Melt-Processed Biodegradable Cellulose Nanofibrils–Thermoplastic Starch Composites for Enhanced-Efficiency Monoammonium Phosphate Fertilizers

The increasing demand for sustainable agricultural practices requires environmentally friendly technologies that improve nutrient use efficiency while reducing environmental losses. In this study, cellulose nanofibril (CNF)–thermoplastic starch (TPS) composites with biodegradation potential were developed as matrices for enhanced-efficiency fertilizer (EEF) formulations based on monoammonium phosphate (MAP). MAP was incorporated into CNF-based microparticles by spray drying and subsequently dispersed in TPS through melt-mixing and extrusion. Structural, morphological, thermal, and processing analyses showed that incorporation of the MAP-CNF microparticles modified their organization and interactions within the TPS matrix without evidence of MAP degradation under the processing conditions employed. Ion-release assays in water demonstrated progressive reduction in nutrient release from MAP to MsMAP and TPS/MsMAP, with complete release occurring after 10, 30, and 50 min, respectively, while extrusion further modified the release behavior of F-TPS/MsMAP. Agronomic validation using Capsicum chinense under protected cultivation showed adequate foliar phosphorus concentrations (4.90–5.57 g kg–1), indicating that phosphorus released from the EEF material remained available for plant uptake. Respirometric analysis also provided evidence of the biodegradation potential of TPS/MsMAP. The results demonstrate the feasibility of integrating spray-dried MAP-CNF microparticles into a thermoplastically processable TPS matrix, modifying nutrient release behavior while maintaining phosphorus availability to plants.