Addition of organic material to sulfuric soil can reduce leaching of protons, iron and aluminium

•Organic materials were mixed into sulfuric soil or placed under the soil in a layer.•Soils were leached four times with reverse osmosis water.•Concentrations of protons, Fe and Al were reduced by up to 90% by organic materials.•Biochars from eucalyptus and wheat were most effective.

Oxidised acid sulfate soils (ASS) with sulfuric horizons (sulfuric soils) can release large amounts of acid and metals, which can be rapidly mobilised when the soils are rewetted or re-flooded. Under flooded conditions, addition of organic materials to ASS has been shown to stimulate microbial sulfate reduction, which results in pH increase. But little is known about the potential of organic materials to reduce leaching of protons and metals from sulfuric soils and how this is influenced by properties of the organic materials and amendment form. Sulfuric material (pH 3.5) was collected from a coastal oxidised ASS at Gillman in the Barker Inlet, South Australia. Eight organic materials (compost, two plant residues and five biochars differing in feed stock and pyrolysis temperature) were applied at 15 g C kg− 1 in two forms: mixed into or placed as a layer under the sulfuric soil. Twenty grammes of soil (dry weight equivalent) was placed in PVC cores, and the cores were leached four times with 45 ml of reverse osmosis water. In the unamended soil, 70–90% of the total leached protons, Fe and Al were released in the first leaching event with only small amounts being released in the three subsequent leachings. In amended treatments release of protons Fe and Al was lower than in unamended soil, the decrease is referred to as retention. The amount of protons, Fe and Al retained in the amended soil compared to the control was highest in the first leaching. Cumulative retention of protons Fe and Al was highest in the soil amended with eucalypt biochar produced at 550 °C and wheat biochar produced at 450 °C, but low in wheat straw and compost. Leachate pH of the unamended soil was 3.5–4, but up to 6.4 in amended soils. In amended soils, proton retention was positively correlated with C concentration of the materials, while Fe and Al retention was positively correlated with percentage of Aryl and O-Aryl groups and negatively correlated with percentage O-Alkyl, Di-O-Alkyl groups. Generally retention of Fe and Al was greater when organic materials were mixed in the soil than if placed as a layer underneath the soil, but there was little difference between amendment forms in proton retention. We conclude that mixing some biochars, particularly eucalypt biochar produced at 550 °C and wheat biochar produced at 450 °C, can strongly reduce leaching of protons and metals in sulfuric soils whereas wheat straw or compost are less effective.