Ectomycorrhizal fungi – potential organic matter decomposers, yet not saprotrophs

Abstract

I.II.III.IV.V.References Summary Although hypothesized for many years, the involvement of ectomycorrhizal fungi in decomposition of soil organic matter remains controversial and has not yet been fully acknowledged as an important factor in the regulation of soil carbon (C) storage. Here, we review recent findings, which support the view that some ectomycorrhizal fungi have the capacity to oxidize organic matter, either by ‘brown‐rot’ Fenton chemistry or using ‘white‐rot’ peroxidases. We propose that ectomycorrhizal fungi benefit from organic matter decomposition primarily through increased nitrogen mobilization rather than through release of metabolic C and question the view that ectomycorrhizal fungi may act as facultative saprotrophs. Finally, we discuss how mycorrhizal decomposition may influence organic matter storage in soils and mediate responses of ecosystem C sequestration to environmental changes.

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

“…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). Thus, ECM fungi may be primed to recycle N from root tissues they colonized and modified pre-mortem, but may be less efficient decomposers than saprotrophs, explaining slower rates of decomposition (Lindahl et al 2002;Langley and Hungate 2003).…”

“…The authors also detected changes in fungal communities colonizing decaying roots when the rhizosphere soil was left intact; ECM fungal taxa including: Boletales, Thelephorales and Cantharellales were detected more frequently in cores than litterbags. Correspondingly, greater release of N and P from roots was strongly correlated with increased abundance of Thelephorales and Cantharellales, hinting at the possibility that ECM fungi are mining nutrients from decaying fine roots (Lindahl and Tunlid 2015;Kuyper 2017). These studies suggest that slower decomposition of lower order roots may be an experimental artifact and draws attention to the change in microbial community composition that can occur when preparing roots for litterbags (Li et al 2015).…”

“…It has been suggested that ECM fungi have saprotrophic capabilities, transforming SOM to acquire organic forms of nutrients (i.e. the nutrient mining by priming hypothesis as discussed by Kuyper 2017; Lindahl and Tunlid 2015). Whether ECM fungi act as Btrueŝ aprotrophs and obtain carbon from SOM for the purpose of building biomass is a point of contention in the literature, complicated by the fact that ECM fungal species vary widely in their ability to decompose organic substrates (Hodge 2017;Kuyper 2017;Pellitier and Zak 2017).…”

“…There is a growing body of evidence showing that some species of ECM fungi produce extracellular enzymes to degrade organic residues, obscuring the distinction between saprotroph and symbiont (Talbot et al 2008;Lindahl and Tunlid 2015). The acquisition of nutrients from organic substrates via extracellular digestion, i.e.…”

The Individual and Interactive Effects of Nitrogen and Phosphorus Enrichment on Coral Reefs

“…This increased variability in EM fungi responses might be expected if EM fungi can scavenge C from the environment via the decomposition of organic matter (Read et al 2004), decoupling the performance of EM from their plant hosts. However, the ability of many EM to acquire biologically meaningful amounts of C through saprotrophy appears limited (Lindahl & Tunlid 2015), and the factors that shape potential differences in the response of AM and EM fungi to nutrient loading will be a fruitful area for future research. Overall however, our data suggest that mutualisms in which heterotrophs are heavily dependent on photosynthetically-derived C are particularly vulnerable to nutrient-induced decline.…”