Effects of pig bone‑derived hydroxyapatite on contaminated manure composting: Humification, copper/zinc...

The high-value utilization of livestock manure remains a critical challenge due to co-contamination with heavy metals (Cu/Zn) and antibiotic resistance genes (ARGs), particularly high-risk ARGs that threaten public health. Here, pig bone-derived bio-based hydroxyapatite was incorporated into aerobic composting to comprehensively explore its effects on humification, heavy metals (Cu/Zn) passivation, and ARGs dissemination, as well as the underlying microbial mechanisms. Results demonstrated that bio-based hydroxyapatite prolonged the thermophilic phase, increased humus and humic acid contents by 27.9% and 31.4% respectively, and significantly up-regulated functional genes involved in carbon degradation (e.g., pox for lignin, amyA for starch, xylA for hemicellulose) and carbon fixation (e.g., accA in HP/HB cycle, acsA in WL pathway). The bioavailable fractions of Cu and Zn were reduced by 38.56% and 13.78% via complexation with humic substances and bio-based hydroxyapatite's surface functional groups. Notably, total ARGs abundance decreased by 26.1%, with Rank I and II high-risk ARGs reduced by 58.74% and 81.82% relative to the control. Bio-based hydroxyapatite also reduced the abundance of mobile genetic elements by 29.73% and inhibited the proliferation of Salmonella, a key ARGs host genus. Mechanistically, bio-based hydroxyapatite promoted stochastic microbial community assembly, enhanced cooperative interspecies interactions, and constrained horizontal gene transfer by alleviating oxidative stress (ROS/SOS pathway) and reducing bacterial motility (chemotaxis and flagellar assembly). These findings elucidated the multi-dimensional regulatory role of bio-based hydroxyapatite in synergistically improving compost quality and mitigating co-occurring ecological risks, providing valuable insights for the safe resource utilization of contaminated livestock manure.