Stormwater biofilter response to high nitrogen loading under transient flow conditions: Ammonium and nitrate fates, and nitrous oxide emissions.

Nitrogen (N) in urban runoff is often treated with green infrastructure including biofilters. However, N fates across biofilters are insufficiently understood because prior studies emphasize low N loading under laboratory conditions, or use "steady-state" flow regimes over short time scales. Here, we tested field scale biofilter N fates during simulated storms delivering realistic transient flows with high N loading. Biofilter outflow ammonium (NH-N) was 60.7 to 92.3% lower than that of the inflow. Yet the characteristic times for nitrification (days to weeks) and denitrification (days) relative to N residence times (7 to 30 h) suggested low N transformation across the biofilters. Still, across 7 successive storms, total outflow nitrate (NO-N) greatly exceeded (3100 to 3900%) inflow nitrate, a result only explainable by biofilter soil N nitrification occurring between storms. Archaeal, and bacterial amoA gene copies (2.1 × 10 to 1.2 × 10 gc g soil), nitrifier presence by16S rRNA gene sequencing, and outflow δO-NO values (-3.0 to 17.1 ‰) reinforced that nitrification was occurring. A ratio of δO-NO to δN-NO of 1.83 for soil eluates indicated additional processes: N assimilation, and N mineralization. Denitrification potential was suggested by enzyme activities and soil denitrifying gene copies (nirK + nirS: 3.0 × 10 to 1.8 × 10; nosZ: 5.0 × 10 to 2.2 × 10 gc g soil). However, nitrous oxide (NO-N) emissions (13.5 to 84.3 μg N m   h  ) and NO export (0.014 g N) were low, and soil nitrification enzyme activities (0.45 to 1.63 mg N kg soilday) exceeded those for denitrification (0.17 to 0.49 mg N kg soil day). Taken together, chemical, bacterial, and isotopic metrics evidenced that storm inflow NHsorbs and, along with mineralized soil N, nitrifies during biofilter dry-down; little denitrification and associated NO emissions ensue, and thus subsequent storms export copious NO-N. As such, pulsed pass-through biofilters require redesign to promote plant assimilation and/or denitrification of mineralized and nitrified N, to minimize NO-N generation and export.