Soil microorganisms play central roles in regulating soil organic carbon (SOC) turnover and sequestration. Protists, an integral component of the soil micro-food web, are an important biotic force driving microbial population dynamics and C biogeochemical cycling. However, how protists regulate microbially mediated soil carbon cycling remains largely unknown. This study investigated the roles of two major functional protistan groups, phototrophs and phagotrophs, in modulating soil carbon cycling. Our results showed that phototrophs played important roles in CO₂ fixation, with up to 21% contribution of phototrophs to SOC storage in paddy soils but less than 15% in forest and upland soils. Phototrophic lineages such as Chlorophyceae and Trebouxiophyceae were better predictors of CO₂ fixation in paddy soils. We further investigated the trophic interactions between phagotrophs, bacteria and fungi, and their impacts on C-cycling processes. Soil aggregate sizes significantly influenced the protistan community and microbial hierarchical interactions. Cercozoan taxa were consistently identified as the keystone species in SOM degradation-related ecological clusters in macroaggregates and silt + clay, indicating the critical roles of phagotrophs in SOM degradation by regulating bacterial and fungal taxa. Due to the limitation of soil pore size, microaggregates restrict the relative abundance of bacterivores and the trophic interactions between protists (especially Cercozoan lineages) and bacteria. Instead, omnivores play dominant roles in driving SOM turnover via influencing the fungal community in microaggregates. However, the associations between microbial trophic interactions and SOM decomposition are decoupled in microaggregates, leading to protection of soil organic carbon. Our findings highlight the importance of protists-driven soil C cycling.

Soil organic matter, Carbon cycling, Soil protists, Trophic interactions