Department of Soil Science

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Browsing Department of Soil Science by browse.metadata.advisor "Francis, Michele Louise"

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    Carbon characteristics and the effect of terrain and cultivation on topsoil carbon and salts of the West Coast heuweltjies
    (Stellenbosch : Stellenbosch University, 2023-03) Sakala, Brian Josat; Clarke, Catherine E; Francis, Michele Louise ; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
    ENGLISH ABSTRACT: The 21ˢᵗ century has been marked by global increases in atmospheric carbon dioxide concentrations from annual emissions from land-use conversion and soil cultivation. In addition, the issue of increased salinization of soils is another problem that poses a threat to agricultural production and environmental health. Large (>20 m diameter) regularly spaced earthen mounds called “heuweltjies” (Afrikaans for “little hills”) occur widely in the West Coast region of South Africa with heights between 1 to 2.5 m. These mounds are characterised by significantly higher carbon and salt contents compared to surrounding soils. Therefore, this study was conducted to determine the total carbon characteristics and the effect of terrain and cultivation on topsoil carbon and salts of the West Coast heuweltjies. Firstly, the total carbon characteristics of three previously excavated heuweltjies in three differing rainfall environments in the Buffels river (mean annual rainfall <100 mm/year), Klawer (mean annual rainfall between 100-200 mm/year) and Piketberg (mean annual rainfall between 300- 400 mm/year), were investigated. The analyses focused on soil bulk density, total carbon (total C), and soil organic carbon (SOC) content as well as carbon stocks. Both rainfall and termite foraging activities affected the horizontal and vertical distribution of carbon in all three areas. The vertical distribution of SOC to soil inorganic carbon (SIC) were inversely related, with SOC higher in the upper horizons and SIC increasing with depth. The influence of termite foraging activities on the depth distribution of SOC was observed in the subsoils of the heuweltjies, which was unlike the usual exponential decrease in SOC with depth in a normal soil profile. The subsoils showed localised elevated contents of SOC with depth, attributed to translocation of organic debris by termites and burrowing animals in the mounds. This signified the importance of subsoil studies in accounting for C stocks in termite affected landscapes. Further, the contribution of heuweltjies to total C and SOC stocks was calculated based on the area-weighted land coverage of the mound and inter-mound soils of the Klawer heuweltjie. The amount of total C stored was found to be 14.6 Mg C per mound and SOC was estimated to be 1.1 Mg C per mound. Thus, for the Klawer region, the total carbon stored per hectare is 102 Mg and 7.5 Mg of SOC, suggesting heuweltjies are an important store of atmospheric carbon in global carbon sequestration processes. Secondly, the effects of cultivation on topsoil pH, salinity and carbon were investigated across two rainfall zones in Trawal (mean annual rainfall between 50-200 mm/year) and Koringberg (mean annual rainfall between 300-400 mm/year). The analyses focused on pH, EC, and carbon content in the top 0-10 cm and 0-20 cm at the two study sites, respectively. Adjacent heuweltjies in cultivated and uncultivated land were selected for comparisons. Generally, differences occurred between land use and sampling position. Cultivation practices resulted in an increase in pH of the inter-mound soils by approximately 1.3 units in comparison to the inter-mound soils of uncultivated land. The impact of cultivation on salinity did not provide enough evidence to suggest a lateral distribution of salts within the landscape. The salts were generally higher on than off the mound soils at both study sites, a result consistent in this study. The study also showed that carbon content was negatively affected by cultivation, resulting in approximately 50% loss of total C, SOC, and C stocks in cultivated land. In addition, a lateral uniform distribution of carbon was observed on and off the heuweltjies in cultivated land, suggesting a reduction in termite driven SOM inputs in the mound soils. The carbon stocks in uncultivated soils were found to be generally higher on than off the mounds following the order moderate to low rainfall site. Finally, the effect of terrain on salt load, salt type and distribution of carbon content on and off the heuweltjies across a cultivated hillslope was investigated. The results for the spatial distribution of carbon on and off the heuweltjies showed an inconsistent trend of either higher or lower carbon content with distance downslope. This unclear pattern did not provide enough