Transformation of humic substances in vermicomposting: Structural changes and impact on cowpea soil nutrient dynamics 16

This study assesses the elemental and structural transformation of municipal solid waste (MSW) by vermicomposting at different stages of maturation and evaluates its nutritional potential on rhizosphere soil chemistry compared with NPK fertilizer. The structural characterization of MSW vermicompost at increasing maturity stages was determined by solid-state nuclear magnetic resonance (NMR) and Fourier Transform infrared (FT-IR) spectroscopies, while the elemental analysis was carried out using standard procedures. Thereafter, a field experiment was carried out at the Teaching and Research Farm of Obafemi Awolowo University, Nigeria, to assess the structural and elemental changes of humic acid-like substance extracted from cowpea rhizosphere soil as influenced by MSW vermicompost and NPK mineral fertilizer applied solely and at different proportions. The elemental results show a significant percentage decrease in major soil chemical parameters as the maturation period progresses. However, a notable increase was observed in N, P, and K contents in matured vermicompost. Structural analysis reveals the presence of polysaccharides, alkyl, cyclic, and aromatic compounds in the final vermicompost. Field experiments further substantiate the positive impact of MSW vermicompost on soil health. Plots receiving vermicompost, especially at 100% vermicompost (6 t/ha), had a higher relative percentage of O-Alkyl-C, Methoxyl-C, and carboxyl-C in both planting seasons compared to NPK-treated plots and control. Additionally, all vermicompost-treated plots significantly increased or improved soil pH, cation exchange capacity, soil organic carbon (SOC), and total nitrogen, which recorded the highest in the plot treated with 100% vermicompost. Hence, spectroscopic and elemental results revealed the quality, maturity, and stability of municipal solid waste vermicompost and its positive effect on soil rhizosphere, thus making the use of MSW a viable alternative to NPK fertilizer as a soil amendment. Key words: Vermicompost, rhizosphere soil, characterization, municipal solid waste, humic substance, cowpea.