Periodic redox condition changes in acidic paddy soils substantially induce the biogeochemical redox cycling, and consequently affect Cd availability. However, the underlying biogeochemical mechanisms of the complicated redox processes in paddy soil remain poorly understood. Thus, we investigated the dynamics of Cd fractions under anoxic (0–40 days) and oxic (40–55 days) conditions. The available Cd content was evaluated using the diffusive gradients in thin film (DGT) technique, and the results show it decreased from 6.3 μg L to below 0.1 μg L under anoxic conditions, but rapidly increased to 13.5 μg L under oxic conditions. Both sequential extraction procedures and X-ray absorption spectroscopy (XAS) analyses found that majority of available Cd transformed to organic matter complex and Fe-Mn oxides fractions, rather than bounded with sulfides. The soil chemical properties further evaluated, and the correlation analysis and principal component analysis results suggested that the key factors in soils for the available Cd was the SO, pH, and surface site concentration. The reduction of SO could generate sulfides which may precipitate with dissolved Cd. The biogeochemical redox cycling of Fe/N/S determined the changes of soil pH and consequently influenced adsorption behavior of Cd. The breakdown of the soil aggregations changed the surface site concentration, which may affect the immobilization of Cd. A process-based kinetic model was established, and it found that iron biogeochemical redox cycling dominated the changes of soil pH, and thereby contributed to majority Cd retention by Fe-Mn oxides (34.4 %) and organic matter (33.7 %). In addition, sulfur biogeochemical redox cycling only contributed to 13.6 % of the total Cd contents, because of relatively low Cd solubility in contaminated soils and the competition with other cations for limited sulfides. The findings provide robust targets for interpreting the iron and sulfur biogeochemical processes for Cd availability and would be helpful for developing the precise and effective remediation strategies in Cd-contaminated soils.

中文翻译：

---

铁生物地球化学氧化还原循环主要控制酸性水稻土中镉的有效性


酸性水稻土中氧化还原条件的周期性变化显着诱导生物地球化学氧化还原循环，从而影响镉的有效性。然而，水稻土中复杂的氧化还原过程的潜在生物地球化学机制仍然知之甚少。因此，我们研究了缺氧（0-40 天）和有氧（40-55 天）条件下 Cd 组分的动态。使用薄膜扩散梯度（DGT）技术评估有效Cd含量，结果表明，在缺氧条件下，有效Cd含量从6.3 μg·L-1下降到0.1 μg·L以下，但在有氧条件下迅速增加至13.5 μg·L-1。连续萃取程序和 X 射线吸收光谱 (XAS) 分析发现，大多数可用的 Cd 转化为有机物复合物和 Fe-Mn 氧化物部分，而不是与硫化物结合。进一步评价土壤化学性质，相关分析和主成分分析结果表明，土壤中有效态Cd的关键影响因素是SO、pH和地表位点浓度。 SO 的还原会产生硫化物，硫化物可能与溶解的 Cd 一起沉淀。 Fe/N/S 的生物地球化学氧化还原循环决定了土壤 pH 值的变化，从而影响了 Cd 的吸附行为。土壤团聚体的分解改变了表面位点浓度，这可能会影响 Cd 的固定。建立了基于过程的动力学模型，发现铁生物地球化学氧化还原循环主导了土壤 pH 值的变化，从而有助于 Fe-Mn 氧化物 (34.4%) 和有机质 (33.7%) 保留大部分 Cd。此外，硫生物地球化学氧化还原循环仅贡献了13。总镉含量的 6%，因为镉在污染土壤中的溶解度相对较低，并且与其他阳离子竞争有限的硫化物。这些发现为解释镉可用性的铁和硫生物地球化学过程提供了强有力的目标，并将有助于制定镉污染土壤的精确有效的修复策略。