Volume 55 No 2 April 2007 - New Zealand Society of Soil Science

Denitrification capacity in the vadose zone at three sites in the Lake
Taupo catchment, New Zealand
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 Box 84, Lincoln University, Lincoln, New Zealand.
C Lincoln Environmental Research, Private Bag 3062, Hamilton, 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