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 inv estigated in an eastern Australian estuarine floodplain cropped with sugarc ane. The results show that the acid export was controlled mainly by the com bined effect of soil hydrological and chemical processes. During high evapo transpiration spells, lowering of the watertable allowed air to penetrate i nto the upper part of sulfidic subsoils and the oxidation products of sulfi des were then transferred upwards by capillary action. Since the hydraulic gradient during these periods was from drains towards soils, the upwardly m oved acid materials tended to be temporarily stored in the non-sulfidic upp er soil layer (jarositic layer) in both soluble and buffered forms. During the alternating low evapotranspiration spells, the acid materials accumulat ed in the jarositic layer were exported from the soils to the drainage syst em after heavy rain events. However, the amount of acid exported into the d rainage system during rainfall events appears to be limited, providing a. l arge proportion of acidity stored in the upper soil layer was in buffered f orms through ion adsorption and formation of basic Fe and Al sulfates (pred ominantly 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 d rains in the estuarine floodplains with limited drainage (e.g. Sammut et al ., 1996. Marine Freshwater Research 47, 669-684.). Under sugarcane farming conditions in the study site, the intensive drainage has intensified soil a cidification by increasing the frequency and magnitude of low watertable co nditions which enhanced the oxidation of sulfides in the subsoils, but at t he same time, the creation of highly oxidized conditions in the upper soil layer due to this artificial drainage also caused the accumulation of jaros ite which appears to have an important effect on preventing more extreme ac idification from occurring in the soil-drainage system. (C) 1998 Elsevier S cience Ltd. All rights reserved.