Hydrochar from rice straw as a bio-based slow-release fertilizer: Tuning temperature and oxidation for agronomic performance

Hydrochar from agricultural residues is a promising bio-based fertilizer product. We optimized rice-straw hydrochar slow-release fertilizers (SRFs) by varying hydrothermal temperature (200–280 °C) and post-oxidation (0–8 h), then compared them with a pyrochar SRF and a conventional fertilizer. The optimized hydrochar (H-240 and its oxidized H-6h derivative) showed high nutrient loading at 30.74 wt% of N and rich oxygenated surface groups favorable for nutrient binding. In water, the conventional fertilizer released up to 95 % of N within 1 h, whereas hydrochar SRFs cut the initial burst to 35 % (H-240) and 22 % (H-6h) and sustained release over 5–7 days; in sandy soil, they delayed 80–85 % cumulative N release from day 2 (conventional) to day 4–5. Pot experiments revealed that these optimized hydrochar-based fertilizers increased soil total N and organic matter by up to 63 % and 12 %, respectively, and enhanced nitrate retention by 26 %. Maize plants treated with H-240 and H-6h achieved 23–28 % higher shoot biomass, 21 % thicker stems, and up to 42 % greater root surface area relative to the chemical fertilizer control. Partial least squares structural equation modeling demonsrated that mesoporosity and surface oxidation enhanced nutrient loading, which strongly promoted soil fertility and nutrient retention capacity, and further plant nutrient uptake and growth. These results deliver design rules at 240 °C and 6 h oxidation to convert rice-straw waste into effective, circular SRF products that improve soil fertility and early crop growth. • Optimized hydrochar outperformed pyrochar in nutrient loading and retention capacity. • Hydrochar-based slow-release fertilizer improved soil fertility, maize growth, and nutrient uptake. • Mesoporous structure and surface oxygen groups drive hydrochar’s slow-release behavior.