The relationship between soil organic carbon and soil colour

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Chapter 3 LITERATURE REVIEW

Wetland delineation on the Maputaland Coastal Plain

Very few research studies focusing exclusively on wetland delineation issues exist in South Africa, of which Kotze et al. (1996), Kotze & Marneweck (1999) and Job (2009) are some. However, wetlands are mostly delineated using the guideline produced by the Department of Water Affairs and Forestry (DWAF 2005). The use of four wetland indicators is recommended, namely the terrain unit, vegetation, soil wetness, and soil form. Wetland scientists identify terrain units based on hydrogeomorphic setting. The main hydrogeomorphic (HGM) units are described in Kotze et al. (2005) and are based on Brinson (1993). The use of HGM units hypothesises that wetlands will occur in the bottom parts of the landscape, where water is expected to accumulate. The vegetation indicator may aid to find the boundary of the wetland, as plant communities undergo distinct changes in species composition along the wetness gradient (DWAF 2005). The application of this indicator is supposedly limited due to the dynamic nature of vegetation, and may therefore provide misrepresentation of the actual wetland boundaries. The vegetation composition of wetlands is discussed more thoroughly in Section 3.4.
The soil wetness indicator employs the presence and identification of redoximorphic features (such as iron and manganese) as morphological “signatures” to identify saturated and reduced conditions in soil horizons or –forms (see Section 3.3.5). The use of these features has long been established (Evans and Franzmeier 1988, Vepraskas and Wilding 1983, Veneman et al. 1976). Similarly, many technical publications make use of carbon accumulation and other carbon based morphological features to identify hydric soil (USDA-NRCS 2010), since organic carbon accumulates under saturated conditions (see Section 3.3.1). Furthermore, certain soil forms are regarded as hydromorphic soil forms (DWAF 2005, Kotze et al. 1996). Soil colour is another indicator typically used in delineation practices (USDA-NRCS 2010, Lindboa 2001, Kotze et al. 1996), although this has not been quantified by any studies. The study by Kotze et al. (1996) investigated a number of systems potentially useful for describing wetland water regimes. According to this study the use of the South African soil classification system (Soil Classification Working Group (1991)) is not necessarily recommended since it does not account for depth of waterlogging. Regardless of this, the three-class water regime scheme developed by Kotze et al. (1996) is still currently in use. The different zones (permanent, seasonal and temporary) are delineated according to the soil morphology- and form characteristics, and vegetation indicators. The Natural Resources Conservation Service (NRCS) (2009), Brady & Weil (2007), as well as DWAF (2005) recommends that the presence of hydric soils should be looked for in the top 500 mm of the soil profile.
Not all soils associated with wetlands exhibit the typical hydric soil characteristics. Many publications, especially in US literature, deal with such soil (Vepraskas & Craft 2015, Tiner 1999, Soil Science Society of America 1997, US Army Corps of Engineers 1987). While areas with problematic wetland soil are acknowledged in South Africa, there are very few studies or research contributing data to support these statements or to suggest a solution. On sandy coastal aquifers the delineation procedure recommends the use of the following modified soil criteria (DWAF 2005):
The permanent and seasonal zones are similar as described in Table 3.1, but extremely high organic carbon content can be found in the top soil of these zones as a result of prolonged wetness. The organic carbon content is typically higher than 10% (Champagne soil form), and has a peaty character. The US Army Corps of Engineers Wetlands Delineation Manual (US Army Corps of Engineers 1987) describes three additional soil features that may be used as indicators of sandy hydric soils, namely high organic matter content in the surface horizon, streaking of subsurface horizons by organic matter, and organic pans. In the USA the United States Department of Agriculture & Natural Resources Conservation Service developed a list of field indicators to identify hydric soils. These are broken into three main categories based on the main substrate type (USDA-NRCS 2010). The field indicators rely on morphological features that are formed by reduction when soils are saturated.
The study by Job (2009) on the applicability of the DWAF (2005) guideline showed that in general the principles on which the DWAF guideline is based on were applicable to wetlands in the Western Cape. For the wetlands that were classified as “Specific Cases” (problematic soil) as described in the guideline, the recommended approach for delineation is:
identify whether the site is associated with a stream or other landform or landscape position likely to support wetland,
draw on all other wetland indicators,
presence of a dark surface layer, high in organic carbon, even if only 2 or 3 cm thick,
low chroma matrix, in comparison to adjacent non-wetland soils, site visits in the wet season,
draw upon the expertise of an experienced soil scientist to help interpret the site.
One of the main recommendations is to interpret all the indicators present on the site, together with an interpretation of the influence of the setting and other local conditions, in comparison to adjacent non-wetland areas, so as to build an argument of whether or not an area is a wetland.

