Sources and sinks of nutrients in a New Zealand hill pasture catchment III. Nitrogen

✍ Scribed by James G. Cooke; A. Bryce Cooper

• Publisher: John Wiley and Sons
• Year: 1988
• Tongue: English
• Weight: 1014 KB
• Volume: 2
• Category: Article
• ISSN: 0885-6087
• DOI: 10.1002/hyp.3360020204

✦ Synopsis

A study was made of the nitrogen (N) inputs to, and exports from, a stream draining a pasture catchment near Hamilton, New Zealand, in order to plan measures for minimizing N losses to natural waters.

An estimated 7 kg N ha-' was exported from the catchment during 1981 of which 86 per cent was in reduced forms (Kjeldahl-N, TKN) and the remainder as nitrate-N (N(3-N). Virtually all of the reduced N inputs came from saturated overland flow whereas N03-N inputs were dominantly subsurface derived. Thc TKN exported by individual storm events could be predicted (R' = 0.97) from peak flow and from the peak flow rate in the seven days preceding the storm. A TKN balance for eight events showed that except for large floods (return period approximately a year) the stream system was a net sink for TKN. During large floods, scouring of the organic rich seepage areas resulted in the stream system itself being a net source of TKN.

Microbial assays for nitrification and denitrification activity indicated that the main nitrate source was the well-aerated greywacke and ash soils and that the permanently saturated seepage zones were a significant nitrate sink. An in-stream nitrate addition experiment showed that up to 30mg N m-: h-' was removed from the stream. Simultaneous measurements of in situ denitrification activity demonstrated that only about 1 per cent of this removal could be accounted for by denitrification. It was inferred that plant uptake was responsible for the remainder.

Retention of near-stream seepage areas is suggested as a measure for minimizing N03-N export, whilst removal of stock from seasonally saturated areas during periods of saturation should reduce soil loss and hence TKN inputs to the stream.