Plant Bioindicators of Soil Quality in San Pedro (Southwest Côte d'Ivoire): A Toposequential Approach

Abstract

Agricultural land degradation in sub-Saharan Africa, particularly in Côte d’Ivoire’s ferrallitic soils, is being intensified by unsustainable farming practices, excessive reliance on synthetic inputs, and climate variability. This trend threatens soil health, agricultural productivity, and long-term food security. This study was carried out using a toposequential approach at Goréké (San-Pédro, southwestern Côte d’Ivoire), investigates the potential of plant species as bioindicators for the functional diagnosis of Acrisol quality affected by agricultural degradation dynamics. Conventional soil diagnostic methods, relying on laboratory-based physicochemical analyses, remain costly and scarcely accessible to local stakeholders. In this context, bioindicator plants represent a rapid, cost-effective, and integrative alternative for in-situ soil quality assessment. The experimental setup (2.6 ha) consists of a radial transect structured from a central junction point, encompassing three topographic positions (upper, mid-, and lower slope) representative of smallholder rubber cultivation and post-forest fallows (3-7 years). An integrated approach combining 45 floristic surveys (5 m × 5 m plots) and 9 soil profiles described according to the WRB reference system (IUSS, 2022) down to 1.20 m depth allowed linking the morpho-physicochemical characterization of soils with vegetation data. The bioindicator value of species was quantified using the IndVal index and validated through ROC curves. Among the 68 recorded species (31 families), 12 were identified as robust bioindicators (IndVal ≥ 0.60; AUC ≥ 0.76), enabling the establishment of operational abundance thresholds (3-8 individuals·m⁻²). Degraded soils (C1-C2) are dominated by nitrophilous Poaceae (Imperata cylindrica, Panicum maximum) and ruderal species (Chromolaena odorata, Euphorbia heterophylla), reflecting disturbed conditions and low soil quality. In contrast, intermediate-quality soils (C3) are associated with nitrogen-fixing Fabaceae (Centrosema pubescens, Pueraria phaseoloides) and Arecaceae (Raphia hookeri), indicating a relative improvement in soil nutrient status. These findings led to the development of an operational floristic reference framework, providing a reliable tool for rapid agropedological diagnosis of Acrisols in southwestern Côte d’Ivoire.