BIOCHAR INCLUSION IN FERTILIZERS ALTERS NUTRIENT LEACHING DYNAMICS IN SOIL

Nutrient leaching is a major limitation of mineral fertilizers, leading to off-site losses, reduced nutrient use efficiency, and increased risk of environmental contamination. Biochar-based fertilizers and biochar amendments may alter solute transport and nutrient speciation through changes in physicochemical properties and interactions involving soil organic matter (SOM), potentially redistributing nutrient release over time. This study assessed how biochar-based fertilizer formulations modify nutrient leaching dynamics using a column experiment comparing mineral fertilizer, four biofertilizers formulated with biochar derived from different biomasses (filter cake, poultry litter, sewage sludge, and carbon fines), and a mixture of biochar and mineral fertilizer. Leaching was monitored over 72 days, and nutrient concentrations were analyzed by TOCN and ICP-OES. Monophasic and hybrid (monophasic + linear) models were applied to distinguish rapid and late-release fractions. Phosphorus and potassium leaching were best described by the hybrid model, indicating an initial readily available content, followed by continuous slow release, whereas calcium and magnesium were better represented by a monophasic model truncated at 36 days. Biochar-based formulations reduced the rapidly leachable fraction of P relative to synthetic fertilizer and increased the contribution of late-stage release, resulting in greater retention after leaching, particularly when nutrients were co-formulated rather than simply mixed. For K, Ca, and Mg, formulations promoted higher proportions of late-release, attributed to electrostatic interactions, ion exchange, and mineral associations within the organic matter of biochar matrix. Overall, biochar formulation modulated nutrient release dynamics and temporal redistribution, consistent with enhanced retention processes mediated by mineral–organic interactions and SOM-associated binding sites.