Exploring the role of soil characteristics and plant traits as drivers denitrification in “accidental” urban wetlands in Phoenix, Arizona

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In wetlands, denitrification is an important ecosystem process that permanently removes reactive nitrogen from systems. Much research on denitrification has occurred in non-urban or highly managed urban wetlands. However, in urban landscapes N-rich water 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 nitrate; however, these urban wetlands support different vegetation (with potentially different functions) than their non-urban counterparts. Plant functional traits have emerged as a method to link plant species to ecosystem functions. In this study, we examined whether plant traits or soil characteristics were more important for predicting denitrification potentials in accidental urban wetlands. We performed denitrification enzyme assays (measuring denitrification potential, or DNP) on soil samples from nine wetlands located 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). Samples were taken from three to four vegetation patch types within each wetland and across three seasons differing in rainfall pattern. Soil organic matter, moisture, and extractable nitrate were measured on each soil core. Measured plant traits were aboveground biomass, belowground biomass, and rooting depth. We found that DNP varied among plant patches and was typically highest in patches of Ludwigia peploides, indicating that plant type may mediate within-wetland variations in nitrate removal capacity. Soil moisture, soil organic matter, and rooting depth emerged as significant predictors of denitrification potentials; however, the relative importance of each varied among seasons. Soil organic matter and soil moisture were predictive of DNP during the pre-monsoon season. Soil moisture was predictive during the post-monsoon season. Soil moisture and rooting depth were predictive during the winter season. These results suggest that the plant traits we examined were not the most important for driving DNP; a better understanding of how vegetation affects the soil environment for DNP is required. Drivers of DNP varied among seasons reinforcing the importance of cross-season studies when investigating denitrification in wetlands.