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

Abstract

Tundra ecosystems are global belowground sinks for atmospheric CO 2 . Ongoing warming‐induced encroachment by shrubs and trees risks turning this sink into a CO 2 source, resulting in a positive feedback on climate warming. To advance mechanistic understanding of how shifts in mycorrhizal types affect long‐term carbon (C) and nitrogen (N) stocks, we studied small‐scale soil depth profiles of fungal communities and C–N dynamics across a subarctic‐alpine forest‐heath vegetation gradient. Belowground organic stocks decreased abruptly at the transition from heath to forest, linked to the presence of certain tree‐associated ectomycorrhizal fungi that contribute to decomposition when mining N from organic matter. In contrast, ericoid mycorrhizal plants and fungi were associated with organic matter accumulation and slow decomposition. If climatic controls on arctic‐alpine forest lines are relaxed, increased decomposition will likely outbalance increased plant productivity, decreasing the overall C sink capacity of displaced tundra.

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