Assessment of Soil Microbial Diversity and Function in Response to Long-Term Fertilisation Regimes in South West Nigeria

This study assesses soil microbial community diversity and functional enzyme activities in response to long-term fertilisation regimes in the derived savanna zone of Oyo State, located in Nigeria's South West geopolitical zone. Long-term management influences soil microbial community composition and function, which in turn determines nutrient cycling efficiency and disease suppression, yet the microbial ecology of Nigerian soils under different fertilisation histories has received limited systematic investigation. Using archived plots from a 15-year long-term fertilisation experiment at the International Institute of Tropical Agriculture (IITA), Ibadan, soils from six treatment plots (unfertilised control, NPK mineral fertiliser, farmyard manure, NPK plus farmyard manure, organic compost, and biofertiliser) were sampled and analysed for microbial community composition by 16S rRNA gene amplicon sequencing (Illumina MiSeq), and for functional enzyme activities including urease, phosphatase, dehydrogenase, and beta-glucosidase. Alpha diversity metrics (Shannon index, Chao1 richness) and beta diversity (UniFrac distances) were calculated. Findings revealed that NPK-only treatment significantly reduced fungal to bacterial ratios and microbial diversity relative to manure and compost treatments. Enzyme activity was highest in the farmyard manure and compost treatments, with phosphatase activity showing the greatest sensitivity to management. Archaea were significantly enriched in long-term NPK-only plots, potentially linked to nitrification dynamics. The study provides the first amplicon-sequencing-based assessment of soil microbial responses to long-term fertilisation in South West Nigeria and identifies microbial diversity loss as a hidden cost of exclusive mineral fertiliser use. Keywords: soil microbial diversity, 16S rRNA, fertilisation, enzyme activity, South West Nigeria.