Mycorrhizal guild interactions, rather than the Gadgil effect, slow decomposition of mor-layer humus

Boreal forest soils are dominated by three fungal guilds; ectomycorrhizal fungi associated with canopy-forming trees, ericoid mycorrhizal fungi associated with understory shrubs and free-living saprotrophic fungi. We followed decomposition of pine needle litter and mor-layer humus in a factorial pine root exclusion and shrub removal experiment in a mature pine forest over three years to evaluate fungal guild effects on mass loss. Litter mass loss was 10% faster when ectomycorrhizal fungi were excluded, however this ‘Gadgil effect’ was only found in one of two litter sets, and it was independent of shrub presence. In contrast, humus mass loss was hampered by shrub presence and promoted by ectomycorrhizal fungi, although presence of both guilds resulted in the largest humus mass remaining. This suggests that saprotrophic-ectomycorrhizal interactions are of little significance for early-stage litter decomposition, while ericoid and ectomycorrhizal guilds interact to determine late-stage organic matter balance in boreal forest soils.