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.
Biogeochemistry – Springer Journals
Published: Aug 10, 2017
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