Soil properties in and around acid sulfate soil scalds in the coastal floodplains of New South Wales, Australia

Abstract

Soil profiles in 10 persistently bare areas (i.e. scalds), mainly located in coastal backswamps of New South Wales, Australia, were examined for chromium-reducible sulfur content and selected chemical properties. At 5 of the sites, the adjacent paddocks with vegetation cover were also examined. All of the tested sites had been affected by the extensive drainage of the surrounding acid sulfate soil (ASS) landscapes and the consequent oxidation of pyrite. All sites had low pH values in the surface soil layers and these low pH values extended for up to 150 cm into the underlying unoxidised blue/grey pyritic estuarine gels. This can be attributed to the downward diffusion of acidity, either produced in the overlying oxidised zones of these soils or transported laterally across the landscape to these low-lying areas. Acidified unoxidised pyritic zones 120 cm thick can evidently form within several decades after drainage disturbance. At the scalded sites the depth from the soil surface to the main pyritic zone varied from the surface to >200 cm depth, indicating that this variable is not critical to ASS scald formation. For most of the sites examined, the chromium-reducible sulfur contents in the surface soil layers were appreciably higher than those in the immediately underlying soil layers. In most of the vegetated sites the chromium-reducible sulfur content in the surface layers was considerably higher than for the adjacent scalded site. The conditions necessary for pyrite formation (i.e. adequate supplies of organic matter, soluble iron, sulfate, and waterlogging) were found to exist at all sites, and the pyrite accumulations in these surface soil layers are considered to be neo-formed. The vegetated soil-profile pyrite and pH results were very similar to their scalded counterparts except that they had an extra 20–40 cm layer of vegetation and mulch that was missing from the scalded profiles. This indicates that there is considerable potential for more extensive scalding in these ASS areas.

Contemporary pedogenesis of severely degraded tropical acid sulfate soils after introduction of regular tidal inundation

“…Pyrite formation can be rapid in natural inter-tidal environments (Howarth, 1979). Both pyrite and monosulfides are known to reform in CLASS landscapes due to seasonal shifts in hydrology or the formation of localised, highly reducing sub-environments Rosicky et al, 2004;Burton et al, 2006Burton et al, , 2007. Portnoy and Giblin (1997a) demonstrated that saturating a drained and acidified former saltmarsh with seawater stimulated both Fe(III) and SO 4 2− reduction.…”

Seawater inundation of coastal floodplain sediments: Short-term changes in surface water and sediment geochemistry

“…Extensive scalded areas with low pH commonly occur on these coastal floodplains after extended dry periods and hydrous Fe oxides and oxyhydroxides can accumulate rapidly via upward evaporative flux of metal-bearing porewaters (Rosicky et al, 2004). These scalded areas can also accumulate high concentrations of soluble and exchangeable trace metals and can potentially act as a large source to overlying waters following seawater inundation, as demonstrated in this study.…”

“…Rosicky et al, 2004). While the areal extent of these scalded areas may be small, the results indicate that these areas can potentially contribute high concentrations of acidity and trace metals to overlying surface waters when inundated.…”

“…Conversely, there were no significant differences between treatments in surface water pH at the scald site. This is largely due to dissolution of oxidation products which have formed on the surface of these persistently bare areas which are highly acidic (Rosicky et al, 2004). Mobilisation of acidic metal cations and hydrolysis can act to further release acidic cations in to solution and thereby increase acidity and decrease pH in the overlying waters.…”

“…These drainage networks enhance oxidation of previously reduced sulfidic sediments which leads to degradation of soil and water as well as episodic acute and chronic transport of acidity and associated trace metals into adjacent estuarine receiving waters ( van Breemen, 1975;Sammut et al, 1996;Johnston et al, 2004). In extreme cases, scalding of the soil surface occurs, resulting in bare areas without surface vegetation, very low pH and surface accumulations of Fe(III) minerals (Rosicky et al, 2004;Sullivan and Bush, 2004).…”