Nitrogen on the Land: Overcoming the Worries
Lifting fertilizer
efficiency and preserving land for nonfarming uses
Charles
R. Frink, Paul E. Waggoner, and Jesse H. Ausubel
Pollution Prevention Review11(3):77-82 (Summer 2001)
URL: http://phe.rockefeller.edu/NITROGEN2/
Abstract
Which worries about nitrogen fertilizer are well-founded, and where is
the leverage to help? History does not justify worries about
exponential increase in use or widespread rising deposition from the
atmosphere. Nevertheless, nitrogen leaching and water pollution
justify conservation. We analyse four scenarios for fertilizer use to
the year 2070. If farmers sustain both their lifting of yields and
increasing efficiency of nitrogen use, they can feed future world
population better, while freeing the area of ten Iowas for nature and
eliminating excess nitrogen.
Belief in an exponential rise of synthetic nitrogen (N) fertilizer
use evokes fear of global nitrogen overload (Vitousek et al. 1997;
Moffat 1998). Belief that a tandem widespread rise in nitrogen
deposition from the air is diminishing biodiversity intensifies the
fear of overload. In the background looms the fear that more numerous,
richer humans will demand that farmers grow their food with large
multiples of today’s nitrogen, and thus push Nature aside.
Meanwhile, in recent newspaper articles, readers learn paradoxically
of weak agricultural markets: European farmers demonstrate with their
tractors before ministers in Brussels. American farmers invite hunters
to shoot their pigs. Manufacturers’ capacity to make fertilizer exceeds
demand.
Which fears are well-founded, and where is the leverage
to help? This article briefly considers
these issues, and suggests some answers.
Nitrogen Fertilizer
In 1931, Karl Bosch received the Nobel Prize for making nitrogen
fixation practical. Like many new technologies, synthetic nitrogen
fertilizer enjoyed strong growth, from 1.3 Tg nitrogen in 1930, to 83 Tg
in 1998. (One Tg equals a trillion grams or a million metric tons.
These data refer to the fertilizer year 1 July-30 June. Thus, for
example, 1998 data are for 1998-99.) By “synthetic nitrogen
fertilizer,” we mean commercial product, with almost all the nitrogen
fixed by the Haber-Bosch process -- in contrast to manure, guano, oil
meals, and packing-house waste.
Is the use of nitrogen fertilizer growing exponentially? No. A
constant percentage increase accelerates a curve exponentially. This is
not what is happening with nitrogen fertilizer.
After an explosive annual growth of 14% during the period 1945-56,
global growth of nitrogen fertilizer use slowed to 8% as 1970
approached. Growth then slowed further to 5% yearly during 1976-85, and
to less than 1% during 1986-98. In Europe and the United States, which
adopted the new technology early, growth slowed even sooner. Since
1976, use in Europe and the U.S. has grown 1% or less yearly, and
sometimes has actually fallen.
Like
numerous products that saturate their market, nitrogen fertilizer fits not an
ever-rising exponential growth pattern, but an S-shaped logistic one (Frink et
al. 1999).
Nitrogen Deposition
The nitrogen falling from the atmosphere, mostly as
NO3-nitrogen and NH4-N (ammonium-N), can be
measured by simply collecting it in the open as “bulk deposition.”
“Wet deposition” is collected only during precipitation.
“Throughfall” is nitrogen deposited on forest canopies and eventually
falling or washing down in precipitation. Nineteenth century
agronomists, who were concerned about deposition on crops, collected
bulk deposition. More recently, scientists concerned about acid rain
(caused by sulfur and nitrogen) collected wet deposition, and ecologists
concerned about accumulation of nitrogen in forests collected
throughfall beneath trees.
In order to understand nitrogen deposition, we must compare it with
some scale. Synthetic fertilizer use has increased approximately 100
fold. Concurrently, worldwide NO3-nitrogen from high
temperature combustion in car motors and power plants has risen 10-fold
to about 20 Tg.
Farmers increase crop yields with 100 to 200 kg nitrogen ha-1
(kilograms per hectare). For prevention of eutrophication and
95% safety, Europe has established incremental critical loads of 3-10 kg
ha-1 (with standards of less than 3 and more than 10 in some
places) (Posch et al. 1997).
