Juliane Krenz: History of Carbon erosion in South African rangelands

Background

Rangelands represent two thirds of the world’s agricultural land. Most of these rangelands are in semi-arid regions or dry lands and have suffered from degradation during the past centuries. They are especially vulnerable to soil degradation (mostly a consequence of unsuitable land management, overgrazing or climate changes), which leads to a strong decline in net primary production. Unlike croplands, rangelands are not fertilized. Thus, they receive only a low nutrient input, which leads to a faster decline of carbon and a dynamic replacement in eroding soils. This is associated with a decline in the potential of eroding soils to form organic matter and furthermore affects the soil quality and productivity negatively. Consequently, the question is raised which role erosion and the limited dynamic replacement of soil organic matter on rangelands had on net greenhouse gas emissions since European settlement intensified, coinciding with increasing atmospheric CO2 concentration due to the burning of fossil fuels. In this context also the quantity and pathways of potentially lost carbon are of importance and still uncertain.

Work in the Karoo

As a semi-arid area which only has been used for agricultural purposes since the 18th century the Karoo offers a good opportunity to study the history of land degradation and C erosion. Work undertaken has established a link between land degradation and overgrazing when European settlement began. Ongoing land-use change and shifting rainfall patterns resulted in the development of badlands and gully systems. With the ongoing agricultural intensification many dams and reservoirs have been constructed in the region to provide drinking water for cattle or to facilitate irrigation during dry periods. Nowadays, most of the dams are filled with sediments or even breached, offering the potential to use them as an environmental archive to analyse changes in land management, as well as carbon erosion and deposition within the catchment. Furthermore, the region exhibits characteristics typical for many semi-arid areas, enabling an extrapolation of results to further semi-arid rangelands in tropical and subtropical regions that are threatened by degradation and erosion.