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

Abstract. We used ¹⁵N-labelled nitrate (NO₃⁻) additions to investigate pathways of nitrogen (N) cycling at the whole-reach 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 NO₃⁻ retention (i.e. assimilatory uptake) and removal (i.e. denitrification), (ii) the relative contribution of the different primary uptake compartments to NO₃⁻ retention, and (iii) the regeneration, transformation and export pathways of the retained N.

Streams varied strongly in NO₃⁻ 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 NO₃⁻ uptake length was shortest in the agricultural stream, intermediate in the urban stream, and longest in the forested stream. Conversely, the NO₃⁻ mass-transfer velocity and the areal NO₃⁻ uptake rate were highest in the urban stream. Denitrification was not detectable in the forested stream, but accounted for 9% and 68% of total NO₃⁻ uptake in the urban and the agricultural stream, respectively. The relative contribution of detritus compartments to NO₃⁻ assimilatory uptake was greatest in the forested and lowest in the agricultural stream. In all streams, the retained N was rapidly 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 NO₃⁻.

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 NO₃⁻ removal via denitrification may be enhanced over temporary NO₃⁻ retention via assimilatory uptake in heterotrophic human-altered streams characterized by high NO₃⁻ and low DO concentrations.