Soil mineralogical and nutrient characteristics of forest islands and surrounding ecosystem types in West Africa suggest anthropogenic soil improvement

Summary

Aims: Ecosystem changes in the mesic savannas of West Africa are resulting in the formation of patches of ‘forest islands’ around local communities in an otherwise open savanna landscape. There have been conflicting reports on the origin of these forest islands with a very limited understanding of their biogeochemistry. This study evaluated the soil mineralogical and chemical characteristics of forest islands and their surrounding ecosystems comprising croplands and open savannas in Burkina Faso, Ghana, and Nigeria to provide information on the processes leading to the formation of forest islands. Methods: Soil mineralogy was determined using X-ray diffractometry (XRD) while the soil nutrients were analysed with ICP-OES and the other soil chemical properties were determined using standard conventional methodologies. Results: Overall, we found that quartz, kaolinite with significant quantities of 2:1 silicate minerals dominated the soil matrix irrespective of land use type. The minerals identified in most of the locations were independent of land use type. This suggests that the forest island formation is not directly related to soil mineralogy. Forest islands showed differences in soil nutrient contents, being richer in exchangeable potassium and dibasic cations than their surrounding savannas and agricultural fields. This superior fertility status of the soils could contribute to the luxuriant growth of the vegetation leading to the development of forest island. The soil nutrient characteristics of the ecosystem types reflect the land use practices with the forest island having higher nutrient and organic carbon contents. Conclusions: The study provided insight into how human-originated soil nutrient enhancement has induced forest island establishment in open savanna landscapes.