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Essay
Fertilizing Nature: A Tragedy of Excess in the Commons Allen G. Good, Perrin H. Beatty*
Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada
Abstract: Globally, we are apply- ing excessive nitrogen (N) fertilizers to our agricultural crops, which ultimately causes nitrogen pollu- tion to our ecosphere. The atmo- sphere is polluted by N2O and NOx gases that directly and indirectly increase atmospheric warming and climate change. Nitrogen is also leached from agricultural lands as the water-soluble form NO3
2 ,
which increases nutrient overload in rivers, lakes, and oceans, causing ‘‘dead zones’’, reducing property values and the diversity of aquatic life, and damaging our drinking water and aquatic-associated in- dustries such as fishing and tour- ism. Why do some countries show reductions in fertilizer use while others show increasing use? What N fertilizer application reductions could occur, without compromis- ing crop yields? And what are the economic and environmental ben- efits of using directed nutrient management strategies?
In his 1968 seminal paper, ‘‘The
Tragedy of the Commons,’’ the late
Garrett Hardin argued that individuals,
acting in rational pursuit of their own self-
interest, will sacrifice the long-term viabil-
ity of a shared resource for short-term
gain. ‘‘Ruin is the destination toward
which all men rush, each pursuing his
own best interest in a society that believes
in the freedom of the commons.’’ In the
case of pollution, he wrote, ‘‘Here it is not
a question of taking something out of the
commons, but of putting something in –
sewage, or chemical wastes into water’’
[1]. Perhaps one of the best examples of
this ‘‘over-contribution’’ is nitrogen (N)
fertilizers, where individual rational be-
haviour (i.e., applying high fertilizer rates
to maximize short-term economic yield)
can cause long-range harm to the envi-
ronment. The true cost of applying high
rates of N fertilizers in order to maximize
overall yield is already apparent in the
form of global climate change. The
incentive to over-apply N fertilizers is
likely to continue, as both the Food and
Agriculture Organization (FAO) and the
United Nations (UN) have predicted high
future demand for cereal production,
especially within the developing nations,
due to predicted increases in populations
and dietary shifts.
In developed countries, crop yields have
nearly reached their biological maximum
and increasing fertilizer use is unlikely to
provide any significant additional gains. In
contrast, in developing countries, there is
still a large yield gap. Although we need to
increase crop yields to feed the growing
global population, we also need to do this
in an environmentally sustainable way.
We cannot increase our yields by increas-
ing N fertilizer application (not even in
areas of the world that still have an
exploitable yield gap) at the expense of
the ozone layer or marine life. Certainly,
while regions with an N balance surplus
can reduce N fertilizer application rates
without yield losses (i.e., Denmark), other
regions will need to increase their N use
(i.e., sub-Saharan Africa), but still use best
management practices. N balanced coun-
tries may also be able to reduce N fertilizer
rates without yield loss by employing new
technologies such as improved plant
varieties, region-specific farming practices,
time-release N fertilizer, drip irrigation,
crop rotation, bioinoculants, and similar
approaches.
Nitrogen Is a Key Aquatic and Atmospheric Pollutant
Nitrogen is the key limiting nutrient for
most crops and many aquatic and
terrestrial ecosystems. Unfortunately, the
massive increase in anthropogenic N
introduced into the environment, largely
via N fertilizers, has had significant
negative environmental consequences
[2,3]. The link between agriculture and
nitrate pollution is well established with
impacts on drinking water [4,5] and the
eutrophication of fresh water and marine
ecosystems, including the proliferation of
harmful algal blooms and ‘‘dead zones’’
in coastal marine ecosystems [6]. For
example, in the United States, 89% of
total N inputs into the Mississippi River
come from agricultural runoff and drain-
age [6]. In addition, agriculture plays a
substantial role in the balance of the
three most significant anthropogenic
greenhouse gases (GHGs): carbon diox-
ide (CO2), nitrous oxide (N2O), and
methane (CH4). The global warming
potential (GWP) of these gases can be
expressed in CO2 equivalents. The
GWPs of N2O and CH4 are 296 and 23
times greater, respectively, than a unit of
CO2 [7]. Of these, N2O is the most
important gas emitted by fertilizer use,
because of its large CO2 equivalent
influence on GWP. In the US, agriculture
contributed 68% of the country’s N2O
emissions in 2009, but only 3.6% of the
total US GHG emissions [8].
Essays articulate a specific perspective on a topic of broad interest to scientists.
