Quantifying Denitrification Rates in Rivers and Streams: Study Shows Rates Differed in Streams, Compared to Rivers

     Human activities like crop agriculture, livestock agriculture, and inadequate sewage treatment add nitrogen to streams and rivers. Part of it travels down the streams into rivers and ends up in the ocean. Another portion of it is removed in a natural chemical process known as denitrification. The University of Missouri explains the process:

“Soil microorganisms need oxygen for fuel. When the soil is very wet, water fills in the spaces between soil particles. This leaves very little room for oxygen. Some soil microorganisms can get the oxygen they need from the oxygen portion of the nitrite (NO2-) and nitrate (NO3-) forms of nitrogen. When this happens, nitrogen (N2) and nitrous oxide (N2O) gas are formed. These gases return to the atmosphere, and there is a net cycle in the soil. This is called denitrification.”

Two main factors influence denitrification:

·        The oxygen supply in the soil.

·        The soil microorganisms.

     Factors that influence the rates of denitrification include the amount of organic matter, soil water content, soil oxygen supply, soil temperature, soil nitrate levels, and soil pH. Rates are higher in waterlogged soils. Denitrification can have both positive and negative impacts on water quality. When nitrites and nitrates are converted to nitrogen and nitrous oxide gases, there is an improvement in water quality. However, waterlogged soil can lead to water rich in nitrites and nitrates percolating downward into groundwater aquifers, negatively impacting water quality. Groundwater contamination is most likely where the depth to groundwater is shallow, and the soil is sandy and permeable. Nitrates are particularly dangerous for infants, including animal babies.

     Scientists agree that we need better quantification of streams and rivers, especially rivers. A new study and paper in the Journal of Geophysical Research: Biogeosciences set out to do this. According to Phys.org:

“The researchers took hourly water samples from the Tippecanoe River and the Shatto Ditch in Indiana over 36-hour periods in spring, summer, and fall. They used open-channel metabolism and a membrane inlet mass spectrometry–based model to study how rates of denitrification fluctuated in both waterways as the seasons changed.”

     Their results showed that the stream had higher denitrification rates per square meter than the river in all three seasons. They attributed this finding to the higher nitrate levels in the stream and the higher microbial activity in the stream.

“However, when the researchers scaled up, the denitrification rate in rivers per kilometer of channel length was equal to or even higher than that of streams.”

     The higher seasonal nitrate levels were likely caused by higher fertilizer application rates in spring and early summer. Precipitation levels were also higher in these seasons. In contrast, denitrification rates were the highest for rivers in the fall. They attribute this to higher rates of ecosystem respiration in the fall.  

     As shown below, oxygen and nitrogen levels were determined from oxygen/argon ratios and nitrogen/argon ratios, respectively.

 

References:

 

Denitrification looks different in rivers versus streams. Nathaniel Scharping. Phys.org. January 19, 2026. Denitrification looks different in rivers versus streams

Fluvial Denitrification Rates in an Agricultural River and Its Tributary Vary Due To Size and Season. Abagael N. Pruitt, Jennifer L. Tank, Shannon L. Speir, and Alexander J. Reisinger. October 29, 2025. JGR Biogeosciences. Volume130, Issue11. November 2025. Fluvial Denitrification Rates in an Agricultural River and Its Tributary Vary Due To Size and Season - Pruitt - 2025 - Journal of Geophysical Research: Biogeosciences - Wiley Online Library.

Nitrogen in the Environment: Denitrification. Extension. University of Missouri. November 2022. Nitrogen in the Environment: Denitrification | MU Extension