The ectomycorrhizal symbiosis between plant roots and soil fungi plays an important role in both community assembly and resource cycling in temperate and boreal forests. At the heart of this interaction is a bidirectional exchange of resources, with plants providing carbon compounds to the fungi in exchange for soil resources such as nitrogen and phosphorus. This exchange may function like an economy, with the carbon “cost” of fungal resources varying depending upon the overall availability of those resources in the soil, and the behavior of traders interacting with a given root system. In particular, competition among fungi, and between fungi and roots, may be an important force governing these symbiotic resource economies. In this split-root study, we provided Bishop pine seedlings (Pinus muricata) with two fungal partners (Thelephora terrestris and Suillus pungens), inducing indirect competition among fungi, and two different levels of nitrogen fertilization, to foster competition between fungi and roots. We then used N-15 and C-13 labeling to characterize the resource trading economy under these conditions, calculating an exchange rate that may be an important metric of symbiotic function: plant carbon paid to the fungi per unit fungal nitrogen received by the plant.
Results/Conclusions
We found that the symbiotic carbon for nitrogen exchange rate varied with fertilization level and fungal identity. Overall, we found a saturating relationship for resource exchange: The more fungal nitrogen a plant had already received, the less carbon it paid for additional nitrogen. Thelephora terrestris required less plant carbon per nitrogen transferred to its host than did Suillus pungens, especially under high nitrogen conditions. Hosts also appeared to benefit more from colonization by T. terrestris than S. pungens under high nitrogen fertilization, although the two fungi provided similar growth benefits under low nitrogen. The presence of a heterospecific fungal competitor on the opposite side of the split root system generally decreased the plant carbon paid for fungal nitrogen. With this study, we demonstrate that the ectomycorrhizal symbiotic resource economy is dynamic, with exchange rates responding to changes in resource availability and competitive environment. This resource exchange is a key mechanism of carbon and nitrogen cycling in many forest systems, and may partly control the community assembly of ectomycorrhizal fungi on a plant’s root system. Knowing how this exchange can shift with environmental and competitive context is a key step towards building a mechanistic understanding of these processes in ectomycorrhizal forests.