Potential Denitrification and Nitrous Oxide Production in the Sediments of the Seine River Drainage Network (France)

To investigate bottom sediment denitrification at the scale of the Seine drainage network, a semi‐potential denitrification assay was used in which river sediments (and riparian soils) were incubated for a few hours under anaerobic conditions with non limiting nitrate concentrations. This method allowed the nitrous oxide (N 2 O) concentration in the headspace, as well as the nitrate, nitrite, and ammonium concentrations to be determined during incubation. The rates at which nitrate decreased and N 2 O increased were then used to assess the potential denitrification activity and associated N 2 O production in the Seine River Basin. We observed a longitudinal pattern characterized by a significant increase of the potential rate of denitrification from upstream sectors to large downstream rivers (orders 7–8), from approximately 3.3 to 9.1 μg NO 3 − ‐N g −1 h −1 , respectively, while the N 2 O production rates was the highest both in headwaters and in large order rivers (0.14 and 0.09 N 2 O‐N g −1 h −1 , respectively) and significantly lower in the intermediate sectors (0.01 and 0.03 N 2 O‐N g −1 h −1 ). Consequently, the ratio N 2 O production:NO 3 reduction was found to reach 5% in headstreams, whereas it averaged 1.2% in the rest of the drainage network, an intermediate percentage being found for the riparian soils. Finally, the ignition loss of sediments, together with other redundant variables (particulate organic carbon content: g C 100 g −1 dry weight [dw], moisture: g water 100 g −1 dw, sediment size <50 μm: g material size <50 μm 100 g −1 dw) were found to control these activities. However, the biodegradability of organic matter must be measured to better understand the factor controlling denitrification and its associated N 2 O production.