A mass balance procedure was used to determine rates of nitrate depletion in the riparian zone and stream channel of a small New Zealand headwater stream. In all 12 surveys the majority of nitrate loss (56–100%) occurred in riparian organic soils, despite these soils occupying only 12% of the stream's border. This disproportionate role of the organic soils in depleting nitrate was due to two factors. Firstly, they were located at the base of hollows and consequently a disproportionately high percentage (37–81%) of the groundwater flowed through them in its passage to the stream. Secondly, they were anoxic and high in both denitrifying enzyme concentration and available carbon. Direct estimates of in situ denitrification rate for organic soils near the upslope edge (338 mg N m -2 h -1 ) were much higher than average values estimated for the organic soils as a whole (0.3–2.1 mg N m -2 h -1 ) and suggested that areas of these soils were limited in their denitrification activity by the supply of nitrate. The capacity of these soils to regulate nitrate flux was therefore under-utilized. The majority of stream channel nitrate depletion was apparently due to plant uptake, with estimates of the in situ denitrification rate of stream sediments being less than 15% of the stream channel nitrate depletion rate estimated by mass balance.
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