Litter decomposition regulated by stand ECM dominance but not litter mycorrhizal type in a subtropical mountainous forest

Litter decomposition is a key process regulating terrestrial carbon and nutrient cycling. Mycorrhizal association is a fundamental trait shaping both litter chemistry and root-microbe interactions, yet its role in regulating litter decomposition remains unclear in species-rich subtropical forests. We conducted two in situ litter decomposition experiments to disentangle the effects of mycorrhizal type and stand ectomycorrhizal (ECM) dominance on litter decomposition in a subtropical forest. A 1-year field litter decomposition experiment of 21 arbuscular mycorrhizal (AM) and 6 ECM tree species indicated that AM litter initially decomposed faster than ECM litter, primarily due to lower leaf dry matter content, but this difference disappeared over time. Across the entire year, litter nutrient concentrations were the primary determinants of mass loss. The AM and ECM litter decomposition in plots with a natural gradient of ECM tree dominance showed that decomposition rates of both AM and ECM litter decreased with increasing ECM dominance at the stand level. This pattern was linked to lower soil nitrogen availability, higher fine-root biomass and reduced saprotrophic fungal (SAP) abundance, suggesting that intensified competition between ECM fungi and saprotrophs constrained litter decay. Our findings highlight that the tree mycorrhizal strategy, especially ECM dominance at the community scale, regulates decomposition by coupling above- and belowground traits and processes. This emphasizes the importance of forest mycorrhizal dominance as a determinant of litter turnover and nutrient cycling in subtropical mountainous forests.