Effects of storm size and frequency on nitrogen retention, denitrification, and N₂O production in bioretention swale mesocosms

Reported nitrogen (N) retention efficiencies for bioretention swales vary widely, but reasons for this are not well-understood, in part because almost no studies have measured (or characterized controls on) bioretention swale denitrification. Here, we apply a novel N₂:Ar-based approach, in coordination with more established approaches, to estimate denitrification rates and compare bioretention N dynamics during artificial storms of two sizes (3.05 and 5.08 cm days⁻¹) and following 4 inter-storm periods (initial storm with no prior storm, 1-, 7-, and 13-days). Denitrification rates during storms occurring after 7-days (520 ± 150 µmol N m⁻² h⁻¹) were significantly higher than those during an initialization storm (13 ± 34 µmol N m⁻² h⁻¹) or during a storm occurring one day after a previous storm (-63 ± 65 µmol N m⁻² h⁻¹). No significant differences in N processing were observed between 3.05 and 5.08 cm days⁻¹ storms. Somewhat surprisingly, in all experiments [O₂] remained near saturated, and N₂O emissions were very low or undetectable. Mesocosms were largely a net sink for dissolved inorganic N (DIN) and a net source of dissolved organic N (DON). Denitrification was neither a dominant nor consistent pathway for N removal, accounting for a maximum of 23 ± 11% of DIN removal. Future research should continue to evaluate N assimilation as a N removal pathway in bioretention swales, as well as characterize N dynamics during unsaturated conditions associated with smaller rain events and during periods between the large storms examined here.