Ectomycorrhizal mediation of soil carbon sequestration: from carbon allocation to necromass stabilization and priming effects.

Ectomycorrhizal (ECM) fungi form the dominant symbiosis in many of the world's forest biomes. They exert a seemingly contradictory influence on soil carbon (C), simultaneously promoting C accrual through necromass inputs and aggregate protection, while also driving C loss via enzymatic priming. This review synthesizes current understanding of these dual roles, focusing on: (1) the magnitude and controls of photosynthetic C allocation from host plants to ECM mycelium; (2) the enzymatic mechanisms of SOM decomposition, the rhizosphere priming effect, and the contested universality of the "Gadgil effect" (competitive suppression of free-living saprotrophs); (3) physical and chemical stabilization pathways including the "microbial carbon pump" (necromass accrual) and "mineral carbon pump" (organo-mineral complexation); (4) environmental controls including nitrogen deposition, climate change, and forest management practices; and (5) prevailing research controversies and key methodological constraints-including isotope dilution, spatial heterogeneity of hyphal networks, and uncertain biomarker conversion factors-in quantifying fungal-mediated C fluxes. We identify key knowledge gaps, notably the need for explicit integration of ECM functional traits into ecosystem C models, resolution of the net balance between priming and stabilization under varying edaphic conditions, and a mechanistic understanding of how global change drivers alter ECM-C relationships. Future research should prioritize multi-scale approaches that integrate molecular omics, high-resolution isotope tracing, and process-based modeling to better constrain the role of ECM fungi in forest soil C sequestration and vulnerability. We further highlight that the ecological significance of ECM fungi in the global carbon cycle extends well beyond the well-studied northern forests, encompassing extensive tropical and Southern Hemisphere ECM systems that are subject to fundamentally different nutrient economies and global change pressures.