Sediments along the Baltic Sea coast can contain considerable amounts of metal sulfides that if dredged and the spoils deposited such that they are exposed to air, can release high concentrations of acid and toxic metals into recipient water bodies. Two river estuaries in western Finland were dredged from 2013 to 2018 and the dredge spoils were deposited on land previously covered with agricultural limestone to buffer the pH and mitigate acid and metal release. In this study, the geochemistry and 16S rRNA gene amplicon based bacterial communities were investigated over time to explore whether the application of lime prevented a conversion of the dredge spoils into acid producing and metal releasing soil. The pH of the dredge spoils decreased with time indicating metal sulfide oxidation and resulted in elevated sulfate concentrations along with a concomitant release of metals. However, calculations indicated only approximately 5% of the added lime had been dissolved. The bacterial communities decreased in diversity with the lowering of the pH as taxa most similar to extremely acidophilic sulfur, and in some cases iron, oxidizing Acidithiobacillus species became the dominant characterized genus in the deposited dredge spoils as the oxidation front moved deeper. In addition, other taxa characterized as involved in oxidation of iron or sulfur were identified including Gallionella, Sulfuricurvum, and Sulfurimonas. These data suggest there was a rapid conversion of the dredge spoils to severely acidic soil similar to actual acid sulfate soil and that the lime placed on the land prior to deposition of the spoils, and later ploughed into the dry dredge spoils, was insufficient to halt this process. Hence, future dredging and deposition of dredge spoils containing metal sulfides should not only take into account the amount of lime used for buffering but also its grain size and mixing into the soil.

Acid sulfate soils are sulfide-rich soils that pose a notable environmental risk as their strong acidity and low pH mobilizes metals from soil minerals leading to both acidification and metal contamination of the surrounding environment. In this study a rapid and cost-efficient approach was developed to resolve the main distribution patterns and geochemical features of acid sulfate soils throughout coastal plains stretching for some 2000 km in eastern, southern, and western Sweden. Of the investigated 126 field sites, 47 % had acid sulfate soils including 33 % active, 12 % potential, and 2 % pseudo acid sulfate soils. There were large regional variations in the extent of acid sulfate soils, with overall much higher proportions of these soils along the eastern coastal plains facing the Baltic Sea than the western coastal plains facing the Kattegatt/Skagerrak (Atlantic Ocean). The sulfur concentrations of the soil's parent material, consisting of reduced near-pH neutral sediments, were correlated inversely both with the minimum pH of the soils in situ (rS = −0.65) and the pH after incubation (oxidation) of the reduced sediments (rS = −0.77). This indicated the importance of sulfide levels in terms of both present and potential future acidification. Hence, the higher proportion of acid sulfate soils in the east was largely the result of higher sulfur concentrations in this part of the country. The study showed that the approach was successful in identifying large-scale spatial patterns and geochemical characteristics of importance for environmental assessments related to these environmentally unfriendly soils.

Acid sulfate soils are frequently described as the nastiest soils on Earth, and they pose environmental risks associated with their strong acidity and consequential mobilization of toxic metals present in the soils. Within Sweden, acid sulfate soils have been extensively studied around the northern Baltic coastline and have now been found to occur throughout the area below the maximum Holocene marine limit that stretches for some 2000 km from North to South. This study investigated 20 active acid sulfate soils (field tested pH < 4.0) collected throughout this area that were tested for microbial community composition using 16S rRNA gene amplicons, representing a novel study of microbial communities in ripening zones across a broad regional scale. The microbial community compositions exhibited a north (boreal zone) to south (hemiboreal zone) regional divide, primarily within the oxidized zone (pH < 4.0), to a lesser degree in the transition zone (steep pH gradient), while little differences were observed in the reduced zone (near-neutral pH). For instance, a higher relative abundance of Ktedonobacteraceae was identified in the northern boreal and Gallionellaceae in the southern hemiboreal oxidized zones. In addition, microbial taxa associated with iron and sulphur oxidation and reduction were identified, such as Acidobacteriaceae, Gallionellaceae and Koribacteraceae that have been previously identified in other acid sulfate soils and acid mine drainage settings. The predominant controls of the microbial community differences were the north-south divide indicative of a strong soil-temperature effect followed by soil zones suggesting an influence of the soils' pH and/or redox conditions.