Vegetation enhances nitrogen removal in stormwater ponds in coastal South Carolina

Coastal development and the increase in impervious surfaces lead to greater stormwater volumes, a key contributor to nonpoint source pollution. Stormwater ponds are generally highly efficient in eliminating particle-bound nutrients, such as phosphorus, but are often much less effective at removing dissolved nutrients, like nitrogen. Excess inorganic nitrogen can lead to coastal eutrophication, harmful cyanobacterial blooms, and coastal hypoxia. However, inorganic nitrogen can be removed from the environment through several different microbially facilitated processes. This research aims to estimate the rates of nitrogen removal (i.e., by denitrification), the net N₂ fluxes, and internal recycling (DNRA) across the sediment-water interface in a range of stormwater ponds that differ in both the presence of vegetation or lack thereof and vegetation type/coverage. Rates of nitrogen removal (denitrification) were significantly higher in the vegetated stormwater ponds (36.9 - 774.2 µmol N m^-2 h^-1) compared to the non-vegetated ponds (6.4 - 35.1 µmol N m^-2 h^-1). Moreover, the average net N₂ fluxes from the sediments to the water column were positive in vegetated ponds (4.7 - 85.8 µmol N m^-2 h^-1). In contrast, unvegetated ponds exhibited predominantly negative N₂ fluxes (-70.9 - 4.4 µmol N m^-2 h^-1). These results indicate that the majority of net nitrogen removal occurred in vegetated ponds, while nitrogen fixation, which refers to the input of newly fixed atmospheric nitrogen, was prevalent in unvegetated ponds. Thus, unvegetated ponds not only proved ineffective in eliminating dissolved nitrogen but also typically served as a net source of newly fixed dissolved nitrogen. Planting native vegetation along the perimeters or allowing the growth of floating plants (e.g. lily pads) in stormwater ponds is strongly recommended, as it aids in the permanent removal of dissolved nitrogen before it enters coastal areas.