The characterisation and revegetation of acid sulfate soil scalds in Eastern Australia

Mark A Rosicky

In this thesis, 2 m cores are examined from 10 acid sulfate soil (ASS) scalds located between the Richmond catchment in the north to the Shoalhaven catchment in the south of New South Wales (NSW), Australia. At 5 of the sites, nearby permanently-vegetated paddock cores were also taken for comparison with the scalded cores. The sites were drained coastal floodplain environments, mainly used for cattle grazing. The scalds were usually ≤ 20 cm lower than their adjacent vegetated paddocks. The sites (scalded and vegetated) all had low pH values (≤ 4) in the top 1 m of soil and low chloride:sulfate ratios (< 3), because of drainage and sulfide oxidation. In most cases, significant sulfide layers (> 0.1% oxidisable S) were situated < 1 m below the soil surface. However, one ASS scald was found with no sulfide zone in the top 2 m of the soil profile. All the scalds and vegetated cores had various thicknesses of acidified, yet unoxidised, sulfidic material (up to 150 cm) increasing their environmental risk due to preferential bacterial activity and the presence of soluble oxidised iron (Fe3+). All cores had significant surface-soil sulfide concentrations, increasing their environmental risk in dry periods when surface sulfides readily oxidise and produce toxic oxidation products in the potential (or actual) root zone. Salinity at the tested sites was a constant vegetation stressor, with all in excess of the growth parameters (ECe < 4 dS/m) of the freshwater, dryland introduced vegetation planted at the sites. Half the sites were considered saline (ECe > 10 dS/m) because of connate chloride salinity, although all were being managed as freshwater environments. Soil acidity, salinity and sulfide content were always similar in scalded and adjacent vegetated cores, suggesting that more land was at risk of scalding. Indirect (usually human-induced) factors were found to largely explain the location and extent of scalds. Denudation of sensitive land, underlain by groundwater enriched with sulfide oxidation products, was likely to instigate ASS scalding. Denuding factors included: altered hydrological regimes (salinisation of freshwater sites, saltwater exclusion from saline sites); altered vegetation species (drainage and geomorphology made sites alternately too wet for dryland vegetation and too dry for wetland vegetation); stock trampling; vehicle damage; fires; frost; topsoil removal; flood scouring and surface sulfide oxidation. Revegetation trials were undertaken to test the effectiveness of low-cost, accessible management techniques. Vegetation occurrence, biomass and species were monitored and soil was tested to a depth of 10 cm (acidity, salinity, soil moisture, soluble metals). Techniques included fencing, ridging (R), liming (L) and mulching (M). An initial trial tested a combined treatment of the above-mentioned techniques (RML) compared to a control (C). The treated plots were all less saline, less acidic and moister than the control plots, with significant vegetation growth observed. A more detailed trial was conducted to examine the relative contribution of the separate elements mentioned above (Treatments: C, R, L, RL, M, RM, RML). All mulched sites had significantly more vegetation than the control, reaching 100% coverage in the RML plots. Stock exclusion alone produced minimal results. Ridging alone was counterproductive. Liming without mulching caused proliferation of an insubstantial and transient vegetation species (Isolepis inundata). Most interesting was the different vegetation species encouraged by the different mulch treatments: Treatment M was dominated by the sedge, Eleocharis acuta; Treatment RM was an even mix of Eleocharis acuta and native water-tolerant grasses (Paspalum distichum and Pseudoraphis paradoxa); Treatment RML was dominated by the above-mentioned native grasses. These grasses are highly favoured for both economic (highly palatable to stock) and environmental (thick mulch cover, self seeding) objectives. The results demonstrated that revegetation of ASS scalds was possible, and different treatments influenced vegetation composition. This study has shown that ASS scalds in NSW are highly disturbed and require sensitive management. They had re-formed surface sulfide zones underlain by acidified sub-surface sulfide layers. Drainage had discouraged wetland vegetation and introduced vegetation was unsuitable when waterlogging inevitably occurs. Surface salinity was a major vegetation constraint. Mulching was the single most important revegetation technique. Revegetation of ASS scalds was possible using an array of techniques, particularly a combination of fencing, surface disturbance, liming and mulching.

The characterisation and revegetation of acid sulfate soil scalds in Eastern Australia

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The characterisation and revegetation of acid sulfate soil scalds in Eastern Australia

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