Ectomycorrhizae (ECM) have been historically categorized as a mutualistic relationship. However, the analyses of costs/benefits for both associates, particularly of C and N exchanges, suggest that the mycorrhizal-plant relationship can vary from mutualism to parasitism, and may include saprotrophic functions.  The outcome of symbiotic and mutualistic relationships is dependent on biotic and abiotic conditions, the structure of the food-web and availability of resources. Rapid transfers of newly assimilated C and N were studied in a Pinus sabiniana-ECM system under different soil nitrogen availabilities. We hypothesized that flows of C from the plant to the ECM fungi, and N from the ECM fungi to the plant are linked.  We also suggest that under different nitrogen availability, as a tree physiological response, C allocation to the roots and consequently to the ECM would decrease and therefore ECM N uptake would decrease as well. We performed a dual labeling experiment applying ¹³C in the pine needles and ¹⁵N in the soil. We were able to confirm that the ECM roots are a strong sink for newly incorporated ¹³C and that this flows are linked to ¹⁵N uptake.