Dose-dependent effects of Conocarpus erectus biochar on the physicochemical and fertility properties of industrial acidic soil

Abstract Acidic soils influenced by industrial activities often constrain plant growth due to high exchangeable acidity, low nutrient availability, and poor physical structure. This study examined the short-term, dose-dependent effects of Conocarpus erectus biochar on the physicochemical and fertility properties of an acidic industrial soil using a four-week greenhouse incubation. Biochar was applied at 0, 1, 2, and 4% (w/w), and key soil chemical, physical, and nutrient parameters were evaluated. Increasing biochar rates (0, 1, 2 and 4% w/w) resulted in progressive improvements in key physicochemical and fertility‑related properties, including higher pH, soil organic carbon, cation exchange capacity, water holding capacity and nutrient availability, together with reduced bulk density. Soil pH increased from 5.53 to 6.40, accompanied by a moderate rise in electrical conductivity that remained within non-saline limits. Soil organic carbon was more than double in the 4% biochar treatment (1.924%) relative to the unamended control (0.704%), reflecting the strong contribution of Conocarpus biochar to the soil organic carbon pool. Biochar application also reduced bulk density and improved porosity, surface area, and water-holding capacity. The availability of phosphorus, potassium, mineral nitrogen fractions, and micronutrients (Fe, Mn, Zn, and Cu) increased significantly with increasing biochar dose. Within the tested range, biochar application rates of 2–4% were most effective under controlled conditions. These findings support the potential of Conocarpus erectus biochar as a sustainable amendment for improving acidic soil properties, while highlighting the need for longer-term field validation.