Citation: Good AG, Beatty PH (2011) Fertilizing Nature: A Tragedy of Excess in the Commons. PLoS Biol 9(8): e1001124. doi:10.1371/journal.pbio.1001124
Published August 16, 2011
Copyright: � 2011 Good, Beatty. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This research was funded by grant #2009C001R from the Alberta Crop Industry Development Fund (http://www.acidf.ca/).The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
Abbreviations: BNMP, best nutrient management practices; CAP, common agricultural policy; CT-CC, conventional tillage and continuous corn; EEONR, environmental and economical optimal N rate; EONR; economical optimal N rate; EU, European Union; FAO, Food and Agriculture Organization; GHG, greenhouse gas; GWP, global warming potential; MRTN, maximum return to N; N, nitrogen; NUE, nitrogen use efficiency; UN, United Nations; 4R, right source, right time, right place, right rate.
* E-mail: [email protected]
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Table 1. The N fertilizer costs and consumption of specific countries for past use and future forecasts.
World EU Denmark China
Year
Price
($ Mton-1) Total Consumption (MMt N)
Value
(US$B)
Total Consumption (MMt N)
Consumption (kg ha21)
Total Consumption (MMt N)
Consumption (kg ha21)
Total Consumption (MMt N)
Consumption (kg ha21)
1987 425 75.8 $32.2 30.3 127 0.367 142 18.6 138
1997 610 81.3 $49.6 15.5 101 0.238 120 25.4 185
2007 795 100.6 $80.0 13.3 114 0.172 75 34.8 247
2012 869 103.2 $89.7 13.0 114 0.170 74 37.6 267
2020 a
980 110.7 $108.5 13.0 114 0.170 74 42.4 302
2030 b
1220 126.9 $154.8 13.0 114 0.170 74 54.5 388
2050 c
1500 151.6 $227.4 13.0 114 0.170 74 69.0 487
Italicized text represents future estimates and are based on no increase in consumption (EU and Denmark) or a linear increase in consumption, equivalent to the rate of increase between 1987 and 2007 (world and China) (http://www.fao.org/corp/statistics/; accessed 16 June 2010). aEstimates of World N consumption in 2020 are 110 MMt [55], 112 MMt [56], and 135 MMt [57]. b
Estimates of World N consumption in 2030 are 125 MMt [56]. c Estimates of World N consumption in 2050 are 135 MMt [58] and 236 MMt [57].
MMt, million metric tons. doi:10.1371/journal.pbio.1001124.t001
Table 2. Total N consumption, economic, and environmental costs for the US, China, India, and the world.
Year
Actual/Predicted Consumption (MMt N)
Value
(US$B)
Proposed Reduction (from 2007)
Reduced Consumption (MMt N)
Excess N Applied (MMt N)
Value
(US$B) CO2 GWP (MMt)a
CO2 GWP Value
(US$B)
Environmental Cost
(US$B)
World
1987 75.8 32.2
2007 100.6 80.0
2020 110.7 108.5 10% 90.5 20.2 19.8 94 1.4 8.7
2030 126.9 154.8 20% 80.5 46.4 56.6 215.8 3.2 24.9
2050 151.6 227.4 20% 80.5 71.1 106.7 330.7 5.0 46.9
US
1987 9.5 4.1
2007 14.5 11.5
2020 16.7 16.3 5% 13.8 2.9 2.7 13.5 0.2 1.2
2030 19.9 24.3 10% 13.0 6.9 8.4 32.1 0.5 3.7
2050 23.5 35.2 10% 13.0 10.4 15.7 48.4 0.7 6.9
China
1987 18.6 7.1
2007 34.8 27.6
2020 42.4 42.0 10% 31.3 11.5 11.3 53.5 0.8 5.0
2030 54.5 66.5 20% 27.8 26.7 32.5 124.2 1.9 14.3
2050 69.0 103.5 20% 27.8 41.2 61.7 191.6 2.9 27.1
India
1987 5.7 2.4
2007 14.4 11.5
2020 19.1 18.7 10% 13.0 6.1 6.0 28.4 0.4 2.6
2030 24.7 30.2 20% 11.5 13.2 16.1 61.4 0.9 7.1
2050 31.4 47.1 20% 11.5 19.9 29.8 92.6 1.4 13.1
Economic costs are calculated based on the current and predicted cost of N fertilizer. Environmental costs are equal to 44% of the value of the excess N applied. a The GWP of N2O based on a 1% of excess applied N being lost as N2O-N; excess N applied MMt N x 0.016(44/28) x GWP of N2O (296). CO2 GWP Value (1 Tonne CO2 = US$15). Price of N fertilizer (1987 = US$425; 2007 = US$795; 2020 = US$980; 2030 = US$1,220; 2050 = US$1,500).
