Nitrogen and Phosphorus Dynamics in the Lower Ogun River Catchment Under Agricultural Intensification, Ogun State

This study analyses nitrogen and phosphorus dynamics in the lower Ogun River catchment, Ogun State, South West Nigeria, under conditions of increasing agricultural intensification, empirically quantifying nutrient fluxes, identifying dominant export pathways, and evaluating the implications for reservoir eutrophication in the Ogun Reservoir that supplies water to Lagos. Agricultural expansion in the Ogun catchment has intensified synthetic fertiliser application, creating growing concern about non-point source nutrient loading that has been associated with increased algal growth in the Ogun Reservoir. A monitoring programme was established at twelve catchment stations covering both riverine and tributary sampling points, with fortnightly sampling over twenty-four months capturing seasonal and storm event-driven nutrient concentration patterns. Annual nutrient fluxes were computed by integrating concentration data with continuous flow records. Isotopic tracing using nitrogen-15 and oxygen-18 in nitrate was applied to distinguish agricultural from atmospheric and sewage-derived nitrate sources. Export coefficient modelling was applied to estimate contributions from different land uses. Results show that annual total nitrogen export averages 3.8 kilograms per hectare of catchment area, with agricultural land contributing 61 percent of the load despite covering 44 percent of the catchment. Nitrate is the dominant nitrogen form, and its isotopic signature confirms predominantly agricultural fertiliser origin in the main agricultural sub-catchment. Phosphorus export is episodic, dominated by storm event surface runoff from areas with high soil phosphorus saturation. The study provides quantified evidence for a catchment nutrient management programme. Keywords: nutrient dynamics, non-point source pollution, agricultural catchment, Ogun River, eutrophication.