Denitrification capacity in the vadose zone at three sites in the Lake Taupo catchment, New Zealand

Land use in the Lake Taupo catchment is under scrutiny, as early signs of deteriorating water quality in Lake Taupo have been observed. Although the fate of contaminants in soil and groundwater are comparatively well studied, the transformations in the lower vadose zone, i.e. the zone between the soil and the groundwater, are less well understood. The capacity for NO3-N removal via biological denitrification, based on utilising the resident C substrate, in the vadose zone of the Lake Taupo catchment is quantified in this work. Complete vadose zone profiles were sampled at 3 sites (Rangiatea, Waihora, and Kinloch), from the soil surface down to the watertable in approximately 0.5-m depth increments. Texture, allophane content, pH, and concentrations of extractable NO3-N, NH4-N, and dissolved organic carbon were determined. Incubations were undertaken to determine the denitrification capacity of the vadose zone materials amended with NO3-15N, but no added carbon substrate, and maintained under anaerobic conditions at 28°C. Gas samples were taken from the headspace after 48 h and analysed for N2 and N2O. In soil depths down to about 1.2 m, the denitrification capacity ranged from 0.03 to 9.18 kg N/ha.day, and below this depth it ranged from <0.01 to 0.09 kg N/ha.day. A palaeosol layer in the Waihora profile had an enhanced denitrification capacity compared with the other samples in deeper zones of the profiles. In the surface sampling, at least 99.9% of the gas recovered from the 15N applied was in the form of N2. In contrast, no N2 gas production could be detected in any sample from below the second sampling depth, with only N2O detected. Denitrification capacities of all vadose zone materials were low when compared with other studies. Thus, careful land management is required to avoid groundwater contamination by nitrate leaching from the root-zone of the pasture.