Abstract: Diazotrophs are the primary source of bioavailable nitrogen in natural ecosystems, making the elucidation of their abundance paramount for effective soil nitrogen management. To reveal the response of soil diazotroph abundance to Spartina alterniflora invasion and its key controlling factors, we collected soil samples from eight typical coastal wetlands in Fujian Province and employed real-time quantitative PCR to measure the abundance of nifH genes in bare tidal flats and S. alterniflora invaded wetland soils. The results showed that S. alterniflora invasion increased the contents of soil organic carbon (SOC), ammonium (NH₄⁺-N), nitrate (NO₃^--N), sulfate (SO₄^2-), microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), salinity, and Cl^-, while decreased soil pH. The copy number of nifH gene in S. alterniflora invaded wetland soils was significantly higher than that in bare tidal flat soil, suggesting that S. alterniflora invasion significantly increased soil diazotroph abundance. The abundance of nifH gene was significantly negatively correlated with soil pH and positively correlated with SOC, NH₄⁺-N, NO₃^--N, and MBN. Path analysis indicated that S. alterniflora invasion affected the abundance of nifH genes both directly and indirectly through SOC and MBN. Stepwise regression analysis revealed that MBN and SO₄^2- explained most of the variations in nifH gene abundance, with MBN being the most crucial environmental factor controlling nifH gene abundance. In summary, S. alterniflora invasion significantly increases the abundance of soil nifH genes in typical coastal wetlands of Fujian Province, with MBN being the most critical environmental factor regulating nifH gene abundance.

Key words: coastal wetland, plant invasion, diazotroph, nifH