Ectomycorrhizal communities underlie trade‐offs between greater long‐term tree growth and reduced soil carbon concentrations in boreal forests

Understanding the biological interactions which influence the boreal carbon (C) sink are critical to the development of accurate Earth System Models (ESM). Boreal forests are highly nitrogen (N) limited. To meet N demand, boreal trees associate with highly diverse ectomycorrhizal (ECM) fungal communities, which vary in their capacity to uptake recalcitrant organic forms of N. ECM acquisition of organic N has been suggested to positively influence tree growth, but the consequence of this activity on soil C pools is widely debated, with evidence for their activity both increasing and decreasing soil C. To evaluate the potential for ECM fungi to influence trade‐offs in above‐ and below‐ground C, we linked long‐term tree growth records (~25 years) of Picea glauca with individually paired molecular profiles of fungal communities inhabiting focal tree roots and surrounding soils. We conducted this study across a large climatic gradient in interior Alaska, along which we expected fungal community composition, N acquisitive properties and outcomes on C cycling processes to vary. We observed a ~90‐fold variation in tree growth. After accounting for co‐occurring climatic trends, we found that abundant ECM taxa linked with lower soil C concentrations were simultaneously associated with greater long‐term tree growth. We estimated outcomes of fungal community N acquisitive properties using foliar δ 15 N, finding that fungal community composition and particularly Cortinarius sequence abundance were significantly correlated with foliar δ 15 N. Synthesis . Ultimately, this work presents field‐based evidence consistent with the emerging understanding that ECM fungi influence ecosystem C stocks through enzymatic degradation of SOM and uptake of organic N. Specifically, our results support conceptualizations that ECM acquisition of organic N can contribute to trade‐offs in above‐ and below‐ground carbon storage by stimulating tree growth while reducing soil C accumulation. Efforts to promote a sustained boreal forest C sink require interrogating linkages between ECM community traits and their emergent effects on soil decay and tree growth.