Sandy seepage faces as bioactive nitrate reactors: Biogeochemistry, microbial ecology and metagenomics

•Production and removal of nitrate in a seepage face in the Sanggou Bay.•Nitrate production was driven by heterotrophic decomposition of STN and nitrification.•Nitrate removal occurred in the surface layer, linking to denitrification and biological assimilation.•Sediment microbial community structure influence nitrate turnover rates and isotopic fractionation.•Porewater advection rates driven by hydraulic pressure head also impacted nitrate reaction rates.