Mycorrhizal Type Defines Priming Effects in Organic and Mineral Forest Soils

Trees associated with arbuscular mycorrhizal (AM) or ectomycorrhizal (ECM) fungi create distinct soil environments that influence organic matter accumulation, decomposition, and persistence. As forest composition changes globally, understanding how shifts in mycorrhizal dominance affect soil carbon (C) cycling becomes increasingly important. Priming effects, where fresh C inputs alter the decomposition of existing soil organic matter, represent a critical but poorly understood mechanism, through which tree mycorrhizal associations may regulate soil C storage and loss. To address the role of mycorrhizal associations in regulating priming effects, we collected soils from forest plots differing in their dominance of mycorrhizal-associated tree types in a subtropical forest and quantified priming effects with ¹³C-labeled glucose. In the organic topsoil, priming effects were positive, but strongly decreased (from 19 to 2.3 μg C g^-1 soil after 28 days of incubation) as ECM trees became more dominant. This reduction was linked to higher particulate organic carbon, a greater fungal-to-bacterial ratio, and more genes for degrading recalcitrant C. Conversely, in the mineral subsoil, priming effects were generally negative, with AM-dominated plots (-14 μg C g^-1 soil) showing stronger negative priming than in ECM-dominated plots (-3.2 μg C g^-1 soil after 28 days of incubation). This is primarily because AM dominated soils have greater availability of mineral-associated organic matter and nitrate content, as well as higher microbial carbon use efficiency. Tree mycorrhizal associations thus regulate priming effects through depth-dependent changes in C stability, nitrogen availability, and microbial functional activity. We conclude that forecasting forest C dynamics in response to mycorrhizal compositional shifts must account for these soil depth-specific mechanisms.