The Density, Permanganate Oxidisability, and Stability of Soil Organic Carbon in Wetlands of the South African Highveld

Abstract

Wetland soils are highly effective ecosystems for long term storage of carbon. Prolonged periods of soil saturation stimulate high rates of productivity by appropriately adapted hydrophytic vegetation and cause a depletion of oxygen within the soil that significantly reduces the rate of mineralisation of organic material, the process that results in the release of CO2 back into the atmosphere. As a result, the rate of production exceeds the rate of mineralisation of organic material and leads to a net accumulation of soil organic carbon over time. This study aims to serve as a baseline estimate of carbon storage in wetlands of South Africa. This study was not intended to be on an extensive spatial scale but rather an intensive focus on carbon storage as well as spatial distribution, variation and factors that influence carbon storage. Three aspects of soil organic carbon were assessed for sections of four selected wetlands. Firstly, the soil organic carbon density was estimated in tons per hectare as well as the differences in soil organic carbon density of different wetland hydrological zones. Secondly, the nature and stability of soil organic carbon were assessed based on the proportion of soil organic carbon that was readily oxidised by potassium permanganate (permanganate oxidisable carbon). Lastly, correlations were assessed between soil organic carbon and the elements N, P, K, Ca, Mg, Mn, Fe, Cr, S, and Al to assess the relationships between soil concentrations of these elements and the density of soil organic carbon. Four wetlands were selected including three mineral and one organic wetland containing peat. The mean SOC densities of the three mineral wetlands at a soil thickness of 20 cm were 49.6 t ha-1 (tons per hectare), 54.5 t ha-1 and 413 t ha-1. The mean SOC density of the organic wetland was 127.0 t ha-1. There were significant differences in soil organic carbon density between different wetland hydrological zones, the highest densities found in the permanently saturated zones. A negative correlation between wetland disturbance in general and POXC content was found. Total soil N, P, and S concentration showed the highest correlation between SOC and soil POXC concentration. The soil concentrations of Fe, Mn and S did not seem to have any appreciable relationship with the oxidisability of SOC or SOC density.