Carbon dioxide release from disturbed wetland soils of the Mpumalanga Highveld

Abstract

The release of carbon dioxide from wetland soils is a phenomenon which involves several biochemical processes and environmental conditions. Heterotrophic respiration through the metabolisation of organic material by heterotrophic organisms is one such process. The oxidation of methane to carbon dioxide is another important source of carbon dioxide. The oxidation of methane transforms methane into carbon dioxide, joining the oxidation of carbon with the reduction (electron acceptor) of sulphate and/or nitrate. Acid Mine Drainage (AMD) is an increasing threat to environmental resources in South Africa. AMD originates from sulfide bearing minerals such as pyrite (FeS2) being exposed and reacting with oxygen-bearing water to produce an acidic medium which is increasingly entering natural watercourses such as wetlands. The addition of an acidic solution into wetlands causes, amongst others, a change in pH, change in Electrical Conductivity (EC) as well as a change in redox chemistry. AMD can also cause a change in salinity within wetlands. The salinity that is generated during AMD neutralisation can be the greater long-term problem. The study aims to develop a field method of measuring carbon dioxide emissions from wetlands in the field, as well as to investigate the influence of AMD on the carbon dioxide release of wetland soils, focusing on the low organic carbon soils and high organic carbon wetland soils. The field studies were conducted on three wetland study sites, in which field measurements were taken for pH, Electrical Conductivity (EC), redox potential and CO2 emissions. For each study site, soil classification was done to delineate the different wetland zones. The results indicated that in a low carbon mineral wetland, the permanent zone contained the highest carbon dioxide emission measurements, where the outer terrestrial zone contained the lowest measurements. In the high carbon organic wetland, low carbon dioxide emissions were measured in the permanent zone. The depletion of oxygen due to prolonged saturation and stagnation, results in low oxygen levels which are not enough to sustain respiration of organic matter. The organic wetland is storing its carbon rather than releasing its carbon, thus resulting in an organic wetland. The introduction of synthetic AMD into wetland soils resulted in a short term, rapid increase in the amount of carbon dioxide released. Changes in the soil before and after the addition of AMD were highly significant with respect to pH, EC, CO2 increase and redox potential for both the organic and mineral soil. Upon the addition of AMD, the organic soil increased its original CO2 emissions by 385.6%, with the mineral soil increasing by 5.28%. The “Birch Effect” was proven to be relevant in saturated soils, rather than only in unsaturated soils. This proves that soils with high levels of organic matter and thus a high carbon percentage, pose a greater risk to increased carbon dioxide emissions upon the addition of AMD. These high carbon percentage soils are particularly common in wetland systems, most notably in the permanent zone of the wetland. The introduction of AMD into wetlands has been proved to enhance the “source” component, disrupting the carbon balance and at the same time enhancing atmospheric CO2 concentrations. The results confirmed that AMD contamination to wetlands does not only affect the system directly, but also the larger environment as the carbon stock of the wetland will ultimately decrease, and transfer to another system.