Depth-stratified soil organic carbon dynamics across forest–cultivation gradients in Nkanu East Watershed, Southeast Nigeria: implications for agroecological management

Résumé

Background Forest-to-cultivation conversion in tropical watersheds drives soil degradation through altered carbon dynamics and physical compaction, yet depth-stratified assessments that account for subsoil contributions remain scarce. Objective This study quantified horizon-level changes in soil organic carbon (SOC), bulk density, and physicochemical properties following forest-to-cultivation conversion across two watershed locations (Nkerefi/Ihukulu and Ugbawka/Amauzam) in Enugu area, Southeast Nigeria. Methods Profile pits were excavated to 130 cm depth in April 2023 across a factorial design of two locations, two land uses, and three diagnostic horizons (A, B, C), yielding 24 horizon samples from eight profiles. The cultivated sites had been under continuous cultivation for 40–50 years without fallow periods, organic amendments, or conservation tillage. Soil organic carbon, bulk density, nutrient concentrations, and cation exchange capacity were determined, and horizon-specific carbon stocks were computed. Results Organic carbon declined 37.7–52.8% from A to C horizons across all treatments; the steepest decline (52.8%) occurred under cultivation at Ugbawka. SOC stocks decreased 37.7–52.8% after cultivation relative to forest at equivalent horizons, with location-specific trajectories. Total profile SOC stocks (0–130 cm) ranged from 175,796 to 253,524 kg C ha⁻¹, with Ugbawka exceeding Nkerefi by 44% on average. The A horizon contributed only 27–49% of total profile stocks, indicating that 51–73% of SOC resides in subsoil horizons. Cultivation increased A-horizon bulk density by 31.6% at Nkerefi but had no comparable effect at Ugbawka. Three-way ANOVA confirmed significant Location × Land Use × Horizon interactions for organiccarbon, bulk density, pH, and total nitrogen. Conclusion These findings indicate that surface-only sampling protocols miss the majority of profile carbon stocks and that agroecological management in tropical watersheds must be calibrated to site-specific soil conditions, with location effects dominating over land use effects in determining soil health trajectories.

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