Resumo

Increasing crop productivity in recent years has been associated to higher chemical inputs, specially concentrated fertilizers. Despite the high costs of fertilizer production, many sources of nutrients are non-renewable and becoming scarce at high concentrations, such as rock phosphate. Improvement of nutrient use efficiency by crops and utilization of alternative raw materials are essential for food safety in future. The solubilization of rock phosphate by sulfuric acid resultant from microbial oxidation of elemental sulfur (S⁰) is known for more than a century, however the native sulfur oxidizing bacteria (SOB) community in soil is highly diverse and SOB inoculation under field conditions is not efficient. The aim of this study was to develop a new biofertilizer allowing the use of low grade rock phosphate, S⁰ and SOB immobilized in a polymeric matrix for supplying sulfate (SO₄^2-) and phosphate (PO₄^3-) for crops. One autotrophic (SO1) and two heterotrophic (SO2 and SO3) SOB isolates were encapsulated with 10 g of S⁰ kg^-1 for S⁰ oxidation assay or 5 g of S⁰ kg^-1 plus 50 g of apatite kg^-1 for the rock phosphate solubilization assay. The microcapsules were distributed in glass flasks containing 100 g of coarse sand or clay soils. The microcosms were incubated at 28ºC during 0, 6, 22, 38 and 54 days. SO₄^2- was determined by turbidimetry and pH was measured with a pHmeter. PO₄^3- was determined using ion exchange resin. After 54 days incubation, the encapsulated isolate SO1 showed a significant acidification of the soil, and the highest S⁰ oxidation rate, SO₄^2- and PO₄^3- concentrations in both soils. SO₄^2- concentration reached 4247 and 1269 mg SO₄^2- dm^-3 and PO₄^3- reached 447 and 161 mg P dm^-3 in coarse sand and clay soils, respectively. The heterotrophic SOB isolates showed no statically significant changes in pH and SO₄^2- content after 54 days incubation. P solubilization by SO2 (71 mg P dm^-3) in the clay soil after 54 days of incubation was statically significant. The S⁰ oxidation rate and rock phosphate solubilization were affected by soil texture, being inversely proportional to clay content. The encapsulation of SOB is an alternative for the replacement of SO₄^2- in agricultural soils and for the in situ solubilization of low grade rock phosphate, showing a great potential as a biofertilizer in agriculture. This project was funded by FAPESP.