Soil texture and water retention as spatial predictors of denitrification in urban wetlands

Background/Question/Methods

Nitrogen removal is commonly cited as a rationale behind wetland restoration
projects, since wetlands have demonstrated the ability to prevent movement of excess nitrogen from upland areas into streams. The ability of wetland areas to remove nutrients from surface water is of particular importance in the northeastern United States, where atmospheric nitrogen deposition is high, and dense human populations generate high inorganic nitrogen levels in surface and ground water. Urban wetlands potentially play an important role in nitrate (NO₃^-) removal from stormwater, but the intersection of soil properties optimal for NO₃^- removal with nitrogen loads from the atmosphere and surface water is key to this function. Modifications to soils and water imposed by human use in urban areas may therefore compromise NO₃^- removal capacity. NO₃^- removal via the microbial process of denitrification was examined in an urban brownfield wetland in New Jersey, USA. Soil samples were collected at 118 points and analyzed for soil organic matter and texture fractions. Maps of these soil properties were interpolated across the entire site, and flow paths of stormwater and nearby low-lying areas through the site were digitized from aerial imagery. A subset (17%) of points was more intensively sampled to examine relationships between denitrification rates and soil water retention characteristics. Intact core denitrification rate, potential denitrification rate, available inorganic nitrogen, and water retention curves were characterized for this subset of soils.

Results/Conclusions

The highest intact core denitrification rates occurred in soils located at low elevations, with high macroporosity. High potential denitrification rates corresponded with high available soil NO₃^-. Spatial interpolation of soil properties related to high denitrification rates accurately predicted most locations of denitrification hotspots and cold spots as determined by earlier studies at the same site. Predicted hotspots corresponded to the location of stormwater channels running through the site over 31% of total channel area, indicating that soils at the site may be at least partially reducing total NO₃^- loads to the creek flowing through the site. This study demonstrates that even highly modified and unrestored sites in urban areas may be playing an important role in nitrogen cycling within these ecosystems, and that soil physical properties can be used for predicting the location of potential hotspots of denitrification at the landscape scale.