Sustainable hybrid alginate-based hydrogels for dual controlled release of micronutrients and plant growth-promoting bacteria in soil

The intensification of agriculture to meet rising food demand has caused environmental degradation and climate change, largely due to excessive use of fertilizers and pesticides, which threaten soil and water quality as well as ecosystem stability and food security. Polysaccharide-based hydrogels have emerged as promising carriers for the controlled delivery of agricultural inputs. This study reports the development of hybrid hydrogel spheres based on sodium alginate (SA) and xylan (Xyl) for the simultaneous delivery of ZnO nanoparticles (ZnONPs, ~19.8 nm) and the plant growth-promoting bacterium Azospirillum brasilense. SA spheres showed encapsulation efficiencies of 69.2% for Zn and 72.8% for A. brasilense, while SA/Xyl spheres achieved higher efficiencies of 73.3% and 86.1%, respectively. The spheres (~3 mm) displayed well-defined morphology, and Zn distribution was confirmed by energy-dispersive X-ray spectroscopy (EDS). FTIR indicated interactions among the hybrid matrix components, and DSC revealed changes in thermal behavior. Release kinetics over 60 days showed anomalous transport for Zn and super case II transport for A. brasilense. Encapsulation enhanced bacterial viability, reaching 2.83 × 10⁷ CFU mL^-1 (SA) and 7.17 × 10⁷ CFU mL^-1 (SA/Xyl) after 1400 h, compared to 2.9 × 10⁴ CFU mL^-1 for free bacteria. Biodegradation assays indicated slightly higher degradation for SA/Xyl (~46%) than SA (~43%). Maize assays revealed modest differences among treatments, with SA/Xyl spheres promoting root growth and Zn uptake. These findings demonstrate that the hybrid hydrogel system successfully integrates nanomaterials and beneficial bacteria, improving Zn bioavailability and offering a sustainable approach to enhance agricultural efficiency.