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 (sup32)P 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 10(sup7) at
planting to 10(sup3) CFU g(sup-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 ((sup32)P/(sup31)P) than their comparable
controls, suggesting that the plants used P sources not available in their absence. The inoculated rhizobacteria may
have released phosphate ions ((sup31)P), either from the added RP 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.
PMID: 16535730 [PubMed - as supplied by publisher]
[permaculture] Entrez PubMed Soil Quality & phosphate availability,
Lawrence F. London, Jr., 03/16/2006