Denitrification in streams and riparian zones along an urban to exurban gradient

Background/Question/Methods

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 riparian soils and stream sediments in four different watersheds (urban, suburban, exurban, and forested reference) in and around Baltimore, MD. 

Results/Conclusions

Denitrification potential in both the urban and exurban forested riparian soils were significantly higher when carbon was added but not when nitrogen was added, suggesting that denitrification in these riparian soils was carbon-limited.  We also found significantly higher denitrification potential in the exurban riparian soils than the other sites, which did not differ.  This suggests, that despite the large nitrogen loading into all of the streams, they are effectively denitrifying some of this load.  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.