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www.biogeosciences-discuss.net/5/3307/2008/
doi:10.5194/bgd-5-3307-2008
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Nitrate retention and removal in Mediterranean streams with contrasting land uses: a 15N tracer study
1Limnology Group, Centre d'Estudis Avançats de Blanes, Consejo Superior de Investigaciones Científicas, Blanes, Spain
2Departament d'Ecologia, Facultat de Biología, Universitat de Barcelona, Barcelona, Spain
Abstract. We used 15N-labelled nitrate (NO−3) additions to investigate nitrogen (N) cycling at the whole-reach scale in three Mediterranean streams subjected to contrasting land uses (i.e. forested, urban and agricultural). Our aim was to examine: 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.
The concentration of NO−3 increased and that of dissolved oxygen (DO) decreased from the forested to the agricultural stream, with intermediate values in the urban stream. Standing stocks of primary uptake compartments were similar among streams and dominated by detritus compartments (i.e. fine and coarse benthic organic matter). In agreement, metabolism was net heterotrophic in all streams, although the degree of heterotrophy increased from the forested to the agricultural stream. The NO−3 uptake length decreased along this gradient, whereas 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 highest in the forested and lowest in the agricultural stream. In all streams, the retained N was rapidly transferred to higher trophic levels and 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 evidences fast N cycling in Mediterranean streams. Moreover, results indicate 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.
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Citation: von Schiller, D., Martí, E., and Riera, J. L.: Nitrate retention and removal in Mediterranean streams with contrasting land uses: a 15N tracer study, Biogeosciences Discuss., 5, 3307-3346, doi:10.5194/bgd-5-3307-2008, 2008. Bibtex EndNote Reference Manager XML
