Unraveling assembly processes shaping soil protist community dynamics across successional stages of temperate forests on the Chinese Loess Plateau

Protists, as pivotal regulators of soil food webs, maintain various ecosystem functioning. However, their assembly dynamics during plant succession remain poorly understood, hindering the integration of protist ecology into restoration frameworks. This study investigated the dynamics of soil protist communities along a 160-year forest succession chronosequence in the Ziwuling region of the Chinese Loess Plateau via 18S rRNA sequencing, quantifying assembly processes contributions and identifying key drivers. Results showed that deterministic processes dominated protist community assembly across succession, with homogeneous selection (73.29 %- 76.51 % contribution) decreasing gradually from grasslands to mixed forests (P < 0.05) but increasing moderately in climax forests (P > 0.05), mediated by litter/root inputs, soil properties and understory light. Stochastic processes, particularly dispersal limitation, increased transiently in mixed forests (18.21 % contribution), aligning with maximal plant diversity and heightened environmental heterogeneity, which enhanced protist beta-diversity (Bray-Curtis dissimilarity). Despite stable protist alpha-diversity, functional shifts emerged with succession: consumers declined (75.86 % to 62.40 %) while parasites increased (24.06 % to 37.57 %), with phototrophs suppressed by reduced understory light. Co-occurrence networks transitioned from sparse (grasslands) to densely connected (climax forests), showing increased edge number (519 to 1331) and positive correlations (58.57 % to 90.83 %). Soil nutrients (e.g., readily oxidizable carbon, total nitrogen) and plant-derived resources (litter/root organic carbon) persistently drove protist assembly throughout succession, while abiotic factors like soil bulk density (1.22 g/cm(3)) and moisture (12.83 %) shaped early-succession and biotic regulation (e.g., microbial biomass) dominated late-succession. These findings demonstrate how plant succession restructures protist communities, providing critical insights for soil biodiversity recovery and ecosystem restoration in arid/semiarid regions like the Loess Plateau.