Secondary vegetation succession on the Loess Plateau altered the interaction between arbuscular mycorrhizal fungi and nitrogen-fixing bacteria

Arbuscular mycorrhizal fungi (AMF) and nitrogen-fixing bacteria (expressing the nifH gene) play an important role in maintaining plant growth and soil function, and the reconstruction and restoration of vegetation and degraded ecosystems should focus on these communities and the links between them. However, little is known about changes in these taxa and their interactions during the natural recovery of abandoned land. Here, we investigated the changes in and interactions between AMF and nifH gene-expressing bacteria in farmland (S1), grassland (S2), shrubland (S3), pioneer forests (S4), and climax forests (S5) as five typical stages of vegetation succession on the Loess Plateau, China. We found that vegetation succession significantly influenced the diversity of AMF and nifH communities. With the progression of secondary succession, the diversity of AMF increased from farmland to pioneer forest communities, and decreased in the climax vegetation, while the diversity of the nifH community first decreased and then gradually increased. The community structures of the AMF and nifH communities differed significantly between the five vegetation types. Specifically, the AMF community structure changed significantly during the later stages of succession, while the N-fixing bacteria community tended to be stable during late succession. The interactions between AMF and nifH bacteria gradually increased from the farmland to shrub stages but tended to weaken from the shrub to climax forest stages. Therefore, the relationships between microbial communities during the early and middle stages of vegetation succession following land abandonment on the Loess Plateau are mainly cooperative and mutually beneficial, while competition likely intensifies during later successional stages. Furthermore, we found that soil microbial biomass, enzyme activity, and total phosphorus content were the main drivers of microbial community changes. Overall, this study complements current understanding of the changes and interrelationships of AMF and N-fixing bacteria during vegetation succession in areas prone to soil erosion, and provides new insights into the natural recovery of degraded ecosystems as driven by soil microbial communities.