Depth-dependent microbial roles and associations with soil multifunctionality along an afforestation chronosequence in a hyper-arid desert

Soil multifunctionality refers to the capacity of soil to support multiple ecosystem processes (e.g., nutrient cycling, organic matter stabilization), which is mediated by soil microorganisms. Afforestation is a pivotal strategy for combating desertification in hyper-arid regions, yet how it affects soil multifunctionality via microbial diversity and co-occurrence networks remains poorly understood. We investigated soil microbial diversity, network complexity, and multifunctionality along a Tamarix ramosissima afforestation chronosequence (0–10 years) at two soil depths (0–20 cm and 20–100 cm) in the Taklimakan Desert. Afforestation was associated with a significant increase in archaeal and bacterial diversity at the intermediate (7-year-old) stands, whereas fungal diversity decreased significantly at the 7-year stands. Observed co-occurrence network complexity (edge density) declined for archaea and bacteria (especially in the subsoil), while fungal networks remained relatively stable within this chronosequence. Soil multifunctionality reached its highest value at 7 years. Partial Least Squares Path Modeling revealed that multifunctionality was associated with microbial diversity and network complexity in both layers, with an additional indirect pathway statistically consistent with mediation by complexity in the subsoil. Random Forest analysis identified that the most important predictors (based on %IncMSE) of multifunctionality differed by soil layer: archaeal network complexity and fungal diversity were most important in topsoil, whereas fungal diversity and bacterial network complexity were paramount in subsoil. Our findings further suggested that soil multifunctionality in hyper-arid afforestation systems is linked to taxon-specific microbial responses and depth-dependent network dynamics. These findings suggest the potential value for layer-specific management strategies, which warrant further experimental validation in water-limited ecosystems.