title: The mechanisms to regulate arsenic behaviors in redox transition zones in paddy soils
Creators: name: Yuan, Zhaofeng
Publisher: name: University of Liverpool Repository
Year: 2020

description: Rice (Oryza sativa L.) is the staple food for people especially in Asia, but rice production is threatened by arsenic (As) contamination in paddy soil. Contamination of As in paddy soil is mainly caused by anthropogenic activities, such as mining and irrigation of high As groundwater. External As firstly enters overlying water, and then accumulates in paddy soil. Soil-water interface (SWI) is the gate controlling As exchange between soil and overlying water, and rhizosphere is the inlet of As from soils into rice root. Under natural conditions, a redox transition occurs along both micro interfaces due to atmospheric O2 diffusion or radial O2 loss from root. Arsenic is sensitive to redox conditions and tends to change over space and time across those micro interfaces. However, a deep understanding of As cycling in paddy water-soil-rice system has been hindered to date by techniques available to sample micro interfaces repeatedly in high-resolution. In order to fill this gap, a novel high-resolution porewater sampler was developed in this study. Using the technique, the spatiotemporal control of As was studied at paddy SWI and rhizosphere. A hollow fiber membrane tube (~ 2 mm diameter) was evaluated to sample dissolved elements with passive diffusion mechanism. The results showed quantification of solutes surrounding the tube can be achieved in every ≥ 24 h regardless of pH, ionic strength, and dissolved organic matter conditions. This technique, called In-situ Porewater Iterative (IPI) sampler, was further validated in soils under an anoxic-oxic transition by bubbling N2 and air into overlying water. The results showed that the IPI sampler is a powerful and robust technique in monitoring dynamics of element profile in soil porewater in high-resolution (mm). Moreover, measurement methods in ICP-MS and IC-ICP-MS were optimized to promote the measurement throughput of multi-element in limited samples (μL level) collected by high-resolution porewater samplers (e.g. IPI samplers). Major elements (e.g. iron (Fe) and manganese (Mn), mg•L-1 level) were measured by ICP-MS in extended dynamic range mode to avoid signal overflow, while trace elements (e.g. As, μg•L-1 level) in dynamic reaction cell (O2) mode to alleviate potential polyatomic interferences. Ammonium bicarbonate mobile phase was further demonstrated to simultaneously measure common species of As, phosphorus (P) and sulfur (S) with IC-ICP-MS analysis. With the optimized analytical methods and IPI samplers, the measurement throughput of multi-element and their species were improved up to 10 times compared to traditional methods. Furthermore, the cycling of As across SWI and rhizosphere was studied with the updated IPI sampler and state-of-art analytical techniques. In SWI, profiles of As, Fe and other associated elements in five paddy soils were mapped. The results showed a close coupling of Fe, Mn, As and P in 4 out of 5 paddy soils. However, decoupling of Fe, Mn and As was observed in the oxic-anoxic transition zone of one paddy soil. The study provided in situ evidence showing decoupling of As with Fe and Mn may happen in the oxic-anoxic transition zone of SWI. For rhizosphere, dynamic profiles of Fe and As were mapped by IPI samplers from days after transplanting 0 to 40. The results showed Fe and As change spatiotemporally in rhizosphere. Interestingly, Fe oxides formed in rhizospheric soil, rather than on rice root (Fe plaque), play the key role for immobilizing mobile As from bulk soil. A model of As transport from soil to rice, linking the temporal and spatial regulation of As in paddy soils, was provided to help better understand As cycling in paddy soils.