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 [NO.sub.3]-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 [NO.sub.3]-N, [NH.sub.4]-N, and dissolved organic carbon were determined. Incubations were undertaken to determine the denitrification capacity of the vadose zone materials amended with [NO.sub.3][sup.-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 [N.sub.2] and [N.sub.2]O. In soil depths down to about 1.2 m, the denitrification capacity ranged from 0.03 to 9.18 kgN/ha.day, and below this depth it ranged from Additional keywords: nitrate, groundwater, land-use impacts.
Greg Barkle (A), Tim Clough (B), and Roland Stenger (c) (A)Corresponding author. Aqualinc Research Ltd, PO Box 14-041, Hamilton, New Zealand. Email: G.Barkle@Aqualinc.co.nz (B) Agriculture & Life Sciences Division, PO [...]