Chemical controls on acid discharges from acid sulfate soils under sugarcane cropping in an eastern Australian estuarine floodplain

Chemical controls on acid discharges from acid sulfate soils (ASS) were investigated in an eastern Australian estuarine floodplain cropped with sugarcane. The results show that the acid export was controlled mainly by the combined effect of soil hydrological and chemical processes. During high evapotranspiration spells, lowering of the watertable allowed air to penetrate into the upper part of sulfidic subsoils and the oxidation products of sulfides were then transferred upwards by capillary action. Since the hydraulic gradient during these periods was from drains towards soils, the upwardly moved acid materials tended to be temporarily stored in the non-sulfidic upper soil layer (jarositic layer) in both soluble and buffered forms. During the alternating low evapotranspiration spells, the acid materials accumulated in the jarositic layer were exported from the soils to the drainage system after heavy rain events. However, the amount of acid exported into the drainage system during rainfall events appears to be limited, providing a large proportion of acidity stored in the upper soil layer was in buffered forms through ion adsorption and formation of basic Fe and Al sulfates (predominantly jarosite). Release of this buffered acidity did occur but tended to be very slow under successive water extraction in the laboratory. Field observation showed that the drianwater pH of the study site hardly dropped below 3.5. Such a pH value is greater, compared to that observed from the drains in the estuarine floodplains with limited drainage (e.g.