Abstract

We developed a smart N-fertilizer by integrating a selected N-enriching bacterial consortium within an alginate/maghemite (γ-Fe2O3) nanocomposite functioning as an active microhabitat, which improves microbial delivery and persistence, while γ-Fe2O3 nanoparticles (NPs) provide structural reinforcement and bioavailable iron (Fe) to support nitrogenase-related processes. Diazotrophic bacteria isolated from compost and incipient volcanic soil were screened under soil conditions, linking in vitro functional profiling to in situ N-enhancement, and revealing metabolic trade-offs that guided the assembly of a functionally complementary consortium affiliated with the genera Paraburkholderia, Bacillus, Curtobacterium, and Streptomyces. Synthetized superparamagnetic γ-Fe2O3 NPs (6.49 ± 2.51 nm) were incorporated into alginate beads at two concentrations (0.2% and 1% w/v) for consortium encapsulation (FA and FB, respectively). Scanning electron microscopy analysis showed homogeneous NPs. Soil incubation showed that the nanocomposite modulated consortium performance. FA emerged as the most effective treatment, significantly increasing soil N by 13% after 60 days compared to unfertilized soil and 9% relative to the alginate-encapsulated consortium. In contrast, FB showed slower N release, maintaining stable soil N levels from day 21 to 60. Overall, the designed consortium effectively reflected N inputs, whereas the alginate/γ-Fe2O3 nanocomposite (0.2% w/v) delayed nutrient release, prolonged effectiveness, and increased soil N pools. This approach represents a potential alternative to synthetic N fertilizers. Further research is required to evaluate their effects on plant growth and soil microbial communities.