Hydrologic connectivity amplifies riverine N2O emission hotspots and hot moments across the contiguous United States

Authors: HU, MINPENG - University Of Illinois Urbana-Champaign; YU, ZHONGJIE - University Of Illinois Urbana-Champaign; GRIFFIS, TIMOTHY - University Of Minnesota; AHO, KELLY - Boston University; WANG, YUCANG - Arizona State University; YANG, JIE - University Of Illinois Urbana-Champaign; YANG, WENDY - University Of Illinois Chicago; BERNACCHI, CARL - University Of Illinois Urbana-Champaign; McGrath, Justin; DAHLGREN, RANDY - University Of California, Davis; TIAN, HANQUIN - Boston College; BAKER, JOHN - University Of Minnesota

Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/21/2026
Publication Date: 6/22/2026
Citation: Hu, M., Yu, Z., Griffis, T.J., Aho, K., Wang, Y., Yang, J., Yang, W.H., Bernacchi, C., Mcgrath, J.M., Dahlgren, R.A., Tian, H., Baker, J.M. 2026. Hydrologic connectivity amplifies riverine N2O emission hotspots and hot moments across the contiguous United States. Proceedings of the National Academy of Sciences (PNAS). https://doi.org/10.1073/pnas.2524113123.
DOI: https://doi.org/10.1073/pnas.2524113123

Interpretive Summary: Rivers and streams across the U.S. play an important role in how nitrogen moves through the environment, and nitrogen lost to rivers and streams represents lose of fertilizer and an unnecessary cost to farmers. Using more than 3,600 measurements from across the country, we created the first month-by-month, nationwide estimate of nitrogen released from waterways as nitrous oxide. We found that about 60,000 metric tons of nitrous oxide nitrogen pass from rivers into the air each year. Our study shows that changes in water flow—such as snowmelt, heavy rain, or flooding—greatly influence how nitrogen travels from land into rivers and streams. These events can cause short periods of especially high nitrogen release, particularly in smaller streams with high nutrient levels. The Midwest Corn Belt is a prime example, where late-winter thaws and post-harvest rains carry large amounts of nitrogen into waterways. We also found that intensively farmed watersheds release more nitrous oxide per unit of nitrogen than current reference values suggest. This highlights the importance of accounting for seasonal water movement and land use when developing strategies for managing nitrogen in rivers and streams.

Technical Abstract: Riverine nitrous oxide (N2O) emissions constitute a significant yet uncertain component of global greenhouse gas budgets. Integrating ~3,600 observations across the contiguous United States, we present the first monthly-resolved, national-scale estimate of riverine N2O emissions (60.7 Gg N2O-N yr'¹; 95% CI: 41.9–71.2) using a machine learning framework. Our analysis reveals that enhanced hydrologic connectivity strongly regulates nitrogen and N2O delivery to streams, driving emission hot moments during high-flow periods, especially in nutrient-rich low-order streams. The Midwest Corn Belt is identified as a major emission hotspot, where seasonal increases in connectivity (e.g., late-winter thaws and post harvest rainfall) amplify riverine emissions relative to direct soil emissions. We derived a representative riverine N2O emission factor that exceeds the current IPCC default value by over twofold in intensively managed watersheds. These findings highlight the importance of incorporating hydrologic connectivity and nitrogen transport into climate models and watershed nitrogen management strategies.