Carbon sequestration is related to mycorrhizal fungal community shifts during long‐term succession in boreal forests

Summary Boreal forest soils store a major proportion of the global terrestrial carbon (C) and below‐ground inputs contribute as much as above‐ground plant litter to the total C stored in the soil. A better understanding of the dynamics and drivers of root‐associated fungal communities is essential to predict long‐term soil C storage and climate feedbacks in northern ecosystems. We used 454‐pyrosequencing to identify fungal communities across fine‐scaled soil profiles in a 5000 yr fire‐driven boreal forest chronosequence, with the aim of pinpointing shifts in fungal community composition that may underlie variation in below‐ground C sequestration. In early successional‐stage forests, higher abundance of cord‐forming ectomycorrhizal fungi (such as Cortinarius and Suillus species) was linked to rapid turnover of mycelial biomass and necromass, efficient nitrogen (N) mobilization and low C sequestration. In late successional‐stage forests, cord formers declined, while ericoid mycorrhizal ascomycetes continued to dominate, potentially facilitating long‐term humus build‐up through production of melanized hyphae that resist decomposition. Our results suggest that cord‐forming ectomycorrhizal fungi and ericoid mycorrhizal fungi play opposing roles in below‐ground C storage. We postulate that, by affecting turnover and decomposition of fungal tissues, mycorrhizal fungal identity and growth form are critical determinants of C and N sequestration in boreal forests.