Browsing by Author "Mchunu, Sinethemba Euginia"

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    Distribution and stability of soil carbon in spekboom thicket, Eastern Cape, South Africa
    (Stellenbosch : Stellenbosch University, 2012-03) Mchunu, Sinethemba Euginia; Hardie-Pieters, Ailsa G.; Mills, A. J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
    ENGLISH ABSTRACT: Soils of semi-arid regions generally have low organic matter contents, primarily attributed to prevailing climatic conditions. Yet, the spekboom thicket, located in the semi-arid region of the Eastern Cape (South Africa) has been distinguished to accumulate large amounts of soil carbon. To further understand this remarkable accumulation, a detailed investigation was carried out to evaluate the distribution, speciation and stability of the soil carbon from directly beneath spekboom (Portulacaria afra) and other “nonspekboom” vegetation within the intact thicket, and from adjacent, degraded (overgrazed), open thicket sites. In the first experimental chapter, the distribution of organic and inorganic C, as well as general soil properties (pH, EC, exchangeable and water soluble cations, hydrophobicity, water stable aggregates) were investigated in the intact (spekboom and nonspekboom) and degraded (open) thicket soils. There were no significant differences in the C content (organic or inorganic) between the spekboom and nonspekboom sites in the intact thicket. Soil organic carbon (SOC) content however was greatly influenced by thicket degradation, whereas, inorganic C remained unaffected. Mean organic C (0-50 cm) ranged from 19-34, 22-62, and 19-58 g C kg-1 for open, spekboom, and nonspekboom sites, respectively. Whereas, mean inorganic C ranged from 2-9 g C kg-1 irrespective of sites, representing 4-22 % of total soil C. However, no significant differences were detected between degraded and intact sites for all (total, organic and inorganic) C stocks, attributed to variation in bulk density. Spekboom soils contained significantly higher concentrations of exchangeable and water soluble Mg, Na, and K, and consequently had higher pH and EC compared to the other sites. Soils tended to be hydrophobic especially at shallower depths in the intact thicket irrespective of vegetation type, whereas soils from the adjacent open sites were not significantly hydrophobic. Macro-aggregates were considerably more stable under intact sites compared to open sites. The second experimental chapter investigated the effects of degradation and vegetation type on the stability and structural chemistry of SOC. This involved partitioning the soil organic matter into particulate and mineral-bound (stable) fractions, and then examining the relationships between the stable organic C fraction and various soil properties as an attempt to elucidate the stabilization mechanism(s). Particulate organic C ranged from 7.0, 9.3, and 14.4 g C kg-1 for open, spekboom, and nonspekboom respectively; representing 22-34 % of total SOC. Stable (mineral-bound) organic C accounted for the largest fraction ca. 60-66 % of total SOC; 28.0 and 26.2 g C kg-1 for spekboom and non-spekboom respectively, versus 17.2 g C kg-1 for open sites. It was concluded that SOC stability was influenced by the inherent residue quality (recalcitrance) rather than soil properties, attributed to the aliphatic (lipids and waxes) and highly aromatic nature of the spekboom litter substrate and particulate OM. These research findings are of fundamental significance in understanding soil organic matter stabilization in semi-arid environments.