Livestock and poultry manure influence thallium accumulation in chili peppers (Capsicum annuum) under complex toxic elements contamination in soil

Long-term mining activities in the mercury-thallium mining area of Lanmuchang, southwestern China, resulting in contamination of surrounding soils with complex toxic elements such as Tl, As, Sb, Hg, and Cd. While manure aids in pollution control and soil fertilization, its impact on Tl bioavailability in soil remains incompletely understood. This study explored the physiological changes and Tl accumulation in Capsicum annuum under combined pollution stress and application of livestock and poultry manure near the Lanmuchang mining area. The results showed that chicken manure promoted Tl accumulation in Capsicum annuum, whereas pig and cattle manure inhibited it, with pig manure exhibiting the strongest inhibitory effect. Both pig and cattle manure reduced the transpiration rate of Capsicum annuum, which decreased the driving force for Tl transport to the plant's upper parts. Additionally, manure increased organic acid secretion in Capsicum annuum roots and decreased soil Eh. This led to a reduction in Tl³⁺ to the more stable Tl⁺, which decreased the content of exchangeable Tl form and its activity. Pig manure significantly reduced Tl content in stems, leaves, and fruits by increasing the area of the Casparian bands in Capsicum annuum roots, which hindered Tl accumulation. In contrast, chicken manure had the opposite effect. The findings of this study provide important theoretical support for scientifically guiding agricultural production safety in areas contaminated with the heavy metal Tl.