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Can nitrogen clean up its act on farmlands?

Agricultural nitrogen pollution is widespread in the U.S., but farmers can adopt new practices to prevent it

by Katherine Bourzac, special to C&EN
April 17, 2017 | A version of this story appeared in Volume 95, Issue 16


Satellite image of the Gulf of Mexico showing runoff.
Credit: NASA
In the Gulf of Mexico, discharge of agricultural nutrient runoff and sediment from the Mississippi River (visible as tan and green at the shoreline in this satellite image from May 17, 2011) feeds a summer dead zone that can be as large as 20,000 km2.

A dead zone forms in the deep waters of the Gulf of Mexico every summer. This low-oxygen, or hypoxic, area can stretch for 20,000 km2 some years, with significant consequences for the Gulf’s ecosystems and economy.

Hypoxia puts the multibillion-dollar tourism and fishing industries of the Gulf states at risk, but the problem starts far to the north. The Gulf dead zone feeds on the runoff of nutrients, particularly nitrogen fertilizers, from farms in the Midwest Corn Belt whose rivers and streams feed the Mississippi and Atchafalaya Rivers.

When these nutrients reach the Gulf’s waters, they trigger explosive growth of algae. Eventually, these algae die and aerobic microbes feast on them and on other organic matter, consuming large amounts of oxygen from the water in the process.

When levels of dissolved oxygen fall below 2 mg/L, creatures such as shrimp that can crawl or swim vacate the area, leaving little or no catch for trawling fishers. At 0.2 mg/L dissolved oxygen, the sediment turns black as bacterial mats form on the seafloor and the microbes start oxidizing sulfur. And when the water becomes completely devoid of oxygen, anaerobic bacteria thrive and begin producing hydrogen sulfide, which is toxic to most animals.

The Gulf dead zone may be the most dramatic example of nitrogen pollution in the country, but its effects can be seen elsewhere in the U.S. and across the globe. Nutrient runoff from farmlands can contaminate drinking water wells or pollute streams so that people can’t fish or swim in them. According to a 2012 Environmental Protection Agency fact sheet, 15,000 bodies of water in the U.S. are impaired by nutrient pollution, and 78% of the continental coastline of the country exhibits algae overgrowth.

Over the past five to 10 years, agronomists and environmental activists have begun researching how to ameliorate the nitrogen pollution problem. Many technologies and methods exist to help reduce nitrogen runoff, including chemical fertilizer stabilizers and artificial wetlands. And there is good evidence that these techniques work.

The main challenge is developing solutions tailored to local needs and convincing farmers to adopt them. What works for a corn farmer in southern Iowa will not necessarily be appropriate for someone growing the same crop in the northern part of the state—let alone for a strawberry grower in California. And farmers can be hesitant about implementing such methods; if they use too little fertilizer or spend too much money on a technology that doesn’t improve their yields or the health of their soil, they put their livelihoods and the food supply at risk.

All these concerns end up being as important as developing the techniques in the first place. “If you have the best science and technology in the world, it doesn’t do any good if nobody uses it,” says Patrick Brown, a plant scientist at the University of California, Davis.

Agriculture’s antihero

In 1908, Fritz Haber filed a patent that would transform world agriculture. It also would eventually lead to the nitrogen pollution problem.

Haber’s efficient process for producing ammonia from abundant nitrogen gas, which makes up about 78% of the atmosphere, is essential for making the fertilizer that helps grow food to feed the growing global population. By 2008, nitrogen fertilizers were applied to crops that fed nearly half the people in the world (Nat. Geosci. 2008, DOI: 10.1038/ngeo325).

Corn is one of the most dramatic examples of the benefits of nitrogen: Yields double when farmers apply nitrogen fertilizers. “There’s nothing else you can put on a plant to make it grow like that,” says Stephen Moose, a crop scientist at the University of Illinois, Urbana-Champaign.

