Solubilization of inorganic phosphates and plant growth promotion by Aspergillus niger

Two of 187 fungal isolates (Aspergillus niger 1B and 6A) displaying superior phosphate (P) solubilization and hydrolytic enzyme secretion were studied using P forms of calcium (Ca-P), iron (Fe-P), and aluminum (Al-P). Phosphate solubilization in a sucrose-basal salt (SB) broth was increased and pH decreased by both isolates. In Ca-P medium, solubilization for 6A was approximately 322 μg P mL⁻¹ and pH decreased by 4.2 units to 2.3 in 72 h. However, when pH value of the SB broth was lowered to 2.5 using HCl, 65.3  ±  0.4 μg mL⁻¹ of P was released from Ca-P, whereas trace amounts of P were released from Fe-P and Al-P. Both isolates displayed enhanced Al-P solubilization using NH₄Cl rather than KNO₃ as the N source; final pH values were not significantly different. With Ca-P, gluconic acid was predominantly produced by 1B and 6A, whereas oxalic acid predominated with Fe-P and Al-P. Addition of gluconic acid (final concentration of 8.5 μmol mL⁻¹) to Ca-P-supplemented SB lowered pH (2.9) and solubilized phosphate (146.0 ± 1.0 μg mL⁻¹). Similarly, addition of oxalic acid (final concentration 6.6 μmol mL⁻¹) to Ca-P- and Fe-P-amended media solubilized P (60.2 ± 0.9 and 21.6 ± 2.1 μg mL⁻¹, respectively), although these quantities were significantly lower than those detected in unamended SB. The presence of unidentified P solubilized compound(s) in the dialyzed (MW>500) supernatant warrants further study. In pot experiments, significant increases in plant (Brassica chinensis Linn.) dry weight and N and P contents were observed with the addition of isolate 6A, when a small amount of organic fertilizer together with either rock phosphate (South African apatite) or Ca-P served as the main P sources.