Effects of earthworms and plant growth-promoting rhizobacteria (PGPR) on availability of nitrogen, phosphorus, and potassium in soil

Both earthworms and plant growth-promoting rhizobacteria (PGPR) are ubiquitous and important for promoting circulation of plant macronutrients. Two series of laboratory experiments were conducted to investigate the effects of earthworm casts and activities on the growth of PGPR, and the inoculation of earthworms and PGPR on the availability of N, P, and K in soils, respectively. During a short incubation period (0-34 h), the extracts of earthworm (Pheretima guillelmi)-worked soil significantly (p < 0.05) increased the abundance of the three species of PGPR, including N-fixing bacteria (NFB) (Azotobacter chroococcum HKN-5), phosphate-solubilizing bacteria (PSB) (Bacillus megaterium HKP-1), and K-solubilizing bacteria (KSB) (B. mucilaginous HKK-1), in Luria-Bertani (LB) broth. There were synergistic effects of dual inoculation of earthworms and PGPR on increasing the concentrations of NH, NaHCO₃-extractable P, and NH₄OAc-extractable K in the corresponding soils. Bioavailable N in the dual inoculation was 4 to 24 times those inoculated with earthworms or NFB alone, respectively. The significantly higher concentrations of bioavailable N and P in the dual inoculation of earthworms and NFB or PSB may be due to the higher abundance of PGPR and/or higher activities of urease and acid phosphatase than those of single inoculation of NFB or PSB, respectively. Dual inoculation of earthworms and PGPR would be most effective in reducing the need for chemical fertilizers in agriculture.