Fluvial Denitrification Rates in an Agricultural River and Its Tributary Vary Due To Size and Season

Abstract Anthropogenic impacts such as land use change are increasing dissolved nitrogen (N) loading to streams and rivers, degrading downstream water quality. Denitrification, where nitrate (NO 3 − ‐N) is converted to dinitrogen gas (N 2 ), serves as a permanent N sink. While stream denitrification is well studied, its role in larger rivers remains understudied, despite rivers playing a key role in reach‐scale N retention. Moreover, it remains uncertain how denitrification rates differ throughout the year. We compared stream and river denitrification rates across seasons using an open‐channel N 2 ‐exchange method in the Tippecanoe River and its tributary, Shatto Ditch. Areal denitrification rates were higher in the stream than the river in spring (stream = 148.0, river = 5.8 mg N m −2 h −1 ), summer (76.2 vs. 0.95), and fall (65.5 vs. 18.6). Rates were strongly influenced by NO 3 − ‐N concentrations ( R 2 = 0.93), which were higher in the stream. When scaled per km of channel length, river N removal was comparable or higher to the stream: summer (stream = 167.8, river = 56.7 g N km −1 h −1 ), spring (325.6 vs. 348.0), and fall (144.0 vs. 1116.5). These results confirm that streams have high biogeochemical reactivity, but they also reveal that when we account for size discrepancies, rivers can contribute a similar or greater amount of N removal because of their relatively larger size and volume. This work expands our knowledge of denitrification in fluvial ecosystems and demonstrates that both rivers and streams in agriculturally impacted areas can mitigate excess N loading to downstream ecosystems, which is needed to improve water quality.