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Controlled-release fertilizers (CRFs) are sustainable alternatives as they can increase crop yield and minimize environmental contamination associated with conventional fertilizers. However, there remains a demand for the development of CRFs with high biocompatibility, and tunable morphologies and mechanical properties. Herein, a solvent-free mechanochemical method is developed for synthesizing urea-encapsulated metal-phenolic networks (urea-MPN matrices) as CRFs. The matrices exhibit tunable mechanical resistance, crystallinity, stiffness, and wettability properties via rearranging the internal structure of the MPNs and their subsequent interaction with the encapsulated urea crystals. Sample aging (7 days) leads to a higher degree of complexation of the MPNs, resulting in a material with increased elasticity and melting point relative to the as-synthesized sample. Thermal treatment (60 °C for 6 h) instigates structural reorganization of the urea crystals within the matrix, generating a more robust material with a 51-fold increase in Young’s modulus. As CRFs, the urea-MPN matrices can be tuned to prolong the release of urea for up to 9 days depending on the treatment applied. As the mechanochemical synthesis of MPNs facilitates the tuning of physiochemical properties and has greater practicability for inclusion within large-scale processing, it has potential implementation within a broad range of industries.

Nitrogen losses in agricultural systems can be reduced through enhanced-efficiency fertilizers (EEFs), which control the physicochemical release from fertilizers and biological nitrogen transformations in soils. The adoption of EEFs by farmers requires evidence of consistent performance across soils, crops and climates, paired with information on the economic advantages. Here we show that the benefits of EEFs due to avoided social costs of nitrogen pollution considerably outweigh their costs—and must be incorporated in fertilizer policies. We outline new approaches to the design of EEFs using enzyme inhibitors with modifiable chemical structures and engineered, biodegradable coatings that respond to plant rhizosphere signalling molecules.