Changes in surface water quality after inundation of acid sulfate soils of different vegetation cover

Abstract

Surface soils from an acid sulfate soil (ASS) backswamp were inundated in a temperature controlled environment and surface-water chemistry changes monitored. The soils had contrasting in situ vegetative cover [i.e. 2 grass species, Cynodon dactylon and Pennisetum clandestinum (Poaceae), and litter from Melaleuca quinquenervia (Myrtaceae)]. The different vegetation types had similar biomass and carbon content; however, there were large differences in the quality and lability of that carbon, which strongly influenced decay/redox processes and the chemical composition of surface waters. The grass species had more labile carbon. Their surface waters displayed rapid sustained O2 depletion and sustained low Eh (~0 mV), high dissolved organic carbon (DOC), and moderate pH (5–6). Their soil acidity was partially neutralised, sulfides were re-formed, and reductive dissolution of Fe(III) led to the generation of stored acidity in the water column as Fe2+(aq). In contrast, M. quinquenervia litter was high in decay-resistant compounds. Its surface waters had lower DOC and low pH (<4) and only underwent a short period of low O2/Eh. Soluble Al caused M. quinquenervia surface waters to have higher titratable acidity and soil pH remained consistently low (~3.8–4.0). Concentrations of Cl– and Al in surface waters were strongly correlated to initial soil contents, whereas the behaviour of Fe and SO42– varied according to pH and redox status. This study demonstrates that changes in vegetation communities in ASS backswamps that substantially alter either (a) the pool of labile vegetative organic carbon or (b) the concentration of acidic solutes in surface soil can have profound implications for the chemical characteristics of backswamp surface waters.