Restoring ecological properties of acidic soils contaminated with elemental sulfur

Elemental sulfur (S⁰) accumulates in the environment from anthropogenic sources as a byproduct from oil and gas refining and from trap and skeet shooting targets. Bacteria can oxidize S⁰ to H₂SO₄, which acidifies soil. We explored whether combinations of soil amendments can be used to remediate acidic soils contaminated with S⁰ by restoring soil chemistry, plant growth, and bacterial communities in a greenhouse. Results were compared to a contamination gradient in a field that had been limed with CaMg(CO₃)₂ two years prior. Amendments in the greenhouse included CaCO₃ by itself, and in combination with fertilizer, compost, biochar, and chitin. Amended soils were incubated for one week and half of all containers were planted with Poa nevadensis. We sequenced bacterial DNA from a subset of amended soils and along the field gradient. CaCO₃ additions in the greenhouse initially raised the pH of contaminated soil to values found in uncontaminated soils. However, pH decreased over time, which was likely caused by the oxidation of S⁰ to H₂SO₄. This was also apparent in the field, where CaCO₃ additions raised pH to 4 but not to the desired value of 5 or higher. Plants in the greenhouse failed to grow in the unamended contaminated soil, but CaCO₃ alone reduced concentrations of toxic cations and resulted in more plant growth than in the uncontaminated soil. CaCO₃ also partially restored the bacterial communities in the greenhouse and in the field by increasing richness and diversity to near values found in uncontaminated soil, suggesting that bacteria can be resilient to prolonged acidic conditions. Organic amendments did not provide a significant benefit to restoration. This study demonstrates that acid neutralization alone can restore abiotic and biotic components and productivity of soils contaminated with S⁰, but multiple CaCO₃ applications may be required to avoid future acidification.