Impact of nitrogen enrichment on the functions of extracellular enzymes in ectomycorrhizal community and nutrient status of pine seedlings

Excessive nitrogen (N) alters the nutrient strategies of plant roots, affecting mycorrhizal species dynamics in ecological niches. These changes significantly impact host plant interactions and consequently, the community dynamics and enzyme functionality of ectomycorrhizal (ECM) fungi. Understanding the impact of N addition, especially the critical N load, on ectomycorrhizae's structure and enzyme activity function is vital for forest management. In this study, we applied a greenhouse experiment and exposed four pine species to N addition treatments to investigate the impact on ECM fungal communities. Using Sanger sequencing and fluorescence enzyme assays, we assessed changes in ectomycorrhizae structure across Pinus massoniana, Pinus armandii, Pinus elliottii, and Pinus taeda exposed to five N levels (0, 15, 30, 60, 150 kg N ha- 1 yr- 1). We also explored the relationship between ECM extracellular enzyme function and pine seedlings' nutrient status. Mycorrhizal enzymatic activity showed a threshold at 30 kg N ha- 1 yr- 1 in most tree species, and the activity of some enzymes (such in P. massoniana and P. armandii) still showed an increase even at the highest N addition level (150 kg N ha- 1 yr- 1). The Tomentella group stood out, contributing significantly to enzyme functions in all pine species under various N conditions. Additionally, at a N addition level of 30 kg N ha- 1 yr- 1, the enzymatic functions of ectomycorrhizal communities across various tree species, which formed under N deposition stress, showed similarities. This suggests that the presence of key ECM fungal taxa contributes significantly to maintaining ecological redundancy within these communities.