Soils on the Maputaland Coastal Plain

Since organic soil wetlands (peatlands) dominate the northern hemisphere, international literature specifically tends to focus on permanent wetland systems. There is some literature on seasonal wetlands (the prevailing wetland type in South Africa) (Day et al. 2010), but almost no literature exists on the various zones within a wetland, or the variation of soil properties on a wetness gradient.
A major distinction in hydric soil is made between organic and mineral soil (Job 2009, Reddy & DeLaune 2008, Richardson & Vepraskas 2001, Kotze et al. 1996). Organic matter accumulates in soils as a result of anaerobic conditions caused by continuous saturation, and low temperatures (Richardson & Vepraskas 2001, Kotze et al. 1996). However, organic matter can also accumulate in warm and humid areas such as the MCP, due to a high production rate of organic material in wetlands as a result of high rainfall (Job 2009, Richardson & Vepraskas 2001). Decomposition (mineralisation) is inhibited, which results in high carbon content in the soil. According to the Soil Survey Staff (1975), organic soils contain more than 18% carbon if the dominant texture of the soil is clayey and 12% carbon if the dominant texture of the soil is sandy. The South African Soil Classification System groups the organic soils into a class of their own, known as the Champagne soil form (>10% carbon, and more than 200 mm deep) (Soil Classification Working Group 1991). All other hydric soils are regarded as mineral soils. These soils have a wide range of textures, colours, base status, and pH; lower organic matter content, higher bulk density, and lower porosity than organic soils, and are periodically saturated for sufficient duration to produce chemical and physical soil properties associated with a reducing environment (Reddy & DeLaune 2008, Kotze et al. 1996). The differentiation between these two categories is important for this study, as two of the four systems presented here contain organic soil, while the other two systems are located on a mineral soil. It is also imperative to note that the wetlands containing organic soils only have high SOC levels within the first two zones of the wetland, while the outer zones consist mainly of mineral soil.
Botha and Porat (2007) provide a description of the characteristics of Maputaland soils, which are related to the terrestrial soils of the wetland sites presented in this study. The soils of the Muzi Swamp and Perched Pans (MS and PP Type) are described as loose, apedal sands commonly yellowish to pale brown (10YR 6/6-6/3; 7.5YR 6/8) with few ferruginous mottles, and 0.5 – 10% clay. The pH increases downwards in the profile. Organic matter accumulation results in distinct melanisation and acidification of the A-horizon. Both systems are indicated to exhibit clay-rich, structured, slightly saline and calcareous duplex horizons, as well as hardpan calcrete (Botha & Porat 2007, Matthews et al. 2001). According to Hobday and Orme (1974), this is due to the coastal plain wetlands surrounded by dunes rather than by a coastal barrier beach and lagoonal environment. According to Matthews et al. (2001), the permanently wet soils are characterized by gleying conditions and peat formation. The Moist Grasslands (PL Type) comprises an organic-enriched A-horizon underlain by a grey sandy subsoil with clay enriched horizons in some areas. Abrupt pH, colour and textural change are evident at 1.5 – 4 m depth. Grey clay-enriched or ferruginous mottles and lamellae may be visible. The yellowish Clovelly or grey Fernwood form soils found in areas with high water tables within low-lying interdunal depressions show a sharp reduction of organic carbon to levels of less than 0.5% within 300 mm of the surface (Matthews et al. 2001). The Interdunal Depressions are described to have colours of 7.5YR 6/8, up to 3% clay, and lower pH. It is suggested that the grey sands exhibited on the MCP is a single sand unit in which the upper bleached section is a thick eluvial horizon representing a lengthy pedogenic bleaching episode (Botha & Porat 2007).
According to Fey (2010) the leached nature of hydromorphic soils with E horizons as is found on the MCP is expected to be low in nutrients and with a low CEC and pH. This might change lower down in the profile when the usually more clayey B horizon is reached. A fluctuating water table as is found in seasonal/temporary zones may have a strong influence on nitrogen loss by denitrification. In wetlands where the G horizon is close to the surface such as in the Katspruit form, the soil can be expected to be wet throughout most of the year. The Katspruit form usually has a better reserve of plant nutrients and a higher pH, CEC, and organic matter content than soils of surrounding uplands.

