ESSDEarth System Science DataESSDEarth Syst. Sci. Data1866-3516Copernicus PublicationsGöttingen, Germany10.5194/essd-9-181-2017Global nitrogen and phosphorus fertilizer use for agriculture production in
the past half century: shifted hot spots and nutrient imbalanceLuChaoqunclu@iastate.eduTianHanqinhttps://orcid.org/0000-0002-1806-4091Department of Ecology, Evolution, and Organismal Biology, Iowa State
University, Ames, IA 50011, USAInternational Center for Climate and Global Change Research, and
School of Forestry and Wildlife Sciences, Auburn University, Auburn, AL
36849, USAState Key Laboratory of Urban and Regional Ecology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China
Chaoqun Lu (clu@iastate.edu)2March20179118119222July20168August20166January20179January2017This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://essd.copernicus.org/articles/9/181/2017/essd-9-181-2017.htmlThe full text article is available as a PDF file from https://essd.copernicus.org/articles/9/181/2017/essd-9-181-2017.pdf
In addition to enhancing agricultural productivity, synthetic nitrogen (N) and
phosphorous (P) fertilizer application in croplands dramatically alters
global nutrient budget, water quality, greenhouse gas balance, and their
feedback to the climate system. However, due to the lack of geospatial
fertilizer input data, current Earth system and land surface modeling studies
have to ignore or use oversimplified data (e.g., static, spatially uniform
fertilizer use) to characterize agricultural N and P input over decadal or
century-long periods. In this study, we therefore develop global
time series gridded data of annual synthetic N and P fertilizer use rate in
agricultural lands, matched with HYDE 3.2 historical land use maps, at a resolution
of 0.5∘× 0.5∘ latitude–longitude
during 1961–2013. Our data indicate N and P fertilizer use rates on per unit
cropland area increased by approximately 8 times and 3 times, respectively,
since the year 1961 when IFA (International Fertilizer Industry
Association) and FAO (Food and Agricultural Organization) surveys of
country-level fertilizer input became available. Considering cropland
expansion, the increase in total fertilizer consumption is even
larger. Hotspots of agricultural N fertilizer application shifted from the
US and western Europe in the 1960s to eastern Asia in the early 21st
century. P fertilizer input shows a similar pattern with an additional
current hotspot in Brazil. We found a global increase in fertilizer N / P ratio
by 0.8 g N g-1 P per decade (p<0.05) during 1961–2013, which may have
an important global implication for human impacts on agroecosystem functions
in the long run. Our data can serve as one of critical input drivers for
regional and global models to assess the impacts of nutrient enrichment on
climate system, water resources, food security, etc. Datasets available
at 10.1594/PANGAEA.863323.
Introduction
Agricultural fertilizer use is one of the important land management practices
that has alleviated nitrogen limitation in cropland and substantially increased
crop yield and soil fertility over the past century (Vitousek et al., 1997;
Tilman et al., 2002). Since the generation of the Haber–Bosch process in the
early 20th century, chemical nitrogen (N) fertilizer production has
converted a large amount of unreactive N to reactive forms (Galloway et al.,
2004). Chemical phosphorus (P) fertilizer production was promoted
along
with the phosphorus acid. On the one hand, as a critical component of the “green
revolution”, the dramatic increase in fertilizer production and application
has contributed considerably in raising agricultural productivity and
reducing hunger worldwide (Smil, 2002; Erisman et al., 2008). On the other hand,
excessive fertilizer use is proven to cause a number of environmental and
ecological problems within and outside of farmlands, such as air pollution,
soil acidification and degradation, water eutrophication, crop yield
reduction, and undermining of the sustainability of food and energy production
from agricultural fields (Bouwman et al., 2005; Ju et al., 2009; Vitousek et al.,
2009; Guo et al., 2010; Sutton et al., 2011; Tian et al., 2012; Lu and Tian,
2013).
Large spatial and temporal variations exist in chemical fertilizer use
across the world. China, the US, and India together accounted for
over 50 % of fertilizer consumption globally and they demonstrated
a contrasting changing trend over the past century due to the status of
economic and agricultural development (FAOSTAT, 2015). The rates and
spatiotemporal patterns of N and P fertilizer use are key input
drivers for inventory- and process-based land modeling studies to reliably
estimate agroecosystem production and the greenhouse gas budget (Mosier et al.,
1998; Zaehle et al., 2011; Stocker et al., 2013; Tian et al., 2015).