In infertile soil made responsive by fertilization with other
nutrients, increments as little as 10 kg, but sometimes not less than 54
kg, nitrogen ha-1 decreased biodiversity (Tilman 1987).
Early in the 20th century, bulk deposition in Europe and the
U.S. was 4 to 7 kg ha -1.
Anyone smelling a barnyard knows local deposition of NH4-N
can be high. In the Netherlands, a local throughfall of 100 kg
ha-1 attributed to animals will affect vegetation (Ivens,
1990). The manure km-2 in the Netherlands is five times that
in the quintessential U.S. agricultural state of Iowa. Locally, where
animals are concentrated, throughfall can approach farmers' fertilizer
rates.
However, deciding whether widespread, rather than local, increase of
nitrogen deposition accompanied the use of more nitrogen fertilizer
requires measurements spanning decades at several places. For a
comparison that offers data over a period as long as the growth of
fertilizer use, we must turn to the simple, robust measure of bulk
deposition. In Rothamsted, England, annual deposition rose a total of
about 1 kg ha-1 between 1888 and 1966. Bulk deposition of
8.7 kg (estimated from wet deposition during 1987-96 at nearby Woburn)
confirms an increase, perhaps 5 kg ha-1 during the
century.
In northern Netherlands, bulk deposition increased about 7 kg from
6.7 kg ha-1 at Groningen during 1908-10 to 14.5 kg at nearby
Kollumerwaard during 1994. In Sweden, it changed little from the 5.1 kg
ha-1 at Flahult in 1909 to 7.1 kg in 1996-97 at three nearby
stations (Frink et al. 1999).
At Geneva and at Ithaca, New York, during the first quarter of the
20th century, annual bulk deposition ranged from 4 to 8 kg
ha-1. About 6 kg was also deposited at Mays Point and
Huntington Forest, New York, during 1965-80 and also at Hubbard Brook,
New Hampshire, during 1972-92 (Frink et al. 1999). In the northeastern
United States during the 20th century, bulk deposition changed
little.
One should not be surprised that nitrogen fertilizer use and
widespread increase of nitrogen deposition did not rise in tandem.
Except for escape of some ammonia fertilizer and dust and a little
NO2, no direct path puts fertilizer nitrogen in the air for
deposit. Also, even 80 Tg fertilizer nitrogen plus 20 Tg
NO3-nitrogen from high temperature combustion is insufficient
to increase widespread deposition significantly; in the impossible case
of all 100 Tg being deposited, it would average only about 2 kg
ha-1 on Earth. The low concentration of nitrogen in
Greenland precipitation (Mayewski et al. 1990) corresponds to deposition
far less than that.
Given the moderate likely rise in nitrogen use that we now project,
prospects seem good for any future increase in deposition to continue
making the minor contribution to plant growth that Nadelhoffer et al.
(1999) have calculated is now being made. This should pose little
hazard to biodiversity.
Prospects to 2070
After overcoming worries about exponential increase and widespread
rising deposition, we still come to the lurking fear of burgeoning,
richer humans eventually demanding farmers smother Nature with multiples
of today’s nitrogen in order to grow more food. Although nitrogen from
fertilizer can scarcely increase widespread atmospheric deposition, its
leaching and pollution of water justify conservation.
Foreseeing what humanity will eventually demand requires integrating
forces that drive fertilization (Frink et al. 1999). The change of
fertilizer use (see Exhibit 1) is the sum of changes in four
forces: population, GDP/capita, crop production/GDP, and the Ratio of
nitrogen in fertilizer to nitrogen in crops, which we capitalize to
“Ratio.” (We neglect the small change over time in the nitrogen
composition of the crop mix.) To foresee prospects to 2070, and a
population of roughly 10 billion people, we assume a population growing
at 0.8% and GDP per capita at 1.8%, to lift their sum, the total GDP, to
2.6% per year.
