j is the regulation ratio (unitless) in the year y and
country j.
RMappy,g(r)=RMappy,g×Ray,j,
where RMappy,g(r) is real gridded manure application rate
(kg N km-2 yr-1) in the year y and country j.
Global manure N deposition on pastures and rangelands
To develop global distribution maps of manure N deposition by grazing
animals, we first obtained country-level statistics of “manure left on
pasture” over the period 1961–2016 from the FAOSTAT (2018) “Livestock
manure” domain of FAOSTAT agri-environmental indicators (FAO, 2018). We then
obtained the national-level ratio of manure deposition to production
(Rd2py,j) by combining country-level FAOSTAT datasets of
manure left on pasture and gridded total manure production datasets
based on Zhang et al. (2017). Then, we used spatial distributions of global
permanent meadows and pastures, including pastures and rangelands, based on
HYDE 3.2 grassland data (Klein Goldewijk et al., 2017) and gridded maps of
deposition rates, to spatialize the national-level manure N deposition at
the global scale. For example, we multiplied the Rd2py,j ratio
in grids within which the pastures and rangelands area was larger than zero,
with the time-series gridded spatial distribution maps of manure production
rates in Zhang et al. (2017) during 1961–2014 (Fig. 2).
The above-mentioned processes are represented by the following equations:
Rd2py,j=TMdepy,j∑g=1g=nincountryj(RMprody,g×Ag),
where year (y) is from 1961 to 2016 and country number (j) is 157.
Rd2py,j is the ratio (unitless) of manure deposition to
production in the year y and country j. TMdepy,j is national total
manure deposition amount (kg N yr-1) derived from the FAOSTAT database
for each year. RMprody,g is the gridded manure N production
rate (kg N km-2 yr-1) in the year y and grid g.
RMdepy,g=Rd2py,j×RMprody,g,
where RMdepy,g is the gridded manure deposition rate
(kg N km-2 yr-1) in the year y and country j.
Finally, we calculated the manure deposition amount for each country through
combining RMdepy,g and grid area to compare with the
national-level deposition amounts from the FAOSTAT database, using the
following equation:
CTMdepy,j=∑g=1g=nincountryj(RMdepy,g×Ag),
where CTMdepy,j (kg N yr-1) is the calculated
national-level manure deposition amount in the year y and country j. If
CTMdepy,j is less or more than TMdepy,j, an
adjustment was made to keep calculated national total amounts consistent
with those from the FAOSTAT database. In this case, CTMdepy,j is
roughly equal to TMdepy,j using Eq. (8); thus no adjustment was
needed.
Since the national-level manure deposition amounts are not available during
1860–1960, we assumed that Rd2py,j is the same as that in
1961. Combining the gridded spatial maps of manure production rates in Zhang
et al. (2017), we generated datasets of spatialized manure deposition rates
on permanent meadows and pastures globally for the period 1860–1960.
Results
Synthetic fertilizer N application to pastures, 1961–2016
The FAO data, combined with the geospatial analysis in this work, show that
the total amount of synthetic N fertilizer applied to pastures increased
from 0.04 to 8.7 Tg N yr-1 during 1961–2016 at an average rate of
∼0.18 Tg N yr-1 (R2=0.98) (Fig. 3a).
Synthetic N fertilizer application rates showed rapid increases across the
globe, with large spatial variations during the study period (Fig. 4b–c).
The global average application rate on pastures was 0.07 kg N ha-1 yr-1
in 1961 and reached 10.9 kg N ha-1 yr-1 in 2016
(increased ∼154-fold) (Table 1).
Temporal patterns of global manure N use, N fertilizer use and
manure deposition in grassland systems: (a) manure N use and N fertilizer
use on global pastures during 1860–2016 and during 1961–2016,
respectively, and (b) manure N deposition to global pastures and rangelands
during 1860–2016.
