Abstract
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.
Highlights
Streams and rivers have the ability to store, transform and remove nutrients during downstream transport, which results in variations in the form and amount of nutrients delivered to downstream ecosystems (Alexander et al, 2000; Peterson et al, 2001; Mulholland et al, 2008)
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 dissolved oxygen (DO) concentrations
Retention of NO−3 occurs via assimilatory uptake by stream benthic organisms, often referred to as primary uptake compartments
Summary
Streams and rivers have the ability to store, transform and remove nutrients during downstream transport, which results in variations in the form and amount of nutrients delivered to downstream ecosystems (Alexander et al, 2000; Peterson et al, 2001; Mulholland et al, 2008). Retention of NO−3 occurs via assimilatory uptake by stream benthic organisms, often referred to as primary uptake compartments (i.e. those compartments that take up dissolved nutrients directly from the water column). D. von Schiller et al.: Nitrate retention and removal in Mediterranean streams to consumers at higher trophic levels and regenerated back to the water column as ammonium (NH+4 ) via mineralization and excretion. As it is transported downstream, the regenerated NH+4 is again taken up by the biota or transformed to NO−3 via nitrification, completing a whole cycle or spiral (Newbold, 1996)
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