Introduction

Soil organic matter (SOM) is known to be the largest C pool in terrestrial ecosystems, ranging from 1500 to 2000 Pg in the top meter. This equals two to three times the amount of C stored in the vegetation, which is approximately 500 Pg (Amundson, 2001; Butler & Day, 1998; Dixon et al., 1994). Among the various sources of SOM in forest ecosystems, dead mycelia are probably among the largest ones. It has been suggested that the majority of C stored in the deeper horizons of boreal forest soils originates from the allocation of photosynthate to ectomycorrhizal fungi (Clemmensen et al., 2013). The annual allocation of carbon into mycorrhizal fungi can constitute up to 22% of the annual net primary productivity (Hobbie & Hobbie, 2006). This major contribution suggests the profound importance of these fungi in the overall allocation of carbon in both litter and soil compartments. Estimates of ectomycorrhizal (EM) biomass, which constitutes the major fraction of soil fungal biomass, typically range from 100 to 600 kg ha-1 (Hendricks, Mitchell, Kuehn, & Pecot, 2016; Wallander, Göransson, & Rosengren, 2004) and, together with roots, produces more than 50% of the dissolved organic carbon in forest’s soil (Högberg & Högberg, 2002). Significant amounts of mycelia are produced in soil layers below the depth of 10 cm (Boström, Comstedt, & Ekblad, 2007) and in the litter, where the dry fungal biomass per g substrate can be more than 10-fold that of soil (Baldrian et al., 2013). In forest soil and litter, which are often N limited, fungal mycelia are more attractive than plant residues as a target for decomposers, given their high content of N, which can be 3 up to 30 times greater than in plant litter (Koide & Malcolm, 2009; Mouginot et al., 2014). Due to the large fluxes of C and nutrients into the soil through fungal necromass, a better understanding of the decomposition dynamics of fresh fungal litter may improve our understanding of SOM dynamics, and how these two are related.
A major driving factor of litter decomposition is the initial litter “quality”, i.e. litter chemical composition (Berg, 1984). In general, decomposition is favoured by relatively low litter C:N ratios (Cleveland & Liptzin, 2007), which might help to maintain the low C:N ratio of microbial cells (Manzoni, Trofymow, Jackson, & Porporato, 2010). Yet, there is increasing evidence that other aspects of chemical litter quality beyond basic stoichiometry also affect decomposition rates (Wilkinson, Alexander, & Johnson, 2011).
To date, the number of observations about the decomposition rates of EM fungi is limited, as well as our knowledge on their main drivers. It has been hypothesised that EM fungal necromass decomposition is driven mainly by its necromass “quality”, which, similarly to plant litter, refers to fungal necromass chemistry and morphology. Previous research found a significant positive relationship between the N concentration in the fungal tissues and its decomposition (Christopher W Fernandez & Koide, 2014; Koide & Malcolm, 2009).
The chemical composition of ectomycorrhizal fungal mycelium varies widely among fungal species and possibly contributes to the variation in decomposition rates of EM fungal necromass. While the cytoplasmic fraction does not likely play a significant role in the decomposition of EM necromass due to its high lability (Drigo, Anderson, Kannangara, Cairney, & Johnson, 2012), it seems that the composition of the cell wall cell cel fraction drives the long-term decomposition of EM extramatrical necromass. The chemical composition of the cell wall of EM fungi varies with age, genotype, taxon, and environment (Bowman & Free, 2006; Feofilova, 2010). EM fungal cell walls are composed mainly by polysaccharides such as glucans and chitin (70-90% of dry biomass), proteins, which contains from 40 to 60% of cell wall total N, and melanins. The latter, despite being a minor component and highly variable in concentration across different taxa (Butler & Day, 1998a), is linked to the tolerance of various environmental stressors (Christopher W Fernandez & Koide, 2013; Rosas & Casadevall, 1997; Wang & Casadevall, 1994).
During the last decades, several studies suggested that differences in initial concentrations of the main classes of chemical fungal cell-wall compounds (proteins, carbohydrates, melanin) underpin necromass degradation (Christopher W Fernandez & Koide, 2012, 2014). Yet, there are many uncertainties concerning the mechanisms behind this relationship. These uncertainties range from the dynamics of secondary compounds contents during the decomposition process to the actual interaction between the components themselves. Moreover, we may not assume that the initial concentration of cell-wall compounds (Berg, 2000; Melillo, Aber, & Muratore, 1982), is a dominant predictor of the decomposition rate and dynamics. The fungal wall structure is highly variable. In contrast to plant litter, the fungal cell wall retains considerable plasticity despite the presence of a relatively rigid scaffold formed by chitin fibrils and α-1,3-glucans. This plasticity allows to easily reshape the molecular architecture of mycelia to survive through different external stress. In addition, the fungal cell walls contain an extensive covalent cross-linking of glucans and chitin (Cairney, 2012; Kang et al., 2018). These dissimilarities suggest that we might need to consider different drivers to predict its decomposition compared to plants. So far, no previous research has explored the role of the interaction between cell-wall components in the decomposition of EM necromass. Nor did previous research consider the impacts of changes in these cell wall components during decomposition. Since all these components are highly interconnected and each one has a specific role in maintaining the cell wall structure and function, an extensive temporal analysis of dynamics of their composition through the fungal litter degradation process will improve our understanding of the mechanisms behind fungal necromass decomposition.
In this paper we examine the dynamics in the most abundant cell-wall compounds (chitin, glucans and melanin) from EMF mycelia necromass across 6 EM fungi species, seeking an answer to the question of whether the dynamics in composition and concentration of these compounds is linked to mycelium decomposition dynamics. Hereto we seek an answer to three research questions: 1) Do the initial concentrations of the cell-components alone predict fungal litter decomposition?; 2) Can the concentration dynamics of chitin, glucans and melanin during the decomposition process predict the dynamics of fungal litter mass loss?; and 3) Do distinct EM fungal species exhibit a similar temporal dynamics of losing cell wall components during the decomposition process?