Stormwater detention basins are primarily designed to detain large volumes of storm runoff and trap suspended sediments and associated pollutants. Detaining and retaining nutrients are often not a design focus. The combination of variable hydrologic patterns in stormwater basins along with potential nutrient influxes may make these basins hotspots for nitrogen transformations such as denitrification, as well as potential sources of greenhouse gases nitrous oxide (N2O) and methane (CH4). In this study, we focused on detention basins in upstate New York with differing hydrologic regimes- two fast-draining or ‘dry’ basins and two slow-draining or ‘wet’ basins. Nitrous oxide and CH4 emissions were measured using static chambers in the basins. Denitrification potential of soils collected from these basins was measured using the denitrification enzyme assay (DEA). Soil samples from each basin were also used to extract DNA for metagenomics analysis of genes in the denitrification pathway. In two of the basins (one wet and one dry) we also monitored inflows and outflows to quantify the net removal of nitrogen.
Results/Conclusions
While N2O emissions were low overall, CH4 emissions were higher in both wet basins, averaging 5667 μg C m−2 h−1 in the wettest basin. Denitrification potential was higher in the wet basins (2.27 mg N kg−1 h−1) compared to the dry basins (0.23 mg N kg−1 h−1). We also found higher abundance of genes from the denitrification pathway in the wet basins, including a higher abundance of nos, which promotes complete denitrification to N2. When examining net removal of nitrogen, the dry basin had much higher retention, given that it infiltrated much more of the inflowing stormwater, but it is unclear what the ultimate fate of this infiltrated nitrate was. Overall, wet detention basins had higher greenhouse gas emissions due to CH4 but also had higher potential for complete nitrate removal via denitrification. We suggest that it is important to go beyond the traditional ‘inflow versus outflow’ comparison to investigate specific nutrient fates in these engineered basins, and that designing future stormwater control measures to maintain a subsurface saturated zone rather than fully saturated soils should be considered to promote denitrification while also reducing CH4 emissions at the surface.