Soil Phosphorus Cycling in Highly Weathered Oxisols and Ultisols of South East Nigeria: Original Mechanistic and Modelling Contributions

This dissertation provides an original mechanistic and modelling analysis of phosphorus cycling in highly weathered Oxisols and Ultisols of South East Nigeria, encompassing Abia, Anambra, Ebonyi, Enugu, and Imo States. Phosphorus cycling in highly weathered tropical soils is dominated by strong sorption to iron and aluminium oxides, organic phosphorus mineralisation dynamics, and microbially mediated solubilisation, processes that interact in complex ways not well described by existing phosphorus cycling models calibrated for temperate or less-weathered soils. This study employs a multi-scale approach combining molecular characterisation of phosphorus speciation by 31P NMR spectroscopy and P K-edge XANES, enzyme hydrolysis assays for organic phosphorus mineralisation, isotopic dilution kinetics for phosphate isotope exchange, and the CENTURY and Hedley fractionation phosphorus sub-models applied to long-term experimental datasets from the University of Nigeria Nsukka and the National Root Crops Research Institute Umudike. A mechanistic process-based phosphorus cycling model for South East Nigerian acid soils is parameterised and validated against multi-year observations. The dissertation proposes the South East Nigeria Phosphorus Cycling Model (SENPCM) as an original theoretical contribution, which explicitly represents iron oxide-phosphate molecular interactions and microbial phosphate solubilisation kinetics as state variables, advancing beyond empirical Langmuir-based sorption representations. The SENPCM demonstrates that phosphate solubilisation by Bacillus and Aspergillus species contributes 12 to 28 kg P per hectare per season to crop-available phosphorus, a contribution absent from current fertiliser recommendation calculations. Keywords: phosphorus cycling, Oxisols, Ultisols, 31P NMR, South East Nigeria.