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University of the Witwatersrand (1997)

Processes controlling carbon and nitrogen dynamics across vegetation types and land uses in selected South African sites

Custers, Mark John

Titre : Processes controlling carbon and nitrogen dynamics across vegetation types and land uses in selected South African sites

Auteur : Custers MJ

Université de soutenance : University of the Witwatersrand

Grade : Master of Science (MS) 1997

Résumé
An understanding of the biogeochemistry of carbon and nitrogen in ecosystems is necessary for the sustainability of system function. Transformations, including different land uses, disrupt the natural input:output of soil organic matter and often result in changes in the cycling of carbon and nitrogen. Consequently it is imperative to know how different land uses are likely to alter the pool sizes, flux rates and turnover of carbon and nitrogen in the soil. The savanna and grassland biomes of South Africa include large areas which have been transformed by man and are the main sites of primary and secondary production. Sites in these biomes along a vegetation and soil type gradient have been investigated. Soil samples from a conserved area, a cultivated area and a livestock area have been sampled. A range of soil properties including the potential rate of nitrogen mineralization, total soil carbon and nitrogen, microbial carbon and nitrogen, soil texture, bulk density, pH and standing dead herbaceous biomass have been quantified. These along with values reported in the literature have been used to validate the CENTURY model, which simulates the turnover of ecosystem attributes on the basis of soil organic matter inputs and outputs. Results show that the soil organic matter pool sizes for the sites and land uses were positively correlated with the percentage fines (silt-plus-clay) and site aridity. Sites which were moist and had a percentage of fines greater than 45% tended to have 3 times more C and N. Land use, especially cultivation, reduced the amount of SOM at sites by 50% mainly because of the effects on the light fraction mass. The potential rate of N mineralization was not significantly different between sites but the cultivated land use led to the immobilization of N. Possible reasons for this included the negative impact that cultivation has on soil macroaggregates, the lower (< 1.0 g/kg) input of light fraction, and the low (< 10%) percentage fines at these sites. Simulations of the SOM fractions using the CENTURY model for six functional types indicate that similar trends emerged but the model greatly overestimated absolute amounts of SOM. In conclusion, the absolute quantities of soil carbon and nitrogen are influenced by climate, soil texture, and land use ; but the proportion of soil organic matter fractions do not appear to differ per biome or per land use indicating similar turnover times.

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