High Potential Nitrate Removal by Urban Accidental Wetlands in a Desert City: Limitations and Spatiotemporal Patterns

Urban areas are typically considered to be net exporters of reactive nitrogen. As a result, much effort has gone into creating or restoring areas supporting microbial denitrification, which permanently removes nitrate from urban ecosystems. However, denitrification is a facultative process, with complex spatiotemporal drivers and limitations, making it difficult to predict where or when denitrification will occur. This is particularly true in urban systems, where drivers and limitations can differ greatly from those of native systems. In this study, we examine novel urban ecosystems in a unique geographic setting, investigating limitations and spatiotemporal drivers of denitrification in accidental wetlands (AW) located in a desert city (Phoenix, AZ). These wetlands were unintentionally created by runoff generated in Phoenix and exiting storm pipes into a dry riverbed. Previous work in native, nonurban Arizona wetlands (NW) found that monsoon floods and plant patches are important spatiotemporal drivers of denitrification. While we found that AW had high potential to process nitrate, denitrification patterns in AW exhibit different drivers from NW. As predicted, denitrification potential in AW was greater under plant patches, but surprisingly, this was not only due to the plants alleviating carbon limitation as both vegetated and unvegetated patches were not carbon limited. Contrary to predictions, monsoon floods did not increase denitrification potential, and perennially inundated AW had the highest denitrification potential, suggesting less temporal variation in denitrification in AW than in NW. Together, these findings offer novel insights into the complex interactions shaping spatiotemporal patterns of nitrate processing in arid urban regions.