Complex lacustrine systems, such as hypersaline lakes located in endorheic basins, are exposed to nitrate (NO3-) pollution. An excellent example of these lakes is the hypersaline lake located in the Pétrola basin (central Spain), where the lake acts as a sink for NO3- from agricultural activities and from sewage from the surrounding area. To better understand the role of the organic carbon (Corg) deposited in the bottom sediment in promoting denitrification, a four-stage flow-through experiment (FTR) and batch experiments using lake bottom sediment were performed. The chemical, multi-isotopic and kinetic characterization of the outflow showed that the intrinsic NO3- attenuation potential of the lake bottom sediment was able to remove 95% of the NO3- input over 296 days under different flow conditions. The NO3- attenuation was mainly linked with denitrification but some dissimilatory nitrate reduction to ammonium was observed at early days favored by the high C/N ratio and salinity. Sulfate reduction could be neither confirmed nor discarded during the experiments because the sediment leaching masked the chemical and isotopic signatures of this reaction. The average nitrogen reduction rate (NRR) obtained was 1.25 mmol d- 1 kg- 1 and was independent of the flow rate employed. The amount of reactive Corg from the bottom sediment consumed during denitrification was 28.8 mmol, representing approximately 10% of the total Corg of the sediment (1.2%). Denitrification was produced coupled with an increase in the isotopic composition of both d15N and d18O. The isotopic fractionations (e of 15N-NO3- and 18O-NO3-) produced during denitrification were calculated using batch and vertical profile samples. The results were - 14.7‰ for eN and - 14.5‰ for eO.

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