doi:10.1371/journal.pbio.1001124.t002
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Rather than try to fix the consequences of
N fertilizer overuse, a better solution would
be to employ better management strategies,
such as tillage type, rate and timing of N
fertilizer application, better sources of N
fertilizer such as timed-release N fertilizer,
bioinoculants or biological N fixation, and
more N-efficient crop plants. Although
reducing or eliminating anthropomorphic
N pollution will necessitate a multi-dimen-
sional approach, we will focus mainly on one
approach, N fertilizer application reduction.
Table 3. Improvements in nitrogen use efficiency in crop plants during field trials.
Crop Year
N Fertilizer Rate (kg N ha
21 )
% Decr. in N Fertilizer
% Incr. in Yield PFPN
a % Incr. PFPN
a N Management Ref.
High Low From High Rate
From Low Rate
Americas
Maize 2000–03 191 0 60 55 72 31 BMP versus Illinois state averages
[49]
Maize 1980–00 145 0 35 42 57 36 Improved BMP and use of modern hybrids
[36]
Wheat 1994–96 250 180 28 21 20 34 67 Farmers’ practice versus BNMP in Northern Mexico
[59]
Barley 2007 169 107 37 NC b
46 71 54 Field trials in Alberta, Canada
[46]
Maize 1998–99 250 187 25 NCb 43 57 33 Improving N management Kansas & Nebraska, US
[60]
Maize 2001–03 134 101 25 NCb 50 67 34 Michigan, N2O emissions double after using more N than the low rate
[37]
Maize 2007–08 180 135 25 NC b
44 59 25 Michigan, using the lower N rate reduces N2O emission by 44%
[38]
Maize 2005–07 150 90 40 28 c
69 105 34 Eastern Canada, N2O emission at low N rate were half that of high N rate
[41]
Europe
Wheat 2003–07 200 174 13 1 27 30 11 High rate versus optimal N rate at the N:grain price ratio = 5
[42]
Cereals 1985–02 na na na 36 44 22 Average UK cereal NUE declining use of N fertilizers
[61]
Wheat Prior to 2001 200 160 20 NC b
52 64 24 Using balanced nutrients, ‘‘Law of Minimum’’
[13]
Asia
Rice 1987–99 108 76 30 NCb 45 66 47 Fertilizer rate reduced, NUE variety in Japan
[62]
Rice 1998–99 167 133 20 8 37 49 32 FFP versus SSNMd at 21 farms in China
[63]
Rice 1997–99 117 112 4 7 49 52 6 FFP versus SSNMd at 179 sites in Asia
[33]
Rice 2003–06 300 200 33 3 27 41 52 East China farmers’ N practice versus opt N fertilization
[18]
Rice 1995–98 70 53 24 NC b
30 40 33. On-farm studies, increasing plant population density
[32]
Wheat 2003–06 325 128 61 5 18 47 161 North China Plain farmers’ N practice versus opt N fertilization
[18]
Maize 2003–06 263 158 40 5 32 56 75 North China Plain farmers’ N practice versus optimum N fertilization
[18]
aPFPN, kg grain per kg kg N applied. b
NC, no change in yield. c Slight decrease in yield at the low N fertilizer rate versus high N fertilizer rate.
dFFP, farmers’ fertilizer practice; SSNM, site-specific nutrient management. doi:10.1371/journal.pbio.1001124.t003
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The Value and Consumption of N Fertilizers Are Both Rising
The global value of N fertilizers has
increased from US$32B annually in 1987
to over US$80B annually, and even
conservative estimates project it to in-
crease to US$150B by 2030 (Table 1).
Overall global consumption has increased
18% over the past 20 years, due in most
countries to an increase in cereal produc-
tion [9]. The N balance within countries
and regions reflects the N input to output
ratio as surplus, neutral, or deficit. A
surplus N balance leads to NH3, N2O,
NO3 2
, and or NO pollution while a deficit
N balance leads to low soil fertility from
depleting soil nutrient pools, resulting in
poor crop yield [10]. While overall some
countries, such as the US, have become
fairly balanced in their N input to output,
with little increase in overall N fertilizer
consumption since 1975, there are still
major areas of cropland that are rated as
having high N balances, resulting in soils
highly susceptible to losses of N2O to the
atmosphere [11]. Other regions, such as
the European Union (EU), have had
significant fertilizer N consumption reduc-
tions [12]. The large reduction in N
Box 1. Reducing Fertilizer Applications
Rice: For China, it was suggested that a reduction in N fertilizer application of 30% to 60% could be implemented for wheat, maize, and rice while still maintaining current crop yields [18]. The authors argued that reductions in N fertilizer usage would cause no significant reduction in yields in the rice/wheat and wheat/maize double-cropping systems in eastern and northern China, respectively. This is because the current N fertilizer application rates are upwards of 600 kg N ha-1 and much of this N is lost from the crop-soil system by leaching into the aquatic environment and atmospheric emissions ([18] and references within).
