Millimeter-scale topsoil layer blocks arsenic migration in flooded paddy soil
Millimeter-scale topsoil layer blocks arsenic migration in flooded paddy soil
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DOI：
10.1016/j.gca.2020.01.038
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发表时间：
2020-04
影响因子：
5
通讯作者：
S. Mitsunobu;Misato Toda;N. Hamamura;F. Shiraishi;Yurika Tominaga;M. Sakata
中科院分区：
文献类型：
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作者：
S. Mitsunobu;Misato Toda;N. Hamamura;F. Shiraishi;Yurika Tominaga;M. Sakata
采用微X射线荧光光谱-微X射线吸收精细结构（μ XRF-μ XAFS）、微电极和定量聚合酶链反应（qPCR）等技术，研究了砷（As）在实验室培养的水稻土缺氧-缺氧界面中的微观行为。15 d后，表层土壤毫米层（0~4.6 mm）的氧化还原度随深度的变化呈陡峭的梯度变化，表层土壤处于氧化状态，深层土壤处于还原状态。微尺度元素定位结果表明，培养15 d后，系统中大量的As（约占总量的10%）在表层3mm的土壤中强烈积累。扩展X射线吸收精细结构（EXAFS）的直接形态分析表明，在积累层中的主机相的砷是铁（III）的氢氧化物包括在土壤中，这是占主导地位的铁物种在该层。微区X射线吸收近边结构（μ XANES）直接形态分析表明，富集带和贫化带中As的主要形态发生了明显变化。具体而言，氧化物种As（V）在As积累（顶表面）层中增加，并且还原物种As（III）在As耗尽（更深）层中减少。直接Fe形态也表明，在积累层中最丰富的部分是Fe（III）的氢氧化物，如水铁矿，最优选的吸附剂砷。这些结果表明，在研究中观察到的砷积累与砷的还原和氧化在好氧-缺氧界面的水稻土。具体而言，更不稳定的As（III）形成的还原性较深的层迁移到上层，迁移的As（III）被氧化的表面毫米层的As（V），然后清除的As（V）的Fe（III）的氢氧化物，具有高亲和力的As（V），这是一个可能的机制，在本研究中观察到的强烈积累的表层。此外，锰（Mn）和铁（Fe）μ XANES分析和微生物As（III）氧化酶基因（aioA）定量qPCR表明，作为As积累的触发反应，在顶层的As（III）氧化可能是由化学和微生物氧化过程诱导的。本研究的结果表明，水稻土系统可能具有很强的阻止As迁移到洪水中的能力。我们的研究结果还表明，毫米级的表层土壤在淹水的水稻土可以影响整个砷在稻田的行为的生态地球化学。因此，要全面了解和预测As在稻田中的归趋和迁移，必须对稻田土壤的物理、化学和生物学特性进行研究。
In this study, we investigated the microscale behavior of arsenic (As) in an oxic-anoxic interface in laboratory-incubated paddy soil by combining multiple techniques, micro X-ray fluorescence spectrometry combined with micro X-ray absorption fine structure (μXRF-μXAFS), microelectrodes, and quantitative polymerase chain reaction (qPCR) analyses. A steep redox gradient with depth was observed in the topsoil millimeter layer (depth: 0–4.6 mm) after 15 days, oxidative condition in the surface soil and reductive condition in deeper soil, due to the limited oxygen supply caused by flooding. Microscale elemental mapping of the top soil layer by μXRF showed that a large amount of the As (about 10% of total) in the system strongly accumulated within the top 3 mm soil layer after 15 days of incubation. Direct As speciation by extended X-ray absorption fine structure (EXAFS) indicated that the host phases of As in the accumulation layer were the Fe(III) hydroxides included in the soil, which were the dominant Fe species in the layer. Direct As speciation by micro X-ray absorption near edge structure (μXANES) showed that the dominant As species in the As accumulation and depletion zones were clearly changed. Specifically, the oxidized species As(V) increased in the As accumulation (top surface) layer, and the reduced species As(III) decreased in the As depletion (deeper) layer. Direct Fe speciation also showed that the most abundant fraction of Fe in the accumulation layer was the Fe(III) hydroxides such as ferrihydrite, the most preferable sorbent for As. These findings suggest that the As accumulations observed in the study are significantly associated with As reduction and oxidation in the oxic-anoxic interface of paddy soil. Specifically, more labile As(III) formed in the reductive deeper layer migrates to the upper layer, the migrated As(III) is oxidized to As(V) in oxidative surface millimeter layer, and the As(V) is then scavenged by Fe(III) hydroxides that have high affinity for As(V); this is a possible mechanism for the strong accumulation of As observed in the topsoil layer in this study. In addition, manganese (Mn) and iron (Fe) μXANES analyses and microbial As(III) oxidase gene (aioA) quantification by qPCR suggested that As(III) oxidation in the top layer, which acted as a trigger reaction for As accumulation, might be induced by both chemical and microbial oxidation processes. The findings in this study indicate that the paddy soil system could potentially have a strong ability to prevent As migration to the floodwater. Our findings also indicate that the millimeter-scale topsoil layer in the flooded paddy soil could biogeochemically influence the whole As behavior in the paddy fields. Thus, to fully understand and predict As fate and migration in the paddy fields, we should investigate localized physical, chemical, and, biological properties of paddy soil.