Abstract
In transitional environments, the role of sediments biogeochemistry and denitrification is crucial for establishing their buffer potential against nitrate (NO3−) pollution. The Po River (Northern Italy) is a worldwide hotspot of eutrophication. However, benthic N dynamics and the relevance of denitrification in its delta have not yet been described. The aim of the present study was to quantify the contribution of denitrification in attenuating the NO3− loading transported to the sea during summer. Benthic fluxes of dissolved inorganic nitrogen (N) and denitrification rates were measured in laboratory incubations of intact sediment cores collected, along a salinity gradient, at three sections of the Po di Goro, the southernmost arm of the Po Delta. The correlation between NO3− consumption and N2 production rates demonstrated that denitrification was the main process responsible for reactive N removal. Denitrification was stimulated by both NO3− availability in the Po River water and organic enrichment of sediment likely determined by salinity-induced flocculation of particulate organic load, and inhibited by increasing salinity, along the river–sea gradient. Overall, denitrification represented a sink of approximately 30% of the daily N loading transported in middle summer, highlighting a previously underestimated role of the Po River Delta.
Highlights
Rivers draining agricultural and urban basins export high quantities of reactive nitrogen (N) causing eutrophication-related phenomena in the coastal zones
Denitrification represented a sink of approximately 30% of the daily N loading transported in middle summer, highlighting a previously underestimated role of the Po River Delta
The highest Dtot rates were measured at the M site and were almost equivalent to the rates of N2 production and NO3 − consumption (422 ± 99 and −424 ± 103 μmol N m− 2 h− 1, respectively), exceeding, by a factor of four on average, the rates measured at Lanterna Vecchia (LV), the site nearest to the sea
Summary
Rivers draining agricultural and urban basins export high quantities of reactive nitrogen (N) causing eutrophication-related phenomena in the coastal zones. Denitrification, the stepwise reduction of NO3 − to nitrogen gas (N2 ) under anaerobic conditions, is widely recognized as the dominant biogeochemical process responsible for permanent N removal in rivers and transitional environments [5,6,7]. Distributed global models have demonstrated that denitrification occurring in river networks may remove, on average, 20–50% of the total land-based N input, indicating they are important filters for the N loadings transported toward the sea and play a considerable role in mitigating eutrophication effects [7,10]. The denitrification capacity of the river networks has been recognized, most studies on the regulation of denitrification have addressed headwater streams because of their intense water–streambed interactions (i.e., a high ratio between bioreactive surfaces and water volumes), sustaining significant in-stream N retention [11,12]. The nitrate (NO3 − ) supply to the benthic compartment is mainly controlled by diffusion from the water column; ,
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