Biogeochemical Cycling of Nitrogen in Agricultural Soils of the Derived Savanna, Benue State, North Central Nigeria

Nitrogen cycling in tropical agricultural soils of the derived savanna is complex, influenced by land management practices, seasonal moisture dynamics, and microbial community composition. This study examined biogeochemical nitrogen cycling processes in smallholder agricultural soils under contrasting management practices in the derived savanna of Benue State. Five management types were compared: continuous cropping without fertiliser, inorganic nitrogen fertilisation, legume rotation, integrated crop-livestock management, and secondary fallow. Soil samples were collected from 0 to 15 and 15 to 30 centimetre depths during wet and dry seasons. Mineralisation rates, nitrification potential, denitrification enzyme activity, and nitrogen leaching losses were measured using established incubation and extraction methods. Soil microbial biomass nitrogen and abundance of ammonia-oxidising bacteria were determined using fumigation extraction and quantitative polymerase chain reaction methods. Results showed legume rotation and integrated crop-livestock systems supported highest nitrogen mineralisation rates and microbial biomass, while continuous inorganic fertiliser use stimulated nitrification but suppressed denitrification enzyme activity. Nitrogen leaching losses were highest in inorganic-fertilised plots during early wet season storms. Fallow plots showed the most active and diverse nitrogen cycling microbial communities. The study advances understanding of management-specific nitrogen cycling dynamics in derived savanna soils and provides empirical evidence for promoting biologically integrated nutrient management in Benue State smallholder farming systems. Keywords: nitrogen cycling, agricultural soils, derived savanna, Benue State, soil microbiology.