Because plants can’t use it all, the excess makes its way into groundwater and washes into waterways where it contaminates drinking water and creates vast dead zones in oceans and lakes. Excess nitrogen is also released into the air as nitrous oxide, a greenhouse gas that is 300 times more powerful than carbon dioxide in its ability to drive climate change.

In 2018, experts on a United Nations panel aimed at addressing the issue concluded that the world needs to cut the amount of nitrogen making its way into the environment in half to avoid disastrous consequences.

“It’s an issue that has received a lot of attention over the years, but we are still struggling to gain traction on it,” said Eric D. Roy, a professor who leads the Nutrient Cycling and Ecological Design Lab at the University of Vermont (UVM). Now, Roy and his team are hoping their research will contribute to changing that.

In a new study published in the journal Environmental Research Letters, they identified 20 nitrogen “hotspots” across the U.S. The clusters of counties not only represent areas where a large proportion of surplus nitrogen is being applied; they are also places where researchers say there is significant potential to reverse the trend—to the benefit of farmers, local residents, and the environment.

In recent years, governments, universities, and advocacy organizations have worked to develop effective ways to address nitrogen pollution across the country, by incentivizing practices that reduce runoff—like planting cover crops, buffer strips, and more diverse rotations of crops—and by implementing limited regulations. More ambitious efforts are working to transition commodity-scale farmers to more holistic systems that keep nitrogen in balance and on the farm.

“We’re trying to identify opportunities that we see across the landscape and to provide some ability to target areas where progress seems more achievable,” Roy said. “This isn’t to say we don’t need to address this problem everywhere, but there are advantages, when resources are limited, to focusing on where we can make change most effectively.”

The study comes at an opportune time, as the Biden administration turns its focus to agriculture’s role in fighting climate change, and conversations about scaling up and targeting farm conservation programs are increasing.

Mapping Excess Nitrogen in the U.S.

Farmers in the U.S. have done better. They improved nitrogen use efficiency to 68 percent in 2010, but that still means more than 30 percent is washing into waterways and ending up in the atmosphere. As a result, dead zones occur annually in the Chesapeake Bay and the Gulf of Mexico, and drinking water contamination is widespread in agricultural communities.

In a report published in January, Rebecca Boehm, an economist at the Union of Concerned Scientists, estimated that Iowa alone will spend between $41 million and $330 million treating public drinking water contaminated with nitrates over the next five years if runoff continues at the same rate.

Much of the attention around this issue is paid to the Mississippi River Basin, because hundreds of tributaries across 31 states, many of them flush with active farmland, drain into the river, and subsequently into the Gulf of Mexico, leading to the largest and most impactful dead zones in the country. But the UVM study zooms out from the heartland to look at the issue of nitrogen pollution across the country.

To map out the hotspots, researchers looked at county-level data on three factors. First, they identified places where nitrogen balances—measurements that tell you how much extra nitrogen was applied that plants didn’t use—were high. The 759 counties that contain the 20 hotspots account for 24 percent of U.S. cropland but 63 percent of the country’s surplus nitrogen balance.

Next, they did what they called a breakpoint analysis, which helped them identify areas where yields are unlikely to be affected by reducing nitrogen. “One of the primary reasons farmers will be reluctant to reduce their nitrogen inputs is due to concerns over yield declines,” Roy said.

Boehm said this part of the analysis squared with research she’s done on the economic aspects of the same equation. Economists can chart at what point nitrogen application stops providing a financial benefit to farmers and turns into an unnecessary cost.

“In Iowa, we found that most counties, on average, were way over that point,” she said. “And it’s because nitrogen is sort of an insurance policy. Putting on more is not going to hurt anything on your farm . . . and it’s going to ensure that the crops grow.”

In a 2020 report that looked at the economic impacts of the annual dead zone on the Gulf region’s fisheries, Boehm estimated that since 1990, 31 million tons of nitrogen from corn and soy fields in the corn belt contributed to the dead zone, and those losses cost farmers across the watershed an average of $400 million in unnecessary fertilizer purchases per year.

Finally, the researchers analyzed data on the farms in the regions, looking at socioeconomic factors that correlated to nitrogen balances, including operating expenses, climate change beliefs, and soil productivity to create a “potential for improvement score.” For example, if farmers in a certain region already have highly productive soil and are concerned about climate change, they might be more interested in reducing fertilizer use and implementing practices like cover crops and conservation tillage to reduce runoff.

