Nitrate retention and removal in Mediterranean streams bordered by contrasting land uses: a 15N tracer study

We used 15N-labelled nitrate (NO−3 ) additions to investigate pathways of nitrogen (N) cycling at the wholereach scale in three stream reaches with adjacent forested, urban and agricultural land areas. Our aim was to explore among-stream differences in: (i) the magnitude and relative importance of NO−3 retention (i.e. assimilatory uptake) and removal (i.e. denitrification), (ii) the relative contribution of the different primary uptake compartments to NO−3 retention, and (iii) the regeneration, transformation and export pathways of the retained N. Streams varied strongly in NO−3 concentration, which was highest in the agricultural stream and lowest in the forested stream. The agricultural stream also showed the lowest dissolved oxygen (DO) concentration and discharge. Standing stocks of primary uptake compartments were similar among streams and dominated by detritus compartments (i.e. fine and coarse benthic organic matter). Metabolism was net heterotrophic in all streams, although the degree of heterotrophy was highest in the agricultural stream. The NO−3 uptake length was shortest in the agricultural stream, intermediate in the urban stream, and longest in the forested stream. Conversely, the NO−3 mass-transfer velocity and the areal NO−3 uptake rate were highest in the urban stream. Denitrification was not detectable in the forested stream, but accounted for 9% and 68% of total NO−3 uptake in the urban and the agricultural stream, respectively. The relative contribution of detritus compartments to NO−3 assimilatory uptake was greatest in the forested and lowest in the agricultural stream. In all streams, the retained N was rapidly Correspondence to: D. von Schiller ([email protected]) regenerated back to the water column. Due to a strong coupling between regeneration and nitrification, most retained N was exported from the experimental reaches in the form of NO−3 . This study provides evidence of fast in-stream N cycling, although the relative importance of N retention and removal varied considerably among streams. Results suggest that permanent NO−3 removal via denitrification may be enhanced over temporary NO−3 retention via assimilatory uptake in heterotrophic human-altered streams characterized by high NO−3 and low DO concentrations.