Nutrient input-related processes affect a wide variety of plant
physiological, biogeochemical, and hydrological variables (e.g., crop
productivity, yield, evapotranspiration, N2O emission, N and P leaching
from agricultural runoff, and land-to-water nutrient export) and their
responses to other environmental drivers (e.g., CO2 fertilization
effect). However, there is still a lack of data for describing long-term
spatially explicit agricultural input of N and P through chemical fertilizer
use across the globe.
Diagram of the workflow for developing the database of global
annual N and P fertilizer use rate during the period 1961–2013. Oval boxes
indicate data elements, among which the gray ones show the raw data involved
in fertilizer data development and the white ones are the middle and final data
products. Diamond boxes indicate process elements.
The International Fertilizer Industry Association (IFA) and the Food and Agricultural Organization (FAO) provided data for annual fertilizer consumption amount since 1961,
which is the most complete country-level record of fertilizer use over a
long period of time, and the data have been used in multiple bottom-up nutrient budget
analyses (e.g., Mosier et al., 1998; Galloway et al., 2004; Bouwman et al.,
2005, 2013a). By assuming uniform fertilizer application rate nationwide,
many process-based modeling studies considering management practices (Zaehle
et al., 2011; Stocker et al., 2013) have used this data set as an important
driver for agroecosystems; however, the spatial variations in fertilizer use
within countries have been overlooked. Tian et al. (2015) updated
FAO-based fertilizer use data by using detailed regional information in
China, India, and the US to replace country-uniform data and keeping the
other
countries the same as FAO statistics. They partially demonstrated
within-country variations through province-level census in China and
state-level census in India and US (Tian et al., 2010; Lu and Tian, 2013;
Banger et al., 2015). Based on country-level crop-specific fertilizer record
(“Fertilizer Use by Crop 2002”, from IFADATA) and a global distribution map
of 175 crops (Monfreda et al., 2008), Potter et al. (2010) generated annual
N and P fertilizer application data across the globe at a spatial resolution
of 0.5∘ latitude by longitude. These data contain most
crop-specific variations in N and P fertilizer use over space, but they only
represent average fertilizer application pattern in the period from 1994 to
2001 and cannot meet the timescale of long-term land surface modeling.
Likewise, Mueller et al. (2012) used a similar approach to distribute crop-
and crop-group-specific fertilizer use rates and combine multisource
national and subnational nutrient consumption data to harmonize fertilizer
use rate. However, their data only represent the status around 2000.
Therefore, in this study, we develop a spatially explicit time series
agricultural N and P fertilizer use data set by combining country-level
fertilizer use records, crop-specific fertilizer use data, global maps of
annual cropland area, and spatial distributions of crop types at a
0.5∘ resolution during the period 1961–2013. This
newly developed data set displayed within-country heterogeneity of
agricultural fertilizer use while keeping the country-level total fertilizer
consumption amount consistent with IFA data. It has been recently
incorporated as one of the key environmental drivers for global model simulation
studies and model–model intercomparison projects (e.g., N2O-MIP; Tian et
al., personal communication, 2016). To facilitate Earth system modeling
and inventory-based studies, these global N and P fertilizer use data will be
updated annually based on the most recent IFA–FAO country-level statistics
data and historical land use maps.
Methods
The basic principle is to spatialize the country-level N and P fertilizer
use amount to gridded maps of fertilizer use rate on per unit area of
cropland in the period 1961–2013 (Fig. 1), during which IFA and FAO have
annual records for most countries. Here we adopt “grand total N and
P2O5” from IFA statistical data in thousands of tons of
nutrients per country. The grand total amount includes nutrients
from straight and compound forms. We convert grams of P2O5 in the IFA
database and Heffer (2013) to grams of P by multiplying by the ratio of 62/142.
