3, Department of Chemical Engineering, Federal University of São Carlos, São Carlos, São Paulo, Brazil
2, EPAGRI-EEI, Empresa de Pesquisa Agropecuária e Extensão Rural de Santa Catarina, Itajaí, Santa Catarina, Brazil
Many commercial fertilizers are based on the application of elemental sulfur (S°) and oxides metallic to supply sulfate and micronutrients to plants. However, the soil ability to solubilize oxides and promote the S-oxidation is low, since it is dependent of the presence of microorganisms capable of carrying out such biological reactions. To overcome this limitation, different strategies have been explored to improve the availability of sources of nutrients of low solubility, such as oxides and S°. An interesting alternative is the application of microorganisms to promote the oxides solubilization and the S-oxidation in the field. The filamentous fungi Aspergillus niger has been applied to promote the phosphorus solubilization from phosphate minerals as well as bioremediation and bioleaching of metals from mining ores. Besides, previous studies also revealed that A. niger can promote the biological oxidation of S° as well as the oxides solbulization. However, the major challenge is to develop a material that would favor the integration of both oxides particles and microorganisms in a single granule to promote oxide solubilzation that could be processed and stored, thus when applied in the field the granule would be activated to allow microorganism growth to solubilize the oxide, this is an innovative concept of "bioreactor in granule". Herein, we proposed to produce a composite (granule) based on the dispersion of nanoparticles of S° and oxides in a polysaccharide matrix (polymeric gelatinized starch) with simultaneous encapsulation of A. niger spores. Three oxides model (ZnO, MnO and CuO) besides S° were used to produce different configurations of composites. The oxides were physical-chemical characterized as well as the composites produced. Experiments were performed to evaluate the effect of bio-activation of the composite by A. niger and consequent oxides solubilization and S-oxidation in a liquid medium and in soil. The results showed that all the different composites produced (St/Zn, St/Mn, St/Cu, St/Mix and St/Mix+S) increased solubility of the dispersed material. The effects of dispersion of the oxides and S° particles in starch matrix was analyzed using field emission gun scanning electron microscopy and X-ray microtomography, a high dispersion of the particulate materials was observed with low agglomerates formation. Nevertheless, the St/Mix+S show the best results for oxides solubilization, besides being a source of multi-nutrient (S, Zn, Mn and Cu). The St/Mix+S also showed considerable shelf time and when evaluated under greenhouse conditions, it showed an efficiency comparable to the experiment carried out with soluble commercial fertilizer for the cultivation of Italian ryegrass (Lolium multiflorum Lam.). This strategy opens a new route for development of smart fertilizer capable of making feasible the use of source of micronutrients of low solubility for plant nutrition.