Japanese rice farmers use less N fertilizer currently on their crops than in the past, with no loss in yield. In the early 1990s, a fall in rice prices induced rice farmers to decrease their N fertilizer application rates from 109 kg N ha
-1 in 1985 to 80 kg N ha
-1 in
1997, while still maintaining rice yields. This success was attributed to the reduction of excessive fertilizer application and the use of an N use efficient rice variety called Koshihikari, which maintains high yield under a lower N regime [30]. Currently in Japan, nitrogen use efficiency (NUE) of rice has increased over 30% from 1985 to recent years [31].
On a broader scale, it has been demonstrated [32] that there was no correlation between countries that had high levels of yields for rice and the NUE of that country. For example, Japan had high rice yields and high NUE, whereas China had high yields but low NUE. In a multinational field trial program (179 farm sites in seven countries) for intensive rice production organized by the International Rice Research Institute (IRRI), rice grain yield was increased by 7% by balanced fertilizer use, although less N was applied [33].
Maize: There have been many N fertilization studies conducted with maize in the US, and some selected examples are shared here (Table 3). One study found that both the currently recommended N application rate (168 kg ha-1) and the farmers’ use (197 kg ha-1) exceeded the profit maximizing level of N by a minimum of 35% [34]. Minnesota farmers were able to reduce nitrogen use in corn by 21% without any reduction in crop yield [35]. Based on US Department of Agriculture statistics for US maize yield and fertilizer N used for corn production, from 1980 to 2000 US maize yields increased by 35% without significant increases in N fertilization levels [36]. A three-year Michigan corn study using different fertilizers, different fertilizer management strategies, and nine N fertilizer application rates (from 0 to 292 kg N ha-1) showed that using 101 kg N ha-1 maximized grain yield while minimizing N2O emissions, whereas using 134 kg N ha
-1 or more increased N2O emissions significantly [37]. These authors
concluded that N2O emissions could be reduced, without a yield penalty, by reducing N fertilizer inputs to a level that just satisfies the crops N requirement. A study conducted from 2007 to 2008 in Michigan at multiple commercial corn farms examining N2O response to six different N fertilizer rates (0-225 kg N ha
-1 ) showed that high rates of N fertilization led to (on
average) nonlinear increasing rates of N2O loss without economic yield gains [38]. When old versus modern maize hybrids were examined, modern hybrids had an optimal N application rate that was 18% less than older hybrids (160 kg ha-1 versus 195 kg ha-1), despite the fact that the modern varieties also had significant improvements in yield, in the range of 20% [39]. The research and education/extension programs of many of the land grant universities have been effective at reducing excess applications of N fertilizers; however, even The Economist was quick to point out that ‘‘Western countries have complacently cut back on the work done in universities and international institutions. It was a huge mistake. Basic farm research helps the whole world—and is a bargain’’ [40]. However, there have been studies to suggest that farmers applying BNMPs or new fertilizer technologies can reduce their N fertilizer application with no loss in yield [23,38]. A seven site-year study conducted on corn farms in Ontario, Canada, determined the effects of N fertilizer rate and timing on yield and N2O emissions [41]. The authors determined that although there was a slight increase in yield when fertilizer rate increased from 90 to 150 kg N ha-1, cumulative N2O emissions also doubled.
Wheat: There have been a number of studies that have demonstrated that modern wheat varieties have improved NUE (Table 3). Modern UK wheat varieties have shown a 14.6% to 18% increase in NUE, depending on the N conditions [42], while modern Spanish wheat varieties had a 24% to 29% increase in NUE (as measured by PFPN; [43]). A number of other UK wheat varieties have been evaluated and significant differences were determined in total N uptake and grain N uptake efficiency, depending on the N application rate [44]. These differences in NUE were primarily determined by greater yield, not increased concentrations of N in the plant material.
