Dynamics of soil chemical properties along oil palm chronosequences and natural forests in Central Sulawesi, Indonesia

Land use conversion from tropical forests to oil palm plantations substantially impairs soil fertility, yet the trajectory of soil chemical change across successive plantation ages remains poorly understood. This study investigated changes in soil organic carbon (SOC), nutrient availability, cation exchange capacity (CEC), and micronutrient dynamics along a chronosequence comprising primary forest, secondary forest, and oil palm plantations aged 6, 8, 10, and 12 years in Central Sulawesi, Indonesia. SOC declined consistently following forest conversion and exhibited no meaningful recovery after 12 years of cultivation. Soil pH remained strongly acidic across all land-use categories, with progressive acidification becoming more pronounced in older plantation stands. Available phosphorus exhibited considerable spatial variability, largely governed by Fe-P interactions under acidic conditions, thereby constraining phosphorus use efficiency despite external fertilisation inputs. Exchangeable base cations, particularly Ca, accumulated progressively with advancing plantation age, yielding elevated base saturation values; however, this apparent improvement in chemical fertility was not accompanied by corresponding increases in SOC or CEC, the latter exhibiting a declining trend in older plantation soils, reflecting diminished nutrient retention capacity over time. Micronutrient analysis revealed progressive accumulation of Fe, Mn, and Cu in long-term plantation soils, suggesting potential micronutrient imbalance and phytotoxicity risks. Collectively, the chronosequence analysis indicates a discernible transition from biologically regulated forest soils to input-dependent plantation systems characterised by persistently low SOC, declining CEC, and imbalanced nutrient dynamics. These findings underscore that conventional oil palm management practices are insufficient to restore soil chemical quality and may predispose these systems to long-term degradation, necessitating the adoption of sustainable soil management strategies that prioritise the restoration of organic matter and enhanced nutrient use efficiency.