Effect of varying degrees of water saturation on redox conditions in a yellow brown apedal B soil horizon
Various studies have been conducted into redox potential (Eh), redox indicators and the measured soil water contents in soil (Franzmeier et al., 1983; Schwertmann & Fanning, 1976; Veneman et al., 1976). Although a measure of success has come from these studies, there are still vast knowledge gaps within this field. The degree of water saturation where reduction in the soil is initiated cannot be determined from literature, although it was approximated that 70% of water saturation (S0.7) was sufficient to initiate reduction (Van Huyssteen et al., 2005). This value will vary for different soil temperatures, varying bulk densities as well as soils with different organic matter contents. This study aimed to determine if it was possible to identify a degree of water saturation at which reduction is initiated for a soil in a closed system. It also aimed to determine the effect of bulk density on reduction. Reduction was defined by a decrease in pe (Eh) of a soil and an increase in the soluble Fe2+ concentration. There were three key aims to the study: to establish the relationship between the degree of water saturation (s) and the onset of reduction; to establish the relationship between the degree of water saturation (s) and the duration of reduction and to establish the effect of bulk density on the above-mentioned processes. A yellow brown apedal B horizon from an Avalon soil form (profile 234) in the Weatherley catchment was used in this study. A soil core experiment was carried out to determine the effect of degree and duration of water saturation on Eh, pH, Fe2+, Mn2+, Ca2+, Mg2+, K+, and Na+. Soil cores were packed to a bulk density of 1.6 Mg m-3 and individually saturated to S0.6 (60% of the pores saturated with water), S0.7 (70% of the pores saturated with water), S0.8 (80% of the pores saturated with water), and S0.9 (90% of the pores saturated with water). Measurements were done in triplicate. The cores were sealed with a double layer of plastic wrap and stored in a laboratory at 23°C until needed. Analysis started three days after initial water saturation. A set of cores (four degrees of saturation with triplicates of each) was analysed every 3.5 days for the first three months after which a set was analysed once a week for the remaining month of analyses. The experiment was terminated after 121 days. The same soil and experimental setup was used for the bulk density experiment. The experiment consisted of a set of three cores packed to an initial bulk density of 1.4, 1.6 and 1.8 Mg m-3. The cores were all saturated to S0.8, each packed in triplicate. The bulk density experiment was terminated after 23 days. There was a good correlation between an increase in degree of water saturation and pe (R2 = 0.95); Mn2+ (R2 = 0.91) and Fe2+ (R2 = 0.92) concentrations. Eh, pH, Fe2+, Mn2+, Ca2+, Mg2+, K+, and Na+ were significantly affected by duration of water saturation and all except Ca2+ and K+ significantly affected by degree of water saturation. Fe2+ and Mn2+ accumulations and depletions (visible segregations or mottles) occurred within 12 months of water saturation in a separate experiment where cores were packed to a bulk density of 1.6 Mg m-3 in a core saturated to S0.9. It was therefore evident that this soil with 0.22% organic carbon and a bulk density of 1.6 Mg m-3 will produce morphological features due to reduction within a year of water saturation at S0.9. An experiment was set up with cores kept at a constant degree of water saturation (S0.8) with varying bulk densities, namely 1.4, 1.6 and 1.8 Mg m-3. All the factors measured (Eh, pH, Fe2+, Mn2+, Mg2+ and K+) except Ca2+ and Na+ were significantly affected by a variation in bulk density. In another part of the experiment two different water temperatures were used to saturated the cores, namely 23°C and 30°C respectively. It was determined that the temperature difference of 7°C caused the cores to react significantly different to each other.. The higher water temperature caused the Eh to decrease more rapidly and therefore a higher Fe2+ concentration occurred in these cores. It was concluded that for this soil at 23°C, Fe3+ and to a certain extent Mn4+ will start to become reduced at a pe of 6 at S0.78. These findings show that the first approximation of Van Huyssteen et al. (2005) where S0.7 was found to be sufficient for reduction is very similar for this soil.