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Nitrates that accumulate in soil near rivers play an important role in increasing nitrate levels in river water during rainfall, researchers from Nagoya University in Japan report. Their findings, published in the journal Biogeoscience, could help reduce nitrogen pollution and improve water quality in downstream water bodies such as lakes and coastal waters.
Nitrates are an important nutrient for plants and phytoplankton, but high levels of nitrates in rivers can degrade water quality, lead to eutrophication (over-enrichment of water with nutrients), and pose a risk to animal and human health. Although nitrate levels in streams are known to rise when it rains, it is not clear why.
There are two main theories about how nitrate increases when it rains. According to the first theory, atmospheric nitrates dissolve in rainwater and enter directly into streams. The second theory is that when it rains, soil nitrates in the area bordering the river, known as the riparian zone, enter the river water.
To further investigate the source of nitrates, a research team led by Professor Urumu Tsunogai of the Graduate School of Environmental Studies, in collaboration with the Asian Center for Air Pollution Research, conducted a study to analyze changes in the composition of nitrogen and oxygen isotopes in nitrates and during heavy rains. Increasing concentrations of nitrates in rivers.
Previous studies have reported significant increases in nitrate concentrations during storms in a river upstream of the Kaji River in Niigata Prefecture in northwest Japan. The researchers collected water samples from the Kajigawa catchment, including from streams upstream of the river. During three storms, they used autosamplers to sample watershed streams every hour for 24 hours.
The team measured the concentration and isotopic composition of nitrates in the water of the stream, and then compared the results with the concentration and isotopic composition of nitrates in the soil in the coastal zone of the stream. As a result, they found that most of the nitrates come from the soil and not from rainwater.
“We concluded that the washing of coastal soil nitrates into streams due to rising stream levels and groundwater was the main cause of the increase in nitrates in streams during storms,” said Dr. Weitian Ding of Nagoya University, author of the study.
The research team also analyzed the effect of atmospheric nitrate on the increase in nitrate flux during storms. The content of atmospheric nitrates in the river water remained unchanged, despite the increase in precipitation, which indicates a slight influence of sources of atmospheric nitrates.
The researchers also found that coastal soil nitrates are produced by soil microbes. “It is believed that nitrates of microbial origin accumulate in coastal soils only in summer and autumn in Japan,” explains Professor Tsunogai. “From this perspective, we can predict that the increase in nitrates in the river due to rainfall will only occur during these seasons.”
Reference: Dean W, Tsunogai W, Nakagawa F, et al. Tracking the source of nitrates in forest streams showed elevated concentrations during storm events. Biogeoscience. 2022;19(13):3247-3261. doi: 10.5194/bg-19-3247-2022
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