Functional Diversity of Soil Microbial Communities and Its Relationship to Ecosystem Processes in Savanna-Forest Transition Zones of North-Central Nigeria

Soil microbial communities mediate fundamental biogeochemical processes in savanna-forest transition ecosystems, yet the functional diversity of these communities and their contribution to ecosystem processes across the highly dynamic transition zones of north-central Nigeria are essentially uncharacterised, representing a significant gap in understanding how land use change alters below-ground ecosystem functioning. This study characterises the functional diversity of soil microbial communities and investigates their relationships with above-ground ecosystem processes across a savanna-forest transition gradient in Kwara, Niger, and Kogi States. A stratified sampling design incorporated five vegetation types along the savanna-forest continuum, with six replicate plots per vegetation type. Shotgun metagenomics characterised microbial functional gene diversity, while network analysis identified keystone taxa and functional guilds. Soil enzyme activities (cellulase, urease, phosphatase, and arylsulfatase) were assayed as proxies for specific biogeochemical functions. Nitrogen cycling rates were measured through buried bag incubations and isotope dilution techniques. Methane flux was measured using static chamber methodology to quantify greenhouse gas dynamics. Microbial functional gene richness declined from forest to open savanna, but functional redundancy was highest in open savanna communities, suggesting greater resistance to disturbance. Nitrogen mineralisation rates were significantly predicted by microbial functional gene diversity indices, independent of above-ground biomass. Network analysis identified a guild of glomalean mycorrhizal-associated bacteria as keystone connectors in forest community networks, whose disruption under savanna conversion had disproportionate consequences for phosphorus cycling. The study provides the first integrated above-ground and below-ground functional diversity assessment for Nigerian savanna-forest transition ecosystems. Keywords: soil microbial diversity, functional diversity, savanna-forest transition, metagenomics, ecosystem processes