Effect of synthetic amorphous and crystalline iron oxides on redox potential and exchangeable cations in wetland soils of Maputaland Kwazulu-Natal, South Africa

Abstract

Some grey-coloured soils of the Maputaland Coastal Plains in northern KwaZulu-Natal do not exhibit the redox morphology normally associated with seasonal water saturation, resulting in serious limitations in using soil morphology for wetland delineation. One of the hypotheses to explain this phenomenon is that these soils simply do not contain enough iron minerals that can be reduced and re-oxidised to form redox accumulations and depletions. Alternatively, if iron minerals are present, they are too crystalline to be readily reduced as a result of the lower iron activity associated with these oxides and can only be reduced at a slow rate. This investigation aimed to determine if a deficiency in iron oxides could explain the absence of redoximorphic features in the wetland soils. A split-plot experimental design with 78 cores was set up to investigate the effect that varying iron crystallinity had on soil redox potential. The experiment was conducted in a controlled environment in the laboratory with the temperature set at 23°C with fluctuations of ±2°C. Synthetic ferrihydrite and goethite were mixed with soils (0.84 g.kg −1 ) and packed to a bulk density of 1.4 Mg m −3 . The cores were then saturated to 73% of porosity and the pH, Eh, Ca, Mg, Fe, and Mn measured bi-weekly. The ferrihydrite and goethite treatments did not affect ammonium acetate extractable Ca and Mg. The treatments did, however, increase significantly the ammonium acetate extractable Fe, Mn, and pH, while both treatments significantly decreased pe over the 42 day extraction period. The goethite treatment reduced within five days, while ferrihydrite only reduced after 20 days. The Fe o /Fe d ratios for the treatments suggested that ferrihydrite started as less crystalline and increased sharply after saturation, while the crystallinity of the goethite treatment only increased slightly. Results presented here indicate that the absence of redoximorphic features in the soils of Manguzi could be attributed to the low iron oxide content of the soils. Other indicators, such as vegetation or long-term water measurements, therefore need to be engaged to delineate wetlands in the soils of the Maputaland Coastal Plain.