Final Activity Report Summary - CMN - AMF (Cheaters and collaborators in natural plant communities - the impact of a ubiquitous root symbiont on carbon movement among plants) The great majority of plants live in a root symbiosis with arbuscular mycorrhizal (AM) fungi, where the fungi receive carbon from the host plant in return for soil acquired nutrients. Despite the ubiqutious nature of this symbiosis, functional aspects have been researched predominately in the greenhouse with fungal isolates that are culturable. There are several reasons why these studies may poorly reflect conditions in nature. First, molecular analyses have made it increasingly clear that these fungi represents a minor fraction of the total AM fungal diversity. Indeed, our work in natural grassland has shown that the majority of AM fungal are uncultured and do not sporulate, and therefore display very different behaviour from known fungal isolates. In fact, molecular analyses have indicated that one group is sufficiently different from all previously cultured AM fungi to necessitate the creation of a new fungal family! Second, plants in nature are not isolated from each other but are instead connected by multiple fungal taxa in complex networks, where both carbon and soil nutrients have been shown to move among plants within the fungal mycelium. Due to the significance of mycorrhizas on scales ranging from the individual plant to ecosystems, the overall objectives with this research were to quantify carbon movements from plants to fungi in nature, and to identify factors that drive this flow. Labelling plants with non-radioactive carbon isotopes allowed us to trace the carbon movement from plants to AM fungi seasonally. These novel studies showed that plants take up significantly more carbon in July and October relative to February and that the carbon allocation to AM fungi was proportional to the carbon assimilated by the plant. While the fungal structures remained inside the root tissue throughout the year, the carbon reserves changed drastically, indicating that the AM fungi may be carbon limited during times when the plants are less active, and that the plant and fungal carbon status are tightly linked and dynamic. Mycelial connections among plants can never be directly observed in soil. Therefore, to address factors that drive carbon and nutrient flows within the mycorrhizal network, we utilised a controlled system where plants are connected by AM fungi in the laboratory. This study showed that if a plant is carbon limited, for example as a result of shading, fungi colonising its roots are to a greater extent receiving carbon from neighbouring plants. However, plants that supply little carbon to the fungi receive few benefits in terms of nutrients, suggesting a direct link between costs and benefits in the symbiosis. Furthermore, it also indicates that a fungus connected to multiple plants allocates the carbon received from all plants to optimise its own fitness. In this project, we have - for the first time - quantified seasonal carbon flows from plants to associated AM fungi. Combined with the controlled studies, we have shown that the mycorrhizal symbiosis is a dynamic interchange of carbon and nutrients, not only involving a single plant and fungus, but connecting whole plant communities and multiple fungal species. Our results are not only relevant for a better understanding of mycorrhizal dynamics and plant interactions, but is also highly relevant in the global climate change debate because as much as 20% of the carbon fixed during photosynthesis can be allocated to AM fungi.