Solubilization of hardly soluble iron and aluminum phosphates by the fungus Aspergillus niger in the soil

Abstract

Some Brazilian soils present high contents of hardly soluble iron and aluminum phosphates and a high capacity for fixation of soluble phosphates. This study evaluated the ability of the fungus Aspergillus niger F111 isolated from soil to solubilize Fe and Al phosphates. Iron, aluminum or calcium phosphate were added to soil samples and inoculated with the A. niger F111. Sugar-cane molasses (2% v/w) was added as a carbon source on the 1st and 10th day of incubation. Soil samples without molasses, phosphates or fungus were used as control. Soil was incubated at 30 °C for twenty days and samples were removed every 5 days for determination of soil respiration (CO2 production), pH, titratable acidity, soluble phosphate and total carbohydrate contents. Soil respiration increased early on the first day after molasses addition and decreased thereafter to the minimum level. The largest contents of soluble phosphorus were observed on the 5th and 15th day of incubation, with the following sequence of phosphate solubilization: aluminum phosphate > iron phosphate > calcium phosphate > control. Another experiment was performed under the same conditions as described above using only aluminum phosphate as a source of phosphorus, with evaluations performed daily for 15 days. Aluminum phosphate solubilization was related to CO2 evolution, which increased on the 2nd and 12th day of incubation. Soluble phosphate increased on the 2nd and 11th day and titratable acidity increased on the 3rd and 11th day. Carbohydrates decreased after molasses application. The effect of solubilization of insoluble phosphates by the fungus depended on the addition of a carbon source (molasses) but decreased as soon as the carbon source was mineralized in the soil.