Denitrification and soil water chemistry in riparian areas along an urbanization gradient
Humans have significantly altered the landscape in urban areas resulting in increased volume and velocity of runoff following precipitation events. This increased volume causes urban streams to become incised which disconnects them from their riparian areas, which are hot spots for denitrification. Denitrification is an anaerobic microbial process that converts nitrate into nitrogen gas. This process is important to the water quality maintenance function of riparian zones in areas where there is concern about nitrate delivery to receiving waters. In this study we monitored water chemistry (NH4+, NO3-, PO4-3) and water table height monthly from 1999-2011 in wells located 5 m away from streams in four forested riparian areas: urban (Cahill), less urban (Gwynnbrook), suburban (Glyndon), and reference (Oregon ridge). At each site adjacent to the monitoring wells we took two 100 cm long cores that we divided into 4 sections: 0-10 cm, 10-30 cm, 30-50 cm and 50-100 cm. On the sections from one core we measured soil nitrate, ammonium, respiration, potential net N mineralization and potential net N nitrification. On the sections from the other we measured denitrification rates (N2O plus N2) using the Nitrogen-Free Air Recirculation Method (N-FARM).
The water table was consistently closer to the surface and the nitrate concentrations in groundwater were lower in the reference site than at the urban/suburban sites. Soil respiration and soil nitrate decreased with depth at all sites. Compared to the urban and suburban sites soils at the reference sites had: lower nitrate, higher potential net N mineralization, lower potential net N nitrification, and lower denitrification. These data further the idea that physical changes in streams and riparian zones associated with urbanization can convert riparian areas from denitrification “sinks” for nitrogen to sources of reactive nitrogen in urban watersheds.