Plants associating with mutualistic ectomycorrhizal (ECM) fungi may directly obtain nitrogen (N) bound in soil organic matter (N-SOM). However, the contribution of N-SOM to plant growth under field conditions remains poorly constrained. We tested the hypothesis that turnover in ECM communities along soil inorganic N gradients mediates a functional transition from plant reliance on N-SOM in low inorganic N soils, to primarily inorganic N uptake in inorganic N-rich condition soils. We quantified the δ ¹⁵ N of Q. rubra foliage and roots, organic and inorganic soil N pools, and used molecular sequencing to characterize ECM communities, morpho-traits associated with N-foraging, and a community aggregated sporocarp δ ¹⁵ N. In support of our hypothesis, we document the progressive enrichment of root and foliar δ ¹⁵ N with increasing soil inorganic N supply; green leaves ranged from − 5.95 to 0.16‰ as the supply of inorganic N increased. ECM communities inhabiting low inorganic N soils were dominated by the genus Cortinarius, and other fungi forming hyphal morphologies putatively involved in N-SOM acquisition; sporocarp estimates from these communities were enriched (+ 4‰), further supporting fungal N-SOM acquisition. In contrast, trees occurring in high inorganic N soils hosted distinct communities with morpho-traits associated with inorganic N acquisition and depleted sporocarps (+ 0.5‰). Together, our results are consistent with apparent tradeoffs in the foraging cost and contribution of N-SOM to plant growth and demonstrate linkages between ECM community composition, fungal N-foraging potential and foliar δ ¹⁵ N. The functional characteristics of ECM communities represent a mechanistic basis for flexibility in plant nutrient foraging strategies. We conclude that the contribution of N-SOM to plant growth is likely contingent on ECM community composition and local soil nutrient availability.

与互生外生菌根（ECM）真菌相关的植物可能直接获得结合在土壤有机质（N-SOM）中的氮（N）。但是，在田间条件下，N-SOM对植物生长的贡献仍然受到限制。我们测试了以下假设：ECM群落中沿着土壤无机N梯度的周转介导了从低无机N土壤中植物对N-SOM的依赖到主要是富含无机N的土壤中无机N吸收的功能转换。我们量化的δ ¹⁵ N个Q.杨梅叶子和根，有机和无机土壤氮池，以及用于分子测序用N-觅食相关特征分析ECM社区，吗啉-性状，和社区聚集子实体δ ¹⁵ N.在支持我们的假设，我们文档根目录的逐渐富集和叶面δ ¹⁵ n，其中增加土壤无机氮的供应; 随着无机氮供应的增加，绿叶的变化范围为− 5.95至0.16‰。居住在低无机氮土壤上的ECM群落以Cortinarius属为主导，和其他真菌形成菌丝形态，可能参与了N-SOM的获取；这些群落的果皮估计值得到了丰富（+ 4‰），进一步支持了真菌N-SOM的获取。相反，在高无机氮土壤中生长的树木则具有不同的群落，这些群落的形态特征与无机氮的获取和枯萎的子果皮有关（+ 0.5‰）。总之，我们的结果与以N-SOM的觅食成本和贡献植物生长明显权衡一致，表明ECM群落组成之间的联系，真菌的N-觅食潜力和叶面δ ¹⁵ N. ECM社区的功能特征为植物养分觅食策略的灵活性提供了机械基础。我们得出的结论是，N-SOM对植物生长的贡献可能取决于ECM群落组成和当地土壤养分的利用率。