Nitrogen Deposition and Mycorrhizal Network Disruption Synergistically Accelerate Woody Debris Decomposition

Anthropogenic nitrogen (N) deposition and mycorrhizal network disruption are key threats to forest carbon cycling. However, their interactive effects on woody debris decomposition remain poorly understood. We conducted a 730-day factorial field experiment in a temperate Korean pine mixed forest in northeastern China, combining N addition (50 kg N·ha-1·a-1) and root trenching to investigate their individual and synergistic effects on wood decomposition and fungal community dynamics. Nitrogen addition significantly increased mass loss (15.97%) and carbon loss (15.17%) from birch wood while suppressing phosphorus enrichment (26.46%), indicating a stoichiometric shift in nutrient limitation. Root trenching exhibited context-dependent effects: under ambient N conditions, it initially inhibited decomposition at 365 days but promoted it at 730 days, challenging the universality of the Gadgil effect. However, under N-enriched conditions, trenching consistently accelerated decomposition, particularly in later stages, suggesting synergistic interaction between N availability and mycorrhizal disruption. Fungal community analysis revealed a profound functional shift: N addition triggered a 5122% increase in saprotrophic basidiomycetes and a 3950% increase in Mucoromycota, while nearly eliminating ectomycorrhizal fungi. This reflects a transition from nutrient-conserving to nutrient-mining fungal strategies. Temporal dynamics showed clear successional patterns, with β-diversity shifting from species replacement-dominated to co-dominated by replacement and richness differences. Our findings suggest that the Gadgil effect is context-dependent rather than universal, mediated by both N availability and decomposition stage. These results highlight the importance of integrating biotic-abiotic interactions and temporal dynamics when predicting forest carbon cycling under global change, with significant implications for forest management in N-saturated regions.