REGULATORS OF DENITRIFICATION IN AN ORGANIC RIPARIAN SOIL

We investigated microbial denitrification in an organic riparian zone and identified factors which regulated its rate. The riparian zone rec eived nitrate from incoming groundwater draining an upslope forest whi ch was spray irrigated with treated effluent. Soil cores were taken fr om the riparian zone and the following variables were measured: KCl-ex tracted nitrate, water soluble carbon concentration, organic matter co ntent, moisture content, denitrifying enzyme activity, on-site denitri fication rates and natural N2O production. Five sampling surveys were made at a range of field temperatures (12-21-degrees-C). The riparian soil was continually water-saturated and contained an average organic matter content of 26%. Nitrate concentration in groundwater entering t he upslope edge of the riparian zone was generally greater than 5 mg N l-1. In combination, these factors resulted in an ideal environment f or denitrification. Mean and median denitrification rates were found t o be 1.12 and 0.95 g Nm-2 day-1; while mean and median N2O production rates were 73 and 84 mg N m-2 day-1 These rates were 1-3 orders of mag nitude greater than those reported in previous studies of upland soils . Up to 77% of the variation in on-site denitrification rate could be explained by nitrate concentration and denitrifying enzyme activity. T emperature may also have regulated the rate of denitrification; howeve r, insufficient observations at different temperatures were made to fu lly establish a temperature effect. N2O production was found to be mos t highly correlated to on-site denitrification rate. Rates of denitrif ying enzyme activity were also greater than those generally found in u pland soils, the mean and median rates were 810 and 740 ng N g-1 h-1