Environmental and Geochemical Controls on Acid Sulfate Soil Formation Along the Southern Baltic Sea Coast

This study investigates the environmental and geochemical controls on forming and transforming acid sulfate (AS) soils along the southern Baltic Sea coast. Field surveys and laboratory analyses were conducted on a series of coastal soil transects located in hydrologically dynamic environments, including abrasive terraces/beaches, micro-cliffs/beach ridges, and organic-rich depressions. The results revealed a high site-specific variability in AS soil properties driven by topographic position, hydrological regime, and sedimentary history. Hypersulfidic materials, indicative of sulfide accumulation under reducing conditions, were found across all geomorphological settings. Geochemical indicators such as field pH, total organic carbon to total sulfur ratio, chloride, and calcium carbonate content proved effective in assessing the soil variability, including acidification potential. Magnetic susceptibility measurements indicated a predominantly natural origin of potentially toxic elements and the absence of technogenic contamination. However, under changing redox conditions, particularly in carbonate-poor soils, the mobilisation of toxic elements such as chromium, nickel, lead, and zinc cannot be excluded, despite their generally low concentrations. Organic matter, derived from both autochthonous and allochthonous sources, played a key role in sulfidisation processes, although the influence of its humification degree on acidification risk remains unclear. Overall, the study highlights the importance of localised environmental controls in AS soil development and provides a methodological framework for identifying similar systems in other coastal plains of the Baltic Sea.