Rapid formation of iron monosulfide on Indicator of Reduction in Soil devices in S-rich hydric soils

Abstract

Indicator of Reduction in Soil (IRIS) devices are used to identify anaerobic conditions in soils for hydric soil and wetland identification purposes. IRIS devices quantify anaerobic conditions via the visualization of iron (Fe) reduction (e.g., orange Fe³⁺ paint reduces to soluble Fe²⁺, leaving behind the bare, white polyvinyl chloride device). Under stronger reducing conditions, sulfate (SO₄²⁻) reduction can occur, and sulfides (S²⁻ and/or H₂S) can react with the Fe³⁺ paint, resulting in the precipitation of black-colored, reduced iron monosulfide (FeS). While these processes are well known, the rate of FeS formation remains relatively understudied and current IRIS methods may not capture these data accurately. This study investigated FeS formation on IRIS films to identify deployment times that capture the minimum and maximum precipitation of FeS. To determine the timing and magnitude of FeS-precipitation on IRIS films, five replicate films were deployed in a wet, S-rich soil, across 11 periods ranging from 2 min to 30 days.  Results show that FeS precipitated on IRIS films in just 2 min, and the highest average amount of FeS (82%) precipitated in 1 day. After 1 day, the percentage of FeS decreased and a white color change became more apparent on IRIS films. Our results suggest that the recommended 30-day deployment period is too long for accurately measuring FeS-precipitation on IRIS devices deployed in wet, alkaline soils. These considerations should be incorporated into standard IRIS protocols used to quantify anaerobic conditions and other biogeochemical conditions (e.g., pore-water sulfide levels) in S-rich soils.