• The supply of carbon (C) from tree photosynthesis to ectomycorrhizal (ECM) fungi is known to decrease with increasing plant nitrogen (N) supply, but how this affects fungal nutrition and growth remains to be clarified.
• We placed mesh-bags with quartz sand, with or without an organic N (¹⁵N-, ¹³C-labeled) source, in the soil along a natural N supply gradient in boreal forest, to measure growth and use of N and C by ECM extramatrical mycelia.
• Mycelial C : N declined with increasing N supply. Addition of N increased mycelial growth at the low-N end of the gradient. We found an inverse relationship between uptake of added N and C; the use of added N was high when ambient N was low, whereas use of added C was high when C from photosynthesis was low.
• We propose that growth of ECM fungi is N-limited when soil N is scarce and tree belowground C allocation to ECM fungi is high, but is C-limited when N supply is high and tree belowground C allocation is low. This suggests that ECM fungi have a major role in soil N retention in nutrient-poor, but less so in nutrient-rich boreal forests.

中文翻译：

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北方森林天然氮供应梯度下外生菌根菌丝的碳-氮关系

• 已知从树木光合作用到外生菌根 (ECM) 真菌的碳 (C) 供应会随着植物氮 (N) 供应的增加而减少，但这如何影响真菌的营养和生长仍有待阐明。
• 我们将带有或不带有有机氮（¹⁵ N-、¹³ C-标记）源的石英砂网袋放置在沿着北方森林天然氮供应梯度的土壤中，通过以下方式测量氮和碳的生长和使用ECM 胞外菌丝体。
• 菌丝体 C：N 随着 N 供应的增加而下降。添加 N 增加了梯度低 N 端的菌丝体生长。我们发现添加的 N 和 C 的吸收之间存在反比关系；当环境氮含量低时，添加的氮的使用量高，而当光合作用的碳含量低时，添加的碳的使用量高。
• 我们提出，当土壤 N 稀缺且树木地下 C 分配给 ECM 真菌时，ECM 真菌的生长受 N 限制，但当 N 供应高且树木地下 C 分配低时，ECM 真菌的生长受 N 限制。这表明 ECM 真菌在营养贫乏的土壤 N 保留中起主要作用，但在营养丰富的北方森林中作用较小。