![]()
The N input rates, applied/deposited area and total amounts in
global pastures and rangelands in 1860, 1961, 1980, 2000 and 2016 (1 km2=100 ha).
N/A: not available.
1860
1961
1980
2000
2016
Averaged N fertilizer application rate (kg N ha-1 yr-1)
N/A
0.07
3.6
7.8
10.9
Total applied area (Mha)
N/A
623.8
725
797.8
803.1
Total amounts (Tg N yr-1)
N/A
0.04
2.6
6.2
8.7
Average manure N application rate (kg N ha-1 yr-1)
5.3
8.1
9.8
9.5
10.7
Total applied area (Mha)
268.2
623.8
725
797.8
803.1
Total amounts (Tg N yr-1)
1.4
5.0
7.1
7.6
8.6
Average manure N deposition rate (kg N ha-1 yr-1)
11.2
15.4
19.0
20.7
25.3
Total deposited area (Mha)
1250.1
3070.7
3194.2
3398.5
3295
Total amounts (Tg N yr-1)
14.0
47.2
60.7
70.5
83.5
In the 1960s, Europe (0.2 Tg N yr-1) was the largest contributor
(67.8 %) to the total global N fertilizer use, followed by North America
(0.06 Tg N yr-1, 21.8 %) and southern Asia (0.03 Tg N yr-1,
9.9 %) (Fig. 5a). The remaining regions accounted for less than 1 % of
the total N fertilizer application. During 1961–2016, southern Asia showed
a continuous increase of N fertilizer consumption and became the largest
contributor (3.4 Tg N yr-1, 45 %) between 2000 and 2016. In contrast,
Europe's synthetic N fertilizer use and contribution to the global total
has decreased since the 1980s (Fig. 5a). This is a well-known trend, linked to
EU-wide policy directives aimed at minimizing N pollution (Tubiello, 2018).
During 2000–2016, Europe applied 2.1 Tg N yr-1, which accounted for
27 % of the total global N fertilizer use on pastures. There was a slight
increase in the contribution from North America, and the synthetic
fertilizer N use amount increased by 1.6 Tg N yr-1. The remaining
regions accounted for roughly 7 % of the total N fertilizer application on
pastures.
The average synthetic N application rate in Oceania, North America and
southern Asia showed a rapid increase over the period 1961–2016 (Fig. 5d).
Africa and northern Asia showed a slight increase in average N fertilizer
application rates during the study period. Europe exhibited a rapid increase
of N fertilizer application rates from 1961, then these decreased after 2000 and
have started to increase in the last 5 years (Fig. S3).
We identified the top five countries (India, United States, China, France
and Germany) with the highest fertilizer N application to pastures in 2016.
These countries consumed 49 % to 58 % of the total N fertilizer from
1961 to 2016. India (1.5 Tg N yr-1) and the United States
(1.5 Tg N yr-1) were the two largest contributors in 2016, at an increasing rate
of 45 Gg N yr-1 (R2=0.98) during 1980–2016 and
32 Gg N yr-1 (R2=0.99) during 1961–2016,
respectively. China consumed 1.4 Tg N yr-1 in 2016 at an increasing
rate of 34 Gg N yr-1 (R2=0.96) during 1977–2016,
while there was only a slight increase during 1961–1976. In contrast,
fertilizer N use in France peaked in 1999 (0.8 Tg N yr-1), then showed
a rapid decrease until 2016 (0.5 Tg N yr-1). Similarly, in Germany, it
peaked in 1988 (0.8 Tg N yr-1), and showed a continuous decrease until
2016 (0.3 Tg N yr-1).
Manure N application to pastures, 1860–2016
Our results showed that the annual manure N application rates on pastures
increased from 1.4 to 8.6 Tg N yr-1 during 1860–2016 (Fig. 3a).
Manure N application rates showed rapid increases across the globe and
exhibited large spatial variations, shifting the regional use from North
America and Europe to Asia during the study period (Fig. 4d–f). The global
average manure application rate was 5.3 kg N ha-1 yr-1 in the
1860s and roughly doubled by 2016 (10.7 kg N ha-1 yr-1) (Table 1).
