Evaluating Environmental Drivers of Soil Reduction Using IRIS in Field and Laboratory Settings

Abstract

Providing evidence of anaerobic conditions is critical to identify hydric soils for wetland research and mitigation efforts. Indicator of Reduction in Soils (IRIS) devices utilize the reduction of iron (Fe) oxides and manganese (Mn) oxides to document anaerobic conditions. Currently, only Fe IRIS devices are incorporated into the guidelines of the Hydric Soil Technical Standard (HSTS). Even though Fe IRIS have been used for the past two decades, there is a growing need to clarify and compare the effect of early growing season conditions on the performance of Fe and Mn IRIS. In this study, we set out to explore the influence of saturation, temperature, and soil organic carbon content on IRIS performance in 11 mitigation wetland sites as well as in two laboratory microcosm experiments. In addition, the suppressive effect of Mn oxide on Fe reduction was demonstrated in a third microcosm experiment. We observed that increasing the temperature, duration of saturation, and soil organic carbon content all significantly increased the amount of Fe and Mn oxides removed from IRIS films and promoted the mobilization of these metals into microcosm porewater. Furthermore, we demonstrated that the current HSTS threshold for Fe IRIS paint removal of ≥30% adequately identifies anaerobic conditions in warm (≥11 °C) conditions, but it is too conservative in cool conditions (<11 °C) and lowering the threshold to ≥15% minimizes the possibility of false negative results. In contrast, Mn IRIS paint removal is more sensitive than Fe IRIS paint removal. While no HSTS guidelines exist for Mn IRIS, we found a threshold of ≥30% Mn removal best documented anaerobic conditions in cool conditions. However, no Mn IRIS threshold was deemed informative in warm conditions due to the greater reactivity of Mn oxides. This study presents insights into the environmental drivers of soil reduction and provides a more nuanced understanding of the application and interpretation of IRIS films in seasonal conditions and restored wetlands.