Rivers act as natural filters by converting reactive nitrogen from terrestrial runoff into harmless dinitrogen gas. Denitrification is generally considered the principal pathway for this removal, while anammox provides an alternative route under oxygen-limited conditions. Both processes depend on nutrient availability, temperature, organic carbon, sediment characteristics, and nitrogen-transforming microorganisms. At broader scales, altitude, climate, slope, and land use can modify these local conditions by controlling runoff, sediment transport, and nutrient inputs. Previous studies, however, have usually examined either catchment-scale geography or micro-environmental controls separately. Consequently, how these factors interact across spatial scales and seasons to regulate nitrogen removal in relatively undisturbed forest rivers has remained poorly understood.
A study (DOI: 10.48130/nc-0026-0007) published in Nitrogen Cycling on 01 May 2026 by Hao Jiang’s team, Tianjin University, demonstrates that denitrification dominates sedimentary nitrogen removal, while the factors controlling denitrification and anammox vary substantially between summer and winter.
The researchers investigated the Jinshui River, a 730-square-kilometer forested catchment spanning elevations from 416 to 2,904 meters. They collected paired surface-water and sediment samples from 18 locations in July and December 2021, producing 36 water and 36 sediment samples. Remote-sensing data were used to characterize altitude, climate, slope, and land use, while field and laboratory measurements quantified water temperature, pH, dissolved oxygen, oxidation-reduction potential, inorganic nitrogen, sediment nutrients, organic carbon, moisture, and other properties. The team applied nitrogen-15 isotope-pairing techniques to measure denitrification, anammox, and dissimilatory nitrate reduction to ammonium. Quantitative polymerase chain reaction assays measured genes associated with nitrogen cycling, including nirS, nirK, nosZ, and hzsA. Statistical correlations and partial least-squares path modeling were then used to distinguish direct and indirect relationships among geographical factors, environmental conditions, microbial genes, and nitrogen-removal rates. Denitrification averaged 1.68 ± 2.22 milligrams of nitrogen per kilogram of sediment per day in summer, compared with 0.21 ± 0.23 for anammox. In winter, the respective rates declined to 0.67 ± 1.52 and 0.07 ± 0.12 milligrams per kilogram per day. Denitrification contributed about 90% of total nitrogen removal in summer and 95% in winter. Summer denitrification was associated with denitrification-gene abundance, sediment total nitrogen, organic carbon, carbon-to-nitrogen ratio, and moisture. The path model explained 67% of the variation in summer denitrification, identifying water properties and functional genes as major drivers. By contrast, altitude and land use exerted the strongest direct influence on summer anammox, with the model explaining 39% of its variation. Forest cover was positively associated with anammox, possibly because low carbon and nutrient availability reduced competition from denitrifiers. In winter, geographical influences weakened as lower precipitation reduced land-river connectivity. Water temperature, ammonium, and nitrate instead became the principal regulators, with the models explaining 62% of denitrification variation and 54% of anammox variation.
Overall, the study shows that riverine nitrogen removal cannot be explained by either catchment geography or local sediment conditions alone. Its controls form a seasonally changing hierarchy: geographical factors shape summer micro-environments and strongly constrain anammox, whereas water chemistry becomes more influential during winter. Integrating remote sensing, isotope tracing, microbial analysis, and cross-scale modeling therefore provides a more complete picture of how forest rivers process excess nitrogen. This framework may help researchers anticipate changes caused by climate and land-use shifts and enable watershed managers to design nitrogen-control measures suited to different locations and seasons.
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References
DOI
10.48130/nc-0026-0007
Original Source URL
https://doi.org/10.48130/nc-0026-0007
Funding information
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