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Ditching Nutrients: Roadside Drainage Networks are Hotspots for Microbial Nitrogen Removal

Human‐made stormwater control systems are biogeochemical hotspots, but construction and management may result in homogenization of their ecosystem structure. Roadside ditches are a ubiquitous part of the landscape, yet few studies have quantified their biogeochemical potential. We conducted a study to determine (a) nitrate (NO3−) removal potential through rate measurements and (b) microbial community structure using 16S rRNA gene (iTag) sequencing in roadside ditches draining predominantly forested, urban, and agricultural watersheds surrounding Mobile Bay, AL (USA). We found that nitrogen (N) removal rates by denitrification and anammox dominated over N‐retention by dissimilatory nitrate reduction to ammonium, accounting for upwards of 89% of NO3− reduction on average. There were no differences in soil characteristics between land use types, but denitrification potential rates in forested ditches were less than half of those in urban and agricultural ditches, possibly as a result of differences in vegetation management. Microbial alpha and beta diversity were largely homogenous across the three land use types. However, indicator species analysis revealed putative ammonia oxidizers (Nitrososphaeraceae and Nitrosomonadaceae), nitrate reducers (Gaiellales), and nitrous oxide reducers (Myxococcales) as significant groups in urban and agricultural ditches. We conclude that land use effects on N‐removal in these constructed drainage networks are mediated through key microbial groups and ditch vegetation management strategies. Further, roadside ditches have significant potential for reactive N removal in the landscape.