Spatial patterns of N input rates in global pastures and
rangelands in 1860, 1961 and 2016: (a, b, c) N fertilizer application
rates, (d, e, f) manure N application rates and (g, h, i) manure N deposition
rates.
![]()
From the regional perspective (Fig. 5b), in the 1860s Europe (0.8 Tg N yr-1)
was the largest contributor and accounted for 53 %, while
southern Asia (0.25 Tg N yr-1) accounted for 17 % of the global total
manure N application on pastures. South and North America shared the same
proportion (13 %), whereas the remaining regions only shared 4 %.
Conversely during 2000–2016, manure N application on pastures in southern
Asia (2.9 Tg N yr-1) was 10-fold higher than that in the 1860s and
accounted for 36 % of the global total, surpassing Europe, which accounted
for 28 % of the global total. Manure N application amounts in North
America and South America increased, but with different magnitudes. During
2000–2016, North America accounted for 11 %, while South America
accounted for 17 % of the global total. In the remaining regions,
significant increases of annual manure N application on pastures also
occurred, but their contributions to the global total changed only slightly
(8 %) compared to the 1860s.
Nitrogen fertilizer use (a) and rate (d), manure N use (b) and
rate (e) and manure N deposition (c) and rate (f) at regional scales in
the 1860s, 1960s, 1980s and 2000–2016. Error bars represent standard
deviation within each decade.
![]()
The regional average manure N application rate was increasing in southern
Asia and Africa during 1860–2016 (Fig. S3b). South America, Oceania and
North America exhibited a rapid decreasing trend of manure N application
rates from the 1860s to the 1960s and showed continuous increases afterward
until 2016 (Figs. 5e, S3b), which was associated with the substantial
expansion of pasture areas (Table S2). Europe exhibited a rapid increase of
manure N application rates from the 1860s, then these decreased after the 1980s
(Figs. 5e).
In 2016, the top five countries with largest manure N applications on
pastures were China, United States, Brazil, Russia and France. Manure N
application in these countries contributed 43 % to 52 % of global total
use from 1961 to 2016. China (2.5 Tg N yr-1) alone accounted for 30 %
in 2016 at an increasing rate of 42 Gg N yr-1 (R2=0.98) during 1961–2016. Manure N use on pastures in Brazil and the United
States was roughly the same (0.7 Tg N yr-1) in 2016. Both countries
showed a slower increasing trend (Brazil 7 Gg N yr-1 and
United States 3 Gg N yr-1) during 1961–2016. In contrast,
Russian manure N application peaked in 1989 (0.7 Tg N yr-1), then
showed a rapid decrease until 2016 (0.3 Tg N yr-1). Similarly, in
France, it peaked in 1979 (0.45 Tg N yr-1), then showed a continuous
decrease until 2016 (0.28 Tg N yr-1).
Manure N deposition on pastures and rangelands, 1860–2016
Our data show that the total amounts of manure N deposited on pastures and
rangelands increased from 14 to 84 Tg N yr-1 during 1860–2016 (Fig. 3b).
Manure N deposition rates increased steeply across the globe, but
exhibited large spatial variations during the study period (Fig. 4g–i). The
increase was much larger in the eastern world (typically China and India)
and South America compared to the western world. The global average manure
deposition rate was 11 kg N ha-1 yr-1 in 1860 and reached 25 kg N ha-1 yr-1
in 2016 (Table 1).
At the regional scale (Fig. 5c), in the 1860s southern Asia was the region
with the largest manure N deposition on pastures and rangelands (4.4 Tg N yr-1;
30 % of total manure N deposition amounts), followed by Africa
(2.8 Tg N yr-1; 19 %) and South America (2.4 Tg N yr-1; 16 %).
