Impact of chloride on denitrification potential in roadside wetlands

Developed landscapes are exposed to changes in hydrology and water chemistry that limit their ability to mitigate detrimental impacts to coastal water bodies, particularly those that result from stormwater runoff. The elevated level of impervious cover increases not only runoff but also contaminant loading of nutrients, metals, and road salt used for deicing to water bodies. Here we investigate the impact that road salt has on denitrification in roadside environments. Sediments were collected from a series of forested and roadside wetlands and acclimated with a range of Cl− concentrations from 0 to 5000 mg L−1 for 96 h. Denitrification rates were measured by the isotope pairing technique using 15N–NO3−, while denitrifying community structures were compared using terminal restriction fragment length polymorphism (T-RFLP) of nitrous oxide reductase genes (nosZ). Chloride significantly (p < 0.05) inhibited denitrification in forested wetlands at a Cl− dosage of 2500 or 5000 mg L−1, but the decrease in denitrification rates was less and not significant for the roadside wetlands historically exposed to elevated concentrations of Cl−. The difference could not be attributed to other significant changes in conditions, such as DOC concentrations, N species concentrations, or pH levels. Denitrifying communities, as measured by T-RFs of the nosZ gene, in the roadside wetlands with elevated concentration of Cl− were distinctly different and more diverse compared to forested wetlands, and also different in roadside wetlands after 96 h exposures to Cl−. The shifts in denitrifying communities seem to minimize the decrease in denitrification rates in the wetlands previously exposed to Cl. As development results in more Cl− use and exposure to a broad range of natural or manmade wetland structures, an understanding of the seasonal effect of Cl on denitrification processes in these systems would aid in design or mitigation of the effects on N removal rates.