Slow-release biochar-based urea: Formulation, characterization, and nitrogen release kinetics

Conventional urea application poses substantial environmental challenges. The development of efficient slow-release fertilizers is crucial for mitigating these issues. Six types of biochar-based nitrogen fertilizers (BBNFs), comprising different formulations of biochar, urea, and attapulgite through mixing (M) or adsorption (A) methods: M100 (100% urea+100% biochar), M65 (100% urea+65% biochar), M65 +35(100% urea+65% biochar+35% attapulgite), A100, A65, and A65 +35. The results revealed that compared with pristine biochar (BC), the BBNFs decreased the pH (3.47 units) and surface area, but increased the average pore width. Total porosity also decreased except with the M65 +35 treatment. In both hydrostatic testing and soil column experiments, the M groups (M100, M65, and M65 +35) exhibited consistently lower release rates (57.5% and 43.8%, respectively) than the A groups (A100, A65, andA65 +35), which showed release rates of 71.2% and 63.3%, respectively. No significant differences in N release were detected among the M groups, whereas the A groups significantly varied among treatments. The dissolved organic nitrogen (DON) (mainly urea) was the predominant form of N released, followed by NH 4 + –N. In water, N release from BBNFs was best described by the first-order model. In soil, total nitrogen (TN), DON and NH₄⁺–N release were generally better described by the pseudo-second-order model, whereas NO₃⁻–N release was best described by the Elovich model. Among all treatments, M100 and M65 +35 exhibited the most favourable sustained-release properties among all treatments. This study provides a laboratory-scale foundation for designing slow-release fertilizer formulations with reduced N leaching potential.