Potential of yak dung-derived hydrochar as fertilizer: Mechanism and model of controlled release of nitrogen

Highlights: • Yak dung hydrochar and modified hydrochar were prepared and characterized. • HCl modification improves N slow release by rising function groups content. • Acidic groups could combine N through hydrogen bonds or electrostatic interactions. • Phenolic hydroxyl has largest positive correlation with N slow release. • Statistical analysis was conducted to reveal the quantitative relationship. Improving fertilizer efficiency with assistance of biochar has drawn much attention in sustainable agriculture. Promoting slow-release properties of biochar itself with cost-effective production technology is a pressing demand. In this study, hydrochar derived from nutrition-enriched yak dung (HC) and corresponding controlled release nitrogen fertilizer (HCRNF) via HCl modifying were studied, and the slow release performance as well as mechanisms were investigated. The results show that HCRNF presents a better N controlled-release performance with cumulative N release amounts of 56.01%–70.30% compared with 72.60%–78.45% of HC. The specific surface area reached highest 47.161 m²·g⁻¹ in HCRNFs with the pore volume of 0.098 cm³·g⁻¹. Hydrochloric acid modification treatment increases the surface acid group contents such as phenolic hydroxyl group content increasing to 1.48 mmol·g⁻¹ in HCRNF250. Because the porous structure and stable internal force between N and O-containing functional groups are improved, the N desorption from HCRNF is retarded, which shows a controlled release behavior. We concluded that the HCRNF via HCl modification in this work has a great application potential as slow released N fertilizer in sustainable green agriculture.