Although nitrogen fertilizer helps crops grow, much of it is wasted. Only 17% of what farmers apply—whether as ammonia, nitrate, urea, or another form of reactive nitrogen—ends up being consumed by people in the form of fruits, vegetables, grains, dairy products, and meat (Philos. Trans. R. Soc., B 2013, DOI: 10.1098/rstb.2013.0164).

One reason is that farmers fear the consequences of applying too little fertilizer, so they tend to apply more nitrogen than plants need. “If they get it right to the edge, they worry; if they overapply, they know they’re safe,” says Mark Lubell, director of the Center for Environmental Policy & Behavior at UC Davis. For example, farmers in California growing high-value crops such as strawberries and lettuce, which can sell for tens of thousands of dollars per acre, might not blink at an additional hundred dollars per acre of fertilizer.

In addition, the time when it’s convenient for a farmer to apply nitrogen may not be the time the plant is ready to take it up. So if there’s a big rain after farmers fertilize, large amounts of nitrogen can run off the soil before the plants can absorb it.

The complex environmental chemistry of nitrogen also prevents applied fertilizer from reaching plants. “Nitrogen is the most complicated nutrient,” says Brian Lutz, a biogeologist at Climate Corp., a Monsanto-owned precision agriculture company that markets a nitrogen-management software tool to farmers in the Corn Belt. As nitrogen moves through the environment and interacts with different organisms, it forms compounds such as ammonia, nitrates, nitrites, and nitrogen oxides. Some of these are very soluble and readily wash out of the soil.

Crops also must compete with soil bacteria hungry for nitrogen. Some of these bacteria transform ammonia-based fertilizers into nitrates, which contribute to harmful algal blooms that can clog fishes’ gills and feed dead zones. Nitrates can also pose a health risk to people, in particular, infants. According to EPA, more than one-fifth of household wells in agricultural areas have nitrate levels above the drinking water standard of 10 mg/L.

So it seems nitrogen is the antihero of modern agriculture. “We’re not going to get rid of it,” Moose says. The Food & Agriculture Organization of the United Nations estimates that agricultural yields need to increase by 50% by 2050 to feed a projected population of 9.7 billion people. That means even more demand for nitrogen fertilizers. To feed more people without worsening the effects of nitrogen pollution—and possibly even to ameliorate those effects—farmers will need to adopt new practices.

Stabilize it


Bacteria blocker

Credit: Shutterstock
Soil bacteria convert nitrogen fertilizer into nitrates that can leach into the water supply. Fertilizer stabilizers such as nitrapyrin block the key bacterial enzymes so that more fertilizer stays in the soil for crops to absorb.
Diagram showing nitrification of fertilizer interrupted by nitrapyrin, a fertilizer stabilizer.
Credit: Shutterstock
Soil bacteria convert nitrogen fertilizer into nitrates that can leach into the water supply. Fertilizer stabilizers such as nitrapyrin block the key bacterial enzymes so that more fertilizer stays in the soil for crops to absorb.

One way to prevent nitrates and other harmful reactive nitrogen compounds from reaching streams and groundwater is through practices that keep the nitrogen where it’s needed—in farmland soils. Such practices then allow farmers to use less fertilizer.

Several companies make chemical additives designed to keep nitrogen in place. Dow AgroSciences, for example, sells nitrogen stabilizers that can be mixed with ammonia fertilizers, manure, or urea. The products, called N-Serve and Instinct, are based on nitrapyrin, which inhibits soil bacteria from turning ammonia into nitrates. A 2004 meta-analysis showed that stabilizers made by Dow and others kept nitrogen in the root zone of soil around crops, reduced leaching of nitrates by 16%, and boosted yields by 7% (Nutr. Cycling Agroecosyst. 2004, DOI: 10.1023/B:FRES.0000025287.52565.99). “It’s a win-win situation,” says Mark Peterson, global biology leader at Dow AgroSciences.

Jeffrey Wolt, an environmental chemist and former Dow employee who performed the meta-analysis after moving to his current post at Iowa State University, says stabilizers are just one tool among many. “There is no panacea for control of nutrient loss from agricultural soils,” he says. “No one nutrient reduction strategy will work consistently because of the overriding importance of environmental conditions on nutrient fate.”

