Distribution Characteristics of phoD-Harbouring Bacterial Community Structure and Its Roles in Phosphorus Transformation in Steppe Soils in Northern China

The phoD-harbouring bacterial community is responsible for much of the hydrolysis of organic phosphorus (Po) in soils and is therefore significant for the improvement of soil phosphorus (P) availability. However, the distribution of phoD-harbouring bacterial community structure and how it regulates the soil P fractions in steppe soils remain largely unknown. It is necessary to assess these relationships to maintain sustainable development on the steppe. We sampled soils from three steppe types across Inner Mongolia, China. Illumina high-throughput sequencing was used to quantify the bacterial phoD gene. The dominant phoD-harbouring genera were Amycolatopsis (5–11%), Bacillus (6–13%), Bradyrhizobium (3–8%) and Pseudomonas (3–5%) across all steppe soils. The relative abundances of phoD-harbouring Amycolatopsis and Bacillus increased significantly as available P (AP) decreased, while the relative abundances of Bradyrhizobium, Pseudomonas and Methylobacterium were significantly positively correlated with AP content. Redundancy analysis showed that the soil stoichiometric ratio of carbon (C), nitrogen (N) and P had a strong effect on the phoD-harbouring bacterial community structure. Correlation analyses further indicated that only phoD-harbouring Dietzia and Sphingomonas had a significant correlation with alkaline phosphatase activity and that they increased P availability by mineralizing Po. phoD-harbouring Frankia and Methylobacterium were positively correlated with labile-Po and negatively correlated with non-labile inorganic P. Moreover, phoD-harbouring Bacillus and Bradyrhizobium promoted the conversion of the non-labile Po pool into the labile Po pool, which can be attributed to microbial immobilization. Not all bacteria carrying the phoD gene promote soil P availability through mineralization or are induced not only in a P-repressible manner. Members of the phoD-harbouring bacterial community employ flexible P use strategies and can be strongly activated by their nutrient preferences and environmental conditions.