Regarding crop-specific N and P fertilizer use rates, the database of crop-specific
fertilizer use from the IFA (Heffer, 2013) provides the total amount of N and P
fertilizer use in 13 crop groups at country level, which includes 27
selected countries (considering EU-27 as single countries, Fig. 2) in
2010–2011. It accounts for over 94 % of global fertilizer
consumption. M3-crop data, which include detailed distribution maps of crop types developed by Monfreda et al. (2008), depict
the harvested area for 175 crops in the year 2000 at 5 arcmin resolution
for
latitude by longitude. This unit is a proportion of the grid cell area and the
values could be larger than 1 because of multiple cropping. We calculated
the harvested area of these 13 crop groups (i.e., wheat, rice, maize, other
cereal, soybean, oil palm, other soil seed, fiber, sugar, roots, fruit,
vegetable, and others) in the corresponding 26 countries and EU-27. We
obtained country-level crop-specific N and P fertilizer use rates by
dividing crop-specific fertilizer consumption amount by harvested area of
each crop group. Here, by using harvested area, instead of area of arable
land, we consider the effect of multiple cropping on the calculation of N
and P fertilizer use rate to avoid overestimating nutrient input in
cropland. These tabular data were interpolated to generate spatial maps of N
and P fertilizer use rate for each crop group. Combined with harvested area
of each crop, we produced the area-weighted average of N and P fertilizer
use rate in each grid cell, which serves as a baseline map to downscale
country-level fertilizer use.
CNferg‾=∑iCNferi,jAharvi,j×Aharvi,g∑iAharvi,g,
where CNferg‾ is the area-weighted average of crop-specific
nutrient (N and P) fertilizer use rate (i.e., gridded baseline fertilizer
use rate, in the unit of g N or g P m-2 yr-1) at a resolution of
0.5∘× 0.5∘. CNfer and Aharv are
crop-specific N and P fertilizer use amount (g N or g P) and harvested area
(m2), respectively, for crop type i, country j, and
grid cell g (Fig. 1). Here, the harvested areas of
M3-crops were aggregated to a half degree to meet the expected resolution
of our final product.
Countries with and without crop-specific fertilizer use records
from the IFA database in 2010–2011 (Heffer et al., 2013) and those
excluded by IFA data records.
IFA-based national fertilizer use interpolation
The countries included in
IFA country-level time series surveys but excluded by crop-specific
fertilizer use databases (yellow color in Fig. 2) were depicted with
IFA-reported country average fertilizer use rate. To calculate them, we
divided country- and continent-scale annual fertilizer consumption amount
from the IFA by corresponding cropland area calculated from HYDE 3.2 (Klein
Gildewijk, 2016) to get half-degree gridded N and P fertilizer use rates
during 1961–2013. In this step, we assume that the N and P fertilizer is evenly
applied to croplands of each country. In addition, to represent the status
of countries not included in the IFA (green in Fig. 2), the amount of
fertilizer application in IFA-included countries was subtracted from
continental total, and the remaining fertilizer was assumed to be evenly
applied in the rest of the croplands on each continent. These non-IFA countries
together cover ∼ 8 % of global croplands and account for
less than 1 % of global synthetic N and P fertilizer consumption. Several
countries (e.g., former Soviet Union, former Czechoslovakia, former
Yugoslavia) were broken up in the 1990s, and the emergent countries only have
fertilizer use archived thereafter. We use average fertilizer use rate at
per unit cropland area in the former countries to represent new countries'
agricultural nutrient input before their existence. Country boundary maps are
derived from “World Country Admin Boundary Administrative Areas Shapefile
with FIPS codes” by Global Administrative Areas (2012), in which country
administrative boundaries, country names, and two-digit FIPS codes are
provided across the world.
Harmonizing national total and crop-specific fertilizer use rate
In order
to keep the national total N and P fertilizer amounts consistent with the IFA
inventory, we calculated country-level ratios between the time series
(1961–2013) national fertilizer use amount from the IFA and the product of
gridded baseline fertilizer use rate (CNferg‾) and gridded
cropland area delineated by HYDE 3.2. These tabular country-level regulation
ratio data were interpolated into half-degree maps, which were multiplied by
gridded baseline fertilizer use rate (CNferg‾) to generate
maps of “real” N and P fertilizer use rate during 1961–2013. This approach
was only used in the grid cells containing croplands according to HYDE 3.2.
