Denitrification potential of nontidal riparian wetland soils in the Virginia Coastal Plain

Abstract

In the Atlantic Coastal Plain, riparian wetlands are thought to play an important role in the removal of NO − 3 from groundwater. The denitrification potential of the Bibb (coarse-loamy, siliceous, acid, thermic Typic Fluvaquents) series, located in the Nomini Creek Watershed, Virginia, was evaluated in the laboratory using soil columns under saturated flow conditions. Soil columns were infiltrated with synthetic groundwater containing 3.2 ± 0.1 mmol NO − 3-N (44 ± 2.0 mg NO − 3-N l −1). Soil samples were collected from two surface horizons and one subsurface horizon in May (soil temperature 16.4°C), September (19.9°C), and November (13.5°C) of 1993 at 12 sites along the stream. Denitrification rates (N 2O-N evolution in the presence of acetylene) were significantly higher for soils incubated at 19.9°C than at 16.4 or 13.5°C. The highest mean denitrification rates were measured in the ponded surface horizon (0.40 ± 0.17 μmol N 2O-N (g DW) −1 d −1 at 16.4°C, 0.65 ± 0.27 at 19.9°C, 0.40 ± 0.25 at 13.5°C) in comparison to the terrestrial surface (0.17 ± 0.11, 0.28 ± 0.13, 0.16 ± 0.11) and subsurface (0.04 ± 0.02, 0.11 ± 0.06, 0.04 ± 0.01) horizons. Rates of denitrification were significantly correlated with organic carbon for the ponded surface horizon across all three temperatures. For the terrestrial surface and subsurface horizons, organic carbon was related to denitrification rates for only those soils incubated at 13.5°C. Relations between NO − 3 loss and denitrification rates were present for the terrestrial surface horizon across all temperatures, and at 13.5 and 16.4°C for the ponded surface and subsurface horizons. Spatial variability accounted for less than 10% of the variation in denitrification rates, while soil horizon amounted to about 50%. Within the soil columns, the relative NO − 3 concentration decreased rapidly with increasing temperature in the surface horizons but much less so in the subsurface horizon. For the surface horizons, incubated at the higher temperatures, NO − 3 concentration in the effluent was very low, suggesting that denitrification may have been limited by NO − 3 availability. The denitrification potential for the surface horizons was very high, related in part to the high levels of organic carbon present in those horizons.