Solubilização biológica de rocha potássica para aplicação como biofertilizante

Abstract

Potassium (K) plays an important role in several biochemical and physiological processes in plants, but the low concentrations of soluble K in the soil make it necessary to use chemical fertilizers in order to supply the nutritional needs of the plants. However, the production of K fertilizers from the chemical processing of potassic rocks (RK’s) is considered expensive and can cause damage to the environment. In contrast, the biological solubilization of RKs has been shown to be a potential strategy for obtaining biofertilizers, aiming at more sustainable agriculture. In this strategy, microorganisms are used to promote the solubilization of RK, when applied to the soil or from microbial cultivation in vitro, with the production of organic acids in the medium being the main mechanism of solubilization of this process. In this context, this work aimed to study the mechanisms of dissolution of potassium rocks via organic acids and the development of strategies to increase the solubility of K present in RK from biological solubilization by submerged cultivation. This study was carried out using two model RK’s, rock 1 (Yorkshire, United Kingdom) and rock 2 (Minas Gerais, Brazil). The rocks were previously characterized by X-Ray Diffraction, X-Ray Fluorescence, Scanning Electron Microscopy and their Surface Area and Average Particle Size were determined. Subsequently, the RK’s were subjected to dissolution tests for 40 days in water and different organic acids (citric, gluconic and oxalic) to evaluate their effect on the dissolution of rocks. Finally, strategies for promoting the increase in K solubility through biological solubilization of RK from the action of microorganisms producing organic acids via submerged culture were evaluated. In this context, the solubilization potential of different strains of the filamentous fungus Aspergillus (A. niger C, A. niger F12, A. niger 3T5B8 and Aspergillus 763), the effect of concentration (1%, 2.5%, 5 % and 10%) of two carbon sources (glucose and glycerol) in the culture, the effect of the initial pH of the culture medium, and the effect of the interaction between microorganism-rock for a better understanding of the dynamics of solubilization. Finally, a study of the solubilization kinetics with natural RK and with RK was performed after a pretreatment of mechanical activation (grinding), for 10 days. From the results obtained in this work, the phases Polyhalite (rock 1) and K-Feldspar (rock 2) were identified in the characterization step presenting an order of dissolution of K in the organic acids of KCl (control)> RK Polyhalite> RK Feldspar. Organic acids did not significantly influence the dissolution of K present in RK Polyhalite, on the other hand they influenced the release of K from RK Feldspar, as well as the release of other nutrients from both rocks. RK Polyhalite reached 100% dissolution of K in all evaluated acids while RK Feldspar presented the highest percentage of dissolution in oxalic acid (20.05%) after 25 days. Given the lower dissolution of K presented, RK Feldspar was used in the biological solubilization step by A. niger C in which the variables 5% glucose, initial pH 4 and cultivation without the preculture stage were the conditions selected. The physical interaction between microorganism-rock was able to increase about 33% of the K solubilized in the medium compared to tests without the presence of the fungus. The solubilization kinetics of natural RK Feldspar reached a maximum solubilization of 15.87% K after 3 days, while the kinetics with RK Feldspar after pretreatment provided a 35% increase in K solubility, if compared with natural RK, reaching a solubilization of 20.66% of K on the first day. These results show the potentiating effect of biological solubilization on the release of insoluble K present in RK Feldspar.