The purpose of this pioneer approach is to provide novel insights about the interplay between traits of soil fungi, such as decomposer capacity and metabolic efficiency, and ecosystem level processes, such as carbon sequestration and nutrient cycling, in order to increase the predictive capacity of ecosystem models. Boreal forests represent a large carbon sink, containing 32% of the terrestrial carbon on Earth. Due to intensive human activities (e.g. forestry, clear-cutting, land use change, fertilization) occurring today in a vast majority of boreal forest, it is essential to advance our understanding of soil processes, to enable informed future policy decisions about forest management and ensure a maintained, and preferably increased, carbon sink. Soil microorganisms, in particular fungal communities, play a central role in regulation of soil organic matter dynamics in forest ecosystems and are therefore subject to particular attention. Fungi play a pivotal role in boreal forest soils, and mycorrhizal fungi, living in symbiosis with plant roots, are particularly important in regulating carbon sequestration and nutrient turnover. New innovative metatranscriptomics approaches allow us to open the black box of microbial process in soils, by obtaining massive data of expressed genes from entire microbial communities, based on soil samples collected directly in the field. These data enable us to investigate how soil fungi and other organisms regulate soil organic matter dynamics directly in the ecosystem. We target fungal functional traits related to soil fungal community ecology, more specifically in relation to below-ground cycling of carbon and nitrogen. For this purpose, expression of selected gene families involved in the production of enzymes implicated in organic matter degradation, resource assimilation and metabolism is assessed at the ecosystem level.