Green-Engineered Zinc Fertilizers from Soy Proteins: Boosting Crop Nutrition with Low Environmental Impact

Introduction: Micronutrient malnutrition in crops is a persistent obstacle to global food security and the development of sustainable agricultural systems. Among micronutrients, zinc plays an important role in several biological processes affecting plant development, and its limitations in soil are widely known. In this context, the development of soy protein-based matrices as biofertilizers enriched with Zn emerges as an innovative and sustainable solution.

Methods: Soy protein-based biofertilizer matrices enriched with Zn were developed via incorporating (i) zinc sulphate (ZnSO₄-H₂O) as a reference inorganic salt, (ii) chemically synthesized Zn nanoparticles, and (iii) green nanoparticles synthesized with reducing agents of plant origin, using agro-industrial by-products. Nanoparticles were synthesized via colloidal precipitation using ZnCl₂ as a precursor and NaOH (chemical NPs) or a polyphenol-rich extract from pepper waste (green NPs) as reducing agents. The matrices were evaluated for controlled Zn release, physicochemical stability, water retention, and mechanical integrity, and tested in Capsicum annuum cultivation. A cradle-to-grave life cycle analysis (LCA) was also conducted to assess environmental impact.

Results: The green nanoparticle system exhibited superior performance in controlled Zn release, water retention, and mechanical stability. When tested in pepper cultivation, these matrices enhanced foliar Zn content and reduced irrigation needs without compromising plant growth. The LCA revealed that the green formulations had the lowest environmental impact compared to the other two strategies, with low energy demand, reduced Global Warming Potential (GWP), and lower toxicity. This was attributed to the use of eco-compatible synthesis routes and low-impact raw materials.

Conclusions: These results demonstrate that green-engineered soy protein carriers are a promising, scalable technology for crop biofortification. They contribute to more efficient, low-impact farming practices aligned with the goals of sustainable food systems.

Acknowledgments: The authors acknowledge the financial support received from the Spanish Government (MICIU/AEI/10.13039/501100011033/ERDF/EU) through the sponsored project PID2021-124294OB-C21.