Gross phosphorus fluxes in a calcareous soil inoculated with Pseudomonas protegens CHA0 revealed by 33P isotopic dilution

- Reduced respiration in inoculated soil suggests inhibition of P mineralization by inherent microbial biomass.
- Isotopic dilution can detect small changes in gross P fluxes in soil.
- Combination of incubation and plant study allows disentangling direct and indirect effects of an inoculant on plant P uptake.

Inoculation with phosphorus (P) solubilizing bacteria is being proposed to increase P availability for plants by mineralization and solubilization of non-available soil and fertilizer P. Solubilization of inorganic P compounds by bacterial strains has repeatedly been shown on agar plates and in liquid media. However, the effects of inoculation on P availability to plants growing in soils, either in pot or field studies, are inconsistent and do not allow to separate between direct effects on P availability and indirect effects such as improved plant health. This differentiation could be achieved using 33P isotopic labeling. We applied the 33P isotopic dilution method in a pot and in an incubation experiment to study gross P fluxes in a calcareous soil inoculated with the P solubilizing bacteria Pseudomonas protegens CHA0. We hypothesized that the inoculant dilutes the specific activity (33P/31P) in the soil solution or in the plant shoots because of P solubilization beyond the P mobilization by the endogenous microbial biomass. To this end, we conducted a plant growth experiment with Lolium multiflorum var. Gemini and an incubation experiment. In both experiments, the soil was amended or not with a calcium P rich sewage sludge ash, and both treatments were conducted with and without inoculation. The inoculant was able to solubilize P from sewage sludge ash under controlled conditions in liquid media. However, it did not enhance P release from soil or from sewage sludge ash in the incubated soil. Inoculation of the soil reduced organic P mineralization by the soil microbial biomass, which was supported by a simultaneous decrease in soil respiration. Thus, any inorganic P solubilized by the inoculant might have been offset by less basal organic P mineralization. Increased P uptake of inoculated Lolium multiflorum at first harvest was attributed to an indirect effect, since the specific activity in shoots of inoculated Lolium multiflorum was not decreased. Although sewage sludge ash contained very little water-soluble P, an increase in P availability following sewage sludge ash addition could be shown using 33P isotopic dilution, while biological processes remained unchanged. While in this study, the inoculant did not increase P availability, the approach presented here can give insight into the mechanisms underlying beneficial effects of inoculants.