In the other areas, fertilizer use rate is zero.
RNfery,j=CTYNfery,j∑g=1g=nin countryj(CNferg‾×Acropy,g),
where RNfery,j is the regulation ratio (unitless) in the
year y and country j. CTYNfery,j is national
total N fertilizer use amount (unit is g N yr-1 or g P yr-1) derived from the IFA
database in a specific year, and Acropy,g is the area of cropland
(unit is m2) retrieved from the historical land use data (HYDE 3.2) in
the year y and grid g. Here we have aggregated
annual cropland area of HYDE 3.2 into half degrees.
Nfery,g=CNferg‾×RNfery,g,
where real gridded N and P fertilizer use rates (Nfery,g, unit is g N
or P m-2 cropland yr-1) in the year y and grid g are
the product of gridded baseline N fertilizer use rate and the modification
ratio (RNfery,j) in the corresponding year and grid cell.
It is notable that EU-27 has the same crop-specific fertilizer use rate for
each crop group, but the IFA-based country-level fertilizer use amount is
different among countries and years; thus, annual maps of regulation
ratios are different spatially. Therefore, the final product shows spatially
variant N and P fertilizer use rates in the region of EU-27.
Results
Temporal patterns of global nitrogen (N) and phosphorous (P)
fertilizer use in terms of total amount (tot) and average rate on per unit
of cropland area (avg) per year. Pie charts show the proportion of N and P
fertilizer use in the top five fertilizer-consuming countries and others in
the year 2013.
Our data indicate that N fertilizer consumption increased from
11.3 Tg N yr-1
(0.9 g N m-2 cropland yr-1) in 1961 to 107.6 Tg N yr-1 (7.4 g N m-2 cropland yr-1
on average) in 2013 and that P fertilizer consumption increased
from 4.6 to 17.5 Tg P yr-1 (0.4 to 1.2 g P m-2 cropland yr-1 on average)
during the same period (Fig. 3). Increase in global total fertilizer use
amount is derived from both cropland expansion and raised fertilizer
application rate in per unit cropland area. In 2013, the top five
fertilizer-consuming countries (China, India, the US, Brazil, and Pakistan for
N fertilizer and China, India, the US, Brazil, and Canada for P fertilizer)
together accounted for 63 % of global fertilizer consumption. China alone
shared 31 % of global N fertilizer consumption with an annual increasing
rate of 0.7 Tg N y-1 or 0.6 g N m-2 cropland yr-1 (R2= 0.98) during 1961–2013 (Fig. 4), while India showed a much smaller
increasing trend of 0.3 Tg N yr-1 or 0.2 g N m-2 cropland yr-1 per year
(R2= 0.97). N fertilizer use rate in the US increased by
0.4 Tg N yr-1 or 0.2 g N m-2 cropland yr-1 per year during 1961–1980 and
leveled off thereafter. P fertilizer use in these three countries
demonstrated a similar pattern: a more rapid increase in China (0.1 Tg P yr-1)
than in India (0.06 Tg P yr-1) and the US (0.05 Tg P yr-1 during 1961–1980
and leveled off thereafter). Brazil accounted for 3 and 11 % of global
N and P fertilizer consumption, respectively. N fertilizer use rate in
Brazil has gradually increased since the early 1990s and has now reached half of
the agricultural N input level in the US, while its P fertilizer use rate
ranked the global top in 1980, declined thereafter, and regrew from 2000,
demonstrating the second highest per unit cropland P fertilizer use rate
next to China. Pakistan shared 3 % of global total N fertilizer use, but
its average cropland application rate increased dramatically, with an annual
increase rate of 0.3 g N m-2 cropland yr-1
(R2= 0.97), second
only to China (Fig. 4).
Interannual variations in national average N and P fertilizer use
rate (g N or g P m-2 cropland yr-1) in the top five fertilizer-consuming
countries during 1961–2013.