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The biogeochemistry of soil in wetlands

Hydrology is the main controlling factor of the physical, chemical and biological properties (i.e. biogeochemical characteristics and cycles) of wetland soil. According to Reddy and DeLaune (2008), nutrient loading is characteristically greater in wetlands than in uplands due to the topographic setting. Waterlogged soil alters chemical reactions such as pH, redox reactions, electrical conductivity, CEC, and the sorption and desorption of ions (Reddy & DeLaune 2008, Neue et al. 1997). Flooding causes a soil to become anaerobic because air moves 400 times slower through water than through soil (Collins 2005). The magnitude and intensity of soil reduction is controlled by the amount of organic matter, its rate of decomposition, and the amount and types of reducible nitrates, manganese and iron oxides, sulphate and organic substrates (Neue et al. 1997). Wetland soils can act as a sink, source, or transformer of nutrients (Reddy & DeLaune 2008). The aim of this study is not to investigate biogeochemical cycles and inter-relationships of properties in the wetland systems on the MCP and therefore only a broad overview will be given of the various soil properties and how they vary between wetland zones and –systems.

Soil organic carbon

Soil Organic Carbon (SOC) is the primary driver for all biogeochemical processes in wetlands (Bernal & Mitch 2008, Reddy & DeLaune 2008). SOC refers to the carbon in soils originating from the products of photosynthesis and living organisms (organic matter) and is a heterogeneous mix of simple and complex organic carbon compounds (Chan et al. 2008, Reddy et al. 2000). SOC is a dynamic component of an ecosystem, with both internal changes in the vertical and horizontal directions and external exchanges with the atmosphere and the biosphere (Zhang & McGrath 2004).
SOC accumulation in soils is a function of the carbon balance between inputs (organic matter production) and losses (decomposition) (Adhikari et al. 2009, Bernal & Mitch 2008, Schlesinger 1977). Decomposition is prevented by 1) permanent wetness, 2) low temperatures, 3) extreme acidity or lack of nutrients, and 4) high concentrations of electrolytes or organic toxins – all of which slow down microbial oxidation of the organic matter (Fey 2010). Other secondary, inter-related factors controlling decomposition in wetlands include wetland type and hydrogeomorphic setting, hydroperiod, quality, and quantity of the organic matter, microbial communities, electron acceptors supply, low redox potentials, and pH (Reddy & DeLaune 2008, Neue et al. 1997). SOC content and decomposition rates decrease with depth of a soil profile, because most organic residues are incorporated into the soil at the surface and become more recalcitrant and thus difficult to break down (Reddy & DeLaune 2008, Reddy et al. 2000). Soil organic matter is a source of and provides long-term storage for nutrients in the soil (Reddy & DeLaune 2008). SOC is generally higher in clay soils than in sandy soils due to the clay protecting the organic matter against oxidation. The smaller pores of these clayey soils also restrict aeration and reduce the rate of organic matter oxidation. In poorly drained soils, the high moisture supply promotes litter production while the poor aeration inhibits organic matter decomposition.