Manure N deposition in the remaining regions was estimated to be 5.1 Tg N yr-1,
contributing 35 % to the total manure N deposition amount.
During 2000–2016, southern Asia, Africa and South America were still the
three largest contributors: 27 Tg N yr-1 accounted for 34 %, 20 Tg N yr-1
accounted for 26 % and 15 Tg N yr-1 accounted for 20 %
of the global manure N deposition on pastures and rangelands, respectively.
The remaining regions (Oceania, North America and Europe) contributed to
20 % of the global total during 2000–2016. Europe and Oceania saw an
increase in manure N deposition amounts from 1860 to 1960, but since 1980
there has been a significant decrease, partly explained by the onset of N
pollution regulation. Manure N deposition amounts in North America increased
during 1860–1980, but changed slightly from 1960.
Oceania showed a continuously decreasing trend of average manure N
deposition rates in pastures and rangelands over the period 1860–2016.
Manure N deposition rates in South America decreased between 1860 and 1960
and then increased afterward until 2016 (Fig. S3c). The significant contrast
of changes in manure N deposition rates in Oceania and South America between
the 1860s and the 1960s is due to the substantial and rapid increase of
grassland areas (Tables S2, S3). Africa and southern Asia saw continuous
increases in manure N deposition rates from 1860 to 2016, whereas Europe and
North America were found to have decreasing deposition rates from the 1980s
(Figs. 5f, S3c).
In this study, we identified the top 10 countries (China, Brazil, India,
Ethiopia, United States, Australia, Sudan (former), Pakistan, Argentina
and Nigeria) that together contributed to 48 % of the global total manure
N deposition on pastures and rangelands in 2016. Among these countries,
China (17 %) and Brazil (21 %) were the two largest contributors, with
the similar annual rate of increase of ∼125 Gg N yr-1
(R2=0.99) during 1961–2016. India was the third
largest contributor, however, at a small increasing rate of 63 Gg N yr-1
(R2=0.98) during 1961–2016. Annual manure N
deposition in Ethiopia PDR was stable during 1961–2000, but since then
rapidly increased at a rate of 117 Gg N yr-1 (R2=0.96). The United States showed a significant increase of annual manure N
deposition on pastures and rangelands from 1961 to 1975 and then was stable
after 1980. Australia showed a decreasing trend during 1990–2016 at a rate
of 62 Gg N yr-1 (R2=0.92), whereas in the former
Sudan, Pakistan and Nigeria annual manure N deposition amounts to pastures
and rangelands increased at an annual average rate of 68 (R2=0.8), 46
(R2=0.97) and 56 (R2=0.98) Gg N yr-1, respectively. There was no significant change in manure N deposition
amounts in Argentina; the annual deposition from 1961 to 2016 was 2.6 Tg N yr-1.
Discussion
Overview of global N inputs to pastures and rangelands
The global N cycle has been significantly perturbed by human activity since
at least the industrial revolution. Intense agricultural activities, such as
synthetic N fertilizer production and use, and intensive livestock
production, were identified as major drivers of such change. In this
context, improving estimates of global anthropogenic N inputs to pastures
and rangelands and their consequences, including on N2O emissions, is
important (Galloway et al., 2008; Tian et al., 2016; Xu et al., 2017). In
this study, we generated global datasets of fertilizers N inputs from
livestock, both synthetic and from manure, during the period 1860–2016.
Pastures and rangelands experienced substantial land expansion over the
period of 1860–1998 (Klein Goldewijk et al., 2017). The total amount of mineral
and manure N applied to permanent meadows and pastures increased by 573 %
over the study period, from 15 to 101 Tg N yr-1 from 1860 to 2016.
