Résumé
Leaf litter chemistry and ectomycorrhizal (ECM) fungi are key drivers of the below‐ground nitrogen (N) cycling within forest ecosystems. Their combined effects on litter decomposition and N competition between microbial decomposers and plants are still uncertain.We conducted a glasshouse microcosm experiment with low or high ECM‐colonized beech (Fagus sylvatica) saplings, growing with litter collected from old or young beech trees growing on the same loamy soil. After 6 months of growth, we investigated litter decomposition rates, microbial respiration and the N pools within leaf litter, soil (different pools), microbial and plant shoot biomass.We found that the mass loss of the litter from young trees was always higher than the litter from the mature trees. The microbial biomass N per unit soil carbon was low when the litter, especially from mature trees, was added, suggesting that the litter provided recalcitrant compounds and limited soil microbial activity. In contrast to the ‘Gadgil effect’ hypothesis, the high ECM‐colonized roots increased the litter decomposition rate and N immobilization in poorly decomposable litter in comparison with the litter incubated with the low ECM‐colonized roots. Finally, the high ECM‐colonized plants that received the poorly decomposable leaf litter exhibited the highest shoot N amount and biomass and were associated with the lowest microbial biomass N.Two‐way anovas revealed that litter and ECM fungi occurrence together impacted final particulate organic N, microbial biomass N, the amount of shoot N and shoot biomass. Four N pools are key drivers of microbial biomass N and shoot N: particulate organic N, total N, soil ammonium concentration and litter N concentration.Synthesis. Our results support the hypothesis that poorly decomposable leaf litter produced by mature beech trees and ECM fungi together decrease microbial N immobilization but increase tree N acquisition. Increasing N retention within the recalcitrant N forms in soil was identified as a key mechanism by which beech alters soil N cycling with potential positive feedbacks on its acquisition by the plant. Our result emphasizes the importance of considering within‐species litter‐trait variability in litter decomposition.