SOC is accumulated much more effectively in the permanent zones than in the seasonal/temporary zones of a wetland. The fluctuating water table in seasonal and temporary zones results in the oxidation of a considerable portion of the carbon that would have been retained in the soil under saturated conditions (Bernal & Mitch 2008, Phillips & Greenway 1998). Aerobic soils have minimal net retention of organic matter, and what is left consist mainly of highly resistant and stable compounds (Reddy & DeLaune 2008, DeBusk et al. 2001). Anaerobic fermentation plays an important role in the decomposition of reduced carbon, comprising the breakdown of complex substrates before oxidation. This results in an array of substances not found in well-aerated soils (Neue et al. 1997) such as various gases, hydrocarbons, alcohols, carbonyls, volatile fatty acids, nonvolatile fatty acids, phenolic acids, and volatile S compounds (Ponnamperuma 1984).
SOC improves CEC, supplies nutrients such as nitrogen and phosphorous, and affects physical properties including lowered bulk density and increased hydraulic conductivity, infiltration capacity, and water holding capacity (Passoni et al. 2009). Permanent flooding may reduce the availability of some nutrients (Neue et al. 1997). Although the dynamics between the above mentioned soil properties are infinitely complex, it can be expected that these properties will dynamically vary between zones as a result of its dependence on SOC. Because two of the four systems presented in this study are organic soils (some containing peat), a separate section is devoted to peat soils.
Peatlands A peatlands is a very rare wetland type in South Africa (Grundling 2002). Peatlands cover only 3% of the land area worldwide, but contain 30% of the global soil carbon. The International Mire Conservation Group (IMCG) defines peatlands as wetlands with more than 30% organic matter or more than 20% organic carbon, and more than 300 mm of peat. Peat in the South African soil classification system is by default classified as a Champagne soil (Grundling 2010, Soil Classification Working Group 1991).
The sources of, and flow of water though, a peatland has a strong influence on the chemistry of the system (Charman 2002). Both the Muzi Swamp and the Interdunal Depressions in this study classify as fens (driven by groundwater, but also receiving surface runoff water from mineral soils). Fens are regarded minerogenous, because nutrient elements are added to the peatland. While the Muzi Swamp is a soligenous minerotrophic peatland (has an inlet and/or outlet), the Interdunal Depressions are a topogenous minerotrophic peatland (no inlet or outlet; Rydin & Jeglum 2008, Grundling 2002). A rich fen is usually eutrophic with a pH of 6.8 – 8 and rich in vegetation composition, while a poor fen is oligotrophic, with a pH of 4 – 5.5 (Rydin & Jeglum 2008). Peatlands receiving surface runoff from areas overlaying calcareous bedrock such as the Muzi Swamp are particularly rich in carbonates (Charman 2002). This has an effect on soil chemistry, especially redox morphology, and on plant growth.
The elementary composition of peat is strongly related to ecosystem type, peat type, and the eutrophic – oligotrophic gradient. The more minerogenous and open the ecosystem, the less efficient is its ability to trap nutrient and metals (Rydin & Jeglum 2008). The chemical regime of peatlands can be separated into two groups. Firstly the variation in pH (also strongly linked to electrical conductivity (EC) and Ca content) is highly influential in terms of the chemical character of the peatland. Secondly the variability of plant nutrients determines the chemical character, of which N is the key nutrient, but P and K is often more limiting in peatlands then in mineral soil wetlands (Rydin & Jeglum 2008, Charman 2002). Vegetation composition, physical properties, and chemical properties of peatlands are highly interrelated. High correlations have been found between vegetation composition variation and pH and mineral content (Rydin & Jeglum 2008).

