Bioash amendments from biomass energy systems reshape rhizosphere bacterial communities and reduce metal bioavailability in contaminated soils

Biomass-derived ashes (bioash) from energy production are increasingly promoted within circular bioeconomy frameworks as soil amendments that recycle nutrients and mitigate soil acidity. However, their impacts on rhizosphere bacterial communities in metal-contaminated soils (MCS) remain poorly understood. Here, we investigated how four bioash types applied at three rates influence rhizosphere biogeochemistry and bacterial community dynamics of lettuce ( Lactuca sativa L.) grown in MCS. Bioash amendment consistently increased soil pH and electrical conductivity, while significantly decreasing bioavailable Zn and Cd fractions. High-throughput 16S rRNA gene amplicon sequencing revealed limited effects on bacterial alpha diversity, with only slight increases at the highest bioash doses. In contrast, beta diversity analyses demonstrated pronounced, dose- and type-dependent shifts in community composition (PERMANOVA, R² = 0.46, p = 0.009). Multivariate analyses identified rhizosphere pH, salinity, DTPA-extractable K and Cd, and mineral N as the main physicochemical variables associated with bacterial community composition. At the phylum level, bioash application increased the relative abundance of Pseudomonadota, while the relative abundances of Bacillota, Bacteroidota, and Gemmatimonadota declined under high application rates. Differential abundance analysis highlighted a distinct subset of bioash-responsive taxa, with enrichment of genera such as Galbitalea, Dongia , and Caulobacter , alongside depletion of several putatively metal-sensitive groups. Overall, bioashes reduced metal bioavailability and restructured rhizosphere microbiomes, supporting their role in sustainable remediation, while emphasizing the need to optimize application rates to balance soil recovery and microbial ecological integrity.