These three factors were weighted equally to identify the 20 hotspots, where the researchers suggest that interventions to reduce nitrogen pollution could have the greatest impact. Each hotspot is made up of a cluster of counties of varying size, from seven to 80, and while many are in the most concentrated agricultural regions in the middle of the country and California, others are scattered from coast to coast.

The number-one hotspot spans 61 corn- and soy-heavy counties in Illinois, Indiana, Missouri, and Wisconsin, and the next three are similar, in overlapping or neighboring states including Kansas, Nebraska, Iowa, and South Dakota. California’s largest hotspot unsurprisingly includes 21 counties that cover the Central Valley, America’s produce capital. And there are hotspots at the mouth of the Gulf of Mexico and surrounding the Chesapeake Bay.

Others are more surprising, like 11 counties in New England where fields of corn used in dairy cattle feed were the main contributor to the problem and 32 counties in Idaho, Utah, Montana, and Wyoming, where a mix of crops, including hay, alfalfa, wheat, barley, and sugar beets, contributed to overuse.

Places that produce crops for animal feed—like hay, corn, and soybeans—are significantly overrepresented in hotspot locations. But that’s primarily because so much land is dedicated to those crops, not because more fertilizer is typically being applied on those fields compared to other crops. In fact, data shows nitrogen fertilizer rates are typically higher and use can be even less efficient in fruit and vegetable fields.

“What was interesting to me was how they framed it, in that these are hotspots, but the hotspots are opportunities,” said Alison Eagle, a scientist at the Environmental Defense Fund (EDF) who studies nutrient cycling. “Identifying the place where you can make a difference then allows you to start thinking about how you want to make a difference.”

Opportunities for Solutions

She said the UVM study’s approach to targeting is another tool that could work in conjunction with the framework. For example, the study could point those working on solutions to regions they might focus on. Then, within those regions, EDF’s nitrogen-loss framework, which provides farmers with practical tools, could be applied at the farm level. And she thought the inclusion of socioeconomic factors in the researchers’ model was key.

“If you’ve got an environment in which people are interested in change or have the capacity to invest in new management [practices], then you have opportunities to do something. But if there are other challenges in the community, then asking them to do one more thing is just not going to work. Maybe there are factors that are a bigger issue that need to be addressed first,” she said. “We’ve looked at that when we think about where we might want to work as well.” Similarly, she continued, “some states are more open to policy that encourages conservation, and other states might be less open to that.”

At the federal level, advocates and policymakers are talking about expanding conservation programs both immediately and in the near future. Within the past month, senators and representatives brought up expanding the programs repeatedly in hearings on climate change and agriculture in both chambers. “This gives policymakers something to think about in developing agriculture policy,” Boehm said. “The farm bill conversations are already ramping up.”

At the state level, she pointed out that in early March, a bill was introduced in Iowa to add soil health to the state’s conservation goals, which could lead to programs supporting practices that help prevent nitrogen from getting into waterways, like conservation tillage and cover crops.

While there are ways to help farmers more efficiently use nitrogen and practices that can help keep excess from leaching into waterways, she also said the issue points to a broader challenge. “The separation and specialization of livestock and crop production has really driven this imbalance problem,” she said.

Before the invention of synthetic nitrogen fertilizer, farmers developed systems in which animals and plants provided the nitrogen they needed, and crops were planted based on the amount of fertility available. When farm systems are diverse and integrated, they create a closed loop of nitrogen use. In the current specialized industrial system, concentrated animal operations produce large quantities of manure that collects in lagoons which gets sprayed repeatedly on farm fields. Meanwhile, the fertilizer industry aggressively markets its products, while fighting state efforts to reduce and regulate nitrogen pollution.

“So thinking about the longer term,” Boehm said. “Reintegrating crop and livestock production would help bring nitrogen back to being a little more balanced—but that’s a huge shift in the systems that we have.”

A shift like that isn’t likely anytime soon, Roy acknowledged, although many people are working on it. In the meantime, he is also encouraging others thinking about surplus nitrogen and agriculture to consider the study’s model as a template for other targeting efforts.

“My hope is that our study is not just a static document, but the methods we used will eventually help to inform different tracking systems,” he said. “And people could replicate this on a regional scale, with data unique to that region to keep trying to identify: Where are the opportunity zones?”