Organic-inorganic amendment efficiently immobilizes soil heavy metals by synergistically regulating physicochemical properties and microbial ecological networks.

The immobilization of heavy metals (HMs) in contaminated karst farmland is crucial for soil remediation and ecological restoration. This study evaluated the effects of lime (CA), organic fertilizer (F), and their combination (F-CA) on the immobilization of Pb, Cd, Zn, and Cu, alongside associated changes in soil physicochemical properties and microbial communities, in contaminated farmland soil from a lead-zinc mining area. Results from a 60-day field incubation experiment showed that both CA and F-CA markedly reduced the bioavailability of Pb, Cd, Zn, and Cu. Specifically, CA lowered diethylenetriamine pentaacetic acid (DTPA)-extractable Pb, Cd, Zn, and Cu by 50 %-87.2 %, 97.8 %-98.3 %, 89.4 %-92 %, and 8.7 %-66.5 %, respectively, while F-CA achieved reductions of 77.4 %-92.5 %, 70.6 %-90.6 %, 61.8 %-71.6 %, and 22.5 %-66.9 % (P < 0.05). The immobilization of heavy metals was primarily attributed to increases in soil pH, electrical conductivity, and exchangeable Ca and Mg. In contrast to CA, which suppressed microbial diversity and enriched potentially pathogenic fungi, the F-CA treatment enhanced bacterial and fungal diversity, promoted a more complex and stable microbial network, enriched beneficial keystone taxa, and enhanced nitrogen, sulfur, and iron cycling processes. Mantel test, random forest, and partial least squares path model (PLS-PM) analyses confirmed that while abiotic fixation contributed to HM immobilization, the total effect exerted by biotic processes was stronger than that of abiotic factors, highlighting the dominant role of microbial regulation. Overall, the F-CA amendment represents an effective strategy for stabilizing HMs in karst agricultural soils by integrating abiotic and microbial pathways to enhance remediation efficiency and soil ecological health, offering mechanistic insights for improving amendment-assisted bioremediation.