Mechanisms Of Microbe-mediated Soil Organic Phosphorus Turnover In The Hyphosphere

Arbuscular mycorrhizal (AM) fungi and bacteria are significant functional groups in the soil microflora, and their interactions play an important role in the soil phosphorus cycle. Interactions between AM fungi and bacteria are beneficial for soil P turnover and enhance P availability to plants. However, the underlying mechanisms by which these associations influence soil phosphorus turnover, and their ecological function, are not very well understood. In the present study, the interaction of two functional microorganisms (AM fungi and phosphate solubilizing bacteria) and its effects on organic phosphorus mineralization and turnover were studied using a compartmented cultivation system and molecular microbial ecology techniques (T-RFLP,13C-DNA-SIP,454high-throughput sequencing) at both physiological and molecular levels. The main results are listed below point by point:1. Ammonium-induced acidification in the presence of AM fungal hyphae in the hyphosphere improved maize uptake of P from phytin. NH4+treatment in the combined presence of phytin and AM fungal mycelium led to a decrease in hyphosphere pH, enhanced phosphatase activity in the hyphosphere and accelerated mineralization of phytin compared to the NO3-treatment, and improved maize uptake of P from phytin.2. Phosphate solubilizing bacteria (PSB) are involved in the mineralization and turnover of phytate in the hyphosphere, and these organisms assimilated carbon from13C-labeled maize. Co-inoculation of AM fungi and PSB into the hyphosphere chamber caused a marked decline in organic P concentration and an increase in microbial biomass phosphorus concentration in hyphosphere soil, compared to inoculation with AM fungi or PSB alone. The dual inoculation did not contribute to P uptake for maize, and hyphal growth of AM fungi was significantly inhibited under these conditions. Comparison between T-RFLP fingerprints and clone library indicated that Oxalobacteraceae, Streptomycetaceae and Pseudomonadaceae (including the inoculated PSB Pseudomonas alcaligenes strain) were labeled with13C derived from maize assimilates. These results suggest that PSB can utilize the photosynthate from plants via the mycelia of AM fungi, and are involved in soil organic phosphorus mineralization and turnover. The results suggest that PSB competed for available P with the AM hyphae in the hyphosphere, and that mycelial growth was suppressed because phytin was mineralized by PSB primarily for direct assimilation, and not for uptake by AMF and consequent promotion of both mycelial and plant growth..3. The composition of the bacterial community associated with extraradical Funneliformis mossaea mycelia was significantly changed by different forms of phosphorus supplied in the hyphosphere.16S T-RFLP analysis combined with NPMANOVA showed that different phosphorus forms significantly affected the bacterial community in the leek hyphosphere. The predominant phyla in those communities were Firmicutes and Cyanobacteria, as determined by454sequencing. Inorganic P treatment increased the relative abundance of Firmicutes, compared with the control with no added P, but no difference was seen for Cyanobacteria. However, the relative abundances of Firmicutes and Cyanobacteria were respectively decreased and increased after organic P supplementation. The bacterial community composition in the hyphospheres of Medicago truncatula and Lolium rigidum were not affected by the P forms. The results indicated the interactive effects of host plant and P form in determining F. mosseae growth and quantity of hyphal exudates.4. The bacterial community composition in the bulk soil was significantly different from that found in the F. mosseae hyphosphere. However, use of different plant species(Medicago truncatula, Allium porrum and Lolium rigidum) did not change the bacterial community composition associated with extraradical F. mosseae mycelia. These results suggest that the bacterial community composition in the hyphosphere is directly affected by mycelial exudates, but that host plant species do not significantly influence the composition of these exudates.