Studying carbon dynamics of natural and semi-natural catchments in the Drakensberg area

By Rowena Harrison, PhD Student, SAEON Grasslands-Forests-Wetlands Node

In the April 2019 edition of SAEON eNews, fellow student Lindokuhle Dlamini provided a background to the SAEON Grasslands Carbon Project.

He highlighted his PhD studies using the long-term LI-8100A Automated Soil Gas Flux System owned by SAEON to measure the amount of CO₂ released into the air from the soil, and the amount stored. This is known as CO₂ fluxes and their interactions with soil respiration.

As Lindokuhle outlined in his article, the project forms part of a larger international collaboration with the University of Burgundy in France.

Another component of the long-term collaborative carbon project is centred on the identification of interactions between water movement in the soil profile as well as the storage and movement of carbon at a catchment scale into watercourses. The study of the movement of water into and through soil profiles is commonly referred to as hydropedology and this has been linked to its interactions with carbon storage within the landscape and its effects on our water resources.

The University of the Free State and the University of Burgundy are the two academic institutes providing input for this study.

The importance of studying carbon in the landscape

One of the greatest challenges of the 21st Century is managing global water resources. Water resources are under increasing pressure as the global population rises and rainfall events become increasingly variable and uncertain with climate change.

It is therefore essential that water resources are managed sustainably in terms of both their quantity and quality. Improving and protecting water quality, both for human needs and to sustain aquatic ecosystems, is a major challenge.

Wetland systems are water resources that are commonly utilised for their ecosystem services in South Africa. Wetlands are defined by the South African National Water Act (Act 36 of 1998) as: “land which is transitional between terrestrial and aquatic systems where the water table is usually at or near the surface, or the land is periodically covered with shallow water, and which land in normal circumstances supports or would support vegetation typically adapted to life in saturated soil”.

Wetlands in South Africa are the products of the erosional and depositional processes, as well as the presence of geological influences controlled by the variable environment across the country. These factors give rise to a variety of wetland types within the country.

Wetlands offer numerous ecosystem services to humankind, including but not limited to water quality improvement, ﬂood mitigation, and coastal and wildlife protection. Although wetlands comprise only 5–8% of the terrestrial land surface, it is estimated that 20–30% of the Earth’s soil pool of carbon is stored within these dynamic systems.

One type of wetland is known as a hillslope seep. They are located on sloping terrain and are formed by the unidirectional movement of water and soil material downslope.

Hillslope seeps are important units of a catchment and are generally the main filter for water and solute transport from the atmosphere to the stream. As water and material are constantly moving downslope, water inputs are primarily via subsurface flows within the soil profile. This type of water movement is known as interflow.

As hydrological movement through seeps is generally via interflow, the attenuation of water within the landscape is higher within seep systems. This attenuation of water allows for the settling out, or filtration of sediment, soil organic carbon (SOC) as well as other minerals.

Trapping of carbon within the landscape could allow hillslope seeps to contribute positively as a carbon sink (Kotze et al., 2009). In recent years there has been considerable interest from the scientific community to understand the major routes for the transport of SOC as well as identifying the sources and flow mechanisms responsible for SOC to become dissolved organic carbon (DOC), particulate organic carbon (POC) and dissolved inorganic carbon (DIC) in watercourses.

These aspects form the basis of SAEON student Rowena Harrison’s PhD project which she is doing through collaboration with the University of the Free State and the University of Burgundy.


Catchment 3	Catchment 6	Catchment 9

Carbon fluxes at a catchment scale

Interestingly, of the global carbon fluxes to the oceans from the continents, about 18% move in rivers as POC, 37% as DOC and 45% as DIC. These proportions of POC, DOC and DIC in river systems of the world are variable.

The differences are generally due to a number of factors including land management and its degradation; the geology of the area; the formation, types and health of wetland systems; and the flow paths of soil profiles.

If you have ever seen a natural body of water that appears straw, tea or brownish in colour, it likely has a high organic carbon load. This colour arises from the leaching of humic substances from plant and soil organic matter. The organic matter contributes acids to the stream, resulting in the yellow-brown colouration and weathering the soils.


PhD student Rowena Harrison installed piezometers in the three catchments within selected water paths and wetland systems

Why should we study DOC and POC?

Dissolved organic carbon is defined as ‘the organic matter that is able to pass through a filter (filters generally range in size between 0.7 and 0.22 um)’.

While POC is a much larger solute which is filtered out at 0.45um, this study is interested in DOC and POC. The reason for this is they are important components in the carbon cycle and serve as primary food sources for aquatic food webs.

In addition, DOC is an integral water quality component. One of the most common causes of a decline in water quality is the excessive levels of DOC, which can have a range of detrimental effects on water resources.

DOC can make a significant contribution to the acidity of natural waters through the formation of organic acids and can affect biological activity through light adsorption. DOC can also influence nutrient availability through the formation of organic complexes, and control the solubility, transport and toxicity of metals.

In combination, these alterations to water quality can lead to undesirable effects to human health. For example, DOC has been found to be a major precursor in the formation of carcinogenic and mutagenic disinfection by-products. It has also been linked to the transport and reactivity of toxic substances such as mercury.

It is therefore critical to understand how, when and where DOC and POC concentrations are amplified at the catchment scale and how these subsequently move into watercourses. The factors influencing the solubility of organic carbon (DOC and POC) and their sources and pathways need to be identified, characterised and quantified.

Even more important is that the characterisation and quantification of DOC concentrations and fluxes can then be used to predict the impact of land-use and climate change on DOC dynamics.

Methodology

As the carbon project is centered around the Cathedral Peak LTER master site, Rowena is working with both the University of the Free State and the University of Burgundy in three of the catchments in this area. These are named catchments 3, 6 and 9. Each of the catchments has a different land management history and different vegetation patterns.

Rowena’s first task was to identify the water paths and wetlands within the three catchments. She then installed piezometers in the three catchments within selected water paths and wetland systems. Piezometers are open pipes installed into shallow groundwater areas which passively allow water levels to rise and fall inside them and this is recorded to measure the flow path of water within the soil profile.

Rowena collects water from the piezometers on a monthly basis. Furthermore, water is collected from the inlet and outlet of the weirs which are installed at the end of each of the three catchments.

Rowena collects water from the inlet and outlet of the weirs installed at the end of each of the three catchments

The water is filtered to pass through a 0.45um filter for POC and a 0.22um filter for DOC. The samples are then sent to the University of Burgundy in France for analysis. This analysis includes DOC, POC, nitrates, pH, electrical conductivity and temperature.

DOC and POC are measured in the laboratory at the University of Burgundy. DOC is measured by the high temperature combustion method. This involves conversion of inorganic carbon to dissolved CO₂ and purging this from the sample.

The remaining (organic) carbon is then oxidised at a high temperature to CO₂ which can be detected by instruments which use a nondispersive infrared (NDIR) sensor and are directly correlated to total organic carbon (TOC) content. Particulate organic carbon is measured by determining mass lost upon combustion of a sample.

Filters which are pre-measured and placed in a combustion oven in France are sent to the South African team. A known volume of water is passed through the filter from the weir samples. The filters and sediment collected are dried and sent back to the university’s laboratory. Here the dry mass of the filters is measured after it is subjected to combustion via heating the filter to 550^° C.

Long-term collection of these measurement data will help the team to understand the different flow paths of the three catchments; where carbon is stored more frequently in different seeps and wetlands of the catchments; and how much is being removed from the catchments via the watercourses. This will aid in filling in the gap of knowledge scientists currently have of the carbon dynamics of natural and semi-natural catchments in the Drakensberg area.

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