The influence of soil factors on protist community dynamics during plant succession in subalpine natural and planted forests

Protists, as the core component of the soil microbiome, may play an important role in forest succession by altering soil nutrient cycling and plant performance. However, knowledge of how protists change with plant succession is extremely limited. To illustrate protistan community dynamics following plant succession in subalpine natural and planted ecosystems, we used high-throughput sequencing to identify protists in rhizosphere and bulk soils at different successional stages, and the effects of soil abiotic and biotic factors on protists were analysed. Overall, the alpha-diversity and community composition of protists differed significantly between natural and planted ecosystems, but not between rhizosphere and bulk soils. Moving along the successional gradient, the richness and diversity of protists in rhizosphere and bulk soils of natural ecosystems, as well as the richness of functional taxa (i.e., consumers, parasites and phototrophs), gradually increased. In contrast, the richness of protists and functional taxa in the plantations was generally lowest in the old stands (75 years). The community composition of protists in rhizosphere and bulk soils of these two ecosystems varied significantly among successional stages, and their community composition variation was positively correlated with the time interval of successional stages, indicating that community dynamics exhibited directionality along successional gradients. Soil abiotic and biotic factors jointly regulate protistan alpha-diversity and community composition, and forest succession regulates functional taxa through multiple pathways. However, the key influencing factors and specific regulatory pathways are not consistent across different ecosystems, soil types and functional taxa. Therefore, our study has revealed that protistan community assembly was distinct along two separate successional gradients, indicating potential functional differences. These results provide a basis for the development of future forest management measures.