Organic matter supply triggers rapid microbial-driven changes in carbon-nitrogen cycling in subterranean estuary sediments

Excessive carbon and nitrogen emissions from human activities have exerted unpredictable impacts on coastal belts at a global scale. To investigate carbon transformations and the coupling between carbon and nitrate reactivity in subterranean estuaries (STEs), laboratory flow-through reactor (FTR) experiments were conducted with permeable sandy sediments. These included a control and five experimental treatments with different dissolved carbon sources (∼500 μmol/L (equivalent to μM) final concentration), including fucoidan, chitosan, oligosaccharide, Phragmites australis extracts, and Laminaria japonica extracts. After 48 h incubation, ca. 80 μM nitrate (NO₃⁻) was added in all treatments to further identify carbon‑nitrogen coupling in STEs. The modified FTR cells used in this study permitted us to follow changes in both the circulating water and the sediment. The results suggested that the composition and concentration of dissolved organic carbon (DOC) during the tidal cycle could affect NO₃⁻ removal by regulating the sediment microbial community composition. Sandy STEs would preferentially maintain high DOC metabolic rates under aerobic conditions to sensitively regulate chemical oxidation/reduction and microbial community domestication, which enhances the adaptation to NO₃⁻ removal. For example, the degradation of DOC in the oligosaccharide treatment stimulated the NO₃⁻ removal capacity of aerobic denitrifying bacteria ( Chryseobacterium and Delftia ). Sufficient electron donors eliminated nitrogen limitation and increased denitrification capacity in sandy STEs. Domesticated sandy sediments would be better adapted to high NO₃⁻ loads, demonstrating the complexity of carbon‑nitrogen coupling under natural conditions.