Objective The research was conducted to explore the remediation effect of water regulation and biochar on farmland soil near mining areas and improve the safe use of farmland.

Method Taking farmland soil near a lead-zinc mine in Yunnan as the research object, three water conditions: dry-wet alternation (G), flooding (Y) and natural moisture content (Z), and four biochar addition amounts of 0% (BC₀, CK), 2.5% (BC1), 5.0% (BC₂), and 7.5% (BC₃) were set up. Through soil culture experiments, the effects of applying chicken manure biochar under different water regulation measures on the forms of Pb and Zn in heavy metal contaminated soil were studied.

Result Under the three water conditions, the application of biochar could increase soil pH. The concentrations of water-soluble and exchangeable Pb and Zn in soil treated with biochar were significantly lower than that of CK, reducing the bioavailability of Pb and Zn. The mass fractions of oxidizable and residual Pb in soil treated with biochar increased, with ZBC₂ treatment having the highest mass fractions of oxidizable and residual Pb, which were 64.58% and 25.06%, respectively. The mass fractions of residual Zn in soils under various treatments ranked as: dry-wet alternation > flooding > natural moisture content. Under different water conditions, the application of biochar significantly reduced the mass concentrations of TCLP-extracted Pb and Zn, both of which were far below the international limitation standards. Among them, the soil treated with GBC₃ had the lowest concentration of TCLP-extracted Zn, which was 3.45 mg/L. The average mass fraction of Pb and Zn bioavailability by SGET method was lower than that by PBET method and SBET method, while the ZBC₃ treatment by SGET method had the lowest mass fractions of Pb and Zn bioavailability, which were 0.43% and 0.27%.

Conclusion The application of different amounts of biochar under three types of water regulation effectively passivated soil heavy metals Pb and Zn, reducing their harm to the soil environment. Under natural moisture content conditions, the application of high amounts of biochar (5.0%, 7.5%) had the best passivation effect on heavy metal Pb in soil; the optimal passivation effect of 7.5% biochar on heavy metal Zn in soil was observed under dry-wet alternation conditions.