Ectomycorrhizal fungi and mobilisation of organic phosphorus from forest soil: myth or reality?

National audienceForest soils contain a high proportion of phosphorus (P) in organic form (Po) represented by phosphomonoesters (such as glucose 1-P, ATP, phytate, …) or phosphodiesters (such as nucleic acids or phospholipids). To be used by trees, the phosphate group (Pi) must be released by the hydrolysis of the ester bond by phosphatases (Pases). It has been hypothesized that ectomycorrhizal (ECM) fungi, able to release phosphatases in their environment, could play an important role in the recycling of P in forest soil, thus contributing to plant P nutrition. We addressed this question in the context of the maritime pine (Pinus pinaster) forest representing the first planted area in France. This forest is established on sandy podzol very poor in available Pi but high in Po contents relative to total P contents, making P limiting for tree growth. Field studies showed a high degree of mycorrhizal symbiosis. To assess the efficiency of fungal Pases on organic P hydrolysis, we used Hebeloma cylindrosporum, a fungal ECM species isolated from maritime pine plots, able to release significant amounts of Pase activity when grown in vitro in P-starved conditions. Using cation-exchange chromatography, we separated four fractions with AcPase activity. Each AcPase active fraction displayed strong ability in vitro to hydrolyse a wide range of phosphate monoesters, but none of them efficiently hydrolysed phytate. In vitro measurements showed that up to 11% of the NaHCO3-extractable Po from intact soil was phosphatase reactive. This value reached 50% in soils autoclaved previously (Louche et al. 2010, FEMS 73: 323-335). Taken together, these data suggest that AcPases secreted from H. cylindrosporum could be much more efficient in recycling microbial organic P pools that may be delivered by soil autoclaving than other soil organic P