Can simultaneous immobilization of arsenic and cadmium in paddy soils be achieved by liming?

Abstract Liming acidic paddy soils to near-neutral pH is the most cost-effective strategy to minimize cadmium (Cd) accumulation by rice. However, the liming-induced effect on arsenic (As) (im)mobilization remains controversial and is called upon further investigation, particularly for safe utilization of paddy soils co-contaminated with As and Cd. Here we explored As and Cd dissolution along pH gradients in flooded paddy soils and extracted key factors accounting for their release discrepancy with liming. The minimum As and Cd dissolution occurred concurrently at pH 6.5-7.0 in an acidic paddy soil (LY). In contrast, As release minimized at pH < 6 in other acidic two soils (CZ and XX) while the minimum Cd release still appeared at pH 6.5-7.0. Such a discrepancy was determined largely by the relative availability of Fe under overwhelming competition from dissolved organic caron (DOC). A mole ratio of porewater Fe/DOC at pH 6.5-7.0 is suggested as a key indicator of whether co-immobilization of As and Cd can occur in flooded paddy soils with liming. In general, a high mole ratio of porewater Fe/DOC (≥ 0.23 in LY) at pH 6.5-7.0 can endow co-immobilization of As and Cd, regardless of Fe supplement, whereas such a case is not in the other two soils with lower Fe/DOC mole ratios (0.01–0.03 in CZ and XX). Taking an example of LY, introduction of ferrihydrite promoted transformation of metastable As and Cd fractions to more stable ones in the soil during 35 days of flooded incubation, thus meeting a Class I soil for safe rice production. This study demonstrates that the porewater Fe/DOC mole ratio can indicate liming-induced effect on co-(im)mobilization of As and Cd in typical acidic paddy soils, providing new insights into the applicability of liming practice for the paddy soils.