Chitosan/diatomite-coated core-shell biochar-based slow-release urea alleviates salinity stress and improves rice yield.

To tackle the persistently low nitrogen use efficiency of conventional urea in Saline-alkali soils, we developed a new core-shell structured fertilizer (CS/DE-BU). The novelty resides in the synergistic integration of a biochar-diatomite-urea core and a chitosan-diatomite (CS/DE) composite coating, establishing a robust dual-phase slow-release and salinity mitigation mechanism. The chitosan-based CS/DE coating, with chitosan serving as the primary film-forming matrix and embedded diatomite as a functional filler, serves as a primary, tunable physical barrier. Concurrently, the functionalized core acts as a secondary regulator, where the highly porous biochar and diatomite matrix not only adsorbs and retains urea but also actively immobilizes sodium ions via ion exchange, initiating remediation from within the granule. Soil column leaching experiments demonstrated that CS/DE-BU effectively prolonged nitrogen availability, meeting the slow-release criterion (cumulative release <75% within 28 days). Field validation showed that CS/DE-BU raised NUE to 62.81% and grain yield to 8.95 t/ha, substantially exceeding all controls. Simultaneously, it significantly reduced soil exchangeable sodium percentage and salinity, while improving key soil properties like organic matter and cation exchange capacity. An integrated economic analysis further validated its feasibility for large-scale application. This work provides a potent strategy for co-enhancing fertilizer efficiency and ameliorating soil salinity in sustainable agriculture.