Although wealthy people eat better than the poor do, they do not eat
more in fixed proportion to GDP. Hence, the declining ratio of crops to
overall GDP has mirrored the rise of world GDP. Whereas the GDP/Cap
data in Exhibit 1 are above the zero change line, the Crop/GDP are below
the zero change line, signifying a falling ratio of crops to GDP.
Expecting that the world economy will continue to favor computer chips
over potato chips, we project a long decline of 1.0%/year in crop/GDP.
The sum of GDP per capita rising 1.8%, and crop/GDP falling 1.0%, still
raises the crop for all persons by 2070 to the level that rich countries
enjoy now.
Farmers control the fertilizer per crop as they economize inputs.
Globally, the Ratio of fertilizer nitrogen to crop nitrogen recently
plummeted 2.0%/year. In the United States, it fell earlier, and since
the 1970s has fallen some 1.0%/year. For the long pull, a decline of
0.5%/year seems reasonable as farmers continue lifting yields faster
than their nitrogen fertilizer use grows. The sum of the changing
forces (0.8 + 1.8 - 1.0 - 0.5%/year) would raise world nitrogen
fertilizer use 1.1%/year to 2.4 times the 1990 use by 2070.
Concern for sparing only fertilizer would be myopic. We integrate
with the saving of fertilizer the sparing of land, which is preeminent
for sparing Nature. Accordingly, we project cropland taken as well as
fertilizer used (see Exhibit 2).
As a baseline for prospects to 2070, consider the situation in 1990,
when 79 Tg fertilizer and 11% of world land yielded the caloric
equivalent of 1,900 kg ha-1. The 150% Ratio of fertilizer
nitrogen to crop nitrogen shows that nitrogen from fertilizer, plus that
from legumes and manure, far exceeds nitrogen incorporated into crops,
indicating an opportunity for conservation.
Our first projection for 2070 (and assuming a population of ten
billion people) is a situation in which farming has stagnated at 1990
levels in terms of yields and Ratio. The consequent 284 Tg fertilizer
nitrogen, or 3.6 times 1990 use, exceeds the 2.4 times projected above
because the Ratio remains 150%. Under this projection, cropland has
expanded because population and crop per capita has grown, while yield
has stagnated. A scenario assuming use of 284 Tg nitrogen, and 38% of
the land, offers the scary specter of population and wealth eventually
demanding that farmers push Nature aside and smother the land in
fertilizer.
Researchers can forestall that specter becoming reality with at least
two levers: Substitute nitrogen already on farms for nitrogen
fertilizer, and raise yields, thus lowering the Ratio of fertilizer
nitrogen to crop nitrogen.
Farms fix nitrogen in legumes and collect nitrogen in manure.
Although so-called alternative agriculture features nitrogen fixed by
legumes, legumes take land and devour natural habitat. Manure is
another matter. The estimated 80 Tg nitrogen in the world’s manure
matches the 83 Tg fertilizer nitrogen used now and is significant
compared to the projected 284 Tg. Remembering that some manure nitrogen
is already captured in crops, but that animals may multiply by 2070, one
can envision 50 Tg of manure nitrogen substituting for fertilizer
nitrogen, decreasing the fertilizer-nitrogen-to-crop-nitrogen Ratio to
124% with crops still using 38% of the land.
The ancient challenge of saving and storing nitrogen without odor
continues. Concentration of animals has focused attention on odor and
increased the distances for hauling manure to fields. It has also
changed the goal from conserving nitrogen to denitrifying it and
controlling odor. A challenge for pollution prevention is replacing the
negative tasks of regulating odor and returning N2 to the air
with the positive task of discovering profitable and practical ways to
move more of the dilute nitrogen in manure into crops.
Conserving fertilizer can also lower the Ratio of fertilizer nitrogen
to crop nitrogen. Conservation includes testing soil nitrogen and
adjusting application to each site by precision farming and splitting
applications during the season to avoid leaching. Because yields divide
the Ratio, higher yields can lower the Ratio and spare land, too.
Increasing yields to lower the Ratio involves breeding better crop
varieties, plus removing limitations such as other nutrients, water, and
pests so that crops can exploit the nitrogen provided.
