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

Abstract

1. 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. 2.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. 3. 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. 4. 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.

Maintaining connectivity: understanding the role of root order and mycelial networks in fine root decomposition of woody plants

“…In boreal forests which are known to be N limited, ECM and SAP fungi are spatially separated. SAP fungi tend to colonize more recently shed litter at the forest floor surface and mycorrhizal fungi are more abundant in the underlying layers which contain older, more decomposed litter (Lindahl et al 2007;Clemmensen et al 2013;Bödeker et al 2016). Whether this vertical separation is the result of competitive exclusion of saprotrophic fungi by ECM or niche differentiation needs to be addressed (Fernandez and Kennedy 2015).…”

“…Whether this vertical separation is the result of competitive exclusion of saprotrophic fungi by ECM or niche differentiation needs to be addressed (Fernandez and Kennedy 2015). Bödeker et al 2016 tried to address this question by investigating the vertical positions of saprotrophic and ECM fungi in the soil profile and the potential for different fungal guilds to colonize substrates of varying quality. The results from their study support the idea that SAP and ECM fungi have overlapping fundamental niches, in that both were able to colonize the same substrates and their vertical separation in the soil is likely reinforced by competition for N (Bödeker et al 2016).…”

“…Bödeker et al 2016 tried to address this question by investigating the vertical positions of saprotrophic and ECM fungi in the soil profile and the potential for different fungal guilds to colonize substrates of varying quality. The results from their study support the idea that SAP and ECM fungi have overlapping fundamental niches, in that both were able to colonize the same substrates and their vertical separation in the soil is likely reinforced by competition for N (Bödeker et al 2016). ECM fungi may have an advantage in decomposition at lower soil depths through early access to nutrients contained in senescing mycorrhizal roots and enzyme production subsidized by plant sugars (Cairney and Burke 1994;Langley and Hungate 2003;Lindahl and Tunlid 2015).…”

Anthropogenic nitrogen enrichment enhances soil carbon accumulation by impacting saprotrophs rather than ectomycorrhizal fungal activity

“…However, studies have also demonstrated that some EM fungi may have similar oxidative and hydrolytic enzymatic capacities to saprotrophic fungi (Bödeker, Nygren, Taylor, Olson, & Lindahl, 2009;Firoz, 2014;Phillips, Ward, & Jones, 2014), and that gene expression for their oxidative degradation may be regulated by belowground tree C allocation (Rineau et al, 2013;Voříšková, Brabcová, Cajthaml, & Baldrian, 2014). Furthermore, a recent study conducted in a boreal forest suggested that saprotrophic and EM fungi may have overlapping niches with regard to their colonization of different substrate qualities, suggesting that EM fungi may colonize fresh litter as well as humus (Bödeker, Lindahl, Olson, & Clemmensen, 2016). While it is clear that both EM fungi and saprotrophic microbiota (i.e., fungi and other microbes) play an important role in decomposition, less is known about how N enrichment influences the relative contribution of these functional groups to the decomposition of new (fresh litter) versus old substrates (humus).…”

More bang for the buck? Can arbuscular mycorrhizal fungal communities be characterized adequately alongside other fungi using general fungal primers?

“…High throughput sequencing has revolutionized microbial ecology. We now have the ability to characterize both prokaryotic and eukaryotic microbial communities more thoroughly and disentangle relationships among broad organismal groups (Bödeker et al ., ; Morriën et al ., ; Toju et al ., ). Arbuscular mycorrhizal fungi (AMF; subphylum Glomeromycotina, Spatafora et al ., ) are no exception, and high throughput sequencing has revealed greater richness within communities compared with Sanger sequencing (Öpik et al ., ), as well as an increased understanding about global distribution patterns and their potential underlying drivers (Davison et al ., ).…”