Consequences of Sulfate reduction in Floodplain Wetlands

Abstract

This research project has been a laboratory based experimental study on the cycling of sulfur in acid sulfate soils and underlying sediments of floodplain wetlands. Of particular focus was the effect that the reductive transformations of sulfur can have on nitrogen and phosphorus cycling. The objectives of this study were to quantify the rates at which sulfidic material forms, the controls on these rates, and to analyse the nutrient pulse from the sediments under a low redox potential. In-vitro experiments simulating inundation were conducted on sediments collected from three wetlands of the Lower River Murray. Two were salt disposal basins and another was a freshwater wetland. Experimental data collected indicates that significant amounts of nitrogen and phosphorus will be released from the sediments when inundated and anoxia is induced. The pulse of nutrients from the sediments related to the geochemistry of the sediments, as well as the hydrological regime. Partial drying and oxidation of sulfidic material results in the formation of poorly crystalline iron (oxyhydr)oxide minerals, which sorb phosphorus, with reductive dissolution following inundation liberating sorbed phosphorus. The high calcium concentrations in the salt disposal basins can potentially occlude the phosphorus through the formation of apatite minerals. The result is that phosphorus is limiting primary production in salt disposal basins and nitrogen is limiting in the freshwater systems. The pulse of nitrogen and phosphorus has the potential to increase algal growth. Consequently, the increased algal growth and subsequent availability of energy sources for microbial metabolism could increase the rate of sulfide production. This research will enable land and catchment managers to better manage sediment and water quality in the Murray-Darling Basin.