Contribution of Soil Fauna to Nutrient Cycling in Forest-Agriculture Transition Zones of Ondo State, South West Nigeria

This study investigates the contribution of soil macrofauna and mesofauna communities to nutrient cycling in forest-agriculture transition zones of Ondo State, located in Nigeria's South West geopolitical zone. Soil fauna play critical roles in organic matter fragmentation, microbial community structuring, and nutrient mineralisation, yet their functional contribution to nutrient dynamics in Nigerian forest margin agroecosystems is almost entirely unstudied, representing a significant research gap in soil ecology. Using a comparative field design, soil fauna communities were characterised in secondary forest, cocoa agroforest, and sole food crop plots across six sites in Akure, Idanre, and Owo local government areas. Macrofauna were sampled by monolith extraction (25 by 25 by 30 cm blocks), and mesofauna by Berlese-Tullgren funnel extraction. Earthworm biomass was measured separately. Fauna-mediated nutrient fluxes were estimated by combining fauna community biomass with published metabolic activity coefficients. Nitrogen mineralisation rates under fauna-excluding and fauna-present mesocosms were compared in a controlled incubation experiment. Findings revealed that earthworm biomass was highest in cocoa agroforest (mean 62 g/m2) and lowest in sole crop plots (11 g/m2). Estimated earthworm-mediated nitrogen mineralisation contributed 22 to 35 kg N per hectare per year in agroforest systems, equivalent to 18 to 29 percent of total measured mineralisation. Termite-mound soils showed significantly elevated potassium and phosphorus compared to surrounding soils, indicating nutrient concentration by termite activity. The study establishes quantitative baseline data on soil fauna nutrient cycling in Ondo State transition zones. Keywords: soil fauna, nutrient cycling, earthworms, agroforest, Ondo State.