Soil along streams is a greater source of stream nitrate than rainwater

Researchers at Nagoya University in Japan reported that nitrates accumulated in the soil bordering streams play an important role in raising nitrate levels in stream water during the rainy season. Their results are published in the journal Biogeological sciencescan help reduce nitrogen pollution and improve water quality in downstream bodies of water such as lakes and coastal waters.

Nitrate is an essential nutrient for plants and phytoplankton, but excessive nitrate levels in flows can damage water quality, cause eutrophication (excessive enrichment of water with nutrients) and pose a danger to the health of animals and people. Although it is known nitrate levels in streams rise when it rains, the reason for this is not clear.

There are two main theories as to how nitrates rise during rain. According to the first theory, nitrates in the atmosphere dissolve in rainwater and enter streams directly. While the second theory is that when it rains, soil nitrates in the area bordering the stream, known as the riparian zone, flow into the stream water.

To further investigate the source of the nitrates, a Nagoya University research team led by Prof. Urumu Tsunogai of the Graduate School of Environmental Studies, in collaboration with the Asian Air Pollution Research Center, conducted a study to analyze changes in the isotopic composition of nitrogen and oxygen in nitrates and the increase in nitrate concentration in the stream during rainstorms.

Previous studies have reported that nitrate concentrations increase significantly during storms in the headwaters of the Kajikawa River in Niigata, northwestern Japan. The researchers collected water samples from the Kajikawa catchment, which includes the upper reaches of the river. They used an automatic sampler to sample catchment water at one-hour intervals over a 24-hour period during three storms.

By measuring the concentration and isotopic composition of nitrates in the stream water, the team compared the results to the concentrations and isotopic composition of soil nitrates in the riparian zone of the stream. As a result, they found that most of the nitrates come from the soil in the area, not from rainwater.

“We concluded that riparian soil nitrate leaching into the stream due to rising stream water levels and groundwater was primarily responsible for the increase in stream nitrate during the storm,” said Dr. Weitian Ding of Nagoya University. , corresponding author of the study.

The research team also analyzed the effects of atmospheric nitrates on increased nitrate flux during storms. Despite the increase in precipitation, the nitrate content in the atmosphere stream of water remained unchanged, indicating minimal influence of atmospheric sources of nitrates.

The researchers also found that soil nitrates in the coastal zone are produced by microorganisms that live in the soil. “In Japan, microbially derived nitrates are believed to accumulate in the soil in the coastal zone only in summer and autumn,” Professor Tsunogai explained. “From this perspective, we can predict that the increase in nitrate in the stream due to rain only occurs during these seasons.” Understanding seasonality nitrate increase may be an important finding to ensure safe fresh water.