evidence to support the hypothesis of a reduction in the carbon content on the mounds from crest to footslope. The spatial distribution of salts showed a trend of higher salts on mounds in comparison to inter-mound soils reaffirming earlier results suggesting that cultivation does not have a major influence on salt movement across the landscape. This trend did not support lateral movement of salts downslope. Furthermore, based on the comparison of the heuweltjie salt signature to that of the seawater dilution line, and despite many decades of cultivation and reduced termite activity, the heuweltjies still maintained a marine signature, adding more evidence to the hypothesis suggesting that the salts are derived from marine deposited aerosols. Concentrated hotspots of ions and minerals within the biogenic features of mound soils suggested that burrowing fauna are responsible for accumulating marine-derived salts in heuweltjie soils. The results of this study, based on the difference in SOC concentrations of the heuweltjies in cultivated and uncultivated land, have shown that heuweltjies are an important store of carbon, which when cultivated become a source of atmospheric carbon dioxide (CO2). In addition, the results presented are relevant for land-use change and management strategies that promote carbon storage in drylands. Also, the results showed that tillage has no major influence on salt distribution across the landscape, or that shallow lateral flow is an important contributor to dryland salinity.
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    Structure and chemistry of two heuweltjies in areas with contrasting aridity in the Olifants/Doorn catchment: evidence for downward salt movement
    (Stellenbosch : Stellenbosch University, 2022-04) Hattingh, Magdaleen; Clarke, Catherine E.; Francis, Michele Louise; Miller, Jodie A.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
    ENGLISH ABSTRACT: The Olifants/Doorn catchment in the West Coast region of South Africa is variably affected by saline groundwater. Other areas along the coast of southern Africa experience similar mean annual precipitation rates but do not display the same variability in saline groundwater. This suggests additional contributions to groundwater salinisation in the West Coast. Regularly spaced earthen mounds, termed heuweltjies (up to 2 m high and about 32 m wide), occur in abundance in the Olifants/Doorn catchment area. These mounds are characterised by significantly higher salinity levels compared to surrounding soils. Variable saline groundwater seems to spatially correspond with salt affected heuweltjies, suggesting that mound salts might be contributing to groundwater salinisation in the region. Two heuweltjies, one in a semi-arid climate (Klawer) and the another in a more Mediterranean climate (Piketberg), were excavated to determine the morphological properties and distribution of soluble salts and ions within the mounds. The study was conducted to determine and compare if salts in heuweltjies with different mean annual precipitation rates could be contributing to groundwater salinisation of the Olifants/Doorn catchment. The mineralogy, soil texture, electrical conductivity, pH, anion and cation profiles and dissolved silica was analysed to determine if these heuweltjies are potentially contributing to the groundwater chemistry. Dominant salts exclusively present in heuweltjie soils were identified in both the mineralogy and modelling of ion concentrations. Calcite (in both heuweltjies) and gypsum (in the mound in Klawer) were identified to be enriched in mound soils. The less soluble calcite was saturated at closer to the surface compared to more soluble gypsum at greater depths in both mounds. This sequence of precipitation of increasing soluble salts suggested that the net direction of water movement occurs downward in mounds of both high rainfall and lower rainfall areas. The clay mineralogy did not support the parent material as a provenance. Additionally, increased coarse soil texture with depth and large macropores of heuweltjies indicated that groundwater is an unlikely cause of mound salinity. The chemical signature of salts indicated a marine origin. Concentrated hotspots of ions and minerals within biogenic features of mound soils suggested that burrowing fauna are responsible for accumulating marine-derived salts in heuweltjie soils. Preferential flow paths that aid solute movement were observed in gravelly dorbank, fractured platy dorbank and as termite channels and rodent burrows in the mound centres of both Piketberg and Klawer. This study reinforced the hypothesis that heuweltjie salts are generated within mounds, possibly through biological activity, and that salts are potentially translocated to the groundwater through preferential flow paths in mounds of the Olifants/Doorn catchment.