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Forest soils store large amounts of carbon (C) and nitrogen (N), yet how predicted shifts in forest composition will impact long-term C and N persistence remains poorly understood. A recent hypothesis predicts that soils under trees associated with arbuscular mycorrhizas (AM) store less C than soils dominated by trees associated with ectomycorrhizas (ECM), due to slower decomposition in ECM-dominated forests. However, an incipient hypothesis predicts that systems with rapid decomposition-e.g. most AM-dominated forests-enhance soil organic matter (SOM) stabilization by accelerating the production of microbial residues. To address these contrasting predictions, we quantified soil C and N to 1 m depth across gradients of ECM-dominance in three temperate forests. By focusing on sites where AM- and ECM-plants co-occur, our analysis controls for climatic factors that covary with mycorrhizal dominance across broad scales. We found that while ECM stands contain more SOM in topsoil, AM stands contain more SOM when subsoil to 1 m depth is included. Biomarkers and soil fractionations reveal that these patterns are driven by an accumulation of microbial residues in AM-dominated soils. Collectively, our results support emerging theory on SOM formation, demonstrate the importance of subsurface soils in mediating plant effects on soil C and N, and indicate that shifts in the mycorrhizal composition of temperate forests may alter the stabilization of SOM., (© 2018 John Wiley & Sons Ltd.)