Extraradical hyphae exhibit more plastic nutrient-acquisition strategies than roots under nitrogen enrichment in ectomycorrhiza-dominated forests

Ectomycorrhizal (ECM) functional traits related to nutrient acquisition are impacted by nitrogen (N) deposition. However, less is known about whether these nutrient-acquisition traits associated with roots and hyphae differentially respond to increased N deposition in ECM-dominated forests with different initial N status. We conducted a chronic N addition experiment (25 kg N ha-1 yr-1) in two ECM-dominated forests with contrasting initial N status, i.e. a Pinus armandii forest (with relatively low N availability) and a Picea asperata forest (with relatively high N availability), to assess nutrient-mining and -foraging strategies associated with roots and hyphae under N addition. We show that nutrient-acquisition strategies of roots and hyphae differently respond to increased N addition. Root nutrient-acquisition strategies showed a consistent response to N addition, regardless of initial forest nutrient status, shifting from organic N mining toward inorganic N foraging. In contrast, the hyphal nutrient-acquisition strategy showed diverse responses to N addition depending on initial forest N status. In the Pinus armandii forest, trees increased belowground carbon (C) allocation to ECM fungi thus enhancing hyphal N-mining capacity under increased N availability. By comparison, in the Picea asperata forest, ECM fungi enhanced both capacities of P foraging and P mining in response to N-induced P limitation. In conclusion, our results demonstrate that ECM fungal hyphae exhibit greater plasticity in nutrient-mining and -foraging strategies than roots do in response to changes in nutrient status induced by N deposition. This study highlights the importance of ECM associations in tree acclimation and forest function stability under changing environments.

外生菌根（Ectomycorrhizal, ECM）与养分获取相关的功能性状会受到氮（N）沉降的影响。然而，目前尚不清楚，在初始氮素状态各异的外生菌根优势森林中，与根和菌丝相关的养分获取性状是否会对氮沉降增加产生差异化响应。我们在两种初始氮素状态迥异的外生菌根优势森林中开展了长期氮添加试验（施氮速率为25 kg N·ha⁻¹·yr⁻¹），分别为华山松（Pinus armandii）林（氮素有效性相对较低）和粗枝云杉（Picea asperata）林（氮素有效性相对较高），以评估氮添加条件下根与菌丝相关的养分挖掘与觅食策略。研究结果显示，根与菌丝的养分获取策略对氮添加的响应存在显著差异。无论初始森林养分状态如何，根系的养分获取策略均呈现一致的响应模式：从有机氮挖掘转向无机氮觅食。与之相反，菌丝的养分获取策略对氮添加的响应则随初始森林氮素状态的不同而呈现多样化差异。在华山松林中，树木会增加向外生菌根真菌的地下碳分配，从而在氮素有效性提升的条件下增强菌丝的氮挖掘能力。相比之下，在粗枝云杉林中，外生菌根真菌会同时增强磷觅食与磷挖掘能力，以响应氮沉降诱导的磷限制。综上，本研究结果表明，相较于植物根系，外生菌根真菌菌丝在应对氮沉降引发的养分状态变化时，其养分挖掘与觅食策略具有更高的可塑性。本研究凸显了环境变化背景下，外生菌根共生关系对于树木适应以及森林功能稳定性的重要意义。