Differential Controls of Denitrification in Riparian Zones and Streams along an Urban to Exurban Gradient

The large increase in impervious surface cover due to urbanization leads to “flashier” storm flows and increased runoff. This altered hydrology causes channel incision in urban stream channels and lower water tables in the riparian (next to the stream) zone. In turn, these physical changes alter many stream processes, including denitrification. Denitrification is the anaerobic microbial process that transforms nitrate (NO3) to N2 gas. It is particularly important, as it is one of the primary sinks of nitrogen, and excess nitrogen loading can lead to eutrophication and hypoxia in coastal areas. We might expect there to be more sources of nitrogen from exurban areas than urban areas, as exurban land use is often agricultural, with substantial fertilizer runoff, and houses in these areas are on septic tanks, which are another large nitrogen source. Depending on the extent of these nitrogen sources, and on the availability of carbon, denitrification will either be carbon or nitrogen limited. This study, therefore, sought to determine if denitrification in riparian zones and streams was limited by carbon or nitrogen and whether these limitations varied over an urban to exurban gradient. Denitrification potential and a suite of related variables (microbial biomass carbon and nitrogen content, potential net nitrogen mineralization and nitrification, microbial respiration, inorganic nitrogen pools) were measured in forested and herbaceous riparian soils and stream sediments from debris dams and pools in four different watersheds (urban, suburban, exurban, and forested reference) in and around Baltimore, MD. Denitrification potential was highest in herbaceous riparian soils and lowest in pool sediments. In general, we observed highest denitrification rates in the exurban habitats, while the other watersheds did not differ. Finally, denitrification potential was carbon-limited in all exurban habitats, where rates were significantly higher when carbon was added but not when nitrogen was added. These data suggest that despite the large nitrogen loading from the exurban watershed, some of the load is effectively being denitrified. Further, these results show that stream restoration efforts should focus on bringing more carbon into these systems, which should result in even higher denitrification rates.