DENITRIFICATION POTENTIAL OF NONTIDAL RIPARIAN WETLAND SOILS IN THE VIRGINIA COASTAL-PLAIN

the Atlantic Coastal Plain, riparian wetlands are thought to play an i mportant role in the removal of NO3- from groundwater. The denitrifica tion potential of the Bibb (coarse-loamy, siliceous, acid, thermic Typ ic Fluvaquents) series, located in the Nomini Creek Watershed, Virgini a, was evaluated in the laboratory using soil columns under saturated flow conditions. Soil columns were infiltrated with synthetic groundwa ter containing 3.2 +/- 0.1 mmol NO3--N (44 +/- 2.0 mg NO3--N l(-1)). S oil samples were collected from two surface horizons and one subsurfac e horizon in May (soil temperature 16.4 degrees C), September (19.9 de grees C), and November (13.5 degrees C) of 1993 at 12 sites along the stream. Denitrification rates (N2O-N evolution in the presence of acet ylene) were significantly higher for soils incubated at 19.9 degrees C than at 16.4 or 13.5 degrees C. The highest mean denitrification rate s were measured in the ponded surface horizon (0.40 +/- 0.17 mu mol N2 O-N (g DW)(-1) d(-1) at 16.4 degrees C, 0.65 +/- 0.27 at 19.9 degrees C, 0.40 +/- 0.25 at 13.5 degrees C) in comparison to the terrestrial s urface (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 denitri fication were significantly correlated with organic carbon for the pon ded surface horizon across all three temperatures. For the terrestrial surface and subsurface horizons, organic carbon was related to denitr ification rates for only those soils incubated at 13.5 degrees C. Rela tions between NO3- loss and denitrification rates were present for the terrestrial surface horizon across all temperatures, and at 13.5 and 16.4 degrees C for the ponded surface and subsurface horizons. Spatial variability accounted for less than 10% of the variation in denitrifi cation rates, while soil horizon amounted to about 50%. Within the soi l columns, the relative NO3- concentration decreased rapidly with incr easing temperature in the surface horizons but much less so in the sub surface horizon. For the surface horizons, incubated at the higher tem peratures, NO3- concentration in the effluent was very low, suggesting that denitrification may have been limited by NO3- availability. The denitrification potential for the surface horizons was very high, rela ted in part to the high levels of organic carbon present in those hori zons. Copyright (C) 1996 Elsevier Science Ltd