Researchers from Nagoya University in Japan have reported that nitrate accumulated in soil bordering streams is essential in increasing nitrate levels in stream water when it rains. Their findings, published in the journal Biogeosciences, may help reduce nitrogen pollution and improve water quality in downstream bodies of water, such as lakes and nearshore waters.
Nitrate is an essential nutrient for plants and phytoplankton. Excessive levels in streams can damage water quality, cause eutrophication (the over-enrichment of water by nutrients), and pose health risks to animals and humans. The reason for these increased levels in streams during rain is unclear.
There are two leading theories of how the stream nitrate increases when it rains. According to the first theory, it dissolves in rainwater from the atmosphere and falls directly into streams. The second theory is that soil nitrate in riparian areas flows into the water when it rains.
To further investigate the source, a Nagoya University research team led by Professor Urumu Tsunogai at the Graduate School of Environmental Studies, collaborating with the Asia Center for Air Pollution Research, conducted a study to analyse changes in the isotopic compositions of nitrogen and oxygen in nitrate. They also sought to determine the increased concentration in a stream during rainstorms.
Previous studies have reported that nitrate concentrations significantly increase during storms in the streams of the upper Kajikawa River in Niigata, northwestern Japan. The researchers collected water samples from the Kajikawa catchment area, comprising the streams of the upper part of the river. They used an automatic water sampler to take samples from the catchment water at one-hour intervals for 24 hours during three storms.
What do nitrate levels mean for water quality?
Measuring the concentrations and the isotopic compositions of nitrate in the stream water, the group next compared the results with the concentrations and the isotopic compositions in the riparian zone. They found that it mostly came from the soil rather than rainwater.
“We concluded that the flushing of soil nitrate in the riparian zone into the stream due to the rising of both water and groundwater levels was primarily responsible for the increase during the storm event,” said Dr Weitian Ding of Nagoya University, the corresponding author of the study.
The researchers also found that soil-dwelling microorganisms produced soil nitrate in the riparian zone. “Microbe-derived nitrate is thought to be accumulated in the soil in the riparian zone only in summer and autumn in Japan,” explained Professor Tsunogai. Understanding the seasonality of nitrate increases could be an essential finding to help ensure safe, fresh water.
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