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University of the Free State (2009)

ESTIMATING ORGANIC CARBON STOCKS IN SOUTH AFRICAN SOILS

Ruth, Rantoa Nthatuoa

Titre : ESTIMATING ORGANIC CARBON STOCKS IN SOUTH AFRICAN SOILS

Auteur : Ruth, Rantoa Nthatuoa

Université de soutenance : University of the Free State

Grade : Magister Scientiae Agriculturae degree in Soil Science 2009

Résumé partiel
The organic carbon stock in South African soils was estimated using existing data with reference to master horizons, diagnostic horizons, soil forms, and land cover classes. The data used for this study was taken from the land type survey which started in 1970 covering the whole of South Africa. Approximately 2 200 modal profiles representing were analysed for physical and chemical properties including organic carbon. The results showed that the organic carbon content in the master horizons ranged on average from 16% in the O horizon to 0.3% in the C horizons. In the diagnostic horizons, the highest organic carbon was recorded in the topsoils and ranged on average from 21% in the organic O to 1.4% in the orthic A horizons. However, the organic carbon content in the diagnostic subsoil horizons ranged from 1.2% in the podzol B to 0.2% in the dorbank B horizons. The organic carbon content was related to the soil forming factors namely climate (rainfall, evaporation, and aridity index), topography (terrain morphological units, slope percentage, slope type, and slope aspect) and soil texture (clay). Organic carbon related poorly with climate and topography in both the master and diagnostic horizons, with low correlations. Organic carbon content was positively correlated with rainfall and aridity index in the A, E, B, G, C, and R master horizons and inversely correlated with evaporation in those horizons. Climate had an opposite effect on organic carbon in the O master horizons. A positive relationship between organic carbon and rainfall was found in the pedocutanic B, prismacutanic B, soft plinthic B, red apedal B, yellow-brown apedal B, red structured B, G, unspecified material with signs of wetness, E, neocarbonate B, neocutanic B, regic sand, stratified alluvium, lithocutanic B, hard rock, unconsolidated material without signs of wetness, unspecified dry material, and saprolite. The relationship between organic carbon and evaporation was negative in those diagnostic horizons. Rainfall and aridity index related negatively with organic carbon content and positively with evaporation in the following diagnostic horizons : soft carbonate B, podzol B, hard plinthic B, saprolite, and the unconsolidated material with signs of wetness. The relationship between organic carbon and topography was not very clear in both the master and diagnostic horizons. However, topography seemed to influence the formation of some horizons by restricting their formation to certain slope percentages.

Mots clés : soil form ; master horizon ; land cover class ; diagnostic horizon ; organic matter

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