The contribution of manganese oxides to redox buffering in selected soils of the South African Highveld

Abstract

The definition of a wetland places great importance on inundation and saturated soil conditions. The use of redoximorphic features as a soil wetness indicator is widely practiced. The reduction of Fe3+ to Fe2+, resulting in the formation of these features, is particularly diagnostic for wetland soils. The problem arises when investigating a soil that shows visible wetness but lacks these redoximorphic features. The interpretation of Fe2+ and mottle formation in soils is crucial to correctly assess the hydric status of soils, which has increased the importance of improved delineation guidelines. The study aimed to improve the parameter used to define soil wetness. Different soils where wetlands had been identified were sampled and their difference in expression of redoximorphic features was investigated. The major difference between these soils was the manganese content, with manganiferous soils lacking the expression of mottles. Results from XRF abd chemical extractions indicated that the dolomites had the highest Mn content while the quartzites and granites had negligible Mn contents. It was not the soils with lower Fe contents that lacked redoximorphic features but rather those with higher Mn activity levels than Fe. Total electron demand (TED) appeared to be a more accurate measure of electron demand in the soils contributed by Mn than manganese electron demand (MED) did. The index of redox buffering was therefore TED as this showed the best correlation with Mn content. The quartzite and granite soils with low Mn contents and low TED exhibited mottling in their morphology. The vertic soils with higher Mn and TED showed very poor mottle expression. The dolomite soils with very high Mn contents and TED values lacked the expected redoximorphic features. The andesite soils collected as a sample of intermediary Mn content greater than the quartzite and granite but less than the vertic and dolomite soils also showed an appreciable TED. Results from the study show that the dolomite derived soils, with higher Mn contents, have a much higher oxidative capacity compared to the granite and andesite soils, which lead to the conclusion that this was the main factor that prevented the expression of redoximorphic features in the selected soils investigated. It can therefore be concluded that Mn contributes to redox buffering in the soils investigated, preventing mottle expression where it is expected to occur. These results challenge previous guidelines that use redoximorphic features as signs of wetness indicators to delineate wetlands.