The objectives of this work were: (i) to study the acid monophosphoesterase activity (pH 6.5) of the surface horizons of low-activity tropical clay soils (West Africa, West Indies, Brazil), cultivated or uncultivated, with widely different textures and organic matter (OM) contents, (ii) to measure the phosphatase activity (Pase) in the organic and organo-mineral soil fractions obtained by particle size fractionation. Pase was most significantly correlated to the soil OM content and to a lesser extent to the texture and total P content. Therefore, any modification in soil management resulting in important changes in soil OM contents, leads to important variations in phosphatase activity: Pase decreases after clearing of the native vegetation and continuous cultivation, Pase increase when grass-fallows or meadows succeed to annual cropping. The Pase of all the size fractions (20–2000 μm, 2–20 μm and 0–2 μm) was controled by their OM content. However, the "potential Pase" defined as the ratio Pase/C was higher in the 20- to 2000-μm and 0- to 2-μm fractions than in the 2–20 μm ones This is attributed to a higher Pase renewal by root restitutions in the plant debris fraction (20–2000 μm) and by microbial activity and microbial metabolites in the organo-clay fraction (0–2 μm). More than 50% of the soil total Pase was associated with fractions coarser than 2 μm. Most of the changes in Pase (75–100%) associated with the cultivation were ascribed to these fractions. This illustrate the probably important role of the fractions coarser than 2 μm in the phosphorus dynamics in these soils. Key words: Phosphomonoesterase, organic matter, particle size fractionation, low-activity tropical clay soils
