Ectomycorrhizal fungi supply their plant partners with nitrogen but can also retain substantial amounts. The concentration of nitrogen in the soil and the amount of carbon supplied from the host seem to influence the proportion of N retained by the fungus. In an experiment designed to determine whether differential supply of nitrogen to two plants influenced nitrogen transfer from fungus to plant within a mycorrhizal network, we observed rapid, substantial loss of nitrogen from pine seedlings. The loss occurred when the mycorrhizal fungus experienced a sudden increase in nitrogen supply. We grew Pinus contorta seedlings in association with Suillus tomentosus in low-nitrogen microcosms where some nitrogen was accessible only by hyphae. After 70 days, foliage of some seedlings was treated with nitrogen. Three days later, hyphal nutrient media were replaced with water or a solution containing nitrogen. Foliar treatment did not affect nitrogen transfer by the fungus to shoots, but by day 75, seedling nitrogen contents had dropped by 60% in microcosms where nitrogen had been added to the hyphal compartments. Those seedlings retained only 55% of the nitrogen originally present in the seed. Loss of nitrogen did not occur if water was added or the hyphae were severed. Because of the severing effect, we concluded that S. tomentosus triggered the loss of seedling nitrogen. Nitrogen may have been lost through increased root exudation or transfer to the fungus. Access to nitrogen from nutrient-rich germinants would benefit rhizosphere microorganisms, including ectomycorrhizal fungi colonizing pine from spores after wildfire.

中文翻译：

---

松外生菌根萌发体的氮快速流失是由真菌共生体发出的信号。


外生菌根真菌为它们的植物伙伴提供氮，但也可以保留大量氮。土壤中的氮浓度和宿主提供的碳量似乎会影响真菌保留的氮的比例。在一项旨在确定两种植物的氮供应差异是否影响菌根网络内从真菌到植物的氮转移的实验中，我们观察到松树幼苗的氮快速、大量损失。当菌根真菌的氮供应量突然增加时，就会发生损失。我们在低氮微观环境中与毛牛肝菌一起种植松树幼苗，其中一些氮只能通过菌丝获取。 70天后，用氮处理一些幼苗的叶子。三天后，用水或含氮溶液更换菌丝营养培养基。叶面处理不会影响真菌向芽的氮转移，但到第 75 天，在向菌丝室添加氮的微观世界中，幼苗的氮含量下降了 60%。这些幼苗仅保留了种子中最初存在的氮的 55%。如果添加水或切断菌丝，则不会发生氮损失。由于切断效应，我们得出结论，绒毛S. tomentosus引发了幼苗氮的损失。氮可能因根部分泌物增加或转移至真菌而损失。从营养丰富的萌芽中获取氮将有利于根际微生物，包括野火后通过孢子在松树上定殖的外生菌根真菌。