Accelerated soil erosion holds strong links to excessive land degradation, socioeconomic problems and accelerated climate change, particularly in developing countries such as South Africa. An analysis of the properties of severely eroded soils is undertaken to determine which characteristic properties influence the erodibility of the soils at several gully and badland sites in three catchments of the Ngqushwa rural municipality, which is tagged as one of the area’s most severely affected by soil erosion and degradation in the Eastern Cape. Physical, biological and chemical properties of the soils were analysed, including aggregate stability, soil texture, organic matter- and carbon content, clay mineralogy, sesquioxide content as Fe2O3 and cation composition. The distribution of a number of these properties was also determined, particularly between erosion sites, along hillslope profiles and with depth. The study area comprises three catchments, namely Mgwalana, Bira and Gqora which share common soil parent materials, land use history, exhibit similar topography and advanced degree of erosion. A total of 63 soil samples were strategically collected from identified erosion zones and site controls devoid of active erosion; where stability was evident by means of vegetation cover and soil surface condition. Surface samples were collected above 30cm depth and subsurface samples at depths greater than 30cm. To determine the fate of carbon additional samples were collected from runon and sink zones at the Mgwalana catchment along a transect from top to bottom slope. Laboratory analyses was performed to determine the soil properties, whereby C content was measured by the dry combustion method, Fe2O3- and cation content by Atomic Absorption (AA) Spectrometer, texture by the separation method described by Schlichting, Blume & Stahr (1995), clay mineralogy by X-Ray Diffraction and organic matter content by conversion of total C. In addition to these properties being used to infer aggregate stability, the wet sieving method was also used for this purpose and for validation of the other soil properties. The results revealed that no one soil property has a greater influence on aggregate stability than the others, but rather that the stability of the soil is dependent on the combinations of these properties in the soil and the interactions that occur between them. Based on this, the results indicated greater stability for runon, sink and control samples, mainly in the topsoil, which were characterised by a relatively higher C- and organic matter content, loam texture and higher concentrations of Ca2+. Furthermore, the less stable eroded samples were characterised by a more clay rich texture particularly in the subsoil, relatively lower C- and organic matter contents and a greater sodicity due to higher concentrations of Na+. The least stable soils belonged to the Gqora catchment, which were found to consist of higher silt content in the topsoil and a higher Fe2O3 content in the subsoil. The clay mineralogy was relatively uniform across all catchments, comprising of primarily illite and secondly kaolinite. The sample with the highest sodicity in terms of ESP contained smectite clay in addition to illite and kaolinite which is assumed to contribute to this increased Na+ concentration. These findings aid in the conclusion that the chemical characteristics of the soils, in association with biological and, to a less extent, physical properties of the soil exacerbate the erosion problem initiated by the extrinsic contributors, such as climate and topography. Investigation into the fate of carbon on eroded lands revealed a topographically driven dynamic whereby the total carbon content was found to be greater at the top and middle slope positions as well as in the sink zone. These hillslope sections were found to have a lower slope gradient and slightly more pronounced concave shape to those sections with lower C values. These topographic variables influence the degree of gully erosion taking place at different hillslope sections, which tends to be greater where slope angle is increased and convexity exists, resulting in the removal of soil C at these positions and its deposition in areas of accumulation, namely the runon and sink zones of low angle concave slope sections. The findings of this research may be used to develop restoration and management strategies with the ultimate goal to reduce the soils vulnerability to erosion by enhancing those soil properties conducive to greater aggregate stability as determined in the present study.

