Assessment and evaluation of soil ecosystem services in forest and adjacent cultivated lands in Nkanu East Microclimate Watershed, Southeastern Nigeria

Résumé

Land-use conversion from tropical forests to agriculture represents one of the most widespread environmental transformations globally, with profound implications for soil ecosystem services. In West Africa, extensive forest conversion has led to soil degradation and loss of critical ecosystem services. This study quantified land-use change impacts on soil ecosystem services using static Soil Function Assessment (SFA) in the Nkanu East watershed, southeastern Nigeria. Sixteen soil profile pits were excavated to 1.5 m depth across four communities (Nkerefi, Ugbawka) underlain by contrasting parent materials (Ajali Formation and Awgu Shale). Eight soil quality indicators—texture, organic carbon, pH, bulk density, hydromorphic stage, stone content, profile depth, and horizon depth—were measured using ISO/IEC 17025:2017 certified laboratory procedures, scored using nonlinear sufficiency functions, and integrated into a Soil Ecosystem Service Function Index (SESFI). Statistical analysis employed ANOVA with effect size calculations (Cohen’s d, partial η²) and bootstrap-derived 95% confidence intervals. Forest soils significantly outperformed cultivated soils across all indicators: organic carbon (2.51 ± 0.13% vs. 2.01 ± 0.11%; Cohen’s d = 2.18, p < 0.001), pH (6.0 ± 0.08 vs. 5.4 ± 0.10; Cohen’s d = 2.12, p < 0.001), and bulk density (1.32 ± 0.03 vs. 1.45 ± 0.04 g cm⁻³; Cohen’s d = 1.85, p < 0.001). These differences yielded a dramatic 24-fold SESFI difference (forest: 0.259 ± 0.024 vs. cultivated: 0.011 ± 0.003; Cohen’s d = 3.45, p < 0.001; partial η² = 0.89), representing 96% multifunctionality loss. Forest soils demonstrated 40–60% higher infiltration capacity, superior nutrient cycling, and enhanced carbon sequestration potential. All degradation is attributable to management impacts—tillage-induced compaction, organic matter depletion, and acidification—rather than inherent limitations. The magnitude of ecosystem service loss (96% SESFI reduction) underscores critical needs for forest conservation and soil rehabilitation. Integrated management approaches, including organic matter enrichment (5–10 Mg ha⁻¹ yr⁻¹), conservation tillage, and agroforestry, are projected, based on SESFI sufficiency-curve behaviour and published long-term trials in comparable tropical systems, to restore on the order of ~15–20% of the lost SESFI within a decade. This trajectory represents a model-based projection rather than a measured outcome at this site, and it requires validation through longitudinal monitoring. The standardized SFA methodology enables cross-regional comparisons and supports evidence-based land-use planning for tropical watersheds facing similar transformation pressures.

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