Carbon investment into mobilization of mineral and organic phosphorus by arbuscular mycorrhiza

To overcome phosphorus (P) deficiency, about 80% of plant species establish symbiosis with arbuscular mycorrhizal fungi(AMF), which in return constitute a major sink of photosynthates. Information on whether plant carbon (C) allocation towardsAMF increases with declining availability of the P source is limited. We offered orthophosphate (OP), apatite (AP), or phytic acid(PA) as the only P source available to arbuscular mycorrhiza (AM) (Solanum lycopersicum x Rhizophagus irregularis)in a mesocosm experiment, where the fungi had exclusive access to each P source. After exposure, we determined P contents in the plant, related these to the overall C budget of the system, including the organic C (OC) contents, the respired CO₂, the phospholipid fatty acid (PLFA) 16:1ω5c (extraradical mycelium), and the neutral fatty acid (NLFA) 16:1ω5c (energy storage)at the fungal compartment. Arbuscular mycorrhizal (AM) plants incorporated P derived from the three P sources through the mycorrhizal pathway, but did this with differing C-P trading costs. The mobilization of PA and AP by the AM plant entailed larger mycelium infrastructure and significantly larger respiratory losses of CO₂, in comparison with the utilization of the readilysoluble OP. Our study thus suggests that AM plants invest larger C amounts into their fungal partners at lower P availability. This larger C flux to the AM fungi might also lead to larger soil organic C contents, in the course of forming larger AM biomass under P-limiting conditions.