Comparative Analysis of Nutrient Cycling Patterns in Native and Exotic Tree Plantation Stands in the Derived Savanna of Oyo State

Native and exotic plantation tree species differ in litter quality and decomposition dynamics, with significant consequences for nutrient cycling and soil fertility in the derived savanna transition zone of Oyo State, South West Nigeria. This study comparatively analysed nutrient cycling patterns (nitrogen, phosphorus, potassium, and carbon dynamics) in plantations of four species: two native (Terminalia superba, Khaya ivorensis) and two exotic (Tectona grandis, Eucalyptus camaldulensis), at the University of Ibadan Research Farm managed by the Department of Forest Resources Management, Oyo State. Litterfall was collected monthly using 0.5m2 litter traps (5 traps per stand) over 18 months. Leaf litter nutrient concentrations were determined by standard laboratory analysis. Decomposition rates and nutrient release were assessed using the litterbag method over 12 months. Soil nutrient concentrations (0 to 15cm depth) were compared across plantation types. Annual litterfall production ranged from 4.1 Mg/ha/year (Eucalyptus) to 6.8 Mg/ha/year (Terminalia). Native species litter decomposed significantly faster than exotic species, with Khaya achieving 50% mass loss in 42 days compared to 89 days for Eucalyptus. Nitrogen returned to soil was highest under native species plots (mean 68 kg N/ha/year vs. 34 kg N/ha/year under Eucalyptus). Soil organic carbon was significantly higher under native species plots after 18 months. The study concludes that native species plantations support superior nutrient cycling compared to Eucalyptus monocultures in Oyo State's derived savanna and recommends replacing Eucalyptus-dominated blocks with native hardwood species in commercial forestry and environmental planting programmes.

Keywords: nutrient cycling, litterfall, plantation forestry, native versus exotic trees, Oyo State