Objective The aim was to explore the effects of different Bacillus treatments on the availability and morphological transformation of soil phosphorus (P) in red soil.

Method The experiment was divided into three groups, namely single application of Bacillus mucilaginosus (A), single application of Bacillus megaterium (B) and combined application of two bacteria (C). The inoculum doses of each group were 2.5%, 5%, 10% and 15%, respectively (quality score). The inoculum doses of single application were recorded as A1, A2, A3, A4, B1, B2, B3 and B4, respectively. The mixed applications of two bacteria were recorded as C1, C2, C3 and C4, and a blank control (CK) was added, thus the total treatments were 13. The effects of different bacterial treatments on soil pH value, organic matter, exchange P, organic P and inorganic P components were analyzed and compared.

Result ① The soil pH and content of organic matter were increased in different degrees by culturing with the target bacillus. The soil pH of two mixture bacteria treatment increased by 0.55 units in appeared in C3 treatment cultured to 45 days compared with the control. The contents of organic matter were higher in C3 and C4 treatments, which increased by 1.47 and 1.53 times, respectively. ② The contents of exchange P in soil increased during 30 ~ 60 days and 15 ~ 60 days, for two single treatments of Bacillus mucilaginosus and Bacillus megaterium, respectively. Compared with the single microbial agent treatment, the mixed treatment of both microbial agents showed a more significant effect on enhancing P availability. During the cultivation of 15 ~ 60 days, the mixed treatment demonstrated a more pronounced activation of soil insoluble P. The content of exchange P increase by 97.07% compared with the control on the 60^th day of cultivation, which showed a strong activation for insoluble P. ③ Compared with the control, the optimal inoculation treatment (C3) significantly increased the contents of liable-active organic P (LOP) and medium liable-active organic P (MLOP) on the 75^th day of cultivation, while decreased the medium-resistant organic P (MROP). ④ Correlation analysis of different soil indices showed that the inorganic P in test soil was mainly composed of Al-P and Fe-P. Fe-P is the correlation with the exchange P, showing that Fe-P is P source for available ones in soil.

Conclusion The effectiveness of mixed inoculation with Bacillus mucilaginosus and Bacillus megaterium at 10% inoculum size is the best one in all treatments, which could increase soil pH value and reduce soil acidity, as well as increased the content of exchange P, active organic P and inorganic P, prolonging the phosphorite-dissolving time. Fe-P is an available P source in soil. Those results provide important evidences for the efficient utilization of soil P.