Ectomycorrhizal trees rely on nitrogen resorption less than arbuscular mycorrhizal trees globally

Nitrogen (N) resorption is an important pathway of N conservation, contributing to a proportion of plant N requirement. However, whether the ratio of N resorption to N requirement would be affected by environmental factors, mycorrhizal types or atmospheric CO2 concentration remains unclear. Here, we conducted a meta-analysis on the impacts of environmental factors and mycorrhizal types on this ratio. We found this ratio in ectomycorrhizal (EM) trees decreased with mean annual precipitation (MAP), mean annual temperature (MAT), soil total N content (TN) and atmospheric CO2 concentration and was significantly lower than that in arbuscular mycorrhizal (AM) trees. An in situ 15N tracing experiment further confirmed this stronger reliance on N resorption for AM trees than EM trees. Our study suggests that AM and EM trees potentially have different strategies for alleviation of progressive N limitation, highlighting the necessity of incorporating plant mycorrhizal types into Earth System Models.氮回收（N resorption）是植物氮素保存的重要途径之一，可满足植物部分氮素需求。然而，氮回收与氮需求的比值是否会受环境因子、菌根类型以及大气CO₂浓度的影响，目前仍不明确。本研究针对环境因子与菌根类型对该比值的影响开展了荟萃分析。研究发现，外生菌根（ectomycorrhizal, EM）树木的该比值随年平均降水量（mean annual precipitation, MAP）、年平均气温（mean annual temperature, MAT）、土壤全氮含量（soil total N content, TN）以及大气CO₂浓度升高而降低，且显著低于丛枝菌根（arbuscular mycorrhizal, AM）树木。后续开展的原位¹⁵N示踪实验进一步证实，相较于外生菌根树木，丛枝菌根树木对氮回收的依赖程度更强。本研究表明，丛枝菌根与外生菌根树木在缓解渐进式氮限制方面可能存在不同策略，这凸显了将植物菌根类型纳入地球系统模型（Earth System Models）的必要性。