List of tables
Chapter 3 Study Area
3.5 Soil
Sodium-rich, deep, duplex soil on the footslopes is typical of granite-derived, semi-arid landscapes (Venter et al., 2003; Grant & Scholes, 2006; Siebert & Eckhardt, 2008). These soils tend to be sparsely vegetated with herbaceous cover interspersed by extensive bare patches with significant crusting (Venter et al., 2003; Khomo & Rogers, 2005; Alard, 2009).
Sodic soil contains dispersed clay particles together with a high concentration of available nutrients including nitrogen (N), phosphorus (P) and sodium (Na) (Qadir & Schubert, 2002;
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Khomo & Rogers, 2005; Grant & Scholes, 2006; Alard, 2009). The duplex structure of sodic soil comprises of two distinct layers, namely (i) a coarsely textured sandy loam A-horizon (<
15 cm) which is capable of supporting a variety of low trophic level organisms (soil microbes, detritivores and plants), overlying a (ii) compacted B-horizon of almost impervious calcium- rich heavy clay which often contributes towards vegetation mortality and limiting rooting of woody species (Qadir & Schubert, 2002; Khomo & Rogers, 2005; Grant & Scholes, 2006).
Sodic soil is characterised by a disproportionately high concentration of sodium (i.e. sodium adsorption ratio above 13) in the cation exchange complex (CEC) originating from sodium releasing parent material such as granite (Qadir & Schubert, 2002; Khomo & Rogers, 2005;
Grant & Scholes, 2006). Sodic soil is further characterised by a pH greater than 8.5 and electrical conductivity (EC) of < 4.0 dS/m. The accumulation of sodium can potentially displace various essential elemental cations (e.g. magnesium, calcium (Ca) and potassium, resulting in stronger competition between soil microbes and plants for available nutrients (Qadir &
Schubert, 2002; Khomo & Rogers, 2005; Kaspari et al., 2014).
Hyper-accumulation of exchangeable sodium often results in poor physical soil conditions that are typically associated with sodic soil (Qadir & Schubert, 2002; Khomo & Rogers, 2005; Alard, 2009). In soils with a high sodicity, clay particles tend to expand when water is applied, causing dispersion of these particles which extensively weakens soil aggregation, resulting in structural failure and closing-off of soil pores (Qadir & Schubert, 2002; Venter et al., 2003;
Khomo & Rogers, 2005). This property of sodic soil restricts the movement of water and gasses through the soil (hydraulic conductivity), having an adverse effect on soil-based diffusion of water and nutrients as well as the osmotic capability of microbial organisms (Qadir
& Schubert, 2002; Khomo & Rogers, 2005; Eldridge et al., 2019). The closing of soil pores can affect soil moisture of sodic soil in two fundamental ways: 1) a slow infiltration rate leading to soil moisture content dropping below that of vegetation and microbial requirements (Qadir
& Schubert, 2002; Khomo & Rogers, 2005) and 2) slow internal drainage that results in soil becoming susceptible to waterlogging and in turn creating an anoxic environment (Qadir &
Schubert, 2002; Khomo & Rogers, 2005).
Weakened soil aggregates are prone to erosion, since such erosion events are triggered and enhanced by water- and wind-flow, anthropogenic or animal activity as well as a shortage of soil-binding root-systems and low-density microbial biofilm (Qadir & Schubert, 2002; Venter et al., 2003; Khomo & Rogers, 2005; Alard, 2009). Surface erosion often leads to the loss of the original A-horizon which contains the highest concentration of SOM and is better suited for organismal activity (Qadir & Schubert, 2002; Venter et al., 2003; Khomo & Rogers, 2005).
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40 3.6 Fauna and Flora
The study site is located within the Lowveld Bioregion of the Savanna Biome and forms part of the Granite Lowveld vegetation unit (SVI 3) (Figure 11) (Mucina and Rutherford, 2006). The Granite Lowveld is characterised by tall shrubland with few trees and a herbaceous layer predominantly composed of C4graminoid species suited to tolerate the hot growing season (Mucina & Rutherford, 2006; Siebert & Eckhardt, 2008). Dominant graminoids are characterised as palatable, nutritious bulk growers (Venter et al., 2003; Mucina & Rutherford, 2006; Alard, 2009).
