Soil Nutrients Rather than Heavy Metals Dominate the Succession of Microbial Communities in Subtropical Coal Gangue Piles: A Case Study from a Successional Gradient in Southern China

The natural revegetation of coal gangue piles relies on microbial community succession, yet limiting factors in subtropical regions remain unclear, hindering the acceleration of natural revegetation. This study examined coal gangue tailings of varying successional ages in southern China to identify the factors that limits on microbial succession. Soil properties, microbial diversity, community structure, network complexity, and heavy metal fractions were analyzed across successional stages. Results show a progressive soil acidification anda shift from carbon/nitrogen limitation to phosphorus limitation as succession progresses. Bacterial richness peaked at middle stage and then declined, while fungal richness shows no significant difference among different stages. Bacterial community structure changed sharply in early successional stage while fungal structure in the mid-stages. The complexity of microbial cooccurance network initially decreased then increased. Heavy metals mostly accumulated in stable residual fractions during succession, which has only limited toxic effects. The nutrient limitation that shift from C/N to P was the dominant factor shaping core functional modules and community composition, though dissolved organic carbon and few heavy metal fractions were also the main driving factors of microbial network complexity. These findings indicate that microbial succession is constrained more by nutrient availability than by heavy metal toxicity, with early stages limited by C/N sources and later stages limited by phosphorus. This provides a basis for targeted management to accelerate natural recovery of coal gangue piles.