Low phosphorus (P) availability in Ferralsols of the Malagasy Highlands is a major limitation to crop growth. Direct seeding mulch-based cropping practices which were adopted in the region to improve and sustain soil fertility are known to favour earthworms' presence. The mesocosm study aims to analyse the effect of an endogeic geophageous earthworm species on the soil P status. Total P content (P t), NaOH-extractable P content, P ions (Pi) concentration (C p) in solution and rapid and slow reactions of Pi in solution with solid phase were determined in two Malagasy Ferralsols. Both C p and reactions rates were assessed in laboratory batch experiments using pdpoPi labelling and isotopic exchange kinetics (IEK). The P t values were 836 and 349 mg P gp# in a clayey soil and a sandy-clayey soil, respectively. For both soils, NaOH-extractable organic P was significantly higher in earthworm casts than in parent soils, whereas Pt was unchanged. Also, the effect of earthworm ingestion significantly changed parameters of the IEK. In casts compared with the soil from which they were derived, the immediate isotopically exchangeable Pi (E min) increased by 116, whereas relative rates of Pi release at the solid-to-solution with time were slightly lowered. The effect of earthworm ingestion on IEK corresponded to a transfer of slowly exchangeable Pi towards quicker Pi pools of exchange. However, according to the literature, the increase in E min remained below the critical level for optimal growth, stating that the soils remained P-deficient even in the presence of active and numerous earthworms.
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