Processes controlling carbon and nitrogen dynamics across vegetation types and land uses in selected South African sites.
Date
1997
Authors
Custers, Mark John
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Abstract
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 glkg) 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.
Description
A project report submitted to the Faculty of Science, University of the Witwatersrand,
Johannesburg, in partial fulfillment of the requirements for the degree Masters of Science
Resource Conservation Biology.
Keywords
BIOGEOCHEMISTRY., BIOTIC COMMUNITIES., ECOLOGY.