Nitrogen

Organic matter dynamics is tightly coupled to the biogeochemical cycles of nitrogen by the processes of decomposition, mineralization and plant uptake (Chen & Twilley 1999). High nitrogen levels in the soil can increase decomposition rates and thus inhibit carbon accumulation (Lu et al. 2007.).
Nitrogen is often the most limiting nutrient in flooded soils (Bai et al. 2005, Reddy et al. 2000). As with SOC, nitrogen occurs as a complex mixture of organic and inorganic forms in any ecosystem. Nitrogen is a very mobile element and the relative proportion of each form depends on the sources of nitrogen entering the system and the relative rates and turnover times of these compounds (Rydin & Jeglum 2008, Charman 2002).
The major nitrogen inputs to wetlands are point and non-point sources (such as floodwater), precipitation, and decomposition and mineralization of organic matter. Nitrogen losses from wetlands occur as a result of plant uptake, immobilization, leaching, ammonia volatilization and denitrificaton (Reddy & DeLaune 2008, Chen & Twilley 1999). Redox reactions control a large portion of the nitrogen cycle processes. The availability of nitrogen in a wetland is therefore influenced by temperature, hydrological fluctuations, water depth, electron acceptors availability and microbial activity.
As with SOC, saturated soil conditions and a fluctuating water table have a huge influence on the dynamic cycling of these elements. Inorganic nitrogen as well as the reduction of nitrates is a function of anaerobic conditions, pH, and redox conditions. Since N20 is released into the atmosphere as a result of reduction, nitrogen is lost from the wetland.

pH

The concept of pH is defined as pH = -log [H+], where [H+] is the activity of H+ ion in solution. At high pH, solutions have low H+ activity and compounds are not protonated. pH is a very dynamic wetland soil property. It can fluctuate daily or seasonally, along with the hydroperiod of the wetland, and/or with the depth of a soil profile. It can also vary over very small distances (<5 mm). Along with SOC and nitrogen, pH is one of the most important electrochemical properties affected by saturated soil conditions (Reddy & DeLaune 2008).
A strong relationship exists between soil pH and SOC content (Reddy & DeLaune 2008, Lu 2007, Bai et al. 2005, Noordwijk 1997). The study showed that between the pH of 5 – 6 the lowest SOC content can be expected (Noordwijk 1997), although this differs from wetland system to system (Bai et al. 2005). Below a pH of 5, reduced biological activity and the increase in toxic cations may reduce the decomposition rates of organic matter. Soils with a higher organic matter content tend to have a lower pH, lower nutrient availability, and are more reduced than mineral soils. Since wetlands are usually acidic, saturated soil conditions generally increase the pH (Reddy & DeLaune 2008, Phillips & Greenway 1998). Flooded alkaline wetland soil tends to approach a neutral pH under flooded conditions. The increase of pH in acid soils depends on the activities of oxidants (nitrates, iron and manganese oxides, and sulfate) and proton consumption during reduction of these oxidants under flooded conditions. If these acid soils are low in SOC and reactive iron content, the pH is very slow to rise after flooding, but usually stabilizes after a few weeks. In alkaline soils pH is controlled by the accumulation of dissolved CO2 and organic acids. This phenomenon implies that waterlogged soils are buffered around neutrality by substances consumed/produced during redox reactions (Reddy & DeLaune 2008). According to data published by Faulkner & Richardson (1989), pH in different wetland types usually vary between 3.9 and 6, but may go up to 8 when including freshwater wetlands located in limestone areas.
Wetland soil containing high levels of SOC result in large increases in CEC with increasing pH under saturated soil conditions. However, since the CEC of organic matter is pH dependant, subsequent decreases in pH will lead to a reduction in CEC and release the balancing cations into the soil solution (Phillips & Greenway 1998). Contradictory to this Fey (2010) reports that soils high in SOC have a low CEC due to the prevalent undecomposed organic matter. Increases in CEC of waterlogged sandy soils are quite low in comparison with other soils since sandy soils contain only small quantities of materials with variable charge. Saturated soil conditions can therefore enhance the ability of soils with variably charged colloids to retain nutrients through increases in CEC.
Soil properties and environmental variables such as temperature can significantly influence the fluctuation of pH (Reddy & DeLaune 2008). Richardson and Vepraskas (2001) states that pH can either increase or decrease organic matter decomposition rates. Salt content is a common cause of high soil pH. Visible redoximorphic features do not easily form in saturated soils with a high pH (Vepraskas 2001). SOC content was also shown to be correlated with soil clay and silt content (Noordwijk 1997).
Many peatlands are strongly acidic due to the hydrological regime, although fens tend to be less acidic than bogs (rainwater-fed; Charman 2002). Certain geological formation may override the effect of water on pH, because pH depends on the properties of soil and bedrock that the water has passed over. Rich fens often occur in areas with calcareous soil (in the field plant indicators are used to recognize levels of richness; Rydin & Jeglum 2008). The pH of the Sibaya peatlands has been shown to vary between 3.1 and 6.9. This is typical of minerotrophic peatlands with a groundwater influence (Grundling 2002). Other controls on pH include exchange of cations in the water and the release of organic acids through decay (Charman 2002).

