Thesis (M. Sc. Agriculture (Soil Science)) -- University of Limpopo, 2021 / Globally and in most parts of South Africa, there is a trend of increasing shrub
encroachment in savanna rangelands. A number of studies have investigated the impact
of shrub encroachment on soil organic carbon content (SOC) and soil organic carbon
stocks (SOCs) in savannas. So far there is no clear consensus on whether shrub
encroachment increases or decreases the level of SOC and SOCs, especially in semi arid savanna grasslands. Furthermore, knowledge on the effects of shrub encroachment
on SOCs is largely restricted to the topsoil, as this is the part of the profile influenced by
inputs and losses of soil organic matter. How shrub encroachment affects the vertical
distribution of SOCs is rarely considered in the existing literature and the edaphic factors
controlling SOCs with depth are poorly understood.
The objectives of this study were (i) to quantify the vertical distribution of SOC and SOCs
and (ii) to identify the edaphic factors controlling the vertical distribution of SOC and SOCs
in a shrub-encroached savanna grassland sited on shallow plinthic soil. To achieve the
objectives, a vegetation and soil survey of the savanna grassland was conducted
whereby sampling areas were demarcated and characterized into open and shrub encroached grassland plots. In each encroachment level, three pits were randomly dug
to the limiting layer on plots sited on the same soil type and similar topographic position.
Soil samples were collected from the pits at depth intervals of 0-10, 10-20, 20-30, 30-40,
40-50, 50-60 and 60-70 cm. The collected soil samples were analyzed for chemical and
physical properties in the laboratory. Correlation analysis was carried out to determine
the relationship between SOC and SOCs, which were the variables of interest in this study
and related controlling soil physicochemical properties.
The results showed that SOC was significantly greater (P<0.05) in the shrub-encroached
grassland compared to open grassland. Furthermore, the results revealed that SOC was
on average 19 and 13% greater in the topsoil (0-20 cm) and subsoil (20-70 cm) of shrub encroached grassland compared to open grassland. The greater SOC in the topsoil of
the shrub-encroached grassland was mainly attributed to higher SOC inputs from plant
litter and detritus derived from trees and grasses, which are the dominant plant life forms
in savannas. In the topsoil, SOC and SOCs were positively correlated with extractable
phosphorus (P) (r = 0.60; P < 0.05), while in the subsoil they were positively correlated
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with extractable phosphorus (r = 0.54), soil porosity (r = 0.52), extractable copper (r =
0.46), extractable zinc (r = 0.41), exchangeable calcium (r = 0.37) and negatively
correlated with mean weight diameter (r = -0.43). Overall, SOC and SOCs under both
shrub-encroached and open grasslands vertically decreased with soil depth.
The results obtained highlighted that the factors controlling the level of SOC and SOCs
differs in the topsoil and subsoil of the studied shrub encroached grassland. These
findings suggest that in the shallow plinthic soil investigated in this study, SOC in the
topsoil is controlled by the macronutrient P, while in the subsoil it is physically protected
by soil aggregates and chemically stabilized via complexation interactions with
exchangeable cations and heavy metals. In-depth understanding of the physico-chemical
factors controlling SOC storage is critical to foster management practices that will improve
the cycling of SOC in shrub-encroached savanna grasslands.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ul/oai:ulspace.ul.ac.za:10386/3740 |
| Date | January 2021 |
| Creators | Masotla, Abel Lesetja |
| Contributors | Dlamini, P. E. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | ix, 52 leaves |
| Relation |
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