Small and Large-scale Drivers of Denitrification Patterns in “Accidental” Urban Wetlands in Phoenix, Arizona

Abstract:

Understanding spatial and temporal patterns of microbial conversion of nitrate (NO₃^-) to nitrogen (N) gas (denitrification) is important for predicting permanent losses of reactive N from systems. In many landscapes, wetlands serve as hotpots of denitrification by providing optimal condition for denitrifiers (sub-oxic, carbon-rich sediments). Much research on denitrification has occurred in non-urban or highly managed urban wetlands. However, in urban landscapes N-rich stormwater is often discharged into areas not designed or managed to reduce N loads. “Accidental” wetlands forming at these outfalls may have the capacity to remove NO₃^-; however, these “accidental” urban wetlands can contain novel soils and vegetation, and are subject to unique hydrologic conditions that could create spatial and temporal patterns of denitrification that differ from those predicted in non-urban counterparts. We performed denitrification enzyme assays (measuring denitrification potential, or DP) on soil samples taken from nine wetlands forming at storm drain outfalls in Phoenix, AZ. The wetlands ranged from perennially flooded, to intermittently flooded (~9 months/year), to ephemerally flooded (2-3 weeks/year). To assess spatial variation in carbon availability to denitrifiers, samples were taken from 3-4 dominant vegetation patch types within each wetland. To assess temporal variation in DP, samples were taken across three seasons differing in rainfall pattern. We found small- and large-scale spatiotemporal patterns in DP that have important implications for management of urban wetlands for stormwater quality. DP varied among plant patches and was typically highest in patches of Ludwigia peploides, indicating that plant species type may mediate within-wetland variations in carbon availability, and therefore NO₃^- removal capacity. We found a range of responses in DP among wetlands to season, which appeared to be driven in part by flood regime: DP in perennially-flooded wetlands was largely unchanged across seasons, DP in intermittently-flooded wetlands generally increased in summer monsoons and decreased in winter, and ephemerally-flooded wetlands had a variable response in DP to season. This pattern indicates temporally variable controls on NO₃^- removal capacity at the whole-wetland scale.