Index Suggests That Half of Nitrogen Applied to Crops Is Lost
Food production is becoming less efficient at using nitrogen
fertilizer, according to a review of global values. Excess nitrogen
damages the environment and the climate.
The power of nitrogen to keep crops healthy can be seen in these corn
hybrids. The adult hybrid plants at left, from 2015, keep nitrogen in
their leaves longer than the 1958 hybrids at right do. Credit: Tony
Vyn
“If we don’t deal with our nitrogen challenge, then dealing with
pretty much any other environmental or human health challenge becomes
significantly harder.”
Nitrogen use efficiency, an indicator that describes how much
fertilizer reaches a harvested crop, has decreased by 22% since 1961,
according to new findings by an international group of researchers who
compared and averaged global data sets.
Excess nitrogen from fertilizer and manure pollutes water and air,
eats away ozone in the atmosphere, and harms plants and animals.
Excess nitrogen can also react to become nitrous oxide, a greenhouse
gas that is 300
times more potent than carbon dioxide.
Significant disagreements remain about the exact value of nitrogen use
efficiency, but current estimates are used by governments and in
international negotiations to regulate agricultural pollution.
“If we don’t deal with our nitrogen challenge, then dealing with
pretty much any other environmental or human health challenge becomes
significantly harder,” David
Kanter, an environmental scientist at New York University and
vice-chair of the International Nitrogen Initiative, told New
Scientist in May. Sri Lanka and the United Nations Environment
Programme called for countries to halve nitrogen waste by 2030 in theColombo
Declaration.
Whereas the global average shows a decline, nitrogen fertilizing has
become more efficient in developed economies thanks to technologies
and regulations, and new results out last month from the University of
Minnesota as well as field trials by the International Fertilizer
Development Center are just two examples of ongoing research to limit
nitrogen pollution without jeopardizing yield.
Too Much of
a Good Thing
Nitrogen is an essential nutrient for plant growth: It
is a vital aspect of amino acids for proteins, chlorophyll for
photosynthesis, DNA, and adenosine triphosphate, a compound that
releases energy.
Chemist Fritz
Haber invented an industrial process to create nitrogen fertilizer
in 1918, and the practice spread. Since the 1960s, nitrogen inputs on
crops have quadrupled. In 2008, food production from nitrogen fed half
the world’s population.
“One of the things that is evident in nitrogen
management, generally, is that there seems to be a tendency to avoid
the risk of too low an application rate.”
said Tony
Vyn,
an agronomist at Purdue University.
Yet nitrogen applied to crops often ends up elsewhere. Fertilizer
placed away from a plant’s roots means that some nitrogen gets washed
away or converts into a gas before the plant can use it. Fertilizer
applied at an inopportune moment in a plant’s growth cycle goes to
waste. At a certain point, adding more fertilizer won’t boost yield:
There’s a limit to how much a plant can produce based on nitrogen
alone.
In many parts of the world, cheap subsidized fertilizer is critical
for producing enough food. But gone unchecked, subsidies incentivize
farmers to apply more than they need. And according to plant
scientist Rajiv
Khosla at Colorado State University, who studies precision
agriculture, farmers struggle to apply just the right amount of
fertilizer probably90%
of the time.
The 90%
Efficiency Goal
According to an average of 13 global databases from 10 data sources,
in 2010, 161 teragrams of nitrogen were applied to agricultural crops,
but only 73 teragrams of nitrogen made it to the harvested crop. A
total of 86 teragrams of nitrogen was wasted, perhaps ending up in the
water, air, or soil. The new research was published in the journal Nature
Food in July.
Globally, nitrogen use efficiency is 46%, but the ratio should be much
closer to 100%, said environmental scientist Xin
Zhang at the University of Maryland, who led the latest study. The
crops with the lowest nitrogen efficiency are fruits and vegetables,
at around 14%, said Zhang. In contrast, soybeans, which arenatural
nitrogen fixers, have a high efficiency of 80%.
The United States has similarly cut losses by improving management and
technology. For instance, even though the amount of nitrogen
fertilizer per acre applied to cornfields was stable from 1980 to 2010
in the United States, the average crop grain yields increased by 60%
in that period, said Vyn. Those gains can be hidden in broad-stroke
indices like global nitrogen use efficiency.
“The most urgent places will be in China and India because they are
two of the top five fertilizer users around the world,” Zhang said.
China set a target for a zero increase in fertilizer use in 2015,
which showed promising
early results.
Cultivating
Solutions
New research from the University of Minnesota using machine
learning–based metamodels suggested that fertilizer amount can be
decreased without hurting the bottom line.
Just a 10% decrease in nitrogen fertilizer led to only a 0.6% yield
reduction and cut nitrous oxide emissions and nitrogen leaching. “Our
analysis revealed hot spots where excessive nitrogen fertilizer can be
cut without yield penalty,” said bioproducts and biosystems engineer Zhenong
Jin at the University of Minnesota.
Applying fertilizer right at the source could help too: A technique
developed by the International Fertilizer Development Center achieved
an efficiency as
high as 80% in field studies around the world using urea deep
placement. The method buries cheap nitrogen fertilizer into the soil,
which feeds nitrogen directly into a plant and reduces losses.
Meanwhile, Vyn said researchers must focus on sharpening scientific
tools to measure nitrogen capture. The differences in nitrogen inputs
in the databases analyzed by the latest study were as high as 33%
between the median values and the outliers.
“The nitrogen surplus story is sometimes too easily captured in a
simple argument of nitrogen in and nitrogen off the field,” Vyn said.
“It’s more complex.” His research aims
to improve nitrogen recovery efficiency by understanding plant
genotypes and management factors.
One of Zhang’s next research steps is to refine the quantification of
nitrogen levels in a crop, which is currently based on simplistic
measurements. “There has been some scattered evidence that as we’re
increasing the yield, the nitrogen content is actually declining. And
that also has a lot of implications in terms of our calculated
efficiency,” Zhang said.
Duncombe, J. (2021), Index suggests that half of nitrogen applied to
crops is lost, Eos,
102, https://doi.org/10.1029/2021EO162300.
Published on 23 August 2021.
