Fungal guild interactions slow decomposition of boreal forest pine litter and humus

Summary Ericaceous understory shrubs and ericoid mycorrhizal fungal communities are ubiquitous in boreal forests, and their interactions with ectomycorrhizal and saprotrophic fungi may determine organic matter dynamics in forest soils. We followed decomposition of pine needle litter and mor‐layer humus over 3 yr in a factorial shrub removal‐ and pine root exclusion experiment in an old‐growth Scots pine ( Pinus sylvestris ) forest, to evaluate effects of fungal guilds on mass loss. Litter mass loss was 23% greater when ectomycorrhizal fungi were excluded suggesting increased saprotrophic activity, independently of ericoid shrub presence. However, this ‘Gadgil effect’ was only found after 17 months following a summer drought. By contrast, humus mass loss was overall stimulated by ectomycorrhizal fungi, while ericoid mycorrhizal shrubs appeared to counteract this effect, potentially caused by simultaneous addition of recalcitrant organic matter and inhibition of ectomycorrhizal decomposers. We conclude that competitive saprotrophic–ectomycorrhizal fungal interactions may slow early‐stage litter decomposition, but this effect was small and inconsistent. Furthermore, interactions between ecto‐ and ericoid mycorrhizal guild members appear to determine the late‐stage organic matter balance of boreal forest humus.