Discussion
Our study for the first time assesses how the temporal dynamics of
secondary components control the decomposition of the extramatrical
mycelium of ectomycorrhizal fungi. We have monitored the concentrations
of the main cell wall secondary components over time to evaluate how
these changes affect the decomposition process. In this way, we
demonstrated how the different loss of these components between species
does not translate into a different overall mass loss from the
decomposing necromass, which was comparable across species after six
weeks. Considering that the structure of our species was very similar,
since they were all cultivated in vitro in absence of external
stressors, we can be sure that our results reflect the actual effects of
fungal chemical composition.
Overall, the magnitude and rate of decomposition of the EM fungal
species were consistent with previous findings, especially so concerning
the rapid mass decline in the first seven to ten days (Drigo et al.,
2012; Ryan, Schreiner, Swenson, Gagne, & Kennedy, 2020). Yet in
contrast to these earlier studies, we found hardly any differences
between fungal species in the decomposition of the necromass. Previous
research suggested that the initial concentrations of secondary
components, and especially of melanin (C. W. Fernandez, Heckman, Kolka,
& Kennedy, 2019; Christopher W Fernandez & Koide, 2014), and chitin
(Fernandes and Koide, 2012) underpin fungal decomposition rate. While
higher concentrations of melanin in fungal litter decreased the
decomposition of EM fungal necromass, a higher concentration of chitin
was shown to be generally positively related to the decomposition rate .
The fungal species here examined did not show a significant difference
in the initial concentrations of these two components, which likely
explains the similar mass loss among species. Yet, initial
concentrations of glucans differed between species. However, these
differences did not result in differences in mass loss during the
decomposition. This happened even though the concentrations in glucans
may translate into differences in nitrogen concentrations given that
glucans do not contain nitrogen
Remarkably, L. deliciosus , the slowest decomposing species in our
species set did not stand out in terms of initial concentration of
cell-wall components. This suggests, in contrast to previous studies
which did not evaluate the full dynamics in secondary compounds during
decomposition, that differences in initial concentrations do not drive
differences in decomposition rates. Instead, and in contrast to the
initial concentration, the dynamics of loss of chitin was related to
fungal species biomass loss. This is in line with the recent findings of
Ryan et al. (2020), who also found that fungal species decomposition was
strongly underpinned by the loss of carbohydrates from fungal biomass.
However, our results indicate that this finding could not be generalized
for all carbohydrates, as there was no significant relationship between
necromass loss and the concentration of glucans or melanin. Moreover,
our results show how the impacts of chitin on decomposition vary over
time. We observed a positive relation between chitin and necromass loss
during the first week of decomposition when its rate is the highest.
During the remainder of the weeks, the necromass loss had a weak
negative relationship to chitin concentration. This might be the result
of the increased nitrogen availability in the previous stage of
decomposition, which might be sufficient for decomposers to support the
production of extracellular enzymes devoted to break down the organic
matter (Schimel & Weintraub, 2003) later on.
The role of melanin in the decomposition process of fungi has been
demonstrated previously (Koide et al 2019), and specifically to a
slow-degrading necromass pool (Ryan et al, 2020). In our study, the loss
of melanin was not related to the necromass loss at all. This might be a
consequence of the small quantity of melanin in our samples. These
consistently low concentrations of melanin among the different species
might be caused by the absence of external stressors during the growth
and growing conditions were optimal in terms of temperature, light and
nutrients.
The patterns of loss of all cell-wall compounds examined in this study
were highly species-specific (Figure 3). Yet, these differences between
species in the pathways of the loss of cell-wall compounds were not
reflected in the inter-specific differences in decomposition patterns.
We hypothesize that the differences across fungal species in the loss of
individual types of polysaccharides such as chitin and glucans, which
constitute 80-90% of the fungal cell wall (Bartnicki-Garcia, 1968), may
compensate each other. This likely happens due to how decomposers forage
on the fungal necromass, attacking labile polysaccharides, without
discriminating between these different compounds. Indeed hydrolytic
enzymes (e.g. endo-chitinase, N-acetylgucosaminidase, b-glucosidase; the
same enzymes that degrade plant litter compounds such as cellulose) are
produced by the majority of bacteria and saprotrophic fungi (Conn &
Dighton, 2000). On the other hand, the oxidative enzymes used mainly for
degrading lignin and melanin, are less abundant because they are almost
exclusively secreted by lignin decomposer fungi (Butler & Day, 1998).
