Chicken manure and cow manure regulate phytoremediation of cadmium-contaminated alkaline soils through distinct mechanisms.

Phytoremediation can completely remove harmful heavy metals from soil; however, it is limited by low efficiency and long cycling times. In this study, to further improve the efficiency of phytoremediation of alkaline cadmium (Cd)-contaminated soils, we investigated the enhanced Cd uptake capacity of Tagetes patula L. through manure application and explored its underlying mechanisms. The results indicate that chicken manure significantly increased soil available Cd through pH reduction, thereby enhancing Cd enrichment in T. patula. In contrast, cow manure did not alter soil pH or available Cd. However, it substantially promoted plant biomass, leading to increased total Cd uptake primarily via a biomass-amplification mechanism. Both manures improved plant physiological status and altered root metabolomic profiles, upregulating Cd-related defense pathways. They also enriched beneficial soil microbial taxa (e.g. Bacteroidota and Patescibacteria) and enhanced soil enzyme activity. Overall, livestock manures can enhance phytoremediation of Cd-contaminated alkaline soils but via distinct mechanisms: chicken manure acts through Cd mobilization and direct uptake, whereas cow manure relies on growth promotion and improved plant tolerance. These findings provide a theoretical basis and application value for optimizing manure-assisted phytoremediation strategies: chicken manure is suitable for short-term Cd removal scenarios, while cow manure is better suited for long-term soil remediation combined with sustainable agricultural practices.