Barley: A number of studies have demonstrated that modern barley varieties have improved NUE (Table 3). Modern UK barley varieties, under optimally applied N conditions, had a 27% increase in NUE [41]. Also, modern Argentinean barley varieties had a 24% to 29% increase in NUE (as measured by PFPN) over older varieties [45]. Eight years of data for different varieties of spring barley grown in Canada were analyzed and the best performing varieties had a 7% to 17% improvement in NUE over the mean for all varieties [46].
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fertilizer consumption was achievable in
these countries because of an initial N
balance surplus that polluted the water
quality to an unacceptable level up to 1987
([12], Table 1). The EU successfully
implemented nutrient reduction programs
by developing best nutrient management
practices (BNMPs; [13])—such as coordi-
nating fertilizer requirements and applica-
tion methods and rates to particular crops,
soils, and soil water status—that have
improved the quality of ground water,
rivers, and lakes [14]. China is an example
of a region with high N balance surpluses
and an N fertilizer consumption rate that
is still on the rise. There are other regions
in the world that display N balance
deficits, for instance the countries of sub-
Saharan Africa, which have chronically
nutrient-poor soils and under-use N fertil-
izers [3].
Given the tremendous costs associated
with N fertilizer over-application, it’s
helpful to consider why the EU has
managed an overall reduction in N
fertilizer while countries such as China
have increased their use, how N fertilizer
usage could be reduced without compro-
mising crop yields, and what the economic
and environmental benefits of directed
nutrient management strategies might be.
Why Has There Been an Overall Decrease in N Fertilizer Application in Some Countries and Not in Others?
Within the EU, there has been a 56%
decrease in total fertilizer use between
1987 and 2007, including a significant
decrease in N application per hectare
(Table 1). In Denmark, for example,
producers have decreased the applied
nitrogen by 52% since 1985, resulting in
a 47% reduction in ammonia emissions
[14]. How was this achieved? Danish
agriculture was forced to employ sustain-
able agricultural methods after the adop-
tion of the Nitrate Directive in 1987
[15,16], which mandated the use of
BNMPs to reduce nitrate levels in drinking
water. After evaluating the most appropri-
ate BNMPs for specific crops, soils, and
different cropping systems and using
nutrient budgeting models (including or-
ganic N sources), Danish regulatory bodies
identified improved agronomic practices
(such as restricting fall N fertilizer appli-
cations, which are often leached as toxic
emissions over winter). From this research,
the government enacted legislation outlin-
ing specific N inputs and management
practices for each crop [14]. EU producers
are now required to provide detailed N
farm budgets before they can receive
Common Agricultural Policy (CAP) subsi-
dy payments [17].
Recent reports on Chinese agricultural
methods, in contrast, indicate that N
fertilizer use is much higher than required
for optimal yield, in some cases up to
600 kg N ha 21
[18]. The government
encouraged producers to use more fertil-
izer to attain higher yields and support
China’s domestic food security. However,
recent estimates of fertilizer usage in
China suggest that a reduction of 30% to
50% in applied fertilizer would not
necessarily reduce yields [18]. Assuming
a conservative reduction of 10% below
current usage, by 2020 China could
reduce its fertilizer use by 11.5 MMt per
year, compared to the predicted increase
in N application. This would result in
savings to Chinese producers of US$11.3B
annually (Table 2).
Reducing N Fertilizer Application without Reducing Yield
In the US, the United Kingdom, and
other countries, rice (Oryza sativa), maize
(Zea mays), wheat (Triticum aestivum), and
barley (Hordeum vulgare) have been grown
experimentally to determine their N re-
sponse to increasing fertilizer applications
(commonly expressed as an N response
curve; Table 3). These long-term studies
demonstrate that implementing BNMPs
can allow for a reduction in N fertilizer
application with no loss to yield, even in N
balanced systems. Also, for those develop-
ing countries that need to increase their N
fertilization rates, there is still a require-
ment to implement local specific manage-
ment strategies to increase yield and
reduce future excessive application rates
(Table 3).
Many field studies have been done in
various regions of the world, analyzing the
optimum BNMPs for the specific region,
including fertilizer rate, for a variety of
crops. All of these studies indicate that
reductions in fertilizer usage, in those
situations where it is being applied in
excess, can occur without any loss in yield
(see Box 1).
Economic and Environmental Benefits of Using Directed Nutrient Management Strategies
The economical optimal N rate
(EONR) is the rate of fertilizer that allows
for the maximum economic yield [19,20].
After the fertilizer price has been included,
a lower N fertilization rate than the
maximum yield rate should be applied.