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Cations and CEC

Cation Exchange Capacity (CEC) is the ability of a soil to hold positively charged ions. H, K, Na, Ca, Mg, and Al and reduced Fe and Mn can be absorbed on the negatively charged soil surfaces. According to Phillips & Greenway (1998), published information on soil nutrients such as soluble and exchangeable cations and anions as well as CEC is scarce, especially for wetlands located on mineral soils.
Organic soils generally have a higher CEC, are believed to be effective ‘traps’ for cations, and therefore have a higher buffering capacity than mineral wetlands (see Section 3.3.1). This is because organic matter produces organic acids, lignin, carboxylic- or phenolic groups, and many other products on decomposition which exhibit exchange properties (Reddy & DeLaune 2008, Rydin & Jeglum 2008, Charman 2002). However, the few existing studies quantifying the fluxes to and from peatlands suggest that fluxes vary between cations and over time (Reddy & DeLaune 2008). A strong correlation exists between CEC and pH, as is discussed in Section 3.3.3.
According to Phillips and Greenway (1998), saturated soil conditions generally increase the concentrations of Ca2+, Mg2+, K+, Na+, and NH4+ in the soil solution. This may be attributed to either a loss of exchange/adsorption sites due to solubilisation of SOC, or displacement from the CEC sites due to increased concentrations of soluble Fe2+ and Mn2+. A heightened solubility of organic matter results in an increase in water-soluble cation concentrations. However, increasing Fe2+ and Mn2+ competing ions, and the possible loss of CEC sites through their dissolution, may also lead to an increase in water-soluble cation concentrations (Phillips & Greenway 1998). Multivalent cations such as Ca2+ and Mg2+ are more easily adsorbed by organic matter than monovalent cations. Low reactivity clays often have a stronger preference for monovalent cations (Phillips et al. 1988). The high Ca concentrations in peats may induce deficiencies in elements such as K+and Mn2+.
Since a major proportion of the CEC arises from the organic carbon fraction, increase in SOC contributes to almost linear increases in soluble Ca2+ and Mg2+ concentrations (Phillips & Greenway 1998, Wolt 1994). Larson et al. (1991) found that the increase in availability of cations such as NH4+ Ca2+, Mg2+, K+, Fe2+, and Mn2+ in the solution phase under saturated soil conditions is especially visible in calcareous wetlands. Potassium has variable patterns in peatlands, although it tends to be highest at the surface. According to Rydin & Jeglum (2008), this is due to nutrient cycling and – conservation in the living portion of peatlands, and also due to the leaching of K from subsurface layers as humification progresses. Proctor (1992) indicates that Na+ and Mg2+ ions varied with distance from coast. Generally, an increase in saturated soil conditions results in an increase of resistance (Reddy & DeLaune 2008). Resistance was measured in this specific study to examine whether distance from the sea influenced the salt content in the soils.

