Abstract
Widespread wetland loss and degradation has resulted in a subsequent loss of the ecosystem services they provide, including the removal of human-produced nitrogen. Human-made stormwater control systems such as roadside ditches are possible hotspots for nitrogen removal in coastal watersheds, yet few studies have quantified their biogeochemical potential. We measured soil nitrogen removal potential and microbial 16S rRNA structure in 96 roadside ditches draining predominately forested, urban, and agricultural catchments surrounding Mobile Bay, AL (USA). Nitrogen removal by denitrification and anammox dominated over nitrogen retention by dissimilatory nitrate reduction to ammonium (DNRA), accounting for upwards of 89% of nitrate reduction on average. There were no differences in soil chemistry, annamox, DNRA, or microbial diversity across land use types. However, denitrification was more than twice as high in urban and agricultural ditches compared to forested ditches, and microbial indicator species analysis selected putative ammonia oxidizers (Nitrososphaeraceae and Nitrosomonadaceae), nitrate reducers (Gaiellales), nitrous oxide reducers (Myxococcales) as significant groups in Urban and Agricultural ditches. Additionally, denitrification and DNRA were positively correlated with plant biomass, which was selected as a key driver of microbial community structure. These results suggest that while land development may influence nitrogen removal in these systems, ditch management practices, such as mowing, may outweigh the effects of land use. Further, 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, and can potentially compensate for a loss of nitrogen removal capacity associated with stream and wetland degradation in the region.
