Boreal forests are N-limited ecosystems storing globally significant amounts of carbon (C) belowground as soil organic matter (SOM). The significant role of mycorrhizal fungi, both ericoid (ERM) and ectomycorrhizal (ECM) in decomposition and building up SOM has been proposed, but it is still largely unknown how these two types of mycorrhiza interact in terms of SOM changes. Here we try to elucidate the combined effect of these mycorrhizal types on the SOM decomposition and accumulation in a pot study with ECM-pine (Pinus sylvestris L.) and ERM-heather (Calluna vulgaris L.). Experimental design included plants growing alone or in a mixture, and a non-planted control. In contrast to the mixture, heather growing alone exerted higher biomass, higher uptake of N from stable SOM pool, slightly higher microbial biomass C and fungal biomass. Moreover, heather soil had the highest dissolved organic carbon and nitrogen. On the contrary, mixed cultures showed lower growth of heather and stable SOM decomposition. Our results suggest that interaction between ERM and ECM plants may lead to stable SOM accumulation in boreal forest soils.