Plant pests influence the movement of plant-fixed carbon and fungal-acquired nutrients through arbuscular mycorrhizal networks

1. Plants typically interact with multiple, co-occurring symbionts, including arbuscular mycorrhizal (AM) fungi which can form networks, connecting neighbouring plants. A characteristic aspect of the mycorrhizal symbiosis is the bidirectional exchange of nutrients between host plants and fungal partners. Concurrent interactions with competing organisms such as aphids or potato cyst nematodes (PCN) can disrupt the carbon-for-nutrient exchange between plants and AM fungi. However, the role of mycorrhizal networks (MNs) in mediating these interactions remains unclear. 2. Using isotope tracing in multi-plant experimental systems, we investigated the movement of plant photosynthates and fungal-acquired soil phosphorus through MNs and the interactive effects of PCN infection on this. 3. We found evidence of preferential allocation of fungal-acquired phosphorus to plants that were not infected by PCN compared with infected neighbours. Contrary to previous findings using single plants, we did not detect a PCN-induced reduction in the amounts of plant carbon delivered to AM fungi in multi-plant systems. However, the MN(s) moved plant-fixed carbon away from PCN-infected host plants, regardless of the PCN infection status of the neighbouring plant host. 4. Our work highlights the responsiveness of MNs to interactions with belowground organisms. It also strengthens the argument for a more mycocentric view of AM–plant symbioses. Experimental designs of increasing ecological complexity are needed for a more comprehensive understanding of the carbon-for-nutrient dynamics in AM fungi–plant networks. This will, in turn, elucidate the role of AM fungi in terrestrial carbon cycling and their function in agricultural systems.