What is now needed is a way to measure
the environmental and economic optimal
N rate (EEONR). This N rate takes into
account the N fertilizer price plus the cost
of the N lost to the environment. The
environmentally optimal N application
rate for maize was recently calculated,
suggesting that a rate of 25 kg ha 21
less
than the economic optimal N application
rate would reduce GHG emissions [21].
The Iowa State University Agronomy
Extension in 2004 recommended another
approach, the maximum return to N
(MRTN), using a range of economical N
inputs for US Midwest corn farmers that
take into account both N fertilizer prices
and corn prices [11]. Although this
approach does not directly take into
account environmental costs, it does
suggest a range of fertilizer rates, on
average, 185 kg N ha 21
(the high profit-
able N rate) to 158 kg N ha 21
(the low
profitable N rate) that are both below the
well-used and recommended yield goal N
Table 4. N losses to the environment and the calculated economic value of these costs for the US.
N Component Average (%) Range (%) Environmental Costs (US $B)
Plant product 40 30–50 NA
Tier 1 N2O-N emission factor 1 0.003–0.03 1.01B a
Leaching and runoff (Nitrate) 20 15–28 3.6B
N2O from volatized ammonia 20 15–25 0.47B b
Lost or denitrified 19 15–83 ND
Total 100 — 5.1B (44%)
Total N applied in the US in 2007 was 14.5 MMt at a value of US$11.5B. a 14.5MMt x 0.016(28+16 g mol21 / 28 g mol21 ) 6 2966$15/t = US$1.01B.
b(14.5 MMt N fertilizer US) / (83 MMt N fertilizer globally) 6(0.6 Tg N2O formed from ammonia volatilization) = 0.11 Tg N2O volatized ammonia in the US per year. 0.11 MMt N2O62966US$15/t = US$0.47B.
doi:10.1371/journal.pbio.1001124.t004
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rate of 250 kg N ha 21
, or more [11]. This
reduction in N fertilizer rate also reduced
N pollution of the ecosystem. Many
studies conducted in the US, especially
through the corn-belt region, show that
loss of N to crops can be reduced by
reduced N fertilizer application, manage-
ment practices, and type of fertilizer used
[22,23]. Nutrient management strategies
take into consideration not only N fertil-
izer application rate, but also factors
including type of tillage, type of N
fertilizer, and rotation with N fixing crops.
N fertilizer is needed to maintain or
increase crop yields; however, depending
on the tillage system and crop rotation
used, a high N application rate can
decrease farmer profits and increase N2O
emissions [23]. For example, corn farmers
in Colorado using a conventional tillage
and continuous corn (CT-CC) manage-
ment system can reduce both GWP and
increase net profits by reducing N fertilizer
application. If those same farmers
switched to a no-till corn-bean rotation
system, they could further reduce GWP
and increase profits but at a higher N
fertilizer rate than for CT-CC [23].
The type of N fertilizer applied can
directly affect N2O emissions as well.
Box 2. The Environmental Cost of Excess N Applications
Global atmospheric N2O concentrations have increased from the pre-industrial level of 270 ppb to 319 ppb in 2005, with agriculture (fertilizer use and animal production) as the primary source of this added N2O. N2O can remain in the atmosphere for approximately 114 years [47]. The FAO has predicted that by 2030 global N2O emissions from fertilizer and manure application will increase by 35% to 60% [38]. For the loss of N by emission of N2O via denitrification, we used the Intergovernmental Panel on Climate Change [47] linear Tier 1 N2O default emission factor of a 1% loss of applied N as N2O-N (1 kg of N2O-N emitted per 100 kg of applied N) which takes into consideration N2O-N emissions from N applied as mineral and organic fertilizers, crop residues and N mineralized from soil due to loss of soil carbon [47]. It should be noted, however, that N2O emissions can vary due to not only N fertilizer rate, but also soil type (texture, drainage, pH), soil organic carbon levels, climate, type of N fertilizer applied, method of fertilizer placement, and crop type grown [47,48]. Several studies conducted in the US and Canada have shown that N2O-N emission rates can be nonlinear, especially at higher N fertilizer rates, showing that higher N fertilizer rates can produce exponential N2O emissions [11,38,41]. Since this 1% N2O-N emission factor is an estimate, it may under-represent the actual N2O emission rate when the N fertilizer rate exceeds the crop or soil uptake ability [22]. Globally in 2005, N fertilizer use was approximately 93 MMt and caused an estimated emission of 1.46 MMt of N2O, equal to 433 MMt of carbon dioxide equivalents (CO2e) [48]. In 2007, in the US, 14.5 MMt of N were applied to crops [9], representing 0.228 MMt of N2O emissions, having the GWP of 67.4 MMt of CO2e. Therefore, the partial environmental cost of soil N2O emissions can be estimated based on the CO2 equivalency. Carbon dioxide credits are traded as commodities on the European and New Zealand CO2 exchanges, so they have a monetary value. When the value for CO2 is taken as US$15/ton, the N2O emissions in the US equates to a value of US$1.01B annually. Although N is also lost as NO2 and N2 (20% of applied N may be lost as N2; [49]) via nitrification and denitrification, there is no directly measurable cost associated with these types of N loss, so we did not include these in our partial estimates of environmental costs. As well, N2 does not have a negative environmental impact on the ecosystem.
