Enhancement of Soil Phosphorus Bioavailability by Arbuscular Mycorrhizae and Earthworms Through Regulating Soil Bacterial Community and Plant Nutrient Balance Under Salt Stress

Aims: Soil salinization is an important factor limiting plant phosphorus (P) uptake and crop production. This study aimed to investigate the effects of arbuscular mycorrhizal fungi (AMFs) and earthworms in enhancing soil P bioavailability by regulating soil salt ions and altering the soil bacterial community under salt stress. Methods Treatments with or without earthworms and with or without AMFs in a high-salinity soil were applied. Results The results showed that the maize biomass and plant P, Ca and Mg contents were significantly increased by earthworms and AMF inoculation, and the highest plant P, Ca and Mg contents were observed with earthworm application alone. Earthworms and AMFs significantly decreased the soil stable inorganic P (hydroxyapatite) proportion and increased the soil available dicalcium phosphate proportion. AMFs significantly increased soil phosphatase activity and inorganic P fraction contents. Earthworms and AMFs significantly increased soil bacterial Chao1 and phylogenetic diversity. Structural equation model analysis showed that the most important driver of soil P mineralization was soil bacterial diversity, followed by soil Ca 2+ and total salt concentration. Network analysis suggested that the response of bacteria to soil Ca 2+ but not salt concentration positively correlated with soil P availability. Earthworms and AMFs could stimulate certain bacteria harbouring the phoX alkaline phosphatase gene to increase soil phosphatase activity and soil P availability. Conclusions In conclusion, earthworms and AMFs could enhance soil P bioavailability by stimulating soil P-cycling bacteria to activate soil stable inorganic P and by improving the plant cation nutrient balance under salt stress.