URBANA, Ill. (U.S.A.) — A new accounting of nitrogen pollution in the Mississippi-Atchafalaya River Basin (MARB) reveals a significant decline in recent decades, suggesting positive momentum for water quality goals in local watersheds and the Gulf of Mexico. Surprisingly, the University of Illinois Urbana-Champaign-led study doesn’t credit the change to reduced fertilizer application, but instead to cleaner air and more efficient nitrogen uptake by modern corn hybrids.

“Between 2000 and 2020, we saw major increases in crop yields, as well as reductions in nitrogen oxide emissions from smokestacks and vehicles as part of the Clean Air Act. Reduced nitrogen oxide emissions result in less biologically available nitrogen coming into soil and water from the atmosphere,” said the study’s lead author, Greg McIsaac, associate professor emeritus in the Department of Natural Resources and Environmental Sciences, part of the College of Agricultural, Consumer and Environmental Sciences at Illinois. “It’s great to see trends moving in the right direction.”

Less nitrogen moving through the MARB should mean a smaller or shorter-lived “dead zone” in the Gulf, an area of oxygen-depleted water that can’t support fish or other marine life. For two decades, the Gulf Hypoxia Task Force and a dozen states bordering the Mississippi River have set goals and incentives to reduce nitrogen flows by 45%.

“Progress toward that goal has been slow, but over the last decade, we have finally started to see nitrogen flows in the Mississippi River decrease,” said study co-author Robert Howarth, Atkinson Professor of Ecology & Environmental Biology in the College of Agriculture and Life Sciences at Cornell University. Howarth led a committee of the National Academy of Sciences in the late 1990s that recommended significant reductions in nutrient flows to all coastal waters.

Howarth and colleagues also developed a widely used nitrogen budgeting tool known as Net Anthropogenic Nitrogen Inputs, or NANI.

“Many researchers have now demonstrated that nitrogen fluxes in large rivers to the coasts throughout North America, Europe, and China are controlled by NANI. More NANI to the watershed, more nitrogen pollution out,” Howarth said. “The NANI framework is pretty simple and includes just four main factors: use of synthetic nitrogen fertilizer, nitrogen fixation by agricultural crops, input of nitrogen from atmospheric deposition, and the net input or export of nitrogen in human and animal food and feeds.”

The new study calculates nitrogen budgets using NANI across 217 watersheds in the MARB from 2000 to 2020. Importantly, the researchers updated how NANI calculates two important metrics — crop nitrogen content and biological nitrogen fixation — more accurately accounting for nitrogen removed via harvest and taken from the air by legume crops like soybeans.

The study shows that high values of NANI in a watershed were associated with high values of flow-normalized riverine nitrate levels. Flow normalization statistically smooths out year-to-year variation in rainfall and river flow fluctuations and is a way of estimating the nitrate load if river flow was near the long-term average.

“The fact that NANI and flow-normalized river nitrate values were so strongly correlated confirms that land-based NANI calculations are a useful predictor of water-based measurements even after accounting for water flow fluctuations,” McIsaac said.

The study also accounts for nitrate load via tile drainage.

“We found that tile drainage is a fairly important factor to include. Nitrate load declined by 5 pounds of nitrogen per acre per year, on average, in watersheds with over 20% tile drainage, where nitrate loads were previously high,” McIsaac said. “Part of that is because farmers have become more efficient at using nitrogen for crop production. Also, atmospheric nitrogen deposition has declined, especially on the Eastern side of the Mississippi. That's good news.”

Howarth added, “When more nitrogen is taken up by crops and then leaves the watershed in exports of food and animal feeds, less nitrogen is left to flow downstream in the river. Similarly, less inputs of nitrogen from the atmosphere means less to flow out in the river.”

The researchers credit the improvement to collaboration among farmers, crop advisors, conservation specialists, and policymakers in each MARB state, as well as to federal policies around air quality and scientific advancements in crop breeding.

“In 2024, nitrogen load to the Gulf achieved an interim goal of a 20% reduction. The next goal is to reach a 45% reduction by 2035, which will require continued investment in conservation and research,” McIsaac said. “Reductions in nitrate loads will improve water quality for local communities within the MARB, as well as in the Gulf.”

The study, “Changes in Terrestrial N Budgets and Riverine Nitrate-N Yields from Mississippi River Basin Watersheds 2000 to 2020,” is published in Ocean-Land-Atmosphere Research [DOI: 10.34133/olar.0131]. Authors include Greg McIsaac, Dennis Swaney, Alejandra Botero-Acosta, and Robert Howarth.

Research in the College of ACES is made possible in part by Hatch funding from USDA’s National Institute of Food and Agriculture. This project was supported by the Walton Family Foundation (grant no. 00108464, 2024) and involved a collaboration of the National Great Rivers Research and Education Center and the National Center for Supercomputing Applications in developing the Great Lakes to the Gulf Virtual Observatory.