Nagoya University unravels why river nitrate spikes after rainfall

A research group including Weitian Ding (a JSPS Postdoctoral Fellow at the time of the study), Professor Urumu Tsunogai and Associate Professor Fumiko Nakagawa of Nagoya University's Graduate School of Environmental Studies, and Department Head Hiroyuki Sase of the Asia Center for Air Pollution Research, has clarified the mechanism behind the sharp increase in nitrate concentrations in river water following rainfall. The researchers identified nitrate accumulated in subsurface forest soils at depths of approximately 10 to 90 cm as the primary source. Because forests cover much of the watershed, this stored nitrate is flushed into rivers during rain events. The study also showed that analyzing the isotopic composition of nitrate carried by rainfall runoff can provide a comprehensive assessment of upstream forest soil conditions across broad areas. These findings are expected to contribute to addressing environmental issues, including impacts on downstream ecosystems and water quality. The results were published in the August 6 issue of Water Resources Research.

Sharp increases in nitrate concentrations in river water following rainfall have long been recognized, but the underlying cause remained unclear. In 2022, the research group conducted observations in a small river in Niigata Prefecture and found that the increase in nitrate concentrations originated from nitrate stored in subsurface forest soils. However, it remained unclear whether the same mechanism operates more generally in other river systems.

To investigate this, the researchers conducted a study in the Lake Ijira watershed in Gifu Prefecture, which covers 298 hectares and is considerably larger than the site examined previously. River water samples were collected every two hours during rainfall events and in the periods before and after rainfall. The team then analyzed changes in the isotopic composition of nitrate associated with rising nitrate concentrations. The results suggest that rising river levels during rainfall promote the release of nitrate, a major nutrient that has accumulated in forest soils.

Analysis of the isotopic composition of nitrate that increased during rainfall further revealed that the nitrate had been produced by microorganisms in the soil. Because microbial activity declines during winter, rainfall-driven increases in nitrate concentrations are expected to occur less readily during that season.

Nutrient inputs stimulate photosynthesis, and excessive nutrient loading into aquatic environments can lead to environmental problems such as eutrophication, red tides, and oxygen depletion. Conversely, a decline in nutrient supply could reduce fishery productivity.

The study also demonstrated that analyzing the isotopic composition of nitrate that increases during rainfall can provide a broad-scale assessment of soil conditions in upstream forested areas. This indicator could enable large-scale evaluations of the health of forest soils and contribute to advances in forestry and soil science.

Tsunogai said, "We believe this study has finally resolved a long-standing mystery that has been debated for many years. Using rainfall-driven increases in nitrate as an indicator of upstream forest soil conditions also represents a novel approach to monitoring forest soils. It could substantially reduce the need for direct field surveys, which traditionally require researchers to enter forests, dig soil pits, and conduct on-site observations. We expect the method to find broader applications and continue to develop in the future."

Journal Information
Publication: Water Resources Research
Title: Flushing of Subsurface Soil Nitrate as the Primary Cause of Increased Stream Nitrate During Storm Events
DOI: 10.1029/2025WR043356