4.2 Elevated temperature and precipitation weaken microbial interactions within the co-occurrence networks
The soil microbiome network plays a key role in helping us understand potential biotic interactions there (Feng et al., 2021; Sanaei et al., 2021; Zhao et al., 2019b). Interspecific interactions among bacterial, fungal, and protist groups were stronger at Guizhou than Guangxi. This finding was contrary to our hypothesis. There were more bacterial nodes (OTUs) than fungal or protist nodes. Hence, there were large, ubiquitous bacterial populations. Readily decomposable organic C should augment bacterial diversity to a greater extent at Guizhou than Guangxi (Poll et al., 2008; Samal et al., 2020). High bacterial diversity fosters linkages with fungi and protists. There were more bacterial nodes at Guizhou than Guangxi. By contrast, fungi and protists had similar numbers of nodes at both locations. These results suggest stronger network interactions at Guizhou than Guangxi primarily because bacterial diversity was greater than fungal or protist diversity at both locations. Similarly, elevated bacterial diversity under cropland resulted in a greater number of network interactions than those under plantation forest or shrubland. Fungi are more tolerant of high soil C/N than bacteria (De Deyn et al., 2008; Wang et al., 2015). Fungi might grow better in plantation forest and shrubland soil than in cropland soil as the latter has comparatively lower C/N (Fig. S1m). Fungi are vital plant biomass decomposers and degrade plant litter containing lignin and cellulose (Fontaine et al., 2011; Zeilinger et al., 2016). At this case, correlations among fungal and protist taxa were stronger under plantation forest and shrubland than cropland. Thus, the fungal community plays a crucial role in vegetation restoration.
Co-occurrence patterns can also identify keystone and indicator taxa (Fournier et al., 2020; Wang et al., 2021). Bacterial phyla Acidobacteria and Proteobacteria, fungal phyla Ascomycota and Basidiomycota, and protist phyla Cercozoa and Lobosa formed the most links with other microbial groups and are, therefore, potentially vital to soil food webs. Protists prey on bacteria and fungi, thereby modifying microbial communities (Geisen et al., 2018). Cercozoa and Lobosa are phagotrophs that consume Acidobacteria, Proteobacteria, and Ascomycota (de Araujo et al., 2018; Seppey et al., 2020). These predator-prey relationships were stronger at Guizhou than Guangxi. Hence, predation of microbial decomposers by protists is more important at low than high temperature (Geisen et al., 2021). Soil moisture content was usually higher at Guizhou than Guangxi because low temperature partially retards evaporation. Protist movement, feeding, and multiplication are all affected by the water films in the soil pore spaces (Ritz, 2011; Stefan et al., 2014). Consequently, predation of bacteria and fungi by protists was strong in Guizhou soil.