Denitrification-nitrification process in permeable coastal sediments: An investigation on the effect of salinity and nitrate availability using flow-through reactors

Permeable coastal sediments act as a reactive node in the littoral zone,transforming nutrients via a wide range of biogeochemical reactions.Reaction rates are controlled by abiotic factors,e.g.,salinity,temperature or solute concentration.Here,a series of incubation experiments,using flow-through reactors,were conducted to simulate the biogeochemical cycling of nitrate (NO_3~?) and phosphorus (P) in permeable sediments under differentNO_3~?availability conditions (factor I) along a salinity gradient (admixture of river and seawater,factor II).In an oligotrophic scenario,i.e.,unamendedNO_3~?concentrations in both river and seawater,sediments acted as a permanent net source ofNO_3~?to the water column.The peak production rate occurred at an intermediate salinity(20).IncreasingNO_3~?availability in river water significantly enhanced netNO_3~?removal rates within the salinity range of 0 to 30,likely via the denitrification pathway based on the sediment microbiota composition.In this scenario,the most active removal was obtained at salinity of 10.When both river and seawater were spiked with NO_3~?,the highest removal rate switched to the highest salinity (36).It suggests the salinity preference of theNO_3~?removal pathway by local denitrifiers (e.g.,Bacillus and Paracoccus) and thatNO_3~?removal in coastal sediments can be significantly constrained by the dilution relatedNO_3~?availability.Compared with the obtained variation forNO_3~?reactions,permeable sediments acted as a sink of soluble reactive P in all treatments,regardless of salinity andNO_3~?input concentrations,indicating a possibility of P-deficiency for coastal water from the intensive cycling in permeable sediments.Furthermore,the net production of dissolved organic carbon (DOC) in all treatments was positively correlated with the measuredNO_3~?reaction rates,indicating that the DOC supply may not be the key factor forNO_3~?removal rates due to the consumption by intensive aerobic respiration.Considering the intensive production of recalcitrant carbon solutes,the active denitrification was assumed to be supported by sedimentary organic matter.