Peatlands are major carbon sinks globally, but it is still unclear what drives their shift from functioning as carbon sink to a source. Fungi rely on soil carbon inputs and play an active role in carbon mobilization and stabilization. Future climate change scenarios predict increases in temperature and lower water tables, which may lead to functional shifts in fungal communities growing in peatlands. Despite their abundance, the impact of fungi on carbon cycling in peatlands is still poorly understood. Therefore, it is crucial to study the dynamics and distribution of fungal communities in pristine peatlands in order to predict the potential changes in peatland ecosystems more accurately. Here, we review the current knowledge about fungal communities in peatlands, including the influence of peatland type, abiotic factors, and temporal and spatial dynamics on fungal diversity. Our overview shows that fungal diversity in peatlands of certain regions such as tropical areas is severely understudied. Furthermore, we examine the ecological roles and the relative contribution to carbon cycling of functional guilds of fungi in peatlands, showing that saprotrophs are the key guild in organic matter decomposition, and ericoid- and ectomycorrhizal fungi have the ability to act as decomposers and vectors of plant carbon input to soil. We suggest that the application of a combination of methodological approaches for taxonomic and functional characterisation of peatland fungal communities is needed to build better understanding about the diversity-functioning relationships in peatland ecosystems. Overall, this review shows that fungi must be considered as important drivers of mechanistic processes underlying biodiversity, plant nutrient uptake and carbon losses in peatlands.
