Monitoring arsenic mobilization in variably flooded rice fields with Fe and Mn indicator of reduction in soil (IRIS) films
Date
2022
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Rice is a vital food source for millions of people, but it is susceptible to arsenic (As) uptake because it is grown in flooded paddy conditions that lead to As mobilization and because rice has an efficient As uptake pathway. The use of alternating wetting and drying (AWD) can help to immobilize As through water drainage, but is difficult to manage. We posit that indicators of reduction in soil (IRIS) films can be used an easy-to-use passive sensor to inform farmers of soil reducing conditions and risk of As mobilization in rice paddies. We used Fe or Mn coated IRIS films to determine when, during the life cycle of rice, these films would best predict the mobilization and uptake of As. We hypothesized that Mn films would have faster rates of paint removal than Fe films, but that Fe films would better predict As mobilization and accumulation in rice grain. We found that Mn films had a more rapid response (i.e., faster paint removal from films) than Fe films and both were predictive of soil redox potential (ORP) when in contact with the soil for 3 and 8 days, respectively. We then installed and removed Fe and Mn IRIS films every 8 and 3 days, respectively, in rice paddies subjected to 6 water managements (flooded, nonflooded and 4 different AWD severities) to determine when IRIS films show the strongest relationship with porewater and grain As. We found that both Fe and Mn IRIS films were equally as predictive as porewater Fe and ORP in predicting porewater and grain As concentrations. Synchrotron X-ray fluorescence imaging also revealed co-localization of As onto the synthesized Fe oxides on Fe IRIS films, as well as the neoformed Fe oxides on Mn films. This finding suggest that IRIS films may also serve as passive As samplers in reducing environments. Moreover, we recommend that IRIS paint removal >70% be used as a marker for risk of As mobilization and rice uptake; this recommendation must be verified in other soils. While future work is still needed to understand their wider application to other soils, this work shows that IRIS films can be an affordable and effective tool for farmers to manage water and mitigate As mobilization
Description
Keywords
Variably flooded, Rice fields, Indicator of reduction in soil films