Effect and Mechanism of Phosphate-Solubilizing Bacterial on Activating of Low-Grade Phosphate Rock Powder in Red Paddy Soil

The biological utilization of phosphorus (P) in low grade phosphate rock powder (PRP) is of great significance. The study explore the effect and mechanism of activating low-grade PRP by phosphate-solubilizing bacterial (PSB) which can provide basis for improving the fertilization effect of PRP on red soil paddy soil.

PRP with different particle sizes (0.18, 0.10 and 0.05 mm) was added to the red paddy soil collected in Hunan Province. Three treatments were set up: inoculated with Acinetobacter calcoaceticus (P1), Acinetobacter pittii (P2) and no inoculation with PBS (P0) as control. Bottles were cultured in a dark incubator at 25 ℃ for 60 days, during which different forms of P and pH were dynamically monitored. On the 60th day, the activity of organic acid, acid and alkaline phosphatase (ACP, ALP) and alkaline phosphatase gene (phoD) were determined to study the activation effect and mechanism of PSB on PRP with different particle sizes.

Both strains of PSB could activate insoluble P, but the activation effect was not significantly different. During the whole culture period, the average available P content of three particle sizes of PRP inoculated with PSB was 13.4-14.7 mg·kg^-1, which was higher than that of P0 treatment and increased by 31.1%-53.1%; The average content of available P increased by 53.1% and 47.5% after inoculation with P1 and P2 bacteria (P<0.05), respectively. The average content of Resin-Pi and NaHCO₃-Pi were 13.9-16.6 mg·kg^-1 and 14.9-16.5 mg·kg^-1, respectively, which were higher than those without inoculation, and increased by 36.4%-78.5% and 13.7%-25.0%, respectively; the increase of average content of Resin-Pi in 0.18 mm PRP was the most obvious, which was 78.5% and 49.5%, respectively (P<0.05). Compared with P0 treatment, the increase of active P in 0.18 mm PRP treatment was the most obvious, ranging from 28.4% to 46.7%, and the decrease of stable P was 2.1%-8.0%. Compared with the P0 treatment, inoculation of PSB significantly reduced pH value by 0.18-0.35 units (P<0.05) and increased acetic acid and propionic acid content by 5.2%-13.7% and 45.9%-127.5% (P<0.05), respectively. ALP content and phoD abundance under P1 treatment increased by 6.5%-13.4% and 24.0%-98.6% (P<0.05), respectively, and ACP in P2 treatment increased by 12.8%-17.2% (P<0.05), which indicated that P1 mainly secreted ALP, while P2 mainly secreted ACP. The results of correlation analysis showed that the two PSB strains dissolved insoluble Conc.HCl-Po, Conc.HCl-Pi and NaOH-Pi by secreting acetic acid and propionic acid, dissolved insoluble Conc.HCl-Po by secreting ACP and ALP, and converted into these insoluble P to Resin-Pi and NaHCO₃-Pi which promoted the turnover of P pool. The structural equation model showed that the addition of small particle size PRP and the inoculation of PSB could directly increase the soil available P content, but the inoculation of PSB had a greater effect on available P.

Inoculation of phosphate- solubilizing bacterial can promote the activation of insoluble phosphorus in phosphate rock powder with the biggest increasement of available phosphorus for particle size of 0.05 mm, and biggest increasement of active phosphorus proportion for particle size of 0.18 mm. The two phosphate-solubilizing bacteria A. calcoaceticus and A. pittii mainly secreted organic acids and phosphatases, such as acetic acid and propionic acid, activated insoluble phosphorus, increased active phosphorus content, and improved the application effect of phosphate rock powder in red paddy soil.