Oligotrophic bacteria and pathotrophic fungi moderate multitrophic interactions in semi-arid and arid environments

Background Future climate projections indicate shifts in intra-annual precipitation patterns, with intensified rainfall events and prolonged dry periods. These changes may alter soil biotic communities and their interactions within food webs, particularly in semi-arid and arid ecosystems. However, the extent to which varying rainfall regimes and semi-arid and arid conditions influence multitrophic associations remains poorly understood. Methods We leveraged a four-year rainfall manipulation experiment across six dryland sites in eastern Australia, representing arid and semi-arid ecosystems with varying, high and low levels of rainfall variability (CV) making three different climatic conditions. Rainfall treatments simulated increased (+ 65%) and reduced (-65%) precipitation relative to ambient conditions. We studied multitrophic co-occurrence network among bacteria, fungi, protists, and nematodes, representing key components of the soil food web, and assess their associated changes to varying rainfall and climatic conditions. Results Climatic differences between arid and semi-arid ecosystems were the primary drivers of soil biotic community composition, whereas rainfall treatments had minimal influence. Multitrophic co-occurrence networks varied significantly across climatic conditions, with increasing aridity promoting more positive associations among bacterial nodes. Bacteria, fungi, and their interactions were central to the belowground multitrophic network structure. In particular, stress-tolerant oligotrophic bacteria and pathotrophic fungi played key roles, with mean annual precipitation (MAP) identified as a critical determinant. Conclusions Our findings suggest that aridity-driven shifts in biotic interactions may restructure belowground food webs in dryland ecosystems. The increasing dominance of oligotrophic bacteria and fungal pathotrophs under arid conditions highlights potential consequences for soil functioning and plant-soil interactions in response to changing precipitation regimes.