Radiocarbon evidence of the role of tree mycorrhizal type in modulating mean soil carbon age

Tree mycorrhizal type is increasingly recognised as a key determinant of the quantity and quality of soil carbon stocks from local to global scales. However, direct evidence linking mycorrhizal associations to the age and persistence of soil carbon pools remains lacking. Here, we leverage radiocarbon (¹⁴C) analysis to investigate the mean age of particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) fractions across temperate forests dominated by arbuscular mycorrhizal (AM) or ectomycorrhizal (EcM) tree species. Our findings reveal significant interactions between mycorrhizal type and soil depth: EcM-dominated soils exhibit younger carbon in the surface organic horizon (higher ∆¹⁴C) but older carbon in the deeper mineral horizons (lower ∆¹⁴C) compared to AM-dominated soils, in both the POC and MAOC fractions. These patterns suggest that EcM associations may suppress surface-layer decomposition, whereas AM-dominated systems promote recent root-derived carbon inputs at greater depths, creating a comparatively broader carbon age gradient under EcM trees. Moreover, carbon persistence mechanisms likely differ between fractions. Specifically, mean age of POC is mainly driven by substrate quality (e.g., C:N ratios), whereas mean carbon age of MAOC is closely tied to microbial processing, as indicated by δ¹⁵N enrichment. These findings provide novel insights into how mycorrhizal-mineral interactions can shape soil carbon storage and persistence and inform forest management strategies aimed at climate change mitigation.