Other nitrogen-management tools include cover crops—plants grown on the land during seasons when food crops can’t grow. These plants continue the work of turning nitrogen in the soil into plant matter instead of allowing bacteria to turn it into nitrates. Cover crops also prevent soil erosion. Together, these properties keep nutrients in the soil, which is good for the land and good for the environment. Before the primary crop is planted, the cover crop is plowed under, acting as a kind of natural fertilizer.

Cover crops can save farmers money on fertilizer, says Lara Bryant, a soil health fellow at the Natural Resources Defense Council. And there are other benefits: Some cover crops, such as winter wheat, can be sold; others can be used to feed grazing animals.

Michael Castellano, a soil scientist at Iowa State University, says the benefits of cover crops depend greatly on where the land is and whether the farmer owns or rents it. “You get less than a pound [about 0.45 kg] of nitrogen retained per acre,” he estimates. That’s good for the long-term health of the soil, but the practice provides farmers little short-term benefit because saving 0.45 kg of fertilizer doesn’t save much money. So the practice has little appeal to farmers who rent the land they work; they don’t care to spend their money and time investing in their landlords’ soil.

And Castellano notes that some places are simply too cold in the winter to sustain cover crops. Farmers in the southern part of his state of Iowa may be able to grow winter cereal rye in the off-season, but those in the northern part of the state cannot.

Aside from methods to keep nitrogen in the soil, some researchers see an opportunity to prevent nitrogen pollution by taking advantage of the hydrology of the Corn Belt. Many farmers use tile drainage to allow them to work land that would otherwise be too wet. These systems are named after the ceramic tiles used since Roman times to drain subsurface water to turn mucky ground into soil in which crops can thrive.

Today, farmers rely on underground piping systems that rapidly conduct water, along with the nutrients dissolved in it, into rivers and streams in the Corn Belt. “We could intercept the tile line and run it through a treatment system” to remove the nutrients before it reaches waterways, says Matthew Helmers, an agricultural engineer also at Iowa State University.

One way to treat this water is to convert a wet patch of farmland into an artificial wetland. Wetlands are effective denitrifiers—so much so that environmental scientists like to call them nature’s kidneys. The most effective artificial wetlands are typically made with gravel and plants such as cattails and reeds, which support denitrifying bacteria. As water flows in, these organisms take up reactive nitrogen compounds and convert them back into benign nitrogen gas. Nitrogen removal rates for artificial wetlands depend on the design and the water source but can reach as high as 78% (Ecol. Appl. 2011, DOI: 10.1890/09-0269.1).

Photo of a wetland habitat in Michigan showing plants and water.
Credit: U.S. Fish & Wildlife Service
Plants and microbes living in wetlands such as this one in Michigan can filter reactive nitrogen compounds from agricultural runoff.

Nitrogen goals


It’s up to farmers to choose which of these methods to use, if any. But nitrogen use is not monitored at a farm-by-farm level in most places. Because the federal Clean Water Act excludes runoff from farms, regulations designed to limit agricultural nitrogen pollution come from the state level. California and states in the Corn Belt provide two examples of how those regulations are playing out.

California is taking a relatively aggressive regulatory approach. In response to a report on nitrates in drinking water in rural agricultural areas prepared by researchers at UC Davis, California’s State Water Resources Control Board has begun requiring farmers to draw up a nitrogen budget. The practice is only a few years old, and it is phasing it in starting with a few water districts.

California is one of the biggest users of nitrogen in the U.S. and will end up setting standards others will follow.
Patrick Brown, plant scientist, University of California, Davis

At the start of a growing season, farmers must report how much nitrogen they plan to apply and their expected crop yield. Then they must report back on how much fertilizer they actually used and what their yields were. Farmers submit these planned and actual yield reports to local water coalitions to determine each farmer’s nitrogen-use efficiency, which then gets compared to that of farmers growing similar crops. The coalition can use these comparisons to find outliers who are wasting nitrogen and ask them to make a plan to correct it. If they fail to do so, they will be fined.