One measure to determine the economic cost of excess nitrate from runoff and leaching would be to look at the economic and social impact excess N has against specific industries. As an example, about 8% of the N applied in the US corn-belt is being directly exported into the Gulf of Mexico via the Mississippi River [50]. This lost N has both a direct economic cost to the agricultural producers, but also has an indirect negative impact on other economic activities. In the Gulf of Mexico, commercial and recreational fisheries currently generate US$2.8B annually. However, one half of the shellfish and many oyster beds have either been permanently closed or declared indefinitely off-limits by health officials as a result of N pollution [51]. Therefore, we estimated the cost to the Gulf marine economy to be US$1.4B annually. An analysis of the economic cost of eutrophication of US freshwaters as it pertains to loss of recreational activities, property value, threatened and endangered species recovery efforts, and drinking water was recently completed [52]. Dodds et al. [52] provide a conservative estimate of the eutrophication cost to be US$2.2B annually for the US fresh waterways. Therefore, in total, a conservative cost estimate for excess runoff and leaching in the US is US$3.6B.
Losses of ammonia from N fertilizer application can be as high as 50% to 80%, depending on climate, type of fertilizer used, application method, and soil type [53]. Livestock manures and urea fertilizers tend to volatize the most ammonia globally at 23% and 21%, respectively [53]. Ammonia is not considered to have a direct GWP, so a direct cost of ammonia volatization is difficult to calculate. However, ammonia emissions affect air, water, and land quality, and can lead to ‘‘acid rain,’’ which causes marine and soil ecosystems to become acidic and in turn contributes to aquatic eutrophication and soil acidification [49]. High levels of ammonia and ammonium can reduce plant diversity, increase plant predation by insects, and cause serious human diseases, including cardiovascular and lung diseases and asthma [53]. Ammonia has a short life span in the atmosphere and is either dry deposited locally to the site of emission or converted in the atmosphere to ammonium (NH4
+), nitric oxides (NOx), and N2O. Ammonium can accumulate in clouds and be wet deposited in regions distant from the site of emission. Globally, synthetic fertilizers and agricultural crops account for 12% of total ammonia emissions [54]. Of the 83 MMt of N fertilizer used globally in 1996, an estimated 0.6 Tg of N2O was formed from atmospheric ammonia oxidation. Assuming similar losses in the US, 0.11 Tg of N2O was formed in 2007, with an indirect GWP cost of US$0.47B.
We determined the environmental costs from excess N to conservatively be 44% of the cost of the total N applied in the US. We then used this value in Table 2 to model the environmental costs associated with excess applied N for the world, the US, China, and India. While these gross cost estimates may not be accurate for any one crop, they provide a starting point for discussion. We fully recognize the challenges of accurately estimating site-specific N losses. However, the important goal is to identify the costs associated with the various types of N pollutants. These cost estimates can then be used to develop economic tools to ensure that the environmental costs are integrated into BNMPs.
.
PLoS Biology | www.plosbiology.org 6 August 2011 | Volume 9 | Issue 8 | e1001124
Research conducted in Colorado for two
years on N2O emission rates from irrigat-
ed no-till-corn grown with enhanced-
efficiency N fertilizers versus conventional
dry urea and liquid urea-ammonium
nitrate showed that the enhanced-efficien-
cy N fertilizers reduced N2O-N emissions
while maintaining yield [24]. Yields of
Minnesota potatoes were maintained
while reducing N2O emission by using
single, pre-plant applications of polymer-
coated urea for N fertilizer compared to
multiple split applications of conventional
uncoated urea [25]. As well as maintaining
yields with fewer N2O emissions, the N
fertilizer costs were reduced due to the
need for only a single application versus
multiple applications with conventional
urea.
