Freshwater and coastal waters may cover just a small portion of the planet, but they turn out to be powerful contributors to nitrogen fixation, a natural process essential to life on Earth.
A new global study, co-authored by a Washington State University Tri-Cities researcher, reveals that these ecosystems produce far more nitrogen than previously recognized, a finding that could have ramifications on forecasting models for climate change, agriculture and pollution.
The study was published in the June 2025 issue of the prestigious journal Science. Sarah Roley, associate professor of environmental science in the College of Arts and Sciences at WSU Tri-Cities, co-authored the paper, as did Carmella Vizza, former WSU Tri-Cities post-doctoral research associate and current assistant professor at Hawai'i Pacific University.
The research demonstrates that lakes, rivers, wetlands, and coastal waters fix approximately 40 million tons of usable nitrogen each year, which is about 15% of the global total. That's a significant contribution, especially considering these environments represent less than 10% of Earth's surface area.
"Nitrogen is essential for life but unusable by most organisms in its atmospheric form," said Roley. "Through nitrogen fixation, specialized microbes convert nitrogen from the air into forms that plants and other organisms can use. This study shows that inland and coastal waters are doing much more of this work than we realized."
Until now, most scientific estimates of global nitrogen fixation focused on terrestrial ecosystems and the open ocean. The new study helps fill a critical gap by quantifying nitrogen fixation in the aquatic environments in between, such as lakes, rivers, wetlands and coastal waters.
The findings suggest that current environmental models may need to be recalibrated. If nitrogen contributions from these systems are undercounted, forecasts for climate change, agriculture, and pollution risks may be off target.
"This research will help improve environmental models and predictions by giving us a more complete picture of where nitrogen is coming from," Roley said. "Often, a lack of nitrogen limits algae and plant growth, especially in coastal ecosystems. These global nitrogen fixation estimates can help us better understand algal growth in freshwater and coastal ecosystems, which are clearly producing more of their own nitrogen than we previously understood."
The research team was part of a Research Coordination Network called Nitrogen Fixation Across Aquascapes, funded by the National Science Foundation. The team compiled 4,793 nitrogen fixation rate measurements from existing studies and synthesized the data during a workshop.
The research team compiled global nitrogen fixation data from water columns and sediments across hundreds of inland and coastal sites. Their analysis revealed that nitrogen fixation is widespread and varies widely by location, with some areas fixing orders of magnitude more nitrogen than others.
"Integration of inland and coastal aquatic ecosystems with global nitrogen estimates has been lacking for several reasons," the authors write. "We anticipate that this study will motivate future research to improve detection techniques, expand monitoring across underrepresented climate zones, and better understand what drives nitrogen fixation in these environments."
