Responses of soil N-fixing bacteria communities to Amaranthus retroflexus invasion under different forms of N deposition

Soil N-fixing bacteria communities (SNB) can increase soil N availability, which can facilitate invasions of many kinds of invasive plant species. Meanwhile, variability in the forms of anthropogenic N deposition can mediate obvious shifts in both soil physicochemical properties and soil microbial communities (especially SNB). This can alter the soil micro-ecological mechanisms of plant invasions via changes in soil characteristics and soil microbial communities, especially SNB. This study uses next-generation high throughput sequencing technology to provide insight into the effects of invasion by Amaranthus retroflexus L. on the structure and diversity of SNB under different forms of simulated N deposition (SND). Soil pH under A. retroflexus invasion under various forms of SND was 8.08% lower than controls under a NH4-N treatment (P < 0.05), 15.15% lower than controls under a NO3-N treatment (P < 0.05), and 6.57% lower than controls under CO(NH2)2-N and Mix-N treatments (P < 0.05). Meanwhile, soil pH was one of the most important environmental variables explaining changes in the structure and diversity of SNB. The total number of species of SNB under A. retroflexus invasion was approximately 13.13% higher than controls for the NH4-N treatment (P > 0.05), 11.00% higher than controls for the NO3-N treatment (P > 0.05), 39.42% higher than controls for the CO(NH2)2-N treatment (P < 0.05), and 27.67 higher than controls for the Mix-N treatment (P < 0.05). The positive effects mediated by the combined treatments of A. retroflexus invasion and SND may be ascribed to enhanced soil nutrient content (especially N), which can enhance the abundance of the nifH gene. The effects of A. retroflexus invasion on the total number of species of SNB under organic and mixed-N addition were greater than under inorganic N addition. This may be a consequence of a disturbance of the balance of inorganic to organic N for SND, as a result of the addition of a single form of inorganic N, which can diminish the stimulatory effects of soil nutrient content on SNB under inorganic N. Another potential cause of this pattern may be stronger acidification under inorganic N addition, which can exert more pronounced effects on SNB. LEfSe analysis showed that some species changed significantly under A. retroflexus invasion in the presence of SND (i.e., Rhodobacteraceae under NH4-N and Proteobacteria under CO(NH2)2-N). Thus, A. retroflexus invasion in the presence of SND caused obvious shifts in certain species of SNB and increased the total number of species of SNB in particular, to facilitate its further invasion.