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

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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−1was 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−2h−1was removed from the stream. Simultaneous measurements ofin situdenitrification 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.