Significant microbial nitrogen loss from denitrification and anammox in the land-sea interface of low permeable sediments

The land-sea interface is a unique site playing an important role in the biogeochemical processes of nutrients, but it is unclear whether it acts as a sink or source for coastal nitrogen loading. In this study, we analyzed the potential activity, microbial abundance, diversity as well as distribution pattern of both denitrification and anammox processes at a land-sea interface of Renshan in order to decipher the microbial nitrogen loss in intertidal low permeable sediments (LPS). Stable isotopic nitrogen tracer assay showed that potential rates of denitrification, which ranged from 0.61 to 13.47 mu mol N kg(-1)h(-1) sediments, were much stronger than that of anammox. Meanwhile, the abundance of denitrifiers was observed obviously higher than that of anammox bacteria by 1-2 order of magnitudes in the LPS. Based on the high-throughput sequencing analysis, high diversity was found for denitrifiers with gamma-proteobacteria, alpha-proteobacteria and beta-proteobacteria as the major members of the microbial community in the region. Anammox bacteria, including Ca. Scalindua, Ca. Kuenenia and Ca. Brocadia, were observed in the same transect. Geochemical and hydrological analyses indicated that hydrological processes, weak ammonium-borne freshwater advection, upper nitrate-borne brackish water diffusion and oxygenated seawater intrusion, may be important factors affecting the microbial nitrogen removal. Collectively, our results suggest that the land-sea interface with low permeable sediment is a microbial active zone with denitrification as the dominant contributor to the total nitrogen loss, resulting in the attenuation of reactive nitrogen from groundwater to the ocean in this region.