Regulators of denitrification in an organic riparian soil

Abstract

We investigated microbial denitrification in an organic riparian zone and identified factors which regulated its rate. The riparian zone received nitrate from incoming groundwater draining an upslope forest which was spray irrigated with treated effluent. Soil cores were taken from the riparian zone and the following variables were measured: KCl-extracted nitrate, water soluble carbon concentration, organic matter content, moisture content, denitrifying enzyme activity, on-site denitrification 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 the upslope edge of the riparian zone was generally greater than 5 mg N l-1. In combination, these factors resulted in an ideal environment for denitrification. Mean and median denitrification rates were found to be 1.12 and 0.95 g N m-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 magnitude 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. Temperature may also have regulated the rate of denitrification; however, insufficient observations at different temperatures were made to fully establish a temperature effect. N2O production was found to be most highly correlated to on-site denitrification rate. Rates of denitrifying enzyme activity were also greater than those generally found in upland soils, the mean and median rates were 810 and 740 ng N g-1 h-1.