Identifying early-stage biogeochemical changes with IRIS films in coastal ecosystems impacted by sea-level rise and saltwater intrusion.

In coastal ecosystems, increased flooding and saltwater intrusion alter soil saturation and salinity, leading to the conversion of upland forests and agricultural lands into tidal marshes. Indicators of reduction in soils (IRIS) films have been previously used to document hydric status, but no studies have tested their ability to detect early-stage signs of biogeochemical shifts due to saltwater intrusion. Using sites established by the Coastal Critical Zone Network, pairs of Mn and Fe IRIS films were installed along four marsh-to-upland transects on the Delmarva Peninsula with contact times of 2 h and 1 week. IRIS films along these previously established salinity and redox gradients were paired with porewater measurements of Fe(II) and sulfide. FeS was the primary indicator observed on IRIS films. With a 2-h contact time, FeS precipitation occurred only in the marsh transect points, and S Pixels (%) were strongly correlated with porewater sulfide. However, with a longer 1-week contact time, IRIS films captured a gradient in reducing conditions and FeS formation. Additionally, IRIS films deployed for 1-week detected FeS even at the most upland transect points, despite porewater sulfide concentrations below our limit of detection. Our results show that IRIS films can capture shifts in reducing conditions and biogeochemistry, making them an effective strategy for documenting early detection of biogeochemical shifts due to saltwater intrusion in coastal ecosystems.