Introduction In New South Wales (NSW), Australia, severe environmental effects occur from the drainage of potential acid sulfate soils (ASS), usually for agriculture, leading to the formation of actual ASS and severe soil and water acidification. Acidification can cause leads to release of toxic amounts of soluble metals (e.g. iron (Fe), aluminium (Al), manganese (Mn)), and mineral deficiencies (e.g. phosphorous (P), copper (Cu)) in plants.

Excess Fe retards root growth, causes mottling and drying of leaves and low yields. Fe flocs can smother aquatic plants and animals. Al phytotoxicity in plants is characterised by altered root morphology (stunted, brown tips, thickened lateral roots, absent fine roots, enhanced fungal infection). Other negative effects of pyrite oxidation include fish disease and mortality; pollution of surrounding waterways; poisoned drinking water; and corrosion of concrete and metal infrastructure.

Areas of chronically bare ground, usually on drained agricultural land, occur along the NSW coast. They have been observed in every major catchment and are associated with ASS formation. These ‘ASS scalds' are an extreme manifestation of ASS-related terrestrial ecosystem degradation, and are agriculturally unproductive and environmentally damaging.

Research A research project was developed to investigate the possibility of revegetating chronically bare ASS scalds. ASS scalds were characterized along the NSW coast, in relation to the main constraints for plant growth (sulfide content, acidity (pH), electrical conductivity (EC), constituent salts (SO4, Cl), soluble metals (Fe, Al)). Soil properties were tested to a depth of 2 m in 10 ASS scalds along the NSW coast, and at 5 of the sites in surrounding permanently-vegetated paddocks. Two field trials were established investigating potential revegetation techniques (fencing, ridging, mulching, liming, and combinations of these individual treatments). The information gathered was used to propose practices that would allow the revegetation of ASS scalds.

Results • Sulfide concentration ranged between 1% – 5%, and pH was < 4 in the top metre of the soil profile. Sulfide layers usually start between 50-100 cm below ground surface. • Proximity of the sulfide zone to the soil surface was not the primary scald-forming parameter in every case. An ASS scald was found which did not have a significant sulfidic zone within 2 m of the soil surface. At this site, sufficient toxic solutes reached the soil surface with groundwater movement from > 2 m depth.

• Larger areas are at risk from ASS scalding. Results from permanently-vegetated soil profiles all showed similarly shallow sulfidic zones, and similar soil-water chemistry, to their adjacent ASS scalds.

• Areas containing ASS scalds have acidified, yet unoxidised, sulfidic zones (ranging from 20-160 cm at different sites) which can oxidise more readily than neutral sulfidic zones.

• All tested sites (vegetated or scalded) exhibited an environmentally-significant surface sulfide layer which can readily oxidise when dried out.

• Soil salinity concentrations were an important vegetation constraint at all ASS scalds. All the tested sites were managed as freshwater vegetation systems. Yet the salinity of most surface and profile samples (scalded and vegetated) were found to be saline (ECe 4 dS/m), with 5 of the 10 ASS scalds considered very saline (ECe > 10 dS/m).

• Management practices contribute to ASS scald formation. Any disturbance or activity that leaves land denuded in areas with groundwater enriched with sulfide oxidation products and connate salinity, can instigate ASS scalding.

Drainage has caused rapidly-changing soil-moisture conditions, surface sulfide oxidation, and increased fires and frost. Other activities included excessive stock and vehicular traffic, saltwater intrusion onto freshwater areas, saltwater exclusion from formerly saline areas, deliberate topsoil removal, flood scouring and prolonged, deep, floodwater inundation which can kill both dry and wetland vegetation.

• ASS scald revegetation is possible, using targeted on-ground works. Stock exclusion alone achieved minimal results. Ridging and furrowing (surface disturbance) alone produced less vegetation response than the control plots. Liming, in the absence of mulching, encouraged small amounts of economically and environmentally inferior plant species (Isolepis inundata).

Mulching was the most important single treatment. Mulching encouraged different vegetation species responses, depending on the other treatment elements utilised. Mulching alone encouraged almost pure stands of Eleocharis acuta, a common wetland sedge. Mulching in combination with ridging and furrowing (surface disturbance) produced a more even mix of Eleocharis acuta and native wetland grasses (Paspalum distichum, Pseudoraphis paradoxa). Mulching in combination with ridging, furrowing and liming produced almost pure stands of the native wetland grasses mentioned above.

Conclusion Areas with ASS scalds will always require sensitive land management. However, in most cases, primary production (mainly cattle grazing) can continue with positive environmental outcomes. Watertables should be kept above sub-surface sulfide layers. Drainage should allow surface water to be shed, but not drain the soil profile. Native pasture grasses and wetland plants should be encouraged, which are able to cope with the generally acidic, intermittently waterlogged nature of these landscapes.