Thesis (MScAgric)--Stellenbosch University, 2012. / 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. / AFRIKAANSE OPSOMMING: Gronde van semi–ariede streke het oor die algemeen 'n lae organiese materiaalinhoud, wat hoofsaaklik aan die heersende klimaatstoestande toegeskryf kan word. Tog, het die spekboomruigtes, geleë in die semi-ariede streek van die Oos-Kaap (Suid Afrika), onderskeidelik groot hoeveelhede grondkoolstof geakkumuleer. Om hierdie merkwaardige akkumulasie verder te verstaan, was 'n gedetailleerde ondersoek uitgevoer om die verspreiding, spesiasie en stabiliteit van die grondkoolstof direk onder spekboom (Portulacaria afra) en ander “nie-spekboom” plantegroei binne die intakte ruigte en van die aangrensende gedegradeer (oorbeweide), oop gras areas te evalueer. In die eerste eksperimentele hoofstuk, is die verspreiding van organiese en anorganiese C, sowel as algemene grondeienskappe (pH, uitruilbare en wateroplosbare katione, hidrofobisiteit, water-stabiele aggregate) was in die intakte (spekboom en nie-spekboom) en degradeerde (oop) ruigtes ondersoek. Daar was geen betekenisvolle verskille in die C-inhoud (organies of anorganies) tussen die spekboom en nie-spekboom gronde in die intakte ruigtes gevind nie. Grond organiese koolstof (GOK) inhoud is grootliks deur die ruigte se agteruitgang beïnvloed, maar anorganiese C-voorrade was egter nie beïnvloed nie. Die gemiddelde organiese C-inhoud (0-50 cm) het van 19-34, 22-62 en 19-58 g C kg-1 onderskeidelik vir 'n oop, spekboom, en nie-spekboom areas gewissel. Die anorganiese C-inhoude het vanaf 2-9 g C kg-1 gewissel, ongeag van ligging, wat 4-22 % van die totale grond C uit maak. Daar is egter geen beduidende verskille tussen vervalle en heel terreine vir almal (totaal, organiese en anorganiese) C aandele opgespoor, toegeskryf word aan die variasie in die massadigtheid. Spekboom gronde het beduidende hoër konsentrasies van uitruilbare en wateroplosbare Mg, Na, en K waardes bevat. Dit het ook hoër pH en EC, in vergelyking met die ander areas gehad. Gronde was geneig om hidrofobies te wees, veral op vlakker dieptes in die intakte ruigte, ongeag van die plantegroei, terwyl die gronde van die aangrensende oop areas nie beduidend hidrofobies was nie. Makro grond aggregate was aansienlik meer stabiel onder intakte ruigtes as in die oop areas. Die tweede eksperimentele hoofstuk het die gevolge van die weiding-geïnduseerde degradasie en tipe plantegroei op die stabiliteit en strukturele chemie van GOK ondersoek. Dit het die skeiding van die grond organiese materiaal in die vrye en mineraalgebonde (stabiele) fraksies behels. Daarna is die verhoudings tussen die stabiele organiese C fraksie en verskeie grondeienskappe ondersoek om die stabiliseringmeganisme(s) uit te lig. Die chemiese aard van die blare, plante oorblyfsels, fyn en minerale gebinde organiese C is met behulp van spektroskopie ondersoek. Vrye organiese C voorraad het van 7.0, 9.3, en 14.4 g C kg-1 vir oop, spekboom, en nie-spekboom onderskeidelik gewissel, wat ongeveer 23-34% van die totale GOK verteenwoordig. Stabiele (minerale gebind) organiese C-voorrade is vir die grootste fraksie ca. 60-66% van die totale GOK verantwoordelik. Dit was 28.0 en 26.2 g C kg-1 onderskeidelik vir spekboom en nie-spekboom, teenoor 17.2 g C kg-1 vir 'n oop area. Die mineraalgebonde GOK het geen noemenswaardige korrelasies met die ondersoekte grondeienskappe getoon nie. Dus is tot die gevolgtrekking gekom dat GOK stabilisering eerder deur die plante oorblyfsel se chemiese kwaliteit beïnvloed word. Strukturele chemiese karakterisering (13C CP-MAS KMR) van die spekboom afval en vrye C fraksie het gewys dat weerspannigheid die mees prominente meganisme van koolstof stabilisering van organiese koolstof in die gronde is. Dit word aan die alifatiese (lipiede en wasse) en sterk aromatiese aard van die spekboom substrate toegeskryf. Hierdie ondersoek het tot die fundamentele kennis van grond organiese materiaalstabilisering in 'n semi-ariede omgewings bygedra.

Although a considerable amount of work has been done on the fauna of the soil of temperate forests and pastures, very little is known about that of tropical and sub-tropical soils. Most of the work on tropical soils concerns the fauna of tropical forests ... . The present investigation represents an attempt to discover the effect of an imported perennial tropical and sub-tropical plant, the Pineapple, on the soil fauna of a semi-arid region (rainfall 15-25 inches). Intro., p. 1-2