Dissolved Inorganic Nitrogen Behavior and Fate in Bioretention Systems: Role of Vegetation and Saturated Zones

Management of inorganic dissolved nitrogen (NH4-N and NO3-N) in urban stormwater is challenging. This study investigated the role of bioretention plants and a saturated zone (SZ) on nitrogen removal and fate, under conditions of varying stormwater hydrology and nutrient loadings. Experiments were conducted using one plant species (Pennisetum alopecuroides), two media types, and with and without a saturated zone and organic carbon addition to examine the fate of NH4-N and NO3-N in stormwater applied to bioretention columns. During 1 year of synthetic stormwater applications, all bioretention treatments effectively captured (>95% concentration and mass) NH4-N from stormwater independent of the presence or absence of plants and SZ. In contrast, both plants and SZs were critical to the effective removal of NO3-N and total nitrogen. Planted systems removed 59.4% of the input dissolved N, compared with 28.9% of the unplanted control. Furthermore, when including plants and a SZ, 95.4% of the input dissolved N was removed, with 57.0% attributed to plant uptake and 33.5% to denitrification. The relative contribution of vegetation and a SZ to nitrogen removal was also controlled by stormwater event magnitude and nutrient loadings. These results suggest that both plants and saturated zones are needed to effectively and consistently remove nitrogen from urban stormwater over a wide range of environmental conditions.