Improvement of arbuscular mycorrhiza development by inoculation of soil with phosphate-solubilizing rhizobacteria to improve rock phosphate bioavailability (32P) and nutrient cycling

Abstract

The interactive effect of phosphate-solubilizing bacteria and arbuscular mycorrhizal (AM) fungi on plant use of soil p sources of low bioavailability (endogenous or added as rock phosphate [RP] material) was evaluated by using soil microcosms which integrated 32P isotopic dilution techniques. The microbial inocula consisted of the AM fungus Glomus intraradices and two phosphate-solubilizing rhizobacterial isolates: Enterobacter sp. and Bacillus subtilis. These rhizobacteria behaved as 'mycorrhiza helper bacteria' promoting establishment of both the indigenous and the introduced AM endophytes despite a gradual decrease in bacterial population size, which dropped from 107 at planting to 103 CFU g-1 of dry rhizosphere soil at harvest. Dual inoculation with G. intraradices and B. subtilis significantly increased biomass and N and P accumulation in plant tissues. Regardless of the rhizobacterium strain and of the addition of RP, AM plants displayed lower specific activity (32P/31P) than their comparable controls, suggesting that the plants used P sources not available in their absence. The inoculated rhizobacteria may have released phosphate ions (31P), either from the added lip or from the less-available indigenous P sources, which were effectively taken up by the external AM mycelium. Soluble Ca deficiency in the test soil may have benefited P solubilization. At least 75% of the P in dually inoculated plants derived from the added RP. It appears that these mycorrhizosphere interactions between bacterial and fungal plant associates contributed to the biogeochemical P cycling, thus promoting a sustainable nutrient supply to plants.