While words about lowering the Ratio of fertilizer nitrogen to crop
nitrogen could be mere anecdotes, statistics show farmers are in fact
doing it (see Exhibit 1). During the 1980s and 1990s the worldwide
Ratio fell. In the United States since the 1970s, the Ratio of
fertilizer nitrogen to crop nitrogen fell about 1%/year. With regard to
a specific locale and crop, the Ratio for Indiana, Iowa, and Nebraska
corn fell 1% to 3% per year during 1980-96 (Frink et al. 1999). Farmers
lifting the world average yield of corn, rice, soybeans, and wheat by an
annual average of 1.6 to 2.3% during 1961-2000 demonstrate that higher
yields also are not mere anecdotes.
A projection involving slower lifting of yields, but conserved
nitrogen, shows that raising yields 1% and lowering the Ratio 0.5%/year
would take 19% of land, while lowering nitrogen fertilizer use to 192
Tg. This is the 2.4-fold increase we deem probable.
With sustained lifting of yields and conserved nitrogen, raising
yields 2% and still lowering the Ratio of fertilizer nitrogen to crop
nitrogen would require the same 192 Tg of nitrogen, but would actually
shrink cropland 144 million hectares, which is about half the area of
India, or ten Iowas.
Today, fertilizer manufacturers have idle capacity. This, combined
with weak agricultural markets in Europe and the U.S., reveal how
groundless are fears that fertilizer use will soon explode. There is
little prospect of a widespread downpour of nitrogen from the air, or
Nature pushed aside to relieve hunger.
Outline for Future Action
For the long run, however, and to relieve “hot spots” of nitrogen
deposition, several things need to be done. On the grand scale,
regulators need to monitor two simple environmental metrics: land
spared from cropland expansion and the Ratio of fertilizer nitrogen to
crop nitrogen.
On the local scale, where action is taken, fertilizer manufacturers
should offer information on how to make fertilizer go further while
farmers, the most hard-pressed actors, struggle to survive.
Researchers, the actors with the most opportunity to effect change,
should devise affordable ways for farmers to utilize manure (now seen as
a nuisance) for higher yielding crops.
Meeting these challenges will temper the use of nitrogen fertilizer,
while sparing more land for Nature.
Exhibits
Exhibit 1. Annual
Global Changes in Population, GDP per Capita, Crop Production per GDP, and the
Ratio of Synthetic Fertilizer Nitrogen to Crop Nitrogen.
Exhibit 2. Prospects to 2070.
References
Frink, C.R.,
Waggoner, P.E., & Ausubel. J.H. (1999). Nitrogen Fertilizer:
Retrospect and Prospect. Proceedings, National Academy of Sciences,
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Ivens, W.P.M.F. (1990). Atmospheric Deposition onto Forests.
Analysis of the Deposition Variability by Means of Throughfall
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Mayewski, P.A., et al. (1990). An Ice-Core Record of Atmospheric
Response to Anthropogenic Sulphate and Nitrate. Nature 346:554-556
Moffat, A.S. (1998). Global Nitrogen Overload Problem Grows
Critical. Science 279:988.
Nadelhoffer, K.J., et al. (1999). Nitrogen Deposition Makes a Minor
Contribution to Carbon Sequestration in Temperate Forests. Nature
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Posch, M., Hettelingh, J.-P., de Smet, P.A.M., & Downing, R.J.
(1997). Calculation and Mapping of Critical Thresholds in Europe. RIVM
Report 259101007.
Tilman, D. (1987). Secondary Succession and the Pattern of Plant
Dominance along Experimental Nitrogen Gradients. Ecological Monographs
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Charles R. Frink (charles.frink@po.state.ct.us)
is a soil expert and scientist emeritus at the Connecticut
Agricultural Experiment Station in New Haven. Paul E. Waggoner
(paul.waggoner@po.state.ct.us) is an agronomist and meteorologist, and
past director of the Connecticut Agricultural Experiment Station.
Jesse H. Ausubel (ausubel@mail.rockefeller.edu)
is an industrial ecologist and director of the Program for the
Human Environment at The Rockefeller University in New York City (http://phe.rockefeller.edu).
