In boreal and northern temperate forests, the moss layer and ectomycorrhizal (ECM) fungi play important roles in carbon and nutrient cycling. ECM mycelium is present in the lower parts of the moss layer, but little is currently known about the interaction between these two key components of northern forest ecosystems. This thesis aims to address this knowledge gap and to improve our understanding of the mechanisms through which the moss layer and ECM fungi influence carbon and nutrient cycling. Nutrient transfer between litter and Scots pine seedlings in symbiosis with the ECM fungus Paxillus involutus (Batsch) Fr. was investigated in highly controlled microcosm experiments using, for the first time, intact moss and pine litter. In addition, moss removal plots were established in a Scots pine forest which allowed measurement of processes involved in carbon (C) and nutrient turnover and related variables. There was a close, reciprocal exchange of carbon and nutrients between the host plant and ECM mycelium colonising moss and pine litter (Chapter 2). This was greatly enhanced by intensive colonisation of moss litter, suggesting that mosses provide a key source of nutrients for ECM fungi and may facilitate transfer of photosynthetic C belowground. During almost 2 years of decomposition, moss tissue released more nitrogen (N) but retained more C than pine litter (Chapter 3), further highlighting the importance of the moss layer in providing nutrients for overstorey trees, and in the accumulation of recalcitrant C in soil. In addition to contributing directly to C cycling through inputs of recalcitrant C in litter, the moss layer can influence C cycling indirectly, by increasing soil microbial activity; CO2 efflux was on average 1.4 times greater from soil under the moss layer than from soil covered only in pine litter (Chapter 3). The results suggest that an indirect influence can occur via two pathways: through an insulating effect of the moss layer on soil temperature, and through inputs of dissolved organic carbon (DOC) leached from moss (Chapter 4), both of which may stimulate activity of soil microbes. These findings demonstrate the importance of both the moss layer and ECM fungi in carbon and nutrient cycling in boreal and northern temperate forests, and indicate that mosses provide a key pathway through which nutrients may bypass sequestration in saprotrophic microbial biomass and be transferred directly from plant tissue to ECM fungi and overstorey trees.