Nitrogen Fixation in the Intertidal Sediments of the Yangtze Estuary: Occurrence and Environmental Implications

Nitrogen fixation is a microbial‐mediated process converting atmospheric dinitrogen gas to biologically available ammonia or other molecules, and it plays an important role in regulating nitrogen budgets in coastal marine ecosystems. In this study, nitrogen fixation in the intertidal sediments of the Yangtze Estuary was investigated using nitrogen isotope tracing technique. The abundance of nitrogen fixation functional gene (nifH) was also quantified. The measured rates of sediment nitrogen fixation ranged from 0.37 to 7.91 nmol N g−1hr−1, while the abundance of nifHgene varied from 2.28 × 106to 1.28 × 108copies g−1in the study area. The benthic nitrogen fixation was correlated closely to the abundance of nifHgene and was affected significantly by salinity, pH, and availability of sediment organic carbon and ammonium. It is estimated that sediment nitrogen fixation contributed approximately 9.3% of the total terrigenous inorganic nitrogen transported annually into the Yangtze estuarine and coastal environment. This result implies that the occurrence of benthic nitrogen fixation acts as an important internal source of reactive nitrogen and to some extent exacerbates nitrogen pollution in this aquatic ecosystem. Nitrogen fixation in the intertidal sediments is associated tightly with the abundance of nifHgeneNitrogen fixation is greatly enhanced by high availability of organic matter, but inhibited by high ammonium concentrationsNitrogen fixation may be coupled to activity of sulfate‐reducing bacteria