During 2000–2016, the global mineral N fertilizer application to
agriculture was significant, reaching 110 Tg N yr-1 in 2016, while
manure N production was 123 Tg N yr-1 (FAO, 2018; FAOSTAT, 2018),
resulting in a total input of 233 Tg N yr-1. Our estimate of total N
inputs (synthetic N fertilizer: 7.5 Tg N yr-1; manure N application:
8.2 Tg N yr-1; manure N deposition: 78.1 Tg N yr-1) to permanent
meadows and pastures (93.8 Tg N yr-1) accounted for 45 % of global
total N production (manure 114.2 Tg N yr-1; synthetic N fertilizer
96.4 Tg N yr-1) during 2000–2016.
Extension of FAO information
Our work extends the relevant FAO national-level statistics in order to
provide input drivers for process-based model simulations (e.g.,
NMIP; Tian et al.,
2018, 2019). We furthermore
separated N application rates between pastures and cropland, based on
previous published work. We likewise extended information available in
FAOSTAT by providing spatialized manure N application rates to pastures and a spatialized national-level manure N deposition dataset from 1860 to 2016.
Comparison with other studies
We compared our datasets with other existing data sources (Table 2). Our
estimate of world total manure N use on pastures was 58 % and 171 %
higher than that estimated by Stehfest and Bouwman (2006) and Liu et
al. (2010), respectively. However, our estimate was 39 % and 87 % lower than
estimates by Bouwman et al. (2002, 2013, respectively). Critically,
pasture area data varied significantly across different studies. For
example, Bowman et al. (2013) divided grasslands into mixed and pastoral
systems, and estimated grasslands area based on the country- or
regional-level grazing intensity (Table 2). In addition, synthetic
fertilizers were applied to the area of mixed agricultural systems
(grassland and cropland) and manure N was assumed to be applied to both
mixed and pastoral systems. The HYDE 3.2 land use dataset divides the global
grazing area into intensively managed grasslands (pastures), and less
intensive and unmanaged grasslands (rangelands) (Klein Goldewijk et al.,
2017). In this study, we rather assumed that all manure N was applied to
pastures, the latter estimated from the HYDE database (798 Mha). Hence,
pasture area defined in Bowman et al. (2013) was more than 4-fold higher
than the data we used. Consequently, the spatial distribution and annual
total N application differed substantially compared with that in Bowman et al. (2013).
Comparison of manure and fertilizer N application
amounts between
this study and published datasets. N/A: not available.
Bouwman et
Stehfest and
Bouwman et
Chang et
Liu et
Lassaletta
al. (2002)a
Bouwman (2006)b
al. (2013)c
al. (2016)a
al. (2010)c
et al. (2014)c
This studyc
Manure N application
12.4
4.8
57.8
12.4
∼2.8
N/A
7.6
(Tg N yr-1)
Applied area (Mha)
625
N/A
3358d
1231
N/A
N/A
798
N fertilizer application
4.3
3.1
N/A
3.1
12.9
6.5
6.2
(Tg N yr-1)
Applied area (Mha)
103
N/A
N/A
39
N/A
N/A
798
a Estimated in 1995.
b National-level fertilizer data for 1998. The total grassland
area for N fertilizer and manure was 677 Mha.
c Estimated in 2000.
d The grassland area includes both mixed and patrol systems.
Similarly, the estimates of N fertilizer use in pastures showed large
variations across studies (Table 1). This study obtained country-level N
fertilizer amounts applied to pastures from the national-level ratios
provided by Lassaletta et al. (2014) and total N amounts applied to soils
provided by FAOSTAT. Thus, the global N fertilizer amount in 2000 was
consistent with that in Lassaletta et al. (2014). Liu et al. (2010) assumed
that 16 % of fertilizer was applied to global grasslands. Their estimate
was roughly twice as high as this study (6.2 Tg N yr-1) for the year
2000. The estimates by Bowman et al. (2002) and Stehfest and Bouwman (2006)
were 31 % and 50 %, respectively, lower than our estimates in the
corresponding years. Klein Goldewijk et al. (2017) divided land used for
grazing into more intensively used pastures, less intensively used or
unmanaged rangelands. In this study, we assumed N fertilizer was applied to
all global pastures and therefore the total area of intensively managed
grassland was significantly different from the area used in
Bowmann et al. (2002) and Chang et al. (2016).
