Sewage sludge biochar-based fertilizers enriched with silicate agrominerals and oxalic acid: from nutrient solubilization to slow-release dynamics in soil

The growing reliance on fertilizers for global food production, combined with low nutrient-use efficiency, highlights the need for new, sustainable fertilizers derived from local sources. This study evaluated the release dynamics and leaching potential of phosphorus (P) and potassium (K) in sewage sludge biochar-based fertilizers (BBF) enriched with mica-schist (Mica) or phonolite (Phon) and varying concentrations of oxalic acid (OA) (0, 0.33, 0.67, and 1 mol L −1 ). Incubation (60 days) and leaching column (6 days) experiments were conducted, and P and K release was described using non-linear kinetic models. The addition of OA increased P and K release in the soil. K release exhibited biphasic behavior—a rapid initial phase followed by gradual release—whereas P was released predominantly during the initial phases. Fertilizers synthesized using biochar produced at 300 °C showed higher P and K release. Despite the increase in the soluble fraction, the BBFs exhibited lower K leaching compared to the conventional fertilizer (KCl), while P leaching was low across all fertilizers due to its limited mobility in the soil. The developed fertilizers demonstrated potential as slow-release nutrient sources, with OA concentration playing a decisive role in modulating release kinetics and the availability of P and K in the soil.