Optimizing biochar-based slow-release fertilizers (BSRFs): Comprehensive impacts of binder types, pyrolysis temperatures, and nutrient formulations on mechanical strength and nutrient release dynamics

This study systematically explored the synergistic effects of binder types, pyrolysis temperatures, and fertilizer compositions on the structural integrity and nutrient-release dynamics of biochar-based slow-release fertilizers (BSRFs) derived from corn stalks. Utilizing calcium bentonite (CB), carboxymethyl cellulose (CMC), soluble starch (SS), and kaolinite (Kao) as binders, BSRFs were prepared at pyrolysis temperatures ranging from 300℃ to 600℃ and integrated with CO(NH 2 ) 2 , (NH 4 ) 2 HPO 4 , or KCl. The results showed that BSRFs bound with CMC exhibited superior compressive strength (16.78 N) and smooth surfaces due to hydrogen bonding and cross-linking, while Kao-bound BSRFs developed porous spherical structures with a loose morphology, and the compressive strength only reached 5.9 N. Pyrolysis temperatures above 500℃ facilitated lignocellulosic decomposition in biochar, merging micropores into meso-/macropores and thinning the carbon skeletons. Kao-bound BSRFs achieved the highest total nutrient content (28.4 %), while CMC formulations optimized nitrogen retention (21.3 %). In soil column leaching tests, over a 10-day period, biochar coatings reduced leaching of urea, P 2 O 5 , and K 2 O by 39.2 %, 15.7 %, and 37.6 %, respectively, adhering to ExpAssoc, Logistic, and Allometric1 models. Principal component analysis identified pyrolysis temperature (550℃) and biochar-binder interactions as key factors influencing nutrient bioavailability. These findings emphasized that binder chemistry and pyrolysis-driven pore evolution were critical to the performance of BSRFs. The study offered valuable design principles for harmonizing nutrient release with crop demand while recycling agricultural residues effectively. • CMC binder ensured highest compressive strength (16.78 N) in BSRFs through cross-linking. • Biochar coatings reduced urea, P 2 O 5 , and K 2 O leaching by 39 %, 16 %, and 38 %. • Kaolinite enhanced P fixation via Al 3 + –PO 4 3- complexation and mesopore formation. • PCA identified 550℃ as optimal for balancing aromaticity and nutrient retention.