Stand-alone or co-occurring invasive plant species do not modify the diversity of the soil N2-fixing bacterial community

ABSTRACT Background: Soil N2-fixing bacterial (SNB) communities may play a vital role in plant invasion. Two or more invasive plant species (invaders) may coexist in the same ecosystem. Thus, it is important to assess the effects of co-invasion on SNB communities to elucidate the ecological mechanisms of plant invasion. Aims: The effects of two co-occurring invaders (Erigeron annuus and Solidago canadensis) with different coverage classes on SNB communities were evaluated via a comparative study. Methods: SNB communities were assessed using a high-throughput sequencing approach. Results: The invasion of E. annuus and/or S. canadensis, regardless of coverage, did not pose pronounced effects on soil physicochemical properties and the diversity of SNB. Soil electrical conductivity was the most important environmental factors explaining the variation in SNB composition. This phenomenon may be chiefly attributed to the fact that the shifts in soil electrical conductivity can recruit obvious variations in the resource utilisation and acquisition patterns of carbon for the metabolism of soil microorganisms. Plant Shannon’s diversity was also one key factor influencing the community structure of SNB, but its direct effects are less powerful than soil electrical conductivity. However, the negatively indirect effects of plant Shannon’s diversity on the Shannon index, ACE index, and Chao1 index of SNB were noticeably greater than soil electrical conductivity. Therefore, the effects of plant Shannon’s diversity on the community structure of SNB may largely be attributed to the variation in soil electrical conductivity via the released root exudates and then modify the resource availability pattern of SNB. Conclusions: SNB diversity is more influenced by changes in soil variables like soil electrical conductivity than by the presence/coverage of invasive plant species.