The litter C/N ratio governs the dual role of ectomycorrhizal fungi in decomposition and plant nutrition absorption

Ectomycorrhizal fungi (EcMF) modulate litter decomposition and facilitate nutrient acquisition through the soil–mycorrhiza–plant pathway. Although litter chemistry has been considered an important factor affecting litter decomposition, how it regulates EcMF‐induced decomposition and subsequent plant growth remains unclear. Using a microcosm experiment with five litter types, we assessed decomposition rates, carbon (C) and nitrogen (N) loss, microbial biomass, enzyme activities and plant growth under ectomycorrhizal and non‐ectomycorrhizal seedlings. Results indicated that effects of EcMF on litter decomposition were litter‐specific and time‐dependent. After 120 days, EcMF enhanced decomposition of low C/N litters ( Carex breviculmis , Lindera glauca and Liquidambar formosana ) by 13%–28%, but suppressed that of high C/N litters ( Quercus acutissima and Pinus massoniana ) by 19%–20%. Initially (30 days), EcMF inhibited low C/N litter decomposition, later shifting to promotion, whereas high C/N litter decomposition was consistently suppressed. Correlation analysis revealed that the hyphal effects on decomposition correlated negatively with litter stage C/N ratio. EcMF significantly increased plant N concentration and biomass under low C/N litters (biomass increased by 20%–30%), but only marginally enhanced biomass under high C/N litters (14%–16%) without improving N concentration. Partial least squares path modelling revealed that EcMF promoted C and N release from low C/N litter, thereby increasing N availability and plant growth. Even under high C/N conditions, EcMF directly supported plant growth despite suppressed decomposition. These findings underscore the dual roles of EcMF, which are regulated by litter C/N ratio, in mediating decomposition and plant nutrient acquisition, highlighting the importance of litter–microbe interactions in forest nutrient cycling and productivity. Read the free Plain Language Summary for this article on the Journal blog.