These overapplying outliers should be able to reduce their fertilizer use without risking their yields. Eileen McLellan, a scientist who studies water in agricultural landscapes at the Environmental Defense Fund, says one advantage of this system is it brings farmers together with their peers. As they pool their data, they may also pool their knowledge.

“California is one of the biggest users of nitrogen in the U.S. and will end up setting standards others will follow,” UC Davis’s Brown predicts.

In the Corn Belt, the physical and political dynamics of nitrogen are different. Through the Mississippi River/Gulf of Mexico Hypoxia Task Force, five federal agencies, agencies from 12 states, and the National Tribal Water Council are working to reduce nitrogen runoff into the watershed. Each state in the task force comes up with its own plan for nutrient management. By 2035, the task force hopes to reduce the five-year average size of the hypoxic zone to 5,000 km2. Some years, the zone is close to that target; other years it can be a few times bigger. To remediate Gulf hypoxia, EPA has recommended, but not required, that annual nitrogen loading into the Mississippi River should be reduced by 45%.

McLellan led a 2015 study that used empirical hydrological models to estimate what it would take to meet the proposed reduction. The study, done in collaboration with researchers from the U.S. Department of Agriculture Agricultural Research Service and the U.S. Geological Survey (USGS), looked at agricultural conservation practices in seven states in the Upper Mississippi-Ohio River Basin and asked how much they reduced nitrogen runoff (J. Am. Water Resour. Assoc. 2014, DOI: 10.1111/jawr.12246).

They concluded that better management of nitrogen alone is not enough. Management strategies include applying fertilizer at the recommended time and rate as well as using nitrogen stabilizers. If these practices were adopted on every farm, they would reduce nitrogen pollution only by 12%. If all farmers in the basin also used cover crops, nitrogen pollution would drop by 30%—not all the way to the EPA goal but a decent reduction.

Meeting the 45% reduction goal will require more sacrifices, but they don’t have to be difficult ones. McLellan says if one-quarter of the region’s farmers adopted nitrogen-management strategies and cover crops, and less than 2% of the region’s croplands were converted to wetlands, the basin would achieve the goal with minimal losses in crop yields.

And the region could meet the 45% goal without individual farmers using the best nitrogen-management practices if 4% of the cropland were converted to wetlands. “There’s usually one place on a farm that’s not that productive, and that could be converted to a wetland,” McLellan says. Since publishing the study, EDF has used funding from the U.S. Department of Agriculture (USDA) to work with farmers on implementing best practices and wetland conversion.

Some evidence shows that voluntary adoption of conservation practices in the Corn Belt is helping reduce nutrient pollution. A 2016 study led by USDA and USGS looked for a link between agricultural conservation and nutrient loading in the upper Mississippi River Basin. The researchers found that nitrogen levels had been reduced by between 5 and 34% (Environ. Sci. Technol. 2016, DOI: 10.1021/acs.est.5b03543).

Being able to show farmers and scientists that these methods are effective in the field is important. Still, there is far to go. With the exception of the nascent program in California, states are not monitoring how much nitrogen farmers apply or what conservation practices they adopt. Iowa State’s Helmers believes water treatment technologies and constructed wetlands in the Corn Belt are rare. “We are orders of magnitude away from the level of adoption we need to meet our goals,” he says.

Nevertheless, environmentally minded agronomists and other scientists are not calling for legislation to implement nationwide nitrogen regulation. Almost all the researchers C&EN talked with grew up on farms, and most said the way forward is for environmental scientists and other academics to build trust with farmers.

Researchers at EDF, UC Davis, Iowa State, and elsewhere are trying an old-fashioned approach: talking with farmers and educating them about the effects of nitrogen on the local environment and what they can do to help. Many are open to it. “We’re starting the conversation,” McLellan says. 

Katherine Bourzac is a freelance journalist in San Francisco.


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