Ditching nutrients: Roadside drainage networks are hotspots for coastal nitrogen removal

Abstract:

Human-made stormwater control systems such as roadside drainage networks are possible hotspots for nitrogen (N) removal in coastal watersheds, yet few studies have quantified their biogeochemical potential. We measured soil N-removal potential and microbial 16S rRNA structure in 96 roadside ditches draining predominately forested, urban, and agricultural catchments surrounding Mobile Bay, AL (USA). We found that N-removal by denitrification and anammox dominated over N-retention by dissimilatory nitrate reduction to ammonium (DNRA), accounting for upwards of 89% of NO₃^- reduction on average. Although there were no differences in soil inorganic N or organic matter concentrations across land use types, denitrification was more than 2X higher in urban and agricultural ditches compared to forested ditches. Microbial community variance was higher in forested ditches, though there were no significant correlations between community composition and environmental characteristics. Random forest analysis identified putative ammonia oxidizers and nitrifiers among the indicator taxa driving community differences between land use types, suggesting that the microbial community could influence N-removal rates by regulating nitrate availability. Our findings show that constructed drainage networks represent areas of considerable nutrient removal potential in the landscape, with denitrification and anammox rates equal to or greater than those measured in “natural” ecosystems. Further, these constructed systems appear to be less sensitive to land use change, potentially compensating for a loss of N-removal capacity associated with stream and wetland degradation in the region.