Threshold effects of soil pH on microbial co-occurrence structure in acidic and alkaline arable lands

Soil pH plays a pivotal role in shaping microbial diversity and community composition in terrestrial ecosystems. Soil microbes clearly do not exist independently and instead form complex interaction networks, but little is known about the effect of soil pH upon those microbial co-occurrence networks. In this study, 62 alkaline and 61 acidic soil samples were collected from the North China Plain to investigate the effect of soil pH on microbial co-occurrence structure. Most bacterial and fungal OTUs in acidic soils were maximally abundant at soil pH of ~5.5, while in alkaline soils the corresponding OTU abundance attainted its maximum at pH ~8.3. Unlike those OTU abundance distributions, the relationship between soil pH and the number of links between OTUs in the co-occurrence network was best fitted by convex curves, with the pH thresholds found at 5.39 for bacteria and 5.60 for fungi in acidic soils. In alkaline soils, a dip in the relationship (only for fungi) between soil pH and the links was also observed, with a pH threshold at 8.04 for bacteria and 8.28 for fungi. From the co-occurrence analysis we identified 37 core bacterial species (mainly in Actinobacteria and Proteobacteria) and 24 core fungal species (mainly in Ascomycota). Together, this study revealed the threshold effects of soil pH upon microbial co-occurrence structure and identified the core microbial species in the North China Plain. These findings provide baseline information in mediating soil microbial co-occurrence structure by adjusting soil pH and core species in croplands.