A Sulfur-Crosslinked Biopolymeric Matrix for Controlled Urea Release Enhances Maize Growth and Reduces Nitrogen Leaching

Modern agriculture faces major challenges driven by rapid population growth, climate change, and environmental concerns. Advanced polymeric architectures for controlled-release fertilizers (CRFs) are essential to mitigate these issues. Urea is one of the most widely used nitrogen fertilizers for field crops; however, its agronomic efficiency is limited by volatilization and leaching losses. In this study, we report a sustainable strategy to encapsulate urea using a matrix derived from industrial sulfur waste and vegetable oil, promoting both improved agronomic efficiency and the valorization of industrial residues and renewable resources. Through inverse vulcanization, we synthesized Bp-SF, a sponge-like polymeric material. Two bio-composites loaded with urea, Bp-SF25U and Bp-SF32U, were also prepared. FT-IR analysis confirmed urea encapsulation and the formation of polymeric structures from sunflower oil. SEM imaging revealed a porous morphology, while contact angle measurements confirmed the hydrophobic nature of the polymer matrix. Release kinetics studies demonstrated slow nitrogen release for more than 77 days, governed by diffusion. Pot experiments with maize showed that Bp-SF32U improved plant growth compared with conventional urea. These sulfur cross-linked biopolymers represent a promising approach to enhance urea efficiency while supporting greener fertilization strategies aligned with circular economy principles.