Department of Soil Science

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Browsing Department of Soil Science by browse.metadata.advisor "Clarke, Catherine E"

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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.