Anthropogenic nitrogen (N) deposition is known to profoundly alter the dynamics of mycelial traits of ectomycorrhizal (ECM) fungi in forest ecosystems. The site-specific change in mycelial traits in forests receiving N deposition led us to hypothesize that growth and functional traits of ECM mycelia are controlled by soil nutrient availability in these forests. In this study, we conducted a N fertilization experiment in two alpine forests (Picea asperata Mast. and Pinus armandii Franch.) on the eastern Tibetan Plateau, China, to assess the responses of mycelial growth traits (biomass, production, density and turnover) and functional traits (exploration type and hydrophobicity) to N addition. We quantified the turnover and production of mycelia by determining mycelial biomass in sequentially harvested in-growth mesh bags and by applying mathematical models. We also captured the changes in mycelium exploration type and hydrophobicity by characterizing the ECM fungal community composition. Nitrogen addition promoted mycelial growth, with 79% more mycelial production, 39% greater biomass, and 73% higher density in the P. armandii stand with lower soil N availability (18 mg mineral N kg⁻¹), but N addition repressed the mycelial growth in the P. asperata stand with higher soil N availability (30 mg mineral N kg⁻¹). Additionally, N deposition increased the abundance of genera belonging to ‘Contact-Short’ and ‘Contact-Medium’ explorers and reduced that of ‘Medium-Long’ explorers in the P. armandii stand. N deposition had opposite effect on mycelial functional traits in the P. asperata stand. Collectively, this confirms that the responses of mycelial dynamics to N deposition in forests largely depend on native soil nutrient availability.