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 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-associateds 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.