According to researchers from Nagoya University in Japan, nitrate buildup in soil near streams contributes significantly to the rise in nitrate levels in stream water after rain. Their research, which was published in the journal Biogeosciences, could help reduce nitrogen pollution and improve the quality of water in lakes and other water bodies upstream.
Although excessive nitrate levels in streams can harm water quality, cause eutrophication (the over-nutrientization of water), and pose health risks to both animals and people, nitrate is an essential nutrient for plants and phytoplankton. The cause of this is unknown. Nitrate concentrations in streams are known to increase when it rains.
There are two main hypotheses for why stream nitrate rises after rain. The first hypothesis proposes that atmospheric nitrate dissolves in rainwater before falling directly into streams. The second theory says that soil nitrate from the riparian zone, which is the area next to a stream, flows into the water of the stream when it rains.
A research team from Nagoya University, led by Professor Urumu Tsunogai at the Graduate School of Environmental Studies, worked with the Asia Center for Air Pollution Research on a study to look at how the nitrogen and oxygen isotopes in nitrates change over time and how the amount of nitrates in a stream rises when it rains.
According to earlier studies, the upper Kajikawa River in Niigata, western Japan, experiences a significant rise in nitrate concentrations during storms. The upper part of the river’s streams make up the Kajikawa catchment area, where the researchers took water samples. Throughout three storms, they collected samples from the catchment stream using an automatic water sampler every hour for 24 hours.
The group then contrasted the findings with the concentrations and isotopic compositions of soil nitrates in the stream’s riparian zone after measuring the concentrations and isotopic compositions of nitrate in the stream water. So, they found that most of the nitrates in the area did not come from rainwater but rather from the soil.
The study’s co-corresponding author, Dr. Weitian Ding of Nagoya University, said, “We came to the conclusion that the main cause of the increase in stream nitrate during the storm event was the flushing of soil nitrates from the riparian zone into the stream because both the stream water level and the groundwater level rose.”
The research team also looked at how increasing stream nitrate during storms was affected by atmospheric nitrates. Even though it rained more, the amount of atmospheric nitrate in stream water didn’t change. This shows that the sources of atmospheric nitrates didn’t have much of an effect.
Additionally, the researchers discovered that soil-dwelling microorganisms were responsible for producing the nitrate in the riparian zone. Professor Tsunogai stated that in Japan, “microbially derived nitrate is thought to be accumulated in soil in the riparian zone only in summer and autumn.” “From this angle, we can forecast that the rise in stream nitrate caused by rain only happens during these seasons.” To help make sure that fresh water is safe, it may be important to know when nitrates levels rise.
This work was paid for by grants from the Yanmar Environmental Sustainability Support Association, the River Fund of The River Foundation, Japan, and the Ministry of Education, Culture, Sports, Science, and Technology of Japan (grant numbers JP17H00780, JP19H04254, and JP00955).