Changes in N inputs hotspots
Overall, southern Asia ranks as a top hotspot of all sources of global N
inputs in pastures and rangelands during the past three decades, causing a
major threat to environmental sustainability and human health in this
region. In the 1860s overall manure N production amounts were similar in
Asia and Europe (Zhang et al., 2017). However, manure N deposition was 2.4
times higher than that in Europe, whereas manure N application was roughly
3 times lower than that in Europe. During 2000–2016, southern Asia
accounted for ∼42 % of global manure N production.
Consequently, manure N deposition and application amounts in southern Asia
were the highest compared to the rest of the regions between 2000 and 2016.
These increases are due to large increases in animal numbers (e.g., cattle,
sheep and goats) since 1950 (Bouwman et al., 2013; Dangal et al., 2017). For
the rest of the regions, the increases of livestock numbers were also found
in South America and Africa from 1860, whereas livestock numbers in Europe
and North America showed a decreasing trend after 1980 (Dangal et al.,
2017). Thus, besides southern Asia, South America and Africa were hotspots
for manure N deposition during 1860–2016, while manure N deposition amount
has decreased in Europe and North America since the 1980s.
Shifting hotspots of N fertilizer application
European countries (e.g., Germany, United Kingdom and Ireland) were
identified as top hotspots of global N fertilizer application in 1961
(Fig. 4b). However, these hotspots have shifted from western Europe towards
southern Asia at the end of the 20th century (Fig. 4c). Southern Asia was
found with the highest N fertilizer application amounts between 2000 and
2016, most concentrated in countries of eastern and southern Asia (e.g., China and
India). China and India together applied 36 % of global total N fertilizer
to pastures and rangelands.
Shifting hotspots of manure N application
Manure application hotspots have moved from European countries to southern Asia
since the 2000s. Between 1860 and 1999, Europe accounted for
50 % of global total manure N application to pastures and experienced a
rapid growth of manure N application, peaking (3.5 Tg N yr-1) in 1986.
In 1860, the highest applications were in the United Kingdom, France and
Germany (Fig. 4d), but by 2016, the highest application was in the North
China Plain (Fig. 4f). China alone applied 29 % of global total manure N
during 2000–2016.
Hotspots of manure N deposition
Southern Asia, as the hotspot of manure N deposition to pastures and
rangelands, has contributed 31 % of the global total amount during the past
157 years. Also, in Africa and South America substantial increases of manure
N deposition during 1860–2016 were observed. In the 1860s, manure N
deposition from southern Asia, Africa and South America contributed to
65 %, whereas Europe accounted only for 12 % of the global total manure
N deposition. In 1860, the highest deposition rates were observed for New
Zealand, Australia and western Europe (Fig. 4g). In 2016, except for the
above-mentioned regions, the highest deposition rates were in southern and
western Asia, China, western and eastern Africa and South America (Fig. 4i). During
2000–2016, manure N deposition from southern Asia, Africa, and South
America contributed to 80 %, while Europe accounted for 5 % of the
global total amount.
Limitations and uncertainties
This study attempts to provide an overall estimate of N inputs to global
rangelands and pastures during the period 1860–2016. However, before
these data are used in global models, uncertainties of these datasets need
to be addressed. First, the different definitions of grassland systems used
by the scientific community introduce uncertainties of the spatial patterns
and annual total amounts of N inputs. Chang et al. (2016) generated global
maps of grassland management intensity from 1901 based on modeled net
primary production and the use of grass biomass generated by Herrero
et al. (2013). Their total grassland area substantially differed from pasture area
developed by HYDE 3.1 (Chang et al., 2016). In this study, we used HYDE 3.2
to generate N inputs to global grasslands, defined more appropriately by
using the FAO land use definition of permanent meadows and pastures. This
dataset exactly followed the FAOSTAT data during 1960–2015, and combined
population density data to reconstruct land use prior to 1960. Pastures and
rangelands defined in HYDE 3.2 were based on the intensity of human
management. Although Bouwman et al. (2013) indicated that grassland areas in
their study were also calculated based on the grazing intensity, their total
area (pastures and rangelands) and spatial patterns were obviously different
from HYDE 3.2 (Table 2). Thus, a better understanding of land use is vital
to reduce the uncertainty of estimating N input rates and amounts in
pastures and rangelands.
