Indicator of redox in soil (IRIS) films as a water management tool for rice farmers

Rice is a crucial part of the world's food supply but is also susceptible to uptake of contaminants including arsenic (As) and cadmium (Cd) depending on the soil redox potential. Careful control of soil redox state by implementing alternate wetting and drying (AWD) water management can decrease mobility of soil As and Cd, but can be difficult to manage. Indicators of reduction in soil (IRIS) tubes and films have been studied by pedologists for wetland delineation; here, we explore the use of the IRIS film technology as passive samplers of soil redox potential in rice paddies. The goal of this study was to test the response time of IRIS films under different water management (i.e., variable soil redox potentials). After paddy soils were exposed to severe or safe AWD, where rice paddies were allowed to dry to >30 cm below the soil surface and 15 cm below the soil surface, respectively, IRIS films, coated with Fe oxide or Mn oxide paint, were installed. Immediately following IRIS film installation, soils were reflooded, and percent removal of Fe or Mn oxides were monitored on films that were removed every 12 h for Fe films, and every 6 h for Mn films. Porewater was collected at installation and every 12 h during the studies to observe correlations between IRIS film paint removal and porewater chemistry. We observed quicker paint removal for Mn films than Fe films, and paint removal varied due to growing season and water management. Moreover, correlations between porewater chemistry and Mn paint removal were observed. While further work is still needed to understand kinetics of IRIS paint removal as it relates to porewater parameters, this work illustrates that IRIS films are a low-cost tool that rice farmers can use to better manage water and we highlight considerations for possible implementation strategies for the future.