Soil hydrology and hydric soil indicators of the Bokong wetlands in Lesotho

[摘要] Wetland hydrology controls the function of the wetland ecosystem and hence it is theprincipal parameter for delineation and management of wetlands. It is defined as the watertable depth, duration, and frequency required for an area to develop anaerobic conditions inthe upper part of the soil profile leading to the formation of iron and manganese based soilfeatures called redoximorphic features. The redoximorphic features must occur at specificdepths in the soil profile with specific thickness and abundance to qualify for a hydric soilindicators. Therefore, hydric soil indicators are used to evaluate the wetland hydrology ifsuch a relationship has been verified. The aims of the study were i) to determine soilvariation and hydric soils indicators along a toposequence, ii) to determine the relationshipsbetween soil water saturation, redox potential and hydric soil morphological properties andiii) to determine the distribution of soil properties and accumulation of soil organic carbon inhydric and non-hydric soils.The study was conducted at the upper head-water catchment of the Bokong wetlands in theMaloti/Drakensberg Mountains, Lesotho. The soil temperature ranged between -10 and23°C. The soils had a melanic A overlying an unspecified material with or without signs ofwetness, or a G horizon. The organic O occurred in small area. Soil profiles were dug alonga toposequence and described to the depth of 1000 mm or shallower if bedrock wasencountered. Redoximorphic features were described using standard soil surveyabundance categories. Soil samples were collected from each horizon and analysed forselected physical and chemical soil properties.The soils had low bulk density ranging from 0.26 in the topsoil to 1.1 Mg m-3 in the subsoil.Significantly low bulk density was observed in the valleys and highest bulk densities wereobserved on the summits. The soil organic carbon content ranged between 0.18% in thesubsoil and 14.9% in the topsoil. The soil also had a high dithionite extractable Fe (mean93±53 g kg-1) and low CEC (mean 26±9 cmolc kg-1). Soil pH and CEC were relatively lowerin the valleys and higher on the summits. Principal component analysis indicated fourprincipal components accounted for 60% of the total variance. The first principal componentthat contributed 23% of the variation showed high coefficients for soil properties related toorganic matter turnover, the second components were related to inherent fertility, the thirdand fourth were related to acidity and textural variation.Hydric indicators identified in Bokong were histisols (A1), histic epipedon (A2), thick darksurfaces (A12), redox dark surfaces (F6), depleted dark surfaces (F7), redox depressions(F8), loamy gleyed matrix (F2) and umbric surfaces (F13). The thick dark surfaces withmany prominent depletions and gley matrix (A12 and F7) occurred in the valleys, while themidslopes and footslopes were dominated by umbric surfaces (A13). The indicators F6, F7and F8 were not common. Indicators that were related to the peat formation (A1, A2 andF13) were frequently observed.The relationship between soil water saturation and redoximorphic features was verified bymonitoring the groundwater table with piezometers, installed in ten representative wetlandsat depths of 50, 250, 500, 750, and 1000 mm for two years from September 2009 to August2011. Redoximorphic feature abundance categories were converted into indices. Strongcorrelations were observed between redoximorphic indices and cumulative saturationpercentage. The depth to chroma 3 and 4 (d_34) and depth to the gley matrix (d_gley)correlations were R2 = 0.77 and R2 = 74 respectively. All redoximorphic indices were poorlycorrelated with average seasonal high water table. Strong correlation were also observedbetween profile darkening index (PDI) and cumulative saturation (R² = 0.88) and weakcorrelations were observed between PDI and average seasonal high water table (R² = 0.63).A paired t test indicated that soil pH, exchangeable Mg and Na, dithionite extractable Fe andAl were significantly different between hydric and non-hydric soils. Hydric soils hadsignificantly higher Mg, Na and Fe content, and significantly low soil pH and Al content.Generally it appeared that soluble phosphorus, Fe and exchangeable bases accumulated inhydric soils, while the soil pH and Al content decreased. The mean soil organic carboncontents were 3.61% in hydric soils and 3.38% in non-hydric soils. However, non-hydric soilrelatively stored more organic carbon (174.4 Mg C ha-1) than hydric soils (155.1 Mg C ha-1).The mean soil organic carbon density of the study area was 166±78.3 Mg C ha-1) and theestimated carbon stored was 21619 Mg C (0.022 Tg C; 1Tg = 1012g) within the 1000 mm soildepth. About 384.9 Mg C was stored in the hydric soils within the study area, which wasabout 1.9% of the total carbon stored in the area to the bedrok or depth of 1000 mm. Amongthe wetland types, bogs had significantly higher organic carbon levels (6.17%) and storedsignificantly higher carbon (179 Mg C ha-1) with at least 44% was store in the A1 horizon.It was concluded that the strong correlation observed between PDI, d_34, d_gley andcumulative saturation representing hydric indicators such as histisols (A1), histic epipedon(A2), umbric surfaces (F13), loamy gleyed matrix (F2) can be used to determine the durationand frequency of the water table in the landscape studied. These hydric indicators can beused to delineate wetlands, however, more indicators can be developed.

[发布机构] University of the Free State