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.

人为氮沉降（Anthropogenic nitrogen deposition）与菌根网络（mycorrhizal network）破坏是森林碳循环的关键威胁。然而，二者对木质残体分解（woody debris decomposition）的交互效应迄今尚未得到充分阐明。本研究在中国东北温带红松混交林（temperate Korean pine mixed forest）中开展了为期730天的析因野外试验（factorial field experiment），设置氮添加（N addition，50 kg N·ha⁻¹·a⁻¹）与根系切挖（root trenching）两个处理，以探究二者对木材分解及真菌群落动态（fungal community dynamics）的单独及协同效应（synergistic effects）。
氮添加显著提升了桦木木材（birch wood）的质量损失（mass loss）率（15.97%）与碳损失（carbon loss）率（15.17%），同时抑制了磷富集（phosphorus enrichment）过程（降幅达26.46%），表明养分限制出现了化学计量偏移（stoichiometric shift）。
根系切挖则表现出情境依赖效应（context-dependent effects）：在背景氮条件（ambient N conditions）下，该处理在试验第365天时抑制了分解过程，但在第730天时反而促进了分解，这一结果对加吉尔效应（Gadgil effect）的普遍性提出了挑战。而在氮富集条件下，根系切挖始终加速了分解进程，尤其在试验后期，这表明氮有效性与菌根网络破坏之间存在协同交互作用。
真菌群落分析揭示了显著的功能类群转变：氮添加使腐生担子菌（saprotrophic basidiomycetes）的相对丰度提升5122%，毛霉菌门（Mucoromycota）提升3950%，同时几乎完全清除了外生菌根真菌（ectomycorrhizal fungi）。这反映出真菌群落从养分保守型（nutrient-conserving）策略向养分获取型（nutrient-mining）策略的转变。
时间动态（temporal dynamics）分析显示出清晰的群落演替模式：β多样性（β-diversity）的驱动因子从单一的物种替换主导，转变为物种替换与丰富度差异（richness differences）共同主导。
本研究结果表明，加吉尔效应具有情境依赖性而非普遍性，其调控机制涉及氮有效性与分解阶段两个维度。上述结果强调，在预测全球变化（global change）背景下的森林碳循环时，需整合生物-非生物交互作用与时间动态特征，该发现对氮饱和区域（N-saturated regions）的森林管理具有重要的指导意义。