Linking organic matter composition in acid sulfate soils to pH recovery after re-submerging

When acid sulfate soils containing hypersulfidic material (pH > 4) dry, oxidation of pyrite causes strong acidification with the formation of sulfuric material (pH < 4), which may release high concentrations of metals and metalloids. Re-submerging of sulfuric material can lead to re-formation of pyrite and pH increase to re-form hypersulfidic and hyposulfidic materials due to the action of sulfate-reducing bacteria. However, low availability and/or low biodegradability of organic carbon (OC) may limit the activity of sulfate reducers in re-saturated sulfuric material. Our study investigated the content and composition of OC with specific emphasis on the proportion of readily available, non mineral-associated OC. Samples were taken from a non-acidifying pasture topsoil with hyposulfidic material and two re-submerged subsoils with hypersulfidic material derived from river sediments in South Australia. The sites experienced drying at depths between 0.5 and 4.5 m with severe acidification (pH < 4) during the Millennium drought from 2007 to early 2010. After re-submerging, sulfuric material at one site recovered to neutral pH values, whereas the other site remained acidic. Samples were analysed for total OC content and the proportion of available, non mineral-associated OC. Chemical composition of bulk soil OC and available fractions was determined by solid-state ¹³C NMR spectroscopy and neutral sugar analyses. The OC composition of re-submerged sulfuric material was generally characterised by small proportions of easily degradable carbohydrates and proteins, but high proportions of hardly degradable lignin and lipids. Lowest amounts of available OC fractions and lowest proportions of carbohydrates and proteins were found in hypersulfidic material which is still acidic. This indicates that slow pH recovery rates can be ascribed to low proportions of biodegradable OC. The OC composition can be explained by: (I) sedimentation of organic materials which were already highly biodegraded during formation of river sediments, and (II) selective preservation of lignin and lipids due to permanent waterlogging. Thus, the organic material is characteristic for wetlands, but hardly usable as substrate for microbes and may retard sulfate reduction and pH neutralisation of re-submerged sulfuric material.