Can phosphorus (P)-releasing bacteria and earthworm (Eisenia fetida L.) co-enhance soil P mobilization and mycorrhizal P uptake by maize (Zea mays L.)?

Purpose Both earthworm (Eisenia fetida L.) and phosphorus (P)-releasing (PR) bacteria, including organic P-mineralizing (OPM) and inorganic P-solubilizing (IPS), are able to convert difficult-to-use P into phytoavailable P. However, there is a undesirable gap between soil P mobilization and root P acquisition when mycorrhization is enhanced. The aim of this study was to investigate the combined effects of earthworm and PR bacteria on soil P mobilization, mycorrhizal colonization, and P acquisition of maize (Zea mays L.). Materials and methods This study consisted of three parallel experiments. Firstly, a 90-day microcosm experiment was carried out to test the diversified effects of earthworm, OPM bacterium Bacillus megaterium, and IPS bacterium Kluyvera intermedia, on available P concentration in an autoclaved soil with or without P amendment. Secondly, a 16-week greenhouse pot experiment was conducted to compare the differential effects of earthworm, B. megaterium, and K. intermedia on P mobilization, mycorrhizal colonization, and P acquisition of maize in the same autoclaved soil receiving chemical fertilizers and Funneliformis caledonium. Thirdly, a microplot experiment was established to verify the combined effects of earthworm and PR bacteria on P mobilization and mycorrhizal P acquisition of maize in a field where the soil used in the former two experiments were collected. Results and discussion In the microcosm experiment, earthworm significantly increased soil available P concentration coupled of declined soil pH regardless of P amendment, while B. megaterium and K. intermedia further enhanced P mobilization in P-fertilized soils. In the greenhouse experiment, PR bacteria and earthworm co-increased mycorrhizal colonization and plant biomass, as well as soil alkaline phosphatase (ALP) activity and available P concentration compared to the control, and individual P acquisition compared to the single inoculation treatments. In the field experiment, both PR bacteria and earthworm significantly increased mycorrhizal colonization and tended to increase soil ALP activity, available P concentration, root biomass, and individual P acquisition, while the combined inoculation significantly increased ALP activity and available P concentration, and thereby individual P acquisition by 29% relative to the control. Conclusions Combined inoculation of PR bacteria and earthworm greatly increased soil available P concentration and maize P acquisition and grain yield. Although the combined inoculation had a lower root mycorrhizal colonization than the single inoculation treatments due to the greatly increased amounts of readily soluble P, it had a higher mycorrhization level than the control. These observations suggest that the combined application could enhance soil P mobilization and be expropriated to elaborate indigenous mycorrhizal fungal function.