Agricultural N fertilizer use rate peaked in the US and western Europe in
the 1960s, and the hot spots gradually moved to western Europe and eastern Asia
in the 1980s and 1990s. They then moved to eastern Asia in the early 21st century
(Fig. 5). Large areas of cropland in eastern and southeastern China stand out
due to extremely high N fertilizer input (e.g., more than 30 g N m-2 yr-1). Northern India and western Europe received 10–20 g N m-2 yr-1
up to now. South America also experienced a rapid increase in N
fertilizer use rate during the past 54 years, particularly for small areas
of Brazil, with N input reaching similar levels as the US. Although
cropland expansion widely occurred in Africa, its average N fertilizer use
rate was enhanced slowly, with most areas still receiving less than 1.5 g N m-2 yr-1
in 2013. Australia demonstrated a similar low level of
agricultural N input (less than 5 g N m-2 yr-1 in 2013). N fertilizer use
in Russia peaked in the 1980s and then declined in the following decades.
It is argued that after 1990 the major reason for fertilizer use drop was a
severe economic depression due to the breakup of the Soviet Union and the
following conversion to market economies (Ivanova and Nosov, 2011).
Spatial distribution of global agricultural nitrogen (N) fertilizer
use in the year 1961, 1980, 1990, and 2013. Colors show N fertilizer use
rate in per square meter cropland of each pixel.
Spatial distribution of global agricultural phosphorus (P)
fertilizer use in the years 1961, 1980, 1990, and 2013. Colors show P
fertilizer use rate in per square meter of cropland in each pixel.
The hot spot of agricultural P fertilizer input centered in Europe before
the 1980s and then shifted to central China and a small area of Brazil, with
an input rate of more than 3 g P m-2 cropland yr-1 in 2013 (Fig. 6). P input
in China showed a significant increasing trend during 1961–2013 (p<0.05), while in Brazil it peaked in the early 1980s and declined
thereafter, growing again since 2000. Most agricultural areas across the
rest of world were characterized by P input of less than 1 g P m-2 cropland yr-1, except India, western Europe, and a small area of the
US,
receiving 1–1.5 g P m-2 cropland yr-1 in 2013. P fertilizer use rate
has remained relatively stable in the US since 1980. Similar to agricultural N
fertilizer use, the increase in total P fertilizer amount in Africa was
primarily driven by cropland expansion, and its input rate per unit of
cropland area was constantly low, less than 0.5 g P m-2 yr-1 during the
past half century. Likewise, P fertilizer use rate in Russia increased in
the 1980s and began to decline after 1990.
We found that the enhancement of N fertilizer use is faster than that of P
fertilizer use, leading to an increase in the N / P ratio in synthetic fertilizer
consumption from 2.4 to 6.2 g N g-1 P (an increase of 0.8 g N g-1 P per decade,
p<0.05) during 1961–2013. This increase mainly took place
in Europe, northern Asia, and small areas of South America and Africa (Fig. 7). However, fertilizer N / P ratio declined in China and India from over 9 g N g-1 P
in 1961 to 5–9 g N g-1 P at present, which is mainly caused by extremely
low P fertilizer input in these two countries before 1980. It has remained
relatively stable in the US and most countries in Africa since 1980. Up to
now, fertilizer N / P ratio in the Northern Hemisphere has generally been higher (more
than 5 g N g-1 P) than that in the Southern Hemisphere.
DiscussionComparison with other studies
In this study, we use M3-crop to
spatialize crop-specific fertilizer use rate and then use HYDE 3.2 to
disaggregate the annual national IFA fertilizer use record to grid cells
with cropland. Therefore, the changes in fertilizer use rate shown in our
data could reflect the comprehensive human disturbances in cropland area and
distribution as well as national total fertilizer inputs at annual time
steps (Figs. 5 and 6). In addition, in spatializing fertilizer data, the
approach we used here based on crop-specific fertilizer use rate is more
reliable than national, provincial, state, or county-based fertilizer
development, which assumes uniform fertilizer input rate in a certain region
(Zaehle et al., 2011; Lu and Tian, 2013; Tian et al., 2015). A regionally
uniform rate overlooks fertilizer use differences among crops. The 13
crop groups we adopted to spatialize national fertilizer input include the
top fertilizer-consuming crops (i.e., wheat, maize, soybean, rice, oil palm)
and aggregate the rest of crops into other cereal, other soil seed, fiber,
sugar, roots, fruit, vegetable, and others, which keeps cross-country
cross-crop heterogeneity of fertilizer use in data development. Overall,
combined with historical land use data (e.g., HYDE 3.2), our half-century-long global fertilizer maps at a 0.5∘× 0.5∘ resolution can be used to force Earth system models for
assessing agroecosystem productivity, greenhouse gas fluxes, N and P export
through agricultural runoff, and their feedbacks to the climate system.
