Reactive heavy metals govern soil microbial communities: Depth-resolved evidence and disentangled metal-specific...

Reactive heavy metals, rather than total concentrations, govern soil microbial community structure, yet their independent effects remain poorly understood across soil depths. This study systematically investigated the effects of reactive heavy metals on soil microbial communities and identified the dominant environmental drivers using 108 samples from two soil horizons (0-15 cm and 15-30 cm). Redundancy analysis and variation partitioning analysis demonstrated that heavy metals played a dominant role in shaping microbial diversity, with their independent contribution (8.29%) exceeding that of traditional soil properties (5.27%). The reactive pool serves as the immediate ecological risk reservoir, directly mediating microbial community assembly. Depth-resolved analysis revealed that the 0-15 cm layer was a biogeochemical hotspot, where elevated reactive heavy metal accumulation and intense microbial activity functionally coupled to drive community assembly. Within this layer, iron (Fe), copper (Cu), lead (Pb), and manganese (Mn) were identified as the key drivers with distinct ecological pressures on microbial communities. Piecewise structural equation modeling revealed that Pb directly triggered structural reassembly of the community, while Cu induced significant diversity loss and homogenization, potentially impairing functional redundancy. Conversely, Mn exhibited a nutritional effect that partially alleviated environmental stress. The findings underscored the pivotal roles of reactive heavy metals in shaping microbial communities in agricultural soils and emphasized the importance of soil heavy metal pollution control to mitigate their potential ecological effects in agricultural production.