Soil aggregate stability of forest islands and adjacent ecosystems in West Africa

Abstract

In the mesic savannas of West Africa, areas around villages of relatively tall and dense forest vegetation are often found. These ‘forest islands’ are presumably the direct outcome of human activity. To better understand these patches with relatively luxuriant vegetation, our study focused on how they influence soil aggregate stability- a key indicator of soil resilience to degradation through erosion. We compared the proportion of stable soil aggregates of the forest islands with nearby croplands and 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 0–5 cm and 5–10 cm depths and stability of soil aggregate groups with diameters: > 500 μm, 500–250 μm and 250–53 μm (viz. “macroaggregates”, “mesoaggregates” and “microaggregates” respectively) determined using the wet sieving method. The results showed significantly (p < 0.05) higher proportion of stable soil meso- and macro-aggregates in forest islands and natural savanna than in agricultural soils. Although there was no effect of land-use type on microaggregate stability, there was a strong tendency for the stable microaggregates across all land use types to increase with increasing precipitation. Soil organic carbon and iron oxides contents were the most important factors influencing meso and macro-aggregate stability in the West African ecosystems. We conclude that formation of stable soil microaggregates in the West African ecosystems was climate or precipitation driven whereas the more labile and larger-size groups of meso-and macro- aggregates was land-use driven. The study provides first insights in soil quality processes in a poorly studied but unique phenomenon of man-made forest islands in West Africa.