Abstract Arsenic (As) mobilization in rice paddy soils under fluctuating redox conditions is influenced by the biogeochemical cycling of redox sensitive elements such as iron (Fe) and manganese (Mn). Arsenic mobility in paddy soils is highly variable, and the influence of Mn abundances and Mn/Fe ratios on As mobility in these soils have received little attention. In this contribution, we developed a complementary set of field and laboratory experiments designed to evaluate the impact of Mn on interconnected Fe and As solubilization in rice paddy soils experiencing wetting-drying cycles through controlled irrigation. Porewater monitoring and synchrotron-based imaging and spectroscopy of thin sections prepared from an Arkansas paddy soil confirmed that As release was primarily governed by reductive dissolution of Fe (oxy)hydroxide phases. Experiments with laboratory soil microcosms amended with the synthetic nanocrystalline Mn oxide, δ-MnO2, showed that higher initial Mn/Fe inhibited Fe and As mobilization into porewater relative to unamended soil by up to 95% and 45%, respectively. Geochemical modeling suggests that pH increases driven by microbial MnO2 reduction, in conjunction with microbial Fe- and sulfate-reduction in carbonate-rich porewater, enhanced the precipitation of siderite (FeCO3(s)), mackinawite (FeS(s)), and potentially a Mn(II) arsenate phase. These secondary mineral phases likely played a greater role in controlling As solubilization than the role of Mn as a redox buffer regulating the redox conditions in the flooded soils. Field and laboratory experiments showed that alternate wetting and drying approaches with a single dry-down can be effective at reducing dissolved As concentrations in porewater through the oxidation of Fe. Differences in soil Mn/Fe ratios had no clear impact on the effectiveness of dry-downs as a strategy to reduce As mobilization.

中文翻译：

---

锰丰度对稻田土壤铁和砷溶解度的影响

摘要 波动氧化还原条件下稻田土壤中砷 (As) 的迁移受氧化还原敏感元素如铁 (Fe) 和锰 (Mn) 的生物地球化学循环的影响。稻田土壤中砷的迁移率变化很大，锰丰度和锰/铁比对这些土壤中砷迁移率的影响很少受到关注。在这项贡献中，我们开发了一套互补的田间和实验室实验，旨在评估 Mn 对通过受控灌溉经历干湿循环的稻田土壤中相互关联的 Fe 和 As 溶解的影响。从阿肯色州稻田制备的薄切片的孔隙水监测和基于同步加速器的成像和光谱证实，砷的释放主要受铁（氧）氢氧化物相的还原溶解控制。用合成纳米晶 Mn 氧化物 δ-MnO2 修正实验室土壤微观世界的实验表明，相对于未经修正的土壤，较高的初始 Mn/Fe 抑制 Fe 和 As 迁移到孔隙水中，分别高达 95% 和 45%。地球化学模型表明，微生物 MnO2 还原驱动的 pH 值升高，结合富含碳酸盐孔隙水中的微生物 Fe 和硫酸盐还原，增强了菱铁矿 (FeCO3(s))、麦基那维 (FeS(s)) 和潜在的Mn(II) 砷酸盐相。这些次生矿物相在控制砷溶解方面的作用可能比锰作为调节淹没土壤中氧化还原条件的氧化还原缓冲剂的作用更大。现场和实验室实验表明，通过单次干燥的交替润湿和干燥方法可以有效地通过氧化铁降低孔隙水中溶解的砷浓度。土壤锰/铁比率的差异对干涸作为减少砷迁移的策略的有效性没有明显影响。