Most phosphorus exists in saline-alkaline soils in the form of insoluble phosphates and the content of available phosphorus is low. Chemical phosphorus fertilizer is often applied in farming to meet the nutritious requirement of crop growing. However, saline-alkaline soils are rich in inorganic salts and poor in organic matter, and are prone to crusting, making application of inorganic fertilizer unsuitable. Phosphate-solubilizing microbes are a class of micoroganisms that transform the insoluble phosphates into available phosphorus, who provide a measure for improvement of soil nutritious properties. Caragana Fabr. are shrubs cultivated to improve saline-alkaline soils, which enhance the availability of nitrogen, potassium and phosphorus in soil.
In our study, the soil sample from rhizospheres of Caragana plants grown on saline-alkaline wasteland was collected and used for culture to screen the phosphate solubilizing fungus and investigate its phosphate solubilizing proprties. The tricalcium phosphate agar medium was used. The colonies with clear halos were considered to be phosphate solubilizing fungus in the screening. One strain of fungus with phosphate-solubilizing ability was isolated in the current paper. The ITS sequence of the strain determined by PCR was of 100% homology with that of Aspergillus niger when compared with the known sequence in NCBI database using pairwise BLAST. The isolated strain was identified as Aspergillus niger on the basis of morphological characteristics and ITS genetic sequence analyses. To study the phosphate-solubilizing ability of the strain, the pH value and available phosphorus content of inorganic phosphorus culture medium, the weight of mycelium and the absorbed amount of phosphorous by the fungus were measured during a culture period of 168 hours. The pH value dropped sharply from 7.0 to 2.0 within the first 36 hours of culture, and then generally leveled off during the rest of the culture. This process provides an acidic condition for the solubilizing of insoluble phosphates and is probably one of the main reasons for the changes in the rhizosphere of Caragana plants. At the same time, the available phosphorus content in culture increased to 4.7mg at the 36th hour of culture and remained at this level for another 96 hours, then underwent a slight increase in the last 24 hours of culture. This indicates a high phosphate solubilizing ability of the strain. The weight of mycelium reached its peak (0.32g) at the 60th hour of culture and stayed at around 0.20g for 60 hours before reached a second peak (0.27g) at 144th hour and then decrease to 0.20g at the end of culture. On the other hand, the absorbed amount of phosphorous by the fungus decreased dramatically from 5.4 mg dropped to 0.5mg during the first 48 hours of culture, then stayed leveled off until the end of culture, indicating that the absorbance and the release of phosphorous later on by Aspergillus niger may be one of the mechanisms for the transformation of insoluble phosphates into available phosphorus by the fungus. These data evidence that Aspergillus niger isolated from the rhizosphere of Caragana plants is able to utilize the insoluble phosphates in soil and transform it into available phosphorus for the plants, thus help with the remediation of soil biology, growth of plants and improvement of soils. The results also provided scientific evidence for the improvement of saline-alkaline soil environment by Caragana plants and the development of biological fertilizer made from Aspergillus niger.