Cellulose nanocrystal-reinforced agar-based polysaccharide hydrogel for dual-barrier slow release of urea

The rapid release of conventional urea fertilizers results in low nutrient utilization efficiency and environmental contamination, highlighting the need for sustainable controlled-release systems. Herein, an agar-based dual-barrier hydrogel fertilizer (ACCMPU) was developed by incorporating polydopamine-modified urea-loaded mesoporous silica nanoparticles (MSNs@PDA-Urea) into a CNCs/CMC-Na-reinforced agar network. The PDA coating and polysaccharide matrix constructed hierarchical diffusion barriers to regulate urea transport at nanoscale and macroscopic levels. The incorporation of CNCs and CMC-Na enhanced the mechanical robustness of the hydrogel, increasing the compressive strength from 0.15 MPa to 0.33 MPa and elastic modulus from 1.15 MPa to 2.75 MPa. Compared with free urea released within 4 h, ACCMPU exhibited sustained nutrient delivery with only 92.56% cumulative urea release after 360 h. The effective diffusion coefficient decreased to 2.41 × 10^-12 m² s^-1, confirming the enhanced diffusion resistance of the dual-barrier structure. Moreover, ACCMPU maintained structural stability during soil degradation and promoted lettuce growth, increasing fresh and dry weights to 171.0 mg and 16.4 mg after 10 days of cultivation. This study provides a promising strategy for designing novel hydrogel slow-release fertilizers by integrating nanoscale encapsulation with agar-based hydrogel networks. Further field studies are required to evaluate long-term agricultural performance and practical scalability.