Functional convergence in the decomposition of fungal necromass in soil and wood

Abstract

Understanding the post-senescent fate of fungal mycelium is critical to accurately quantifying forest carbon and nutrient cycling, but how this organic matter source decomposes in wood remains poorly studied. In this study, we compared the decomposition of dead fungal biomass (a.k.a. necromass) of two species, Mortierella elongata and Meliniomyces bicolor, in paired wood and soil plots in a boreal forest in northern Minnesota, USA. Mass loss was quantified at four time points over an 8-week incubation and the richness and composition of the fungal communities colonizing fungal necromass were characterized using high-throughput sequencing. We found that the structure of fungal decomposer communities in wood and soil differed, but, in both habitats, there was relatively rapid decay (∼30% remaining after 56 days). Mass loss was significantly faster in soil and for high-quality (i.e. high nitrogen and low melanin) fungal necromass. In both habitats, there was a clear trajectory of early colonization by opportunistic fungal taxa followed by colonization of fungi with greater enzymatic capacities to degrade more recalcitrant compounds, including white-rot and ectomycorrhizal fungi. Collectively, our results indicate that patterns emerging regarding substrate quality effects on fungal necromass decomposition in soil and leaf litter can be largely extended to fungal necromass decomposition in wood.

Substrate quality drives fungal necromass decay and decomposer community structure under contrasting vegetation types

“…Specifically, the C:N ratios for the two necromass types were 7 and 13 respectively, which are much lower than C:N ratios typically reported for leaf litter which can range from 20 to 100 (Brabcová et al, 2018; Ferlian, Wirth, & Eisenhauer, 2017; Zhang et al, 2008). In this case, the higher nutrient content of fungal necromass may not demand the same selective enzymatic activity to facilitate decomposition, particularly if initial rates of mass loss are influenced by differences in leaching capacity rather direct microbial degradation (Maillard et al, 2020). It is certainly possible that with more time, differences in the decomposition of the more recalcitrant fraction of the remaining fungal necromass would develop between vegetation types, though the rapid mass loss from our high‐quality necromass is consistent with a similarly fast rate of mass loss recently observed for AM necromass in temperate AM‐dominated forests in Japan (Schäfer, Dannoura, Ataka, & Osawa, 2019).…”

“…As noted above, it is likely that these two traits interact to determine the quality of fungal necromass for decomposers (Fernandez & Koide, 2014); as such, future tests should disentangle the relative importance of each to necromass decay rates. Given the recent documentation of fungal necromass C being disproportionately utilized by bacteria relative to fungi (López‐Mondéjar et al, 2018), but also the significant C and N mining from fungal necromass by EM fungi (Akroume et al, 2019), it will also be important to use isotopic labelling techniques to understand exactly which resources are utilized by which micro‐organisms, particularly in field settings where symbiotic fungi are present (Fernandez & Kennedy, 2018; Maillard et al, 2020; Zeglin, Kluber, & Myrold, 2013). Additionally, like most studies of microbial communities, we relied on relative sequence read counts as proxies for microbial community abundances.…”

“…Two fungal species, Mortierella elongata and Meliniomyces bicolor , which have previously been demonstrated to differ in multiple chemical traits (Maillard, Schilling, Andrews, Schreiner, & Kennedy, 2020; Table 2), were chosen to represent high‐ and low‐quality necromass. M. elongata is a fast‐growing saprotrophic fungus in the phylum Mucromycota, which is frequently found in both forest and agricultural soils (Li et al, 2018).…”

Root presence modifies the long‐term decomposition dynamics of fungal necromass and the associated microbial communities in a boreal forest

“…At both local and regional scales, the decomposition rate of the fungal necromass has been shown to be strongly affected by its biochemical quality (Beidler et al, 2020; Brabcová et al, 2018; Maillard et al, 2020). In particular, fungal necromass decomposition rate is known to be positively correlated with initial N concentration (Brabcová et al, 2018; Fernandez & Koide, 2012, 2014; Koide & Malcolm, 2009).…”

Large-scale genome sequencing of mycorrhizal fungi provides insights into the early evolution of symbiotic traits

“…The diversity of trophic states in extant ectomycorrhizal fungi may be a consequence of their multiple origins from saprotrophic ancestors with varied decay capabilities, including white and brown rot wood decayers, and soil and litter decomposers [3][4][5] . However, the extent to which ectomycorrhizal fungi make use of their secreted plant cell wall degrading enzymes (PCWDEs) and microbial cell wall degrading enzymes (MCWDE) to decay or decompose SOM is not well understood [17][18][19][20][21][22][23][24] .…”