Iron and Manganese

When saturated soil conditions prevail, anaerobic conditions will result in the reduction of Fe3+ and Mn4+ oxides to form reduced Fe2+ and Mn2+, resulting in the accumulation thereof in soil pore water. The cation exchange sites are now dominated by Fe2+ and Mn2+ which displaced the base cations such as Ca2+ and Mg2+. The reduced cations are transported with moving water or along a concentration gradient until an aerobic zone are reached, causing these cations to precipitate again. These ‘mottles’, or redoximorphic accumulations, are an invaluable tool in the determination of the hydrology of wetlands. Reduced Fe2+ and Mn2+ act as reducing agents which, upon donating electrons, are oxidised. Wetlands usually have abundant electron donors, and limited electron acceptors, while non-wetland soils tend to be the opposite (Reddy & DeLaune 2008). Oxidised Fe and Mn provide colour, are insoluble, and immobile in soil, while reduced Fe2+ and Mn2+ are colourless, soluble, and mobile.
The ions Fe2+ and Mn2+ (and subsequent redox reactions) are important in the decomposition of organic matter, nutrient regeneration, are involved in nutrient release in flooded soils, and may decrease the availability of certain plant nutrients through precipitation. Excessive amounts of these nutrients may have an adverse effect on plant growth. It can suppress other microbial processes that regulate organic matter breakdown, alter pH, oxidize toxic organic contaminants and cause mottling and gleying. The stability of Fe and Mn phases in a wetland is regulated by pH and redox potential. A low pH increases the water solubility and exchangeable pool of Fe. Regulators of Fe and Mn reduction include electron donors (organic matter) quality and quantity, bioavailability of Fe and Mn minerals, and soil pH and temperature. Fe and Mn are important electron acceptors, especially in mineral wetland soils (Reddy & DeLaune 2008).

TABLE OF CONTENTS
SUMMARY
ACKNOWLEDGEMENTS
TABLE OF CONTENTS
LIST OF FIGURES
ABBREVIATIONS
Chapter 1 INTRODUCTION
1.1 Background
1.2 Rationale
1.3 Aim
1.4 Objectives
1.5 Thesis exposition
Chapter 2 STUDY AREA
2.1 Locality
2.2 Climate
2.3 Topography
2.4 Geomorphology
2.5 Geology
2.6 Soil
2.7 Hydrology
2.8 Geohydrology
2.9 Vegetation
2.10 Biodiversity
2.11 Land use and conservation
Chapter 3 LITERATURE REVIEW
3.1. Wetland delineation on the Maputaland Coastal Plain
3.2. Soils on the Maputaland Coastal Plain
3.3. The biogeochemistry of soil in wetlands
3.4. Wetland vegetation
3.5. The relationship between soil organic carbon and soil colour
Chapter 4 METHODS
4.1 Site selection and stratification
4.2 Sampling
4.3 Statistical analysis
Chapter 5 SOIL TYPES OF WETLANDS ON THE MCP
5.1 Introduction
5.2 Soil form distribution across wetland types and zones
5.3 Profile description
5.4 Discussion
5.5 Conclusion
Chapter 6 COMPARISON OF WETLAND TYPES AND –ZONES DOWN A TOPOGRAPHICAL GRADIENT
6.1 Introduction
6.2 Comparison of all study sites in terms of soil variables
6.3 Comparison of wetland zones in terms of soil variables
6.4 Discussion
6.5 Conclusion
Chapter 7 VEGETATION AS AN INDICATOR OF WETLAND CONDITIONS ON THE MCP
7.1 Introduction
7.2 Results
7.3 Discussion
7.4 Conclusion
Chapter 8 SOIL COLOUR AS INDICATOR OF SOIL ORGANIC CARBON AND WETLAND BOUNDARIES ON THE MCP
8.1 Introduction
8.2 The correlation between organic carbon and soil colour
8.3 Topsoil colour as indicator of wetland zone boundaries
8.4 Conclusion
Chapter 9 WETLAND DELINEATION ON THE MAPUTALAND COASTAL PLAIN
9.1 Introduction
9.2 Wetland indicators in the wetland types on the MCP
9.3 Comments on wetland delineation in South Africa, and the MCP in particular
Chapter 10 CONCLUSIONS AND RECOMMENDATIONS
Chapter 11 REFERENCES
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