The combined enzyme actions on cell-wall compounds may also explain for
the inversion of the relationship between chitin concentration and the
weekly mass loss as it may impose a structural change in the cell wall
or in the N availability in necromass in general. However, while these
mechanisms may explain the interactions among cell-wall compounds in
affecting fungal litter decomposition and the consequent absence of
significant relationships for individual cell-wall compounds, they do
not fully explain the species-specific decomposition patterns.
To further understand this pattern, not only chitin lability deserves
more attention, but especially glucans, as the most abundant
carbohydrate fraction of the cell wall, should be further investigated.
For instance, distinguishing between alpha- and beta-glucans could be an
interesting line of research, as these compounds serve different
functions in the cell wall compartment. Alpha-glucans contribute to the
cell wall matrix (Sietsma & Wessels, 1994), while the beta-glucans play
a structural role, crosslinking the chitin fibers (Wessels, Mol,
Sietsma, & Vermeulen, 1990). Once we can understand the magnitude and
the rate at which these components are lost from the necromass and later
stored in the soil organic matter, we can more precisely predict what
would be the C:N ratio of the soil organic matter, which in many cases
is essential to assess its chemical ‘lability’ (Cui et al., 2020).
Further insights on this C input from ectomycorrhizal fungi might
provide a more comprehensive view of the contributions of these groups
of microorganisms to soil organic matter. While most of the research to
date has focused on their capacity to mobilize organic C by decomposing
organic matter (Leake et al., 2014; D. Read & Perez‐Moreno, 2003; D. J.
Read, Leake, & Perez-Moreno, 2005), we emphasize the importance of the
decomposition of their necromass as a contribution to soil organic
matter dynamics.
Considering that glucans and chitin are both attacked by hydrolytic
enzymes, it is also worth further investigate the temporal activity of
these enzymes under the influence of external factors. Such analysis of
hydrolytic enzymes could provide more insights into the mechanisms that
drive the ectomycorrhizal decomposition, especially concerning
environmental conditions. The activity of many enzymes depend on soil
abiotic parameters (pH, temperature, moisture), which drastically change
between soil layers and seasonality (Wittmann, Kähkönen, Ilvesniemi,
Kurola, & Salkinoja-Salonen, 2004). This can have a major effect on the
ectomycorrhizal necromass decomposition, which can be modelled more
easily once we can fine-tune them based on environmental conditions.
With the findings presented in this study, we encourage to follow two
main paths to further understand the contribution of ectomycorrhizal
necromass to SOM. First, there is a need to dig deeper into the nature
of interactions between the two most abundant polymers in the cell wall
(glucans and chitin), to see if those interactions may have a
significant effect on the decomposition of both single components and
the overall necromass. In particular, whether these interactions are
prone to change under the influence of environmental conditions or that
they are highly species-specific. Understanding these interactions may
take a long time, in analogy to the complications that arose to
understand the interaction between lignin and cellulose on plant litter
decomposition (Evans, Dutton, Guillén, & Veness, 1994; Talbot &
Treseder, 2012). Those interactions are particularly difficult to study
in natural conditions due to the external biotic factors that are
involved (Cuchietti, Marcotti, Gurvich, Cingolani, & Harguindeguy,
2014). Secondly, when it comes to developing models which aim to predict
SOM dynamic, without knowing the nature of the interactions within
mycorrhizal necromass – as highlighted above, we can still use the
decomposition estimates from our study and others to develop descriptive
models. This would demand considering mycorrhizal necromass as a
separate pool next to plant litter, instead of assuming that both pools
behave similarly. Either way, our results suggest that necromass
decomposition does not behave similarly to plant litter, while it is a
critical component to consider in estimating SOM dynamics.
Conclusions
Our results show that the decomposition
process of necromass of ectomycorrhizal fungal mycelium is not
species-specific overall, as the rate and magnitude of decomposition do
not show significant differences as for the pathway of chemical
component loss from their cell walls. As evidence of this, the
concentration changes of the cell wall compounds over time do not
correspond to the final biomass loss. For this reason, looking at the
concentration of single compounds cannot ensure an accurate prediction
of the necromass loss in ectomycorrhizal fungi. Together, this brings
evidence that distinct EM fungal species affect SOM through distinct
pathways of cell-wall compound loss, even if these fungi show a similar
overall mass loss. These pathways differ remarkably from those of plant
litter and need to be further understood to be able to better predict
the contribution of fungal necromass on soil organic matter dynamics.