Role of nutrient level and defoliation on symbiotic function: experimental evidence by tracing 14C/15N exchange in mycorrhizal birch seedlings

High nutrient availability and defoliation generally reduce ectomycorrhizal colonization levels in trees, but it is not known how this affects the functional aspects of mycorrhizal symbiosis. It was therefore investigated whether (1) defoliation or increasing substrate N availability reduce C allocation from the plant to the fungus and N allocation from the fungus to the plant (symbiotic resource exchange), (2) symbiotic resource exchange depends on relative N and P availability, and (3) fungal N translocation to plant and plant C allocation to fungus are interdependent. Birch (Betula pendula) seedlings were grown in symbiosis with the ectomycorrhizal fungus Paxillus involutus at five times excess N, or at five times excess N and P for 6 weeks. One-half of the plants were defoliated and the plant shoots were allowed to photosynthesize ¹⁴CO₂ while the fungal compartment was exposed to ¹⁴NH₄. After 3 days, the ¹⁴C of plant origin in fungal tissues and ¹⁵N of fungal origin in plant tissues were quantified. Nutrient availability had no observable effect on symbiotic resource exchange in non-defoliated systems. Defoliation reduced symbiotic N acquisition by plants at all levels of nutrient availability, with the reduction being most marked at higher N availability, indicating an increased tendency in the symbiotic system to discontinue resource exchange after defoliation at higher fertility levels. The concentration of ¹⁴C in extramatrical mycelium correlated significantly with the concentration of ¹⁵N in birch shoots. The results support the assumption that N delivery to the host by the mycorrhizal fungus is dependent on C flow from the plant to the fungus, and that exchanges between the partners are reciprocal. No significant reductions in root ¹⁴C content as a response to defoliation were observed, indicating that defoliation specifically reduced allocation to fungus, but not markedly to roots.