Seasonal patterns in sediment nitrification rates and their linkages to ammonium cycling in three agricultural streams

Nitrification, or the microbial transformation of ammonium (NH₄⁺–N) to nitrate, is influenced by NH₄⁺–N and dissolved oxygen availability, water temperature, and carbon-to-nitrogen ratios. Open-canopy agricultural streams receive excess inorganic nitrogen (N) from the surrounding landscape and the mineralization of organic-rich sediments, and the form and timing of these N inputs varies throughout the year. Compared to forested streams, the seasonality of nitrification rates in agricultural streams are not well documented. We conducted nitrification assays on stream sediments to estimate seasonal rates in three agricultural streams from summer 2020 to spring 2021. We documented seasonal variation in nitrification rates and identified changes in environmental controls [e.g., stream temperature, NH₄⁺–N and dissolved organic carbon (DOC) availability, chlorophyll-a]. Nitrification rates were highest in spring (54.4 ± 12.7 mg N m⁻² d⁻¹; p = 0.02), coinciding with elevated NH₄⁺–N and higher stream temperatures relative to winter (p < 0.001). Rates were lowest in autumn (19.9 ± 3.5 mg N m⁻² d⁻¹) when organic carbon concentrations peaked (17.2 ± 10.3 mg C L⁻¹; p = 0.01). Algal senescence in autumn may allow heterotrophs to outcompete nitrifiers for NH₄⁺–N. However, partial least square regression analyses indicated that sediment organic matter (as %OM) is an important positive predictor of nitrification, suggesting carbon can be an indirect positive control on nitrification. In the context of previous studies, agricultural streams had elevated NH₄⁺–N concentrations, but nitrification rates were comparable to those in less impacted systems. Although complex interactions exist among rates and drivers, rates from this study help expand documentation of nitrification in agricultural streams, and provide insight into temporal variation and dominant controls.