Response of soil nitrous oxide reducing bacteria abundance to Spartina alterniflora invasion in coastal wetlands of southern China and its key influencing factors

To investigate the response of soil nitrous oxide (N₂O) reducing bacterial communities to Spartina alterniflora invasion and to identify the key environmental drivers shaping these responses, we conducted a large-scale field survey across 21 coastal wetlands in southern China. Soil samples were collected from both bare tidal flats and adjacent S. alterniflora invaded sites. Using quantitative real-time PCR, we quantified the abundance of nosZ I and nosZ II genes, which encode distinct clades of nitrous oxide reductase enzymes responsible for the microbial reduction of N₂O to dinitrogen (N₂), the final step of the denitrification process. Our results revealed that the copy number of the nosZ II gene was significantly higher than that of nosZ I in both vegetation types, suggesting that nosZ II-type N₂O reducers dominate the microbial N₂O sink in subtropical coastal wetland soils. Notably, both nosZ I and nosZ II gene abundances were significantly elevated in soils invaded by S. alterniflora, indicating that plant invasion enhances the capacity of soil microbial communities to reduce N₂O emissions. This finding is of particular ecological relevance, as coastal wetlands serve as both sources and sinks of N₂O. Stepwise regression analyses identified soil pH and moisture content as the most influential factors controlling the abundance of nosZ I and nosZ II genes, respectively. Specifically, nosZ I gene abundance was positively correlated with pH, soil organic carbon, ammonium and nitrate nitrogen, and microbial biomass nitrogen. In contrast, nosZ II gene abundance showed significant positive relationships with pH, water content, salinity, and clay content, and negative correlations with soil bulk density and sand fraction. These contrasting patterns suggest that nosZ I and nosZ II organisms may occupy distinct ecological niches and respond differently to environmental change. Furthermore, structural equation modeling indicated that S. alterniflora invasion influences nosZ gene abundances through both direct and indirect pathways. While invasion directly stimulated the abundance of both gene clades, it also indirectly influenced them by modifying key soil properties, such as increasing organic carbon availability and altering water retention. Taken together, our study highlights that S. alterniflora invasion enhances the microbial potential for N₂O reduction in coastal wetlands and that soil pH and moisture are critical environmental filters shaping the structure and function of N₂O-reducing communities. These findings underscore the importance of considering plant invasion effects when evaluating greenhouse gas dynamics in coastal ecosystems and suggest that managing vegetation transitions may represent a viable strategy to enhance the soil microbial sink capacity for N₂O under global change scenarios.