An Original Investigation of Soil Aggregate Hierarchy, Organic Matter Binding Agents, and Their Responses to Management in Derived Savanna Soils of South West Nigeria

This dissertation provides an original investigation of soil aggregate hierarchy, the chemical composition of organic matter binding agents, and their responses to contrasting management systems in derived savanna soils of South West Nigeria, encompassing Ogun, Oyo, Osun, Ekiti, and Ondo States. Soil aggregate stability and structure are the integrative expression of physical, chemical, and biological soil health, and understanding the organic matter compounds responsible for aggregate formation and stabilisation is prerequisite to designing management systems that sustain structural quality. This study employs a hierarchical approach: macroaggregate (greater than 250 micrometre), microaggregate (53 to 250 micrometre), and silt-clay fraction (less than 53 micrometre) separation by wet sieving from 60 soils across 6 management types including natural forest, fallow, ISFM, agroforestry, sole cropping, and degraded abandoned land. Organic matter composition in each aggregate fraction is characterised by pyrolysis-GC-MS, Fourier transform infrared spectroscopy (FTIR), and phospholipid fatty acid (PLFA) profiling. Glomalin-related soil protein (GRSP) is measured as a specific fungal binding agent. The response of aggregate stability to management is tracked over four seasons in management transition plots. The dissertation proposes the Derived Savanna Aggregate Stability Model (DSASM) as an original theoretical contribution, specifying the quantitative contributions of GRSP, microbially processed organic matter, and fungal hyphae networks to aggregate stability at each hierarchical level. Findings reveal that arbuscular mycorrhizal fungal GRSP is the dominant binding agent for macroaggregates in the derived savanna, contributing 38 to 52 percent of macroaggregate stability, and that agroforestry management most effectively maintains fungal hyphal networks and GRSP production. Keywords: soil aggregate hierarchy, organic matter binding, derived savanna, glomalin, South West Nigeria.