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The Near-Stream Zone is a Source of Nitrogen in a Swedish Forested Catchment

Most Swedish forests efficiently retain N from deposition and runoff N can be expected to originate from the near-stream zone. Nitrogen leaching from this region is low, but still may be of ecological relevance (e.g., in oligotrophic lakes). In the present study the function of the near-stream zone with respect to N chemistry in stream water of a small pristine catchment in central-south Sweden was investigated. The study included ground water, soil water, and stream water sampling. The aim was to explain the variation in N species in stream water with hydrological flow paths and the spatial and temporal variation in N in ground water and soil water. Nitrate and ammonium concentrations were negligible in ground water and soil water at most locations. When nitrate occurred, it was usually at concentrations below 100 μg N L 1 and related to higher pH and aerobic conditions. Ammonium was detected in anaerobic locations at stable concentrations up to 200 μg N L '. The near-stream zone was the main source of organic N entering the stream where the median concentration was 240 μg L -1 . Nitrate occurred only occasionally in stream water at concentrations around 100 μg N L 1 and then probably originated from near-stream pre-event water. The only significant ammonium concentrations were detected during a spring flow episode and there were indications of flushing of organic matter of microbial origin. The importance of within-catchment variation for understanding leaching processes in forested catchments is discussed.