Spatial Variability of Soil Actual and Potential Acidity in the Mangrove Agroecosystem of West Africa

Spatial variability of soil acidity in coastal lowlands results from a complex interaction of climate, coastal morphology, river hydrology, vegetation, landform, and tidal flooding. This study was conducted to determine whether the causal factors of soil acidification can be related to soil total actual acidity (TAA) and total potential acidity (TPA) in 12 sites selected along four river basins in West Africa. A hierarchical framework was designed corresponding to the scale at which each factor has the greatest influence on acidification. In the dry season of 1991, soil samples to be analyzed for TAA and TPA were taken from three strips within each river basin, perpendicular to the river at different distances from the mouth, following a 40 by 20 m grid, at five soil depths. The contribution of the different causal factors of acidity spatial variability was analyzed with a nested analysis of variance (ANOVA) and related to the hierarchical framework. Geostatistics were used to study spatial variability at the most detailed scale. We defined main ecoregions identified between watersheds at the macroscale, subenvironments identified with distance from the river mouth within watersheds, and zones identified with positions within toposequences. Practical implications for water management in acid sulfate were developed. At the macroscale, a broad subdivision of the study area into two ecoregions was possible, at the mesoscale, site position along rivers significantly predicted a river-dependent longitudinal variability of soil acidity, whereas at the microscale, kriged maps showed different patterns of soil acidity.