When acid sulfate soils with hypersulfidic material dry, oxidation of pyrite can cause strong acidification due to formation of sulfuric (pH < 4) material. Re-saturation of acid sulfate soils containing sulfuric material can lead to re-formation of pyrite and pH increase through activity of sulfate reducing bacteria, which require available organic carbon (OC). In the Lower River Murray region in Australia, a clayey acid sulfate soil had acidified during the severe “Millennium” drought between 2007 and early 2010. We investigated why it has not recovered for over a decade after being reflooded. We hypothesized that the low quality and availability of OC limits the activity of sulfate reducing bacteria.A long-term anoxic incubation experiment was conducted to test if OC additions can help to overcome OC limitation. Small-scale incubation vessels were used, allowing investigating general biochemical phenomena under controlled laboratory conditions. Pre-incubated acid sulfate soil with sulfuric material (approx. pH 3.5) was submerged and pre-adjusted to pH 5.0. We used different rates of wheat straw and cattle manure application to test different organic matter quantities and qualities. Both substrates were added in two portions, at the beginning of the experiment and after 190 days. With every addition, we added two different amounts of organic matter (2 mg g−1 soil and 9 mg g−1 soil), equivalent to approx. 10% and 50% of the native soil OC content. A control treatment without OC addition was also included. CO2 production as well as redox and pH values were monitored weekly over a year. At the start and the end of incubation, we determined OC concentrations and the proportion of available, non mineral-associated OC. OC composition was analyzed by solid-state 13C NMR spectroscopy to assess its chemical degradation.The pH values increased rapidly in treatments with high OC supply, reaching pH ≥ 6.0 within 3 weeks after the second OC addition. Treatments with low OC additions showed slower pH increases, reaching values between pH 5.5 and 6.0 after one year. The control treatment had pH values <5.0 at the end of the experiment. After one year of anoxic incubation, the control treatment lost 10% of the native OC. Treatments with OC additions lost between 13 and 19% of total OC (native + added OC), with higher percentage loss in treatments with high OC additions.Highest losses were observed for the non mineral-associated OC fraction (up to 69% in treatments with high OC additions), with up to 20% being converted to the mineral-associated OC. OC composition changed little compared to the start of the experiment, showing slightly reduced proportions of carbohydrates (≤10% loss) and slightly higher proportions of lipids and lignin.Best remediation success was achieved by adding 50% of the native soil OC as wheat straw, resulting in fast pH neutralization, strongly reducing conditions, and decreased sulfur and iron concentrations in the soil solution. However, the amount of bioavailable OC was reduced to one third after the incubation period. Repeated OC addition is therefore recommended to keep the total amount of bioavailable, undecomposed plant residues high and to ensure long-term remediation success.
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