Bioavailable heavy-metal pressure is linked to microbial functional vulnerability and carbon-use niche contraction in karst agricultural soils.

Heavy-metal contamination threatens microbial processes in karst agricultural soils, but functional responses to the bioavailable metal fraction remain insufficiently resolved. We integrated soil properties, total and DTPA-extractable metals, microbial abundance and community profiles, and Biolog EcoPlate traits to quantify microbial functional vulnerability in Pb-Zn mining-affected agricultural soils. DTPA-extractable metal pressure was positively associated with functional vulnerability (Spearman ρ = 0.626, p < 0.001) and negatively associated with carbon-use niche breadth (ρ = -0.481, p < 0.001). Contraction occurred across several substrate classes and remained consistent in covariate-adjusted and sensitivity analyses. Community reorganization, independent contaminated-soil datasets, and a targeted qPCR microcosm experiment further connected the field pattern with shifts in microbial community structure and functional marker genes. These findings identify carbon-use niche contraction as an informative functional response to bioavailable metal mixtures and support its use alongside chemical indicators when assessing remediation and sustainable management of contaminated karst soils.