Abstract Biochar consistently improves soil physicochemical properties and lowers the bioavailability of Cd and Pb, yet the exact mechanisms controlling metal mobility under field‐relevant flow conditions remain unresolved. We therefore conducted a leaching column experiment to quantify the vertical migration of Cd and Pb in a sandy‐loam paddy soil. The soil was amended with wheat straw biochar and shrub branch and leave‐biochar and leached with two concentrations (5 and 10 mM) of citric acid or sodium humate. The results revealed distinct patterns for Cd and Pb in response to biochar treatments. Biochar accelerated the migration of surface layer Cd but simultaneously reduced exchangeable Cd content by 4.5%–10.7% compared to control treatment. In contrast, Pb migration was retarded, and breakthrough was delayed even though the proportion of bioavailable (reducible + oxidizable) Pb increased by 3.1%–4.7%, reflecting ligand‐enhanced dissolution that was later re‐immobilized along the profile. Organic acid concentration governed the extent of transport. At 5 mM, neither citric acid nor sodium humate appreciably moved Cd or Pb beyond 4 cm. At 10 mM, however, sodium humate doubled leachate velocity and transported both metals to 8–12 cm, enhancing subsoil concentrations and increasing groundwater risk. However, biochar also increased the proportion of residual fractions of both Cd and Pb in the 4‐ to 8‐cm layer, indicating its role in transforming bioavailable forms into stable and non‐bioavailable forms. Collectively, the data show that Cd mobility is controlled primarily by leachate ionic strength, whereas Pb transport is driven by the concentration of water‐soluble organic ligands. Thus, biochar can be an effective long‐term immobilization agent only if coupled with management practices that limit the buildup of strong organic acid solutions in the profile.
Sun et al. (Thu,) studied this question.