Non-linear soil health trajectories in smectitic Vertisols: Coupling sedimentary inheritance to long-term intensive cultivation in Am-Timan, Southeastern Chad

Abstract

Long-term cultivation can substantially reorganize soil physicochemical functioning in semi-arid agroecosystems, particularly in smectitic Vertisols developed from fine-grained alluvial deposits. This study assessed depth-dependent soil quality trajectories and the influence of sedimentary inheritance along a cultivation chronosequence in Southeastern Chad, comprising an uncultivated reference and fields cultivated for approximately 25, 35, and 50 years under continuous flood-recession sorghum production with glyphosate-based weed management. Soil samples were collected at 0–20, 20–40, and 40–60 cm depths and analysed for bulk density, electrical conductivity, particle-size distribution, soil organic matter and total nitrogen, available phosphorus, exchangeable base cations, cation exchange capacity, and derived soil quality indicators. Multivariate analyses, including principal component analysis and hierarchical clustering, revealed non-linear, vertically structured soil quality trajectories rather than a monotonic degradation gradient. The 25-year stage represented an early adjustment phase, characterized by profile-wide compaction, elevated pH, and incomplete structural recovery. The intermediate 35-year stage appeared to correspond to a transient phase exhibiting the strongest coupling between soil structure, base saturation, nutrient redistribution, and organic matter–nitrogen dynamics throughout the profile. In contrast, the 50 years stage showed evidence of long-term alteration, with weaker structure–fertility interactions, progressive, base-cation depletion, potassium limitation, and emerging subsoil constraints despite persistently high inherent cation exchange capacity inherited from the smectitic parent material. These results suggest that soil health in smectitic Vertisol is governed not by clay content or exchange capacity alone, but by the maintenance of effective exchange-site occupancy, aggregation, and nutrient cycling throughout the soil profile. Overall, the findings indicate that long-term cultivation reorganizes rather than overrides the inherited sedimentary template, resulting in non-linear and depth-dependent soil quality trajectories. From a management perspective, sustaining these systems requires residue retention, organic inputs, replenishment of base-cations and potassium, controlled traffic on wet clay soils, and regular monitoring of subsoil properties to preserve both surface and subsurface functioning in Sudano–Sahelian agroecosystems.