Second, the FAOSTAT database provides country-level manure N applied to
soils; however, this dataset could not be directly applied to study N cycles
on pastures since applications to cropland and pasture soils are not
differentiated. In this study, large uncertainty remains due to the fact that we
separated national-level manure N application on pastures simply based on
pasture area over total agricultural area (cropland, pastures and
rangelands). In previous studies, Bouwman et al. (2013) assumed that 50 %
and only 5 % of the available manure was applied to grasslands in most
industrialized countries and in most developing countries, respectively. Liu
et al. (2010) allocated 34 % of the national total solid manure to
pastures in European countries and Canada, 13 % of the national total
manure to pastures in the United States and 10 % of the national total
manure to pastures in developing countries. Chang et al. (2016) assumed that
manure N application rate changes along with changes in the total ruminant
stocking density. Moreover, the spatialization process of N application
rates might introduce large uncertainty. The spatial pattern of gridded
manure N application rates in our study are correlated with manure
production rates in Zhang et al. (2017). The assumptions and uncertainties
mentioned in their study, such as without considering livestock migration,
might cause uncertainty of spatial distribution.
Third, studies used different data sources and made various assumptions of
the annual amount of fertilizer N applied on global pastures (Bouwman et
al., 2002; Chang et al., 2016; Lassaletta et al., 2014; Liu et al., 2010;
Stehfest and Bouwman, 2006). Thus, there is a large uncertainty in the total
N application on permanent meadows and pastures globally. Moreover, N
fertilizer application rates by crops were highly investigated and
documented in previous studies. Hence, N fertilizer application datasets
were generated considering crop-specific fertilizer rates and cropland area
in each grid (Lu and Tian, 2017; Mueller et al., 2012; Nishina et al., 2017;
Potter et al., 2010). In reality, N fertilizer application on pastures of
each country is not homogeneous. In this study, we assumed that N fertilizer
application rate in each country was constant, which means fertilizer was
applied evenly in each grid with a pasture area larger than zero. Last,
inside each relevant land use cell pastures and rangelands may be
characterized by different livestock density and deposition rates, which is
not considered in our current datasets. The final manure N deposition would
be highly affected by the proportion of each type of management in the grid cell.
Thus, it is necessary to consider these in the future research.
Furthermore, other human-induced sources of N inputs to pastures and
rangelands were not included in our study, which may underestimate total N
received globally. For example, biological N fixation was one of the major N
sources in the terrestrial ecosystem in the absence of human influence
(Cleveland et al., 1999). Pastures and rangelands occupy 25 % of the
earth's ice-free land surface across different latitudes with divergent
biological N fixation abilities. Plant production in temperate grasslands is
proximately limited by N supply due to little N via N fixation; however,
tropical savannahs received a large amount of N through leguminous species
(Cleveland et al., 1999; Vitousek et al., 2013). An estimate of potential N
fixation amount by global grassland systems is ∼46.5 Tg N yr-1,
with a range of 26.6–66.5 Tg N yr-1 (Cleveland et al.,
1999). Atmospheric N deposition is another major source of N input to
permanent meadows and pastures globally and increased from 2 to 14 Tg N yr-1
for the period 1860–2016 based on the Chemistry–Climate Model
Initiative N deposition fields (Eyring et al., 2013; Tian et al., 2018, 2019).