A tipping point in carbon storage when forest expands into tundra is related to mycorrhizal recycling of nitrogen

Tundra ecosystems are global belowground sinks for atmospheric CO2. Ongoing warming-induced encroachment by shrubs and trees risks turning this sink into a CO2 source, resulting in positive feedback on climate warming. To advance mechanistic understanding of how shifts mycorrhizal types affect long-term carbon (C) nitrogen (N) stocks, we studied small-scale soil depth profiles fungal communities C–N dynamics across subarctic-alpine forest-heath vegetation gradient. Belowground organic stocks decreased abruptly at the transition from heath to forest, linked presence certain tree-associated ectomycorrhizal fungi that contribute decomposition when mining N matter. In contrast, ericoid plants were associated with matter accumulation slow decomposition. If climatic controls arctic-alpine forest lines relaxed, increased will likely outbalance plant productivity, decreasing overall C capacity displaced tundra.