Soil Aggregate Stability of Forest Islands and Adjacent Ecosystems in West Africa

Abstract. In the more mesic savanna areas of West Africa, significant areas of relatively tall and dense vegetation with a species composition more characteristic of forest than savanna are often found around villages areas. These forest islands may be the direct action of human activity. To better understand the processes leading to the development of these patches with relatively luxuriant vegetation, our study focused on the stability of the soil aggregates of forest islands, nearby areas of natural savanna vegetation across a precipitation transect in West Africa for which mean annual precipitation at the study sites ranges from 0.80 to 1.27 m a⁻¹. Soil samples were taken from 0 to 5 cm and 5 to 10 cm depths and aggregate fractions with diameters: > 500 μm, 500–250 μm and 250–53 μm (viz. macro aggregates, mesoaggregates and microaggregates) determined using the water sieving method. The results showed significant higher proportion of stable meso and macro- aggregates in forest islands and natural savanna compared to agricultural soils (p < 0.05). On the other hand, although there was no effect of land-use type on microaggregates stabilities, there was a strong tendency for the micro-aggregate fraction across all land use types to increase with increasing precipitation. Simple regression analyses showed soil organic carbon and iron oxides contents as the most important factors influencing aggregate stability in West African ecosystems.