Tree species-specific differences in carbon allocation to ectomycorrhizal symbionts tightly relate to ectomycorrhizal fungal community composition

Host trees allocate photosynthates to ectomycorrhizal symbionts (ECMs) in exchange for soil nutrients, and the carbon allocation has profound effects on forest carbon sequestration. However, it is still a challenge to precisely quantify the amount of carbon allocated to ECMs and to identify factors affecting this allocation. Here, we estimated tree species-specific differences and the dynamics of carbon allocation to ECMs by 13CO2 labeling along seedling development, and explored mechanisms underlying these differences from the perspective of ECM fungal community across four tree species (evergreen Picea asperata and Quercus aquifolioides, deciduous Larix gmelinii and Betula albosinensis) and three development stages (early, middle and late of the growing season). Results showed that the proportion of 13C allocated to ECMs (P13CECMs) and ECM fungal community composition varied across tree species and seedling development. P13CECMs was higher in deciduous (3.3 +/- 0.7%) trees than in evergreen trees (1.6 +/- 0.3%), and higher at the late (5.2 +/- 0.7%) than middle (1.8 +/- 0.3%) and early (0.3 +/- 0.1%) growing season. The relative abundance of the four ECM exploration types (contact, short-, medium- and long-distance) was 7.8-14.8%, nearly 50%, 25.2-31.2%, and nearly 10%, respectively, and varied significantly for contact and medium-distance types. The relative abundance of contact type was higher in deciduous than evergreen trees, and higher at the late than mid-growing stage, but the opposite was true for medium-distance type. Across tree species and seedling development, P13CECMs was positively correlated with the relative abundance of contact type, but negatively correlated with that of medium-distance type. The results suggest that carbon allocation to ECMs is tightly associated with ECM fungal community composition and highlight the critical role of ECM fungal community composition in carbon budges of forest ecosystem.