Comparison of synthetic N and P fertilizer use amount between this
study and other existing data sources.
Data sourceOther estimatesThis studyYearSynthetic N fertilizer amount (Tg N yr-1) Van der Hoek and Bouwman (1999)73.670.41994Sheldrick et al. (2002)78.280.31996Boyer et al. (2004)81.176.21995Green et al. (2004)78.3Siebert (2005)72.3Bouwman et al. (2005)82.9Potter et al., 201070.280.12000Mueller et al. (2012)77.8IFA82.1FAO stat80.8IFA110.2107.62013FAO stat99.6Synthetic P fertilizer amount (Tg P yr-1) Sheldrick et al. (2002)12.713.21996Smil (2000)1513.92000Bouwman et al. (2009)13.8Potter et al. (2010)14.3Mueller et al. (2012)13.7IFA14.3FAO stat14.2IFA18.817.52013FAO stat16.7
This newly developed database is based on IFA country-level time series
statistics, and its spatial distribution follows the pattern of crop-specific
fertilizer use rate and gridded harvest area of major crop types. Our data
are comparable to other existing estimates in terms of global N and P fertilizer
consumption amount (Table 1). Our global total is very close to IFA
statistical data. Only a few existing data (e.g., Potter et
al., 2010; Mueller et al., 2012) characterize the spatial heterogeneity and
hot spots of N and P fertilizer use in agricultural land, but none of them
spans long enough to facilitate a modeling study to capture the legacy effects
of historical fertilizer input. Potter et al. (2010) used a similar
approach as we did and developed geospatial data of N and P inputs from
fertilizer and manure across the globe. However, they did not consider annual land
cover change and the resulting changes in spatial patterns of agricultural
fertilizer use by using a one-phase M3-crop map, which represents an average
cropland distribution in the period 1997–2003 (Monfreda et al., 2008).
Likewise, Mueller et al. (2012) revised Potter's approach by incorporating
national and subnational fertilizer application data for crops and crop
groups, harmonizing with FAO consumption record and allocating fertilizer to
crop and pasture areas derived from the M3-crop map. Potter et al. (2010) and
Mueller et al. (2012) both demonstrate total N or P fertilizer use on a per
unit grid cell area. In order to compare them with our data in the year
2000, we converted these two data products to grams of N or P per square meter of
cropland area by dividing grid-level total fertilizer amount by crop areas
from M3-crop data (Fig. 8). We found that the hot spots of global N and P fertilizer
use rate are roughly consistent among these three data sets. The major
differences are likely caused by the following reasons: (1) cropland area and
distribution derived from HYDE 3.2 (used in our study) and M3-crop (used to
delineate fertilizer use area in Potter et al., 2010 and Mueller et al.,
2012) do not match in some areas such as western China, the western US,
central Asian countries, northern Africa, and Australia; (2) the crop-specific
fertilizer use data in 2010–2011 (Fig. 2) used in our study covered more
countries in northern Asia, but less in Africa and South America compared to
IFA data from Fertilizer Use by Crop 2002 in the development of the
other two data products, which led to different spatial details; and (3) the IFA
crop-specific fertilizer use data in our study include 13 crop groups (i.e.,
major crops and groups of others) in each country (Fig. 2), while crop
types range from 2 to over 50 per country. This was reported in the IFA crop
fertilizer use data that is used in Potter et al. (2010) and Mueller et al. (2012). Therefore, our data may to some extent diminish the cross-crop
variations in fertilizer application by using records of crop groups for
these nonmajor crop types.
Spatial distribution and changes in N / P ratio of synthetic
fertilizer application across the world in the years of 1961, 1980, 1990,
and 2013.
