Synergy of Aspergillus niger and Components in Biofertilizer Composites Increases the Availability of Nutrients to Plants.

Intensive fertilization has been required to provide nutrients for plant growth under the current agricultural practices being applied to meet the global food demands. Micronutrients such as zinc, manganese, and copper are required in small quantities when compared to macronutrients (such as nitrogen, phosphorus and potassium), but they are essential for the plant growth cycle and consequently for increasing productivity. Mineral oxides such as ZnO, MnO, and CuO are used in agriculture as micronutrient sources, but their low solubility limits practical applications in plant nutrition. Similarly, elemental sulfur (S) can provide a high-concentration source of sulfate, but its availability is limited by the ability of the soil to promote S oxidation. We propose here the integration of these nutrients in a composite based on a biodegradable starch matrix containing mineral oxides and S in a dispersion that allowed encapsulation of the acidifying agent Aspergillus niger, a native soil fungus. This strategy effectively improved the final nutrient solubility, with the composite starch/S/oxide multi-nutrient fertilizer showing remarkable results for solubilization of the oxides, hence confirming a synergic effect of S oxidation and microbial solubilization. This composite exhibited an extended shelf life and soil-plant experiments with Italian ryegrass (Lolium multiflorum Lam.) confirmed high efficiencies for dry matter production, nutrient uptake, and recovery. These findings can contribute to the development of environmentally friendly fertilizers towards a more sustainable agriculture and could open up new applications for formulations containing poorly soluble oxide sources.