Selection and characterization of soil fungi for an insoluble mineral solubilization and plant growth promoting in biofertilizer development

Abstract

Elements are essential for plant growth and crop production. Only 1 to 5% of elements in the soil are in a soluble form and can be utilized by plants. Some soil fungi play an important role in solubilizing elements to plants. The present study was conducted to isolate, evaluate and develop biofertilizer products from mineral solubilizing fungi for promoting the growth of selected economic plants. Three soil samples were collected from agricultural areas in Mae Wang District, Chiang Mai Province, northern Thailand. A total of 87 fungal strains showed positive phosphate solubilization and 17 strains showed high phosphate solubilization ability. All selected fungal strains were able to solubilize the insoluble mineral form of calcium, cobalt, copper, ferric, magnesium, manganese, zinc, feldspar, and kaolin in the agar plate assay. Consequently, high insoluble mineral solubilization were observed in fungal strain SDBR-CMUI1, SDBR-CMUI4 and SDBR-CMUO2. These fungal strains could solubilize the various insoluble mineral forms into available forms in liquid medium by production of oxalic acid, tartaric acid, and succinic acid. Seven fungal strains were selected and identified using a polyphasic taxonomic approach with multilocus phylogenetic and phenotypic (morphology and extrolite profile) analyses. All selected fungal strains were identified to new species in the genus Aspergillus section Nigri, Aspergillus chiangmaiensis (SDBR-CMUI4 and SDBRCMU15), A. pseudopiperis (SDBR-CMUI1 and SDBR-CMUI7) and A. pseudotubingensis (SDBR-CMUO2, SDBR-CMUO8, and SDBR-CMU20). Besides three new Aspergillus, a new species of mineral solubilizing fungi isolated from soil has been described herein as Apophysomyces thailandensis (SDBR-CMUS24, DBR- CMUS26, and SDBR-CMUS219). Aspergillus chiangmaiensis SDBR-CMUI4, A. pseudopiperis SDBR-CMUIl, and A. pseudotubingensis SDBR-CMUO2) were investigated on plant growth promoting properties. The results revealed that all fungal stains were also able to solubilize the insoluble phosphate sources. Interestingly, A. pseudotubingensis SDBR-CMUO2 could produce indole-3-acetic acid (33.37 µg/mL). All strains were positive in terms of siderophore and extracellular enzymes (amylase, cellulase, laccase, phosphatase, and protease) productions. Moreover, all fungal strains could grow in the presence of 17-18% NaCl for the investigation of salinity tolerances. The optimal conidia production, A. pseudopiperis SDBR-CMUI1 exhibited good production of conidia on oatmeal at pH 7.0, while A. chiangmaiensis SDBR-CMUI4 and A. pseudotubingensis SDBR-CMUO2 on oatmeal at pH 8.0 and the optimum incubation temperature at 35 °C. Additionally, A. pseudopiperis SDBR-CMUI1 and A. pseudotubingensis SDBR-CMUO2 able to tolerate commercial insecticide (methomyl and propargite) at the recommended dosages for field applications. However, A. chiangmaiensis SDBR-CMUI4 could not tolerate any kind of tested agrochemicals at the recommended dosages. The addition of conidia of selected fungi in soil samples supplemented with the insoluble phosphate and potassium under laboratory condition could increase the amount of the available phosphorus and potassium in soils. The development of biofertilizer formulations using carrier materials found that the suitable formulation was a mixture of vermiculite, perlite, and peat moss at a ratio of 5:2:3 (w/w/w). The granule biofertilizer was stored at various temperatures (4 to 50 °C) for five months and shelf life of biofertilizer was evaluated. It was found that the viability of all mineral solubilizing fungi in biofertilizer was higher than 50% at 4 and 20 °C after 3 months of storage. Moreover, the values of pH, electrical conductivity, strength, and density of all granule biofertilizer during storage were in the range of biofertilizer standard. The application of biofertilizer to plant did not cause any disease symptoms. All selected fungal strains significantly increased the leaf number, dried biomass of shoot and root, chlorophyll content, and cellular inorganic phosphate content in Arabidopsis, onion, strawberry, tomato, marigold, and green oak lettuce plants under supplementation with insoluble mineral phosphate. Additionally, the inoculation of selected fungal strains also improved the yield and chemical constituents (total soluble solids, titratable acidity, soluble sugar, ascorbic acid, total anthocyanins, total phenolics, total flavonoids, DPPH radical scavenging activity, quercetin content, lycopene content, carotenoids, lutein content, soluble protein, and nitrate content) of onion bulbs, strawberry fruits, tomato fruits, marigold flowers, and green oak lettuce. Therefore, the biofertilizer produced from the selected fungal strains reveal the potential in plant growth promotion agents that can be applied in many plants in pots and in the field of agriculture.