Effects of pH and ionic strength on elemental sulphur oxidation in soil

Abstract

Elemental S oxidation in soil is a microbially mediated process and is hypothesised to be influenced by changes to soil chemical properties such as acidity and ionic strength, which may arise from co-granulation with macronutrients or elemental S oxidation itself. Soil incubation was conducted with a sandy soil from South Australia to assess the effect of acidification and increased ionic strength on bacterial abundance and community composition and on elemental S oxidation during a 14-week incubation at 25 °C and 70% field capacity. Prior to incubation, the soil was treated with HNO₃ to bring the pH to 6.7–4.4 or with KH₂PO₄ to increase the ionic strength by 0–0.7 M. Elemental S was applied at 200 mg kg⁻¹ air-dried soil. Acidification or increased ionic strength had no or little effect on elemental S oxidation but decreased the abundances of 16S ribosomal deoxyribonucleic acid (rRNA) and soxB genes and changed the bacterial community composition. A second experiment with two other soils also showed that acidification did not, or only slightly, decreased elemental S oxidation, even though acidification strongly reduced 16S rRNA and soxB gene abundances in one of the soils. This study suggests that shifts in bacterial population brought about by temporary changes in pH and ionic strength, as may occur around fertiliser granules, have no or little effect on elemental S oxidation, indicating that the S-oxidising bacterial community in these agricultural soils contains functionally redundant taxa, which responded to changing conditions.