Phosphate-solubilizing capacity of two bacteria strains and its effect on maize growth and the phosphorus fractions in red soil

Objectives Phosphorous (P) is easily immobilized in soil and hardly converted to available P. The capacity of soil P activation and growth promotion of phosphate-solubilizing bacteria strains were studied for solving the problem in this paper.

Methods Burkholderia XQP35 (P35) and Raoultella SQP80 (P80), which have been identified as high P solubilizing strains, were used as test materials. The calcium phosphate in liquid NBRIP medium was replaced by aluminum phosphate, iron phosphate, calcium phytate and lecithin respectively as the phosphorus source treatments, and the pH of the liquid NBRIP medium was adjusted to 4, 5, 6, 7 and 8, respectively. The dissolved P amount was measured at 24, 48, 72, 96, 120 and 144 h of incubation. A soil pot experiment was conducted using maize as test material. P35, P80 and commercial bacteria (EM) were inoculated in soil as treatments, taking no strain as control. The maize growth and soil available P contents were measured at the 20, 40, 60, 80 and 100 days of maize growth. The neutral and acid phosphatase activities, and the contents of P fractions in the soil samples of 100 days were measured.

Results Both strain P35 and P80 were strong in activating calcium phosphate and calcium phytate, and poor in activating aluminum phosphate, iron phosphate and lecithin. The solubilizing capacity was affected by medium pH within 24 h for P35 and 48 h for P80, afterwards, the influence became less and the dissolved P amount reached stable with the incubation time. Strains P35 and P80 improved maize plant height, P uptake and dry weight significantly, and the plant dry weights were increased by 32% and 36%, compared with CK. At the 20, 40, 60, 80 and 100 days, the soil available P contents in both P35 and P80 strain treatments were always higher than those in CK and EM treatments, some time the differences were significant. While the soil available P contents in EM treatment never showed significant differences with CK. At the 100 day, the bacteria strains did not show significant effects on the activities of soil neutral and acid phosphatase, but on the contents of inorganic phosphorus fractions to different extent. P80 significantly increased H₂O-Pi content, and P35 sighificantly in creased NaOH-Pi and NaHCO₃-Pi contents, and the phosphorus activation coefficient was increased by P80 as well.

Conclusions Phosphorous solubilizing strains P35 and P80 have strong solubility to calcium phosphate and calcium phytate and could adapt wide range of environmental pH conditions. P35 is fast in activation of P, which might cause the re-immobilization of P in soil as showed in the increased content of NaOH- Pi and NaHCO₃-Pi at last. P80 is more effective than P35 as its relatively slow activation of P and mainly increase the content of H₂O-Pi, which is more conducive to the absorption and utilization of maize.