| Abstract: | 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?1 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 (NO3-N). Virtually all of the reduced N inputs came from saturated overland flow whereas NO3-N inputs were dominantly subsurface derived. The TKN exported by individual storm events could be predicted (R2 = 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 20mg N m?2 h?1 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 NO3-N export, whilst removal of stock from seasonally saturated areas during periods of saturatior should reduce soil loss and hence TKN inputs to the stream. |
