Assimilatory processes dominate diel inorganic nitrogen cycling in a shallow open-water constructed wetland

Shallow, unit process open water (UPOW) constructed wetlands effectively remove a range of water contaminants. Yet, much remains to be learned about internal carbon and nitrogen cycling within these systems and their diel functionality. The current study focused on light/dark fluctuations in water column and biomat geochemistry and N cycle processes in a field-scale UPOW. Dissolved oxygen concentrations fluctuated daily from 50 to 250 % air saturation, while nitrate (NO₃⁻) and ammonium (NH₃₊₄) concentrations cyclically ranged from 30 to 125 μM and 2–20 μM, respectively. NO₃⁻ concentrations increased at night and decreased during the day, while N isotope values (δ¹⁵N-[NO₃⁻]) varied inversely with concentration. Ammonium concentrations and N isotope values (δ¹⁵N-[NH₃₊₄]) both increased at night, decreasing during the day. Column incubations containing biomat and overlying water, conducted in situ with ¹⁵N-NO₂⁻, determined nearly equivalent rates of nitrite production and denitrification in the dark (6.6 and 7.2 μmol L⁻¹ d⁻¹, respectively), but both decreased 82–84% in daylight. Combined nighttime rates of anammox and dissimilatory NO₃⁻ reduction to NH₃₊₄ (DNRA) were nearly equal to denitrification. A time-step N process model was used to reproduce diel N species concentration and stable isotope changes, and N dissimilatory rates. Best-fit results to field data indicated that N photoassimilation rates far outpaced dissimilatory rates in either light or dark conditions while denitrification and DNRA rates were greater than incubation-measured rates. Redox gradients and potential diatom migration within the biomat, coupled with diatom DNRA respiration, are factors that complicate the diel function and wetland efficacy by short-circuiting N removal as nitrogen gas.