Impact of common reed and complex organic matter on the chemistry of acid sulfate soils

Abstract

Acid sulfate soils (ASS) are naturally occurring soils or sediments formed under reducing conditions that either contain sulfuric acid or have the potentail to form it, in an amount that can have adverse imapcts on the environment. The negative impacts of ASS are associated with the release of acidity produced and the release of toxic metals and metaloids from solubulised soil matricies into the environment. It has been shown recently that addition to ASS of dead plant material as organic matter creates microenvironments for soil microbes to ameliorate sulfuric soil and prevent sulfidic soil oxidation. Initial breakdown of the organic matter results in an oxygen demand that generates anaerobic conditions conducive to the reduction of sulfate to sulfides by sulfate reducing bacteria using the residual organic material as a carbon source and causing the pH to rise. There is also evidence that live plants increase acidification, potentially by aerating the soil. In nature, plants shed dead material as they grow, so that both live and dead organic matter co-exist. It is not known what happens to ASS chemistry, particularly pH, under such natural conditions. In this study, Phragmites australis was used to examine the combined effect of growing plants and incorporated organic matter on ASS chemistry (pH, redox potential and sulfate content) under aerobic and anaerobic conditions. In almost all cases, live plants enhanced sulfuric soil acidity and sulfidic soil oxidation. The mechanism for these changes on ASS chemistry appears to be the facilitation of oxygen penetration into the soil via aerenchymatous tissues in the plant roots.