Nitrogen fixation in surface sediments of the East China Sea: Occurrence and environmental implications

Sediment nitrogen fixation and associated functional gene in the East China Sea were investigated using nitrogen-isotope tracing and molecular techniques. Potential rates of nitrogen fixation were detected, with values of 0.06-5.51 nmol N g^-1 h^-1. Abundance of functional gene (nifH) ranged from 0.36 × 10⁶ to 5.39 × 10⁷ copies g^-1. Nitrogen fixation rates were not related to the abundance of nifH gene but to temperature, salinity, sulfide, iron and C/N, indicating that the sediment properties rather than microbial abundance dominated the nitrogen fixation. It is also estimated that sediment nitrogen fixation annually contributed about 3.43 × 10⁵ to 3.10 × 10⁷ tons nitrogen to the East China Sea, which accounted for 8.2-22.6% of the total inorganic nitrogen input. Overall, this study highlights the importance of benthic nitrogen fixation in controlling nitrogen budget in the East China Sea and improves our knowledge on nitrogen cycling in the coastal marine environments.