Controlled urea release from CMC-based composites: effects of citric acid crosslinking and biochar incorporation

Abstract

Improving nitrogen (N) use efficiency while reducing environmental losses remains a major challenge in agriculture. Here, we show that a single polymeric matrix based on carboxymethyl cellulose (CMC) can be effectively engineered to control urea release when structurally tailored with citric acid as a crosslinking agent and biochar as a physical barrier. The formulation containing 10 wt % citric acid (CMC/U/BC/CA 10) exhibited the best performance, reaching 7500% swelling and maintaining 60% of water retention after 120 h, while providing a significantly delayed release profile (~ 18% after 4 h and 80% at the end of the experiment), whereas pure urea dissolved rapidly. Higher CA levels were associated with lower swelling and greater matrix rigidity and morphological heterogeneity, which may favor preferential diffusion pathways and faster nutrient release (up to 90%). Kinetic analysis indicated that urea release involved an important diffusional contribution coupled with polymer-chain relaxation and formulation-dependent structural changes, rather than a purely Fickian transport mechanism. Under incubation conditions, the composites reduced measured N-NH₃ volatilization relative to pure urea and affected exchangeable N-NH₄⁺ dynamics. The study demonstrates that a single biodegradable polymer matrix can efficiently regulate nutrient release, providing a simple, scalable, and sustainable strategy for the development of advanced nitrogen fertilizers.