Rivers do more than transport water across landscapes. They also act as natural filters, transforming excess nitrogen before it reaches downstream lakes, reservoirs, and coastal waters. A new study of a forest-dominated river in central China reveals that this filtering capacity is controlled by a changing combination of microbial activity, water and sediment conditions, altitude, and surrounding land use.
"Our findings show that riverine nitrogen removal cannot be explained by a single environmental factor," said corresponding author Hao Jiang of the Wuhan Botanical Garden, Chinese Academy of Sciences. "The dominant controls change with the seasons, so effective watershed management must consider both large-scale landscape conditions and the microscopic processes occurring within river sediments."
Excess reactive nitrogen from fertilizer use, sewage, and industrial activity can contribute to eutrophication, harmful algal blooms, oxygen depletion, and biodiversity loss. Rivers help reduce this pollution through microbial processes that convert biologically available nitrogen into harmless nitrogen gas, which is released into the atmosphere.
The researchers focused on two major nitrogen-removal pathways: denitrification and anaerobic ammonium oxidation, commonly known as anammox. They collected water and surface sediment samples from 18 sites along the Jinshui River during summer and winter. The catchment covers approximately 730 square kilometers, spans a large elevation range, and has more than 95% forest cover in much of its upper reaches.
To understand nitrogen cycling across multiple scales, the team combined remote sensing, nitrogen-15 isotope labeling, measurements of water and sediment chemistry, and quantitative polymerase chain reaction analysis of microbial functional genes.
Denitrification was the dominant nitrogen-removal pathway in both seasons. It accounted for about 90% of total measured nitrogen removal in summer and 95% in winter. Average denitrification rates were substantially higher during summer, while anammox contributed a smaller but measurable share of nitrogen removal throughout the year.
The analysis also revealed a strong seasonal shift in the factors controlling these processes. During summer, denitrification was closely associated with microbial functional genes and local sediment conditions, including total nitrogen, organic carbon, carbon-to-nitrogen ratio, and moisture. Water properties indirectly influenced denitrification by shaping sediment conditions and the abundance of nitrogen-cycling microorganisms.
Anammox responded differently. In summer, its activity was influenced more strongly by geographical factors, particularly altitude and land use. Greater forest cover was associated with higher anammox rates, possibly because the relatively low availability of organic carbon in forested areas reduced competition from denitrifying microorganisms.
In winter, water chemistry became the primary control on both denitrification and anammox. Temperature and concentrations of ammonium and nitrate in the overlying water played especially important roles. Lower temperatures reduced microbial activity and weakened the internal recycling of nitrogen within sediments. As a result, nitrogen supplied by the river water became more important as a substrate for microbial removal.
The researchers also found that nitrification and denitrification were closely linked during summer, suggesting that nitrogen compounds produced by one microbial process helped fuel another. This connection weakened during winter, when cold conditions reduced microbial activity and limited the movement of chemical substrates between sediment zones.
The study demonstrates that catchment geography and microscopic sediment processes must be considered together to understand how rivers regulate nitrogen pollution. The findings could help improve models of the global nitrogen cycle and support more seasonally responsive strategies for managing forested watersheds, land-use change, and downstream water quality.
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Journal Reference: Zhang W, Li X, Jiang H, Zhang Q. 2026. Geographical and micro-environmental factors regulate nitrogen removal in a forest-dominated river. Nitrogen Cycling 2: e020 doi: 10.48130/nc-0026-0007
https://www.maxapress.com/article/doi/10.48130/nc-0026-0007
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Nitrogen Cycling (e-ISSN 3069-8111) is a multidisciplinary platform for communicating advances in fundamental and applied research on the nitrogen cycle. It is dedicated to serving as an innovative, efficient, and professional platform for researchers in the field of nitrogen cycling worldwide to deliver findings from this rapidly expanding field of science.
