Naturally occurring sulfide rich deposits are common along the northern Baltic Sea coast that when exposed to air, release large amounts of acid and metals into receiving water bodies. This causes severe environmental implications for agriculture, forestry, and building of infrastructure. In this study, we investigated the efficiency of ultrafine-grained calcium carbonate and peat (both separately and in combination) to mitigate acid and metal release. The experiments were carried out aerobically that mimicked summer conditions when the groundwater level is low and acid sulfate soils are exposed to oxygen, and anaerobically that is similar to autumn to spring conditions. The ultrafine-grained calcium carbonate dissipated well in the soil and its effect alone and when mixed with peat raised the pH and reduced pyrite dissolution while peat alone was similar to the controls and did not halt metal and acid release. High throughput 16S rRNA gene sequencing identified populations most similar to characterized acidophiles in the control and peat treated incubations while the acidophilic like populations were altered in the calcium carbonate alone and calcium carbonate plus peat treated acid sulfate soils. Coupled with the geochemistry data, it was suggested that the acidophiles were inactivated by the high pH in the presence of calcium carbonate but catalyzed pyrite dissolution in the controls and peat incubations. In conclusion, the anaerobic conditions during winter would likely be sufficient to mitigate acid production and metal release from acid sulfate soils and in the summer, treatment with calcium carbonate was the best mitigation method.