Soil microorganisms and their symbiotic relationships with plants are fundamental to nutrient cycling in temperate forest ecosystems. This highly diverse microbiome contains up to a quarter of Earth’s biodiversity, but our understanding of how this affects the function of forests is not well understood. This thesis investigated the role of plant symbionts on the allocation of C to belowground microbial symbionts and to ground vegetation via microbial symbionts. Radio-isotope pulse labelling was used to determine the belowground C dynamics of these highly complex systems by allowing us to quantify pools and fluxes within the plant-microbe-soil continuum. In Chapter 3, the role of arbuscular and ecto-mycorrhizal fungi in belowground allocation of C in three temperate tree species was investigated by destructive harvesting of trees 336 days after a pulse label had been applied. The results suggested that Alnus glutinosa and Betula pendula allocated C belowground to microbes, whereas Castanea sativa transferred the C to the soil where it was sequestered. In Chapter 4, inter- and intra-specific C transfer was studied using trees connected via a common mycorrhizal network (CMN), the results suggested that more C was transferred between inter- than intra-specific species combinations. In Chapter 5, C transferred via three “donor” tree species to the root nodules of A. glutinosa “receiver” tree connected with a CMN was investigated using the methodology pioneered in Chapter 3. The plant: fungal amalgam preferentially allocated C from the donor trees to the root nodules of the receiver A. glutinosa tree. We postulated that this was due to the considerable energetic demands of nitrogen-fixation by Frankia alni in the root nodule creating a strong C sink. In Chapter 6, the transfer of C from 13-year-old coppiced A. glutinosa and C. sativa trees to ground vegetation via CMN was investigated. 14C activity in the ground vegetation under the A. glutinosa trees was expected to be greatest, as A. glutinosa share arbuscular mycorrhizal partnerships with the ground vegetation. No difference in 14C activity was found in the hyphae, soil solution or ground vegetation under A. glutinosa. We postulated that this could be due to root grafting, mycorrhizal types exchanging nutrients, or reabsorption of tree rhizodeposits. Overall this study suggests that the plant: microbe symbiosis that is ubiquitous across the temperate biome is both important for nutrient cycling and C storage, but also that the sharing of resources via CMNs could be altering plant competition dynamics that have previously been based on the assumption that plants are not physically connected and actively sharing resources. Further work to determine how plants or mycorrhizae control belowground resource sharing could lead to a paradigm shift in our understanding of competition and facilitation in plant community dynamics.