N fertilizer (organic and inorganic) that
is not taken up by crop plants can be lost
to the environment through nitrification/
denitrification of ammonium/nitrate (re-
spectively) by soil microbes. N runoff and
leaching of nitrate into waterways (aqui-
fers, rivers, lakes, and oceans) and ammo-
nia volatization into the atmosphere can
also occur. While we recognize that losses
vary dramatically, depending on multiple
variables, we made a number of simple
assumptions to model these N losses from
excess N fertilizer applications and calcu-
late their economic costs to the environ-
ment. For most cereal crops, only 30% to
50% of applied N is actually taken up by
the plant [26,27]. Therefore, we assumed
that plants take up approximately 40% of
the available N with the remaining 60% as
surplus N. The fate of the surplus N can
include becoming an environmental pol-
lutant (Table 4), or held in soil as organic
or inorganic N, depending on the soil and
N type.
We attempted to determine the envi-
ronmental and associated economic costs
of N applications, using the US as an
example (see Box 2). The choice of the US
was based on the fact that there are better
data available. Using both fertilizer use
and price projections, we evaluated the
cost savings associated with reducing N
budgets such that they matched the
appropriate regional fertilizer recommen-
dations (Table 2). All countries analyzed in
Table 2 were assigned a neutral, or
reduction in N fertilizer use (5% to 20%),
based on analysis of their overuse of
fertilizers in the selected literature we have
cited (Table 3). While this analysis includ-
ed only four major regions/countries,
these collectively account for 74% of
global fertilizer use [9]. Based on this
analysis, savings of US$19.8B per year and
US$56B per year are attainable by 2020
and 2030 respectively, assuming no
change in the area of farmed land.
Several Simple Proposals to Reduce N fertilizer Use
It is clear from many studies that when
N application rates are in balance, N losses
via N2O emissions and leached nitrate are
reduced to a minimum, depending on the
cropping system [17,28]. Although dry
land cereal production in Canada is
usually based on a single, pre-planting
application of N fertilizer and mobilization
of the applied N is by rain-fed moisture,
many cropping systems allow revised
application rates, which, along with more
careful monitoring of the 4Rs (right
source, right time, right place, right rate),
can result in significant reductions of N
losses that harm the environment. Clearly,
by using BNMPs, the producer benefits
from reduced costs while everyone benefits
from an improved environment.
In order to successfully optimize the use
of N fertilizers (both agronomically and
environmentally), we propose several sim-
ple approaches. First, fertilizer use require-
ments need to be reassessed in virtually all
agricultural systems, from an economic
and environmental perspective. Second,
economic and environmental models need
to be integrated and be made user-
friendly, particularly in those developed
and developing countries where excessive
N use occurs. Third, countries need to
ensure that government programs do not
discriminate against producers who volun-
tarily choose to use less fertilizer. For
example, crop insurance often requires the
farmer to apply fertilizers at the recom-
mended (but potentially out-dated) rate,
otherwise they will not be compensated for
potential crop losses. Fourth, we need to
find economic tools to better inform and
drive changes in N application rates. It is
easy to say that reducing rates will help
reduce N2O emissions, but the producer
does not benefit economically from that,
unless there is some form of payment for
reducing N applications. This is effectively
providing a global ecological service.
Some countries, such as Austria and
Finland [29], have begun to implement
‘‘green taxes’’ (i.e., taxes on fertilizers and
agrichemicals). However, at a minimum,
we need to eliminate ‘‘negative green
incentives’’, which often provide direct
subsidies to farmers to use fertilizers.
Regardless of the tools used to promote
change (legislative, economic), education
programs need to be put in place imme-
diately to promote the environmental and
economic benefits of the optimal use of N
fertilizer.
In Conclusion
Through a combination of the 4R
BNMPs and advances in fertilizer tech-
nology and plant genetics, it may be
possible to reduce global N application
rates by 20% by 2050, saving US$150B
annually, compared to business as usual.
Unlike many of the challenges faced by
agriculture, reducing excess nutrient ap-
plications (as demonstrated by the EU) is
within our ability. Finally, farmers, scien-
tists, and economists need to communicate
more efficiently to promote the use of the
EEONR and BNMPs while providing
scientific data and leadership to address
this issue.
Acknowledgments
The authors thank Rebecka Carroll, Jayne
D’Entremont, and Juan Wang for their help,
comments, and discussions, and an anonymous
reviewer for valuable input and insights.
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