Comparison of global N and P fertilizer use maps from this study (a),
Potter et al. (2010) (b), and Mueller et al. (2012) (c) in the year 2000. Left panels (La-Lc) indicate N fertilizer use rate and
right panels (Ra-Rc) indicate P fertilizer use in units of g N or P m-2 cropland yr-1.
Change in N and P fertilizer use
Global synthetic N and P
fertilizer use increased by 85 Tg N yr-1 and 10 Tg P yr-1, respectively, between
the 1960s and the last 5 years (2009–2013). Across the region, southern Asia (this region includes East Asia, South Asia, and Southeast Asia; Fig. 9)
accounted for 71 % of the enhanced global N fertilizer use, followed by
North America (11 %), Europe (7 %), and South America (6 %). The other
three continents shared the remaining 5 % increase. Southern Asia is also the
largest contributor (91 %) to global P fertilizer use increase over the
past half century, followed by South America (21 %) and North America
(4 %), while a decrease in P fertilizer consumption (-17 %) is found in
Europe and negligible change is found in other continents. Noticeably, southern Asia
ranks as a top hot spot of global anthropogenic nutrient input, contributing
to a number of ecological and environmental problems, such as increased
agricultural N2O emission, climate warming, nitrate and phosphate
leaching, and coastal eutrophication and hypoxia (Seitzinger et al., 2010;
Bouwman et al., 2013b; Tian et al., 2016).
Changes in N and P fertilizer use (Tg N or Tg P yr-1) between the
1960s and the last 5 years (2009–2013). Upper right panel shows delineation of
seven continents across the globe.
N / P ratio in terrestrial plant species is 12–13 on average, with large
cross-species and cross-site variability (Elser et al., 2000; Knecht and
Goransson, 2004). Human management, such as fertilizer application, can
change N and P supply and modify vegetation and soil properties of the N / P
ratio and their responses to increased N input (Güsewell, 2004). A higher
fertilizer N / P ratio in the Northern Hemisphere (Fig. 7) could be reasonably
explained by N fertilizer increasing faster than P fertilizer in historically
N-limited and P-rich soil in those areas. Particularly in
Europe, P fertilizer use rate declined, while N input continued increasing.
Fertilizer N / P ratio decline in China and India, however, indicates a shift
from nearly zero-synthetic P fertilizer input to a gradually balanced
fertilizer strategy (Zhang et al., 2005). In contrast, South America is
characterized by a lower fertilizer N / P ratio because of its large increase in
both N and P fertilizer use (accounting for 6 vs. 21 % of the global
increase since the 1980s; Fig. 9). In the long term, global increase of
anthropogenic N / P ratio is expected to reduce species richness (Güsewell
et al., 2005), induce the shift from N limitation to P limitation (Elser et
al., 2009; Peñuelas et al., 2012), and increase N loss (e.g., N loads to
downstream aquatic ecosystems, NH3 volatilization and redeposition
elsewhere) due to the limitation of low soil P availability to N
fertilization effect (Carpenter et al., 1998). To better manage
agroecosystem productivity and its sustainability, the dynamic pattern of
anthropogenic N / P input ought to be related to local soil N and P status,
growth demand of different crop species, and historical nutrient inputs.
Differences in historical cropland area between high-resolution
satellite-derived regional LCLUC data (China: Liu and Tian, 2010; India:
Tian et al., 2014) and HYDE 3.2 (Klein Goldewijk, 2016) during 1900–2013.
Uncertainty and future needs
The uncertainties of this database
are mainly from the following aspects: (1) the data of country-level
fertilizer use by crop we used in this study are from the latest estimate (i.e.,
2010–2011; Heffer, 2013), which could reflect current patterns of
crop-specific fertilizer application rate, but in the meantime may bias the
historical allocation of fertilizer use among crop groups. There are no
long-term data indicating how variable the relative contribution of crop
groups is in consuming fertilizer at country level. Here, we assume that the
evolution of global crop production and crop area, rather than crop-specific
fertilizer application rate, is the major reason responsible for the sharing
of fertilizer use amongst crops. (2) The spatial pattern of various crop types
is derived from M3-crop (Monfreda et al., 2008), which is the most complete
and detailed distribution map of 175 crop types so far, though it represents
an average status for 1997–2003. By using the information of distribution
and harvested area for 13 crop groups from M3-crop, we convert crop-specific
fertilizer use amount in each country to gridded agricultural fertilizer use
rate in per unit cropland area. The temporal mismatch between fertilizer and
crop distribution data may cause under- or overestimation of grid-level
fertilizer use rate. (3) We assume that the data of M3-crop can capture the
contemporary area proportion of crop groups well globally and this
proportion is unchanged over time, due to the lack of long-term data sources.
Detailed time series information of grid-level crops and the area ratio
amongst them could help us to generate more accurate spatial maps of
fertilizer use and assess yearly changes due to the changing cropping
system. (4) We use HYDE 3.2 historical cropland percentage to spatialize
country-level fertilizer use amount from the IFA, but HYDE data are proven to
show inconsistent spatial and temporal patterns of cropland area change
compared to satellite-derived land use databases on a regional scale (e.g.,
China (Liu and Tian, 2010) and India (Tian et al., 2014); Fig. 10). Based
on high-resolution satellite images and historical archives, the land use
data from Liu and Tian (2010) show more concentrated cropland distribution
with higher within-grid crop percentage on the North China Plain
compared to HYDE 3.2, although national total cropland area is quite similar
between these two data sets in the recent decade. This might be the reason that our
data fail to capture the extremely high fertilizer use rate on the North
China Plain (more than 40 g N m-2 cropland yr-1 as indicated in Lu and
Tian, 2013, who used land use data from Liu and Tian, 2010). In addition,
the difference in national cropland area between HYDE3.2 and the regional LCLUC
database (Fig. 10) could make our fertilizer data underestimate average
fertilizer use rate per square meter of cropland in India and overestimate
fertilizer use rate before 1990 in China. As a result, the extensive
distribution of cropland and fertilizer use data in China derived from HYDE
3.2 may lead to uncertain estimates in Earth system modeling. (5) We assume that agricultural fertilizer is all consumed in croplands, with no application in grassland or permanent pasture. This assumption overestimates fertilizer use rate in croplands, particularly for those countries with a
large proportion of grassland fertilization (e.g., Ireland; Lassaletta et al., 2014). Therefore, we
call for a continuous survey of crop-specific fertilizer use, development of
dynamic crop-type maps, and updated global land use data with more precise
regional patterns for further improving characterization of geospatial and
temporal patterns of agricultural fertilizer use.
Agricultural nitrogen and phosphorous fertilizer use data are available at 10.1594/PANGAEA.863323 (Lu and Tian,
2016).
Conclusions
Synthetic N and P fertilizer application during agricultural production is a
critical component of anthropogenic nutrient input in the Earth system.
Development of spatially explicit time series N and P fertilizer uses across
global cropland reveals a significant and imbalanced increase in N and P
during the past half century (1961–2013). The hot spots of agricultural nutrient
input shifted from North American and European countries to eastern Asia, which
implies corresponding changes in the spatial pattern of global nutrient
budget, carbon sequestration and storage, greenhouse gas emissions, and
riverine nutrient export to downstream aquatic systems. Meanwhile, Africa is
still characterized by low nutrient input along with expanding cropland
areas. The increased fertilizer N / P ratio is likely to alter the nutrient
limitation status in agricultural land and affect ecosystem responses to
future N enrichment long term. Agricultural management practices
should put emphasis on increasing nutrient use efficiency in those high-input regions, while reducing environmental and ecological consequences of
excessive nutrient loads and enhancing agricultural fertilizer application
to relieve nutrient limitation in low-input regions. In addition to meeting
different fertilizer demands spatially, balanced N : P : K fertilizer
application ought to be promoted depending on local nutrient availability
and crop growth demands.
The authors declare that they have no conflict of interest.
Acknowledgements
This work has been supported by Iowa State University new faculty start-up funds, NSF Grants
(AGS1243232, CNH1210360), and the State Key Laboratory of Urban and Regional Ecology, RCEES,
Chinese Academy of Sciences. We thank the anonymous reviewers for their precious comments and constructive suggestions for improving this paper.
Edited by: D. Carlson
Reviewed by: two anonymous referees
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