Estimating organic carbon stocks in South African soils
Rantoa, Nthatuoa Ruth
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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. The influence of topography on organic carbon content depends on the morphology of the master and diagnostic horizon and underlying material. A regression was done to study the correlation of organic carbon and the independent variables namely: rainfall, evaporation, slope aspect, aridity index, and clay per master and diagnostic horizon. Unfortunately most of the correlation coefficients were too low for the equations to be used to estimate organic carbon content in South African soils. Organic carbon in the soil forms behaved as their diagnostic topsoils. The environmental conditions such as water content and temperature that influenced the amount of organic carbon in the topsoils also determined the amount of organic carbon in the diagnostic subsoil horizons of that specific soil form. Organic carbon stocks were then estimated using three soil bulk density values namely: low = 1.30 g cm-3, average = 1.50 g cm-3, and high 1.70 g cm-3. The results revealed that the organic carbon stocks of South African soils increased from the warmer, drier western to the cooler, wetter eastern parts of the country. The average soil organic carbon stocks is 73 726 kg ha-1 when calculated using a soil bulk density of 1.50 g cm-3. Most soils had an organic carbon content between 30 000 kg ha-1 and 50 000 kg ha-1. The total organic carbon of the soils of South Africa is estimated to be 8.99 ± 0.10 Pg calculated to a depth of 0.30 m which is 0.57% of the world’s carbon stocks. Since the world’s carbon stocks were calculated to 1 m depth this is not a true representative value for the carbon stocks of South Africa in relation to the worlds. Therefore a lower value will be expected if carbon stocks are estimated to a depth of 1 m in South Africa. The organic carbon stocks in the 27 land cover classes ranged from 9 Mg ha-1 in barren rock to 120.2 Mg ha-1 in forest plantations. The highest accumulation of organic carbon per unit area in South African soils was found in the forests plantations > forests > wetlands. However the biggest contribution to the total organic carbon stocks, was reported in the unimproved grassland> thicket and bushland > shrubland and low Fynbos > forests.