Mycorrhizal and saprotrophic fungal guilds compete for the same organic substrates but affect decomposition differently

Summary Communities of litter saprotrophic and root‐associated fungi are vertically separated within boreal forest soil profiles. It is unclear whether this depth partitioning is maintained exclusively by substrate‐mediated niche partitioning (i.e. distinct fundamental niches), or by competition for space and resources (i.e. distinct realized niches). Improved understanding of the mechanisms driving spatial partitioning of these fungal guilds is critical, as they modulate carbon and nutrient cycling in different ways. Under field settings, we tested the effects of substrate quality and the local fungal species pool at various depths in determining the potential of saprotrophic and mycorrhizal fungi to colonize and exploit organic matter. Natural substrates of three qualities – fresh or partly decomposed litter or humus – were incubated in the corresponding organic layers of a boreal forest soil profile in a fully factorial design. After one and two growing seasons, fungal community composition in the substrates was determined by 454‐pyrosequencing and decomposition was analyzed. Fungal community development during the course of the experiment was determined to similar degrees by vertical location of the substrates (24% of explained variation) and by substrate quality (20%), indicating that interference competition is a strong additional driver of the substrate‐dependent depth partitioning of fungal guilds in the system. During the first growing season, litter substrates decomposed slower when colonized by root‐associated communities than when colonized by communities of litter saprotrophs, whereas humus was only slightly decomposed by both fungal guilds. During the second season, certain basidiomycetes from both guilds were particularly efficient in localizing and exploiting their native organic substrates although displaced in the vertical profile. This validates that fungal community composition, rather than microclimatic factors, were responsible for observed depth‐related differences in decomposer activities during the first season. In conclusion, our results suggest that saprotrophic and root‐associated fungal guilds have overlapping fundamental niches with respect to colonization of substrates of different qualities, and that their substrate‐dependent depth partitioning in soils of ectomycorrhiza‐dominated ecosystems is reinforced by interference competition. Through competitive interactions, mycorrhizal fungi can thus indirectly regulate litter decomposition rates by restraining activities of more efficient litter saprotrophs. A lay summary is available for this article.