The role of chitin in the decomposition of ectomycorrhizal fungal litter

Background/Question/Methods

The decomposition of litter is a major driver of biogeochemical cycling in terrestrial ecosystems.  Root biomass accounts for approximately 1/3 of global net primary production and is a major input to litter and thus to soil carbon and nutrient pools. Understanding the controls on the decomposition of root litter remains relatively unclear relative to those on the decomposition of aboveground litter. Ectomycorrhizal fungi (EMF) have a large influence on the biogeochemistry of temperate forests because they comprise the majority of microbial biomass and mediate plant nutrient uptake. Recently the importance of EMF on litter decomposition rates is beginning to be appreciated. For example, as these fungi envelope fine roots with their hyphae they significantly change the chemical composition of the root, which may have implications to the decomposition of this significant litter pool. A recent report has shown a reduction in decomposition of root litter when roots are colonized by EMF and suggested that chitin, a compound found in fungal cell walls, may be relatively recalcitrant and thus reduce the overall decomposability of the root by decreasing the litter quality. This hