Micro-food Web Interactions Involving Bacteria, Nematodes, and Mycorrhiza Enhance Tree P Nutrition in a High P-sorbing Soil Amended with Phytate

Phytate is considered a poorly available plant P source but proved to be useful for particular soil bacteria strains. In soil-free conditions, it has been shown that bacteria locked up the mineralized phosphorus from phytate whereas bacterial grazers like nematodes were able to deliver P to plants. Here, we aimed to determine if the interactions between phytate-mineralizing bacteria, bacterial grazer nematodes, and mycorrhizal fungi could increase plant P acquisition from phytate in high P-adsorbing soils. Pinus pinaster was grown in a Cambisol supplemented with phytate. Plants, whether associated or not associated with the ectomycorrhizal fungus Hebeloma cylindrosporum, were either inoculated or not inoculated with the phytase-releasing bacteria Bacillus subtilis and the bacterial-feeding nematode Rhabditis sp. After 100 days, the dual inoculation of bacteria and nematodes significantly increased net plant P accumulation. We observed that, on average, mycorrhizal plants accumulated more P in their shoots than non-mycorrhizal plants. However, the highest plant P acquisition efficiency was found when the three soil organisms were present in the P. pinaster rhizosphere. We conclude that, in a highly inorganic P-fixing soil, plant P acquisition from phytate strongly depends on the grazing of phytate-mineralizing bacteria. Our results confirm the importance of the soil microbial loop to improve plant P nutrition from phytate, which should be considered a route to improve the utilization of this source of poorly available P by plants.