Relating bush encroachment density of savanna soils in the Kalahari district in Botswana to soil organic matter stocks and quality.

Bush encroachment is a form of rangeland degradation. It is characterized by a shift from herbaceous to woody plant dominance, which has reduced indigenous plant and animal biodiversity and it’s a significant problem in southern African savannas, particularly in Botswana, Namibia and South Africa. Overgrazing by cattle on the one hand and the loss of a large part of large herbivores on the other hand, have been identified as main contributor to imbalance of this delicate ecosystem. The present study aimed to investigate bush encroachment effects on soil organic matter (SOM) in areas of low, medium, and high bush density in a farmed area of the Botswana’s savanna. Bush encroachment is expected to have significant effects on soil properties. It is assumed to decrease soil organic C concentration as well as contents of macro and micro nutrients. To investigate such possible impacts, a sampling was realized during the dry season in various plots with varying encroachment densities (Senegalia mellifera). The analysis included standard soil parameters, (pH, electrical conductivity, cation exchange capacity; elemental composition, and bulk density) the composition of the SOM was characterized by solid-state nuclear magnetic resonance spectroscopy. The preliminary results of these analysis revealed only small differences between sites with low, medium, and high levels of bush encroachment. This suggests that the impact of bush encroachment on savannah’s SOM is lower than expected and may not be directly related to bush density. Further research will compare the obtained data with data obtained during the wet season to obtain a better understanding of the SOM dynamics and the impact of encroachment on soil degradation.

Acknowledgement: The authors would like to express their gratitude to the European Commission for the financial support of this research within the European Framework Program for Research and Innovation Horizon 2020 (Grant No. 101036401).