Figure 11: Location of the Nkuhlu experimental site relative to the different vegetation units within the KNP.
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Plant physiology in this ecosystem resembles a pulse pattern that corresponds with the occurrence of rainfall and the unique wet and dry cycles of the area (Venter et al., 2003; Grant
& Scholes, 2006; Alard, 2009). These pulses of metabolic activity tend to last a few days up to several weeks following the occurrence of rain, after which activity slowly decreases (Venter et al., 2003; Grant & Scholes, 2006; Alard, 2009).
3.6.1 Vegetation composition
Siebert and Eckhardt (2008) described the vegetation of the Nkuhlu exclosures and identified five main plant communities, of which this study was undertaken in the Sporobolus nitens- Euclea divinorum Dry Sodic Savanna which consist of two sub-communities (Figure 12, 1.1 and 1.2).
Figure 12: Vegetation map of the Nkuhlu exclosures indicating the location of the Sabie River,
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seasonal streams, smaller tributaries and the various plant communities relative to the three exclosure units (Adapted from Siebert & Eckhardt, 2008).
The sodic plant community covers approximately 23% of the Nkuhlu exclosures (Siebert &
Eckhardt, 2008). Situated on a relatively fertile substrate the vegetation is generally associated with palatable, nutrient-rich forage that supports congregated herbivore activity (Venter et al., 2003; Grant & Scholes, 2006; Siebert & Eckhardt, 2008; Alard, 2009; Scogings et al., 2014). Sodic patches on the footslopes tend to be dominated by fine-leaved, nitrogen- fixing species such as Vachellia grandicornuta that are known for their mycorrhizal associations and subsequent high nitrogen content (Venter et al., 2003; Siebert & Eckhardt, 2008; Scogings et al., 2014). The herbaceous layer of the sodic zone is dominated by forb species such as Abutilon austro-africanum, Portulaca kermesina and Ocimum americanum while dominant graminoids include, Chloris virgata, Enteropogon monostachyus and Sporobolus nitens (Siebert & Eckhardt, 2008). The sodic plant community is further characterised by a discontinuous woody component which include abundant species such as Vachellia grandicornuta, Euclea divinorum, Spirostachys africana and Pappea capensis (Siebert & Eckhardt, 2008).
Sodic patches facilitate unique vegetation and subsequent herbivore activity that differ from upland crests (Grant & Scholes, 2008; Siebert & Eckhardt, 2008). Sodic vegetation plays a major role in LMH survival as it facilitates nutrient requirements and subsequent body maintenance during the dry season (Grant & Scholes, 2006). The vegetation of the sodic zone exhibits distinct structural patterns across herbivore treatments alluding to the direct influence of herbivory or the loss thereof of on system structure (Grant & Scholes, 2006; Jacobs &
Naiman, 2008; Siebert & Eckhardt, 2008; Alard, 2009; Van Coller et al., 2013).
3.6.2 Herbivore communities
Sodic patches, also referred to as nutrient hot-spots facilitate high-quality forage (i.e. low C:N ratios) and therefore attract congregated LMH communities, particularly grazers (Cape buffalo [Syncerus caffer], hippopotamus [Hippopotamus amphibious], blue wildebeest [Connochaetes taurinus] and plains zebra [Equus quagga]) and mixed feeders (impala [Aepyceros melampus], African elephant [Loxodonta africana], grey duiker [Sylvicapra grimmia] and steenbok [Raphicerus campestris]).These herbivore preferred sites are often over-utilised, which contributes towards the characteristically denuded appearance of sodic patches (Venter et al., 2003; Khomo & Rogers, 2005; Grant & Scholes, 2006; Alard, 2009; Scogings et al., 2012; Van Coller et al., 2013; Kaspari et al., 2014).
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