Carbon flow from plant to arbuscular mycorrhizal fungi is reduced under phosphorus fertilization

Arbuscular mycorrhizal fungi (AMF) are plant root symbionts highly specialized upon phosphorus (P) supply to their hosts. We investigated plants' ability to regulate carbon (C) flow to AMF depending on the soil P supply. Leek (Allium porrum), medic (Medicago truncatula), and ryegrass (Lolium perenne) were subjected to AMF inoculation and/or P fertilization in a glasshouse experiment. The C flows were traced using C-13 pulse labelling. Mycorrhizal P uptake responses were lowered by P fertilization in all tested plant species. Independently from the C flow to the roots, the C flow to AMF-signature fatty acid 16:1 omega 5 were reduced by P fertilization in leek and ryegrass (but not in medic). Calculated mycorrhizal C costs ranged between 0.9% and 10.5% of the plant C budget. Suppression of the C flow from the plants to AMF resulted from both reduced abundance of AMF in the roots and lowered relative C income per unit of AMF biomass in P-fertilized pots. Although inconsistencies amongst different plant species demand caution in making generalizations, these results suggest an active role of host plants in regulating the C flow to AMF.