Above- and belowground effects of ectomycorrhizal dominance on soil carbon and nitrogen in a temperate forest

Abstract Due to global change, the dominance of ectomycorrhizal (ECM) tree species is continually decreasing in temperate forests, which is expected to greatly alter soil carbon and nitrogen dynamics. However, the specific mechanisms through which ECM tree dominance affects soil carbon and nitrogen, particularly via regulating above- and belowground forest properties, remain poorly understood. Here, we investigated the relationships of forest above- (e.g. tree species richness and basal area, leaf nutrient content) and belowground properties (e.g. soil microbial community, enzymatic activity) with soil organic carbon (SOC) and nitrogen along an ECM tree dominance gradient in a temperate forest. We found significant changes in above- (i.e. leaf nutrient content, basal area) and belowground (i.e. fungal community, enzymatic activity) forest properties along the ECM tree dominance gradient. For instance, tree basal area and saprotroph abundance increased with ECM tree dominance, while leaf nitrogen content and enzymes related to soil carbon or nitrogen (β-1,4-glucosidase, cellobiohydrolase, β-N acetylglucosaminidase) decreased. Notably, structural equation modeling suggested that ECM tree dominance negatively affected SOC through regulating aboveground properties. However, ECM tree dominance affected soil nitrogen content and transformation rates by regulating both above- and belowground properties, highlighting different pathways through which soil nitrogen vs. SOC respond to ECM tree dominance change. These results demonstrate that ECM tree dominance can affect soil carbon and nitrogen by distinctively regulating above- and belowground forest properties. Both above- and belowground changes should thus be considered when predicting how temperate forests will respond to the global-change-induced decline in ECM tree dominance.