Occluded phosphorus dominates in highly weathered soils of the Didessa toposequence, Western Ethiopia

Ultisols, Alfisols and Vertisols of humid Western Ethiopia are deficient in bioavailable phosphorus (P) but information on their P reserves is largely lacking. This study quantified the P fractions of four soils (Ferrudalfs, Rhodudalfs, Hapludults, Hapluderts) from four land uses (arable, fallow, grazing and agroforestry) along a 53-km long toposequence in the Didessa watershed at the elevation of 1273–2700 m above sea level. Six P fractions (microbial-P, NaHCO 3 -P, NaOH-P, dilute HCl-P, concentrated HCl-P, and concentrated H 2 SO 4 -H 2 O 2 -P) were sequentially extracted, and Total P (TP) was separately determined. The mean inorganic NaHCO 3 -P was only 5.2 mg kg −1 but the mean TP was high (2020–3138 mg kg −1 ). The trend of TP along the toposequence was neither consistent with landscape positions nor degree of weathering. The sum of occluded and residual P was high both in quantity and proportion (73–80%). An abundance of sparingly soluble P has gradually been built up in these soils during pedogenesis likely as a consequence of leaching of silica and residual accumulation of Fe and Al oxides and P adsorbed onto them. The small proportion of labile P in all land uses and high proportion of more stable P suggest that the abundant recalcitrant P pools sparingly replenish the labile pools but rather scavenge the P released by weathering. This study challenges the pedogenic P depletion paradigm in highly weathered soils. • Phosphorus (P) in Ethiopian Ultisols, Alfisols and Vertisols was investigated. • All soils were rich in total P (2000–3100 mg kg −1 ). • About 80% of total P was found in occluded and residual fraction. • Occlusion is attributed as the process resulting in recalcitrant P accumulation. • This study challenges the pedogenic P depletion paradigm.