Divergent successional responses and environmental drivers of soil bacterial and fungal communities in subalpine forests of the Eastern Qilian Mountains.

INTRODUCTION: Soil microbial communities and extracellular enzyme activities play critical roles in regulating carbon (C), nitrogen (N), and phosphorus (P) cycling in forest ecosystems. However, whether bacterial and fungal communities exhibit similar or divergent successional trajectories, and how environmental factors regulate these responses during long-term subalpine forest succession, remain poorly understood.

METHODS: We investigated a natural successional sequence from broadleaved mixed forest to mature Picea wilsonii forest in Xinglong Mountain, Eastern Qilian Mountains, using high-throughput sequencing combined with measurements of soil physicochemical properties, microbial biomass, and extracellular enzyme activities.

RESULTS: Forest succession significantly altered soil environmental conditions, with surface soil pH increasing from 7.15 at the S1 stage to 7.49 at the S5 stage, whereas total phosphorus (TP) content decreased during early succession and subsequently remained relatively stable. Forest succession significantly influenced bacterial and fungal community composition but did not significantly affect α-diversity. Bacterial communities exhibited stage-specific shifts in the relative abundances of dominant taxa across the successional stages, whereas surface soil fungal communities showed more pronounced compositional reorganization. Mantel tests revealed that bacterial community composition was significantly associated with a broad suite of soil resource variables and environmental conditions, including pH, C and N availability, microbial biomass, and hydrolytic enzyme activities, whereas fungal community composition was primarily associated with soil pH and TP.

DISCUSSION: Collectively, our findings demonstrate that soil bacterial and fungal communities exhibit divergent successional response strategies during subalpine forest development in the Eastern Qilian Mountains. Bacterial communities were shaped by multiple resource-related and environmental factors, whereas fungal communities were more closely linked to soil pH and TP. This study reveals the divergent successional responses of soil bacterial and fungal communities during subalpine forest development under calcareous soil conditions, providing new insights into microbial community assembly processes and environmental regulation during forest succession.