Controlled bush harvesting as a means for restoring soil of encroached savannahs in Southern Africa

Rangelands in Namibia have experienced a shift from herbaceous to woody plant dominance which has reduced indigenous plant and animal biodiversity. It is also altering ecosystem function, and threatening subsistence pastoralism. A common approach to reduce these negative impacts is bush thinning. It is expected that removal of brushes will favorite the development of grasslands with a positive impact on their soil organic matter (SOM) stocks. On the other hand, harvesting bush from those systems removes not only biomass but also nutrients that are stored in it. Such losses can decrease soil fertility which is likely to affect the soil carbon stocks on a long term. In search of a better understanding of the consequences of such a restoration approach, the objective of the present work is to study the impact of bush removal on the quantity, quality and biochemical recalcitrance of SOM as well as nutrient contents in soils of an encroached savannah which was subjected to bush harvesting. Therefore, we sampled a chronosequence of soils with up to fifteen years after bush thinning. Their SOM was characterized by solid-state NMR spectroscopy and composition was related to its biochemical recalcitrance determined by measuring the CO₂ evolution during microbial degradation in a microcosms experiment of 3 months. First results indicate that up to two years after bush harvesting SOM contents of the soil were decreased, although a recovery was observed with increasing time after harvesting. Ongoing analysis of the stable isotopic ratios are performed to identify if this dynamic is caused by lower litter input due to the change of vegetation from bush to grass or by a faster turnover of the SOM, induced by alteration of the environmental conditions due to bush removal (light, soil moisture, temperature etc.).

 

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