Interactions between organic matter and iron oxyhydroxysulfates / iron sulfides during remediation of acid sulfate soils

• Applicant: Dr. Angelika Kölbl
• Subject Area: Soil Sciences
• Term: from 2015 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 274961325

Acid sulfate soils are soils containing iron sulfides. They are widespread throughout the world in coastal and inland areas, particularly in Southern Australia. When acid sulfate soils with hypersulfidic material dry, oxidation of pyrite cause strong acidification (pH <4). Re-saturation of acid sulfate soils can lead to re-formation of pyrite and pH increase due to activity of sulfate reducing bacteria, which also require available organic carbon (OC). However, low availability and/or low biodegradability of OC may limit the activity of sulfate reducers in re-saturated sulfuric material. During the first phase of the project, contents and composition of OC of several acid sulfate soils were investigated, with specific emphasis on the proportion of readily available, non-mineral-associated OC. It was shown that high additions of undecomposed plant residues (50% of native OC) with high proportions of polysaccharides resulted in fast and long-lasting remediation success of a clayey as well as of a sandy acid sulfate soil when soil pH was pre-adjusted to approx. 5. The remediation success includes the removal of Fe and S from the soil solution, which indicates the formation of Fe sulfides. Part of the added OC became associated to the minerals and is likely no longer available to microbes. However, it needs to be investigated which type of minerals are involved in this new formation of mineral-organic complexes under strongly reducing conditions.The overall objective of the renewal proposal is to identify the interaction between different OC sources (plant residues and simple organic molecules like lactate) and the soil mineral assemblage during remediation of acid sulfate soils. The analyses will focus on anoxic laboratory incubation experiments. Particular attention will be paid to the transformation of iron sulfates (jarosite, schwertmannite) into iron sulfides (pyrite) and the formation of mineral-organic complexes after establishing anoxic conditions. It will be tested to which extend Fe-OC associations are formed, and to which extend other mineral-organic associations (Al-OC, clay mineral-OC) are responsible for OC storage in acid sulfate soils. Therefore, a sample set of artificial acid sulfate soils using synthesized or purchased well-defined minerals will be included in the incubation experiment, and supplied with of 13C labeled organic substrates. Changes of the mineral assemblage and concurrent changes in amount and composition of OC will be investigated several times during the incubation experiment with repeated oxic-anoxic cycles. By combining techniques to unravel changes of the specific mineral composition (XRD, Mössbauer) with techniques to quantify and to qualify the fate of OC (13C labeling, solid-state 13C-NMR), it will be possible to identify what kind of mineral-organic associations are formed during the reduction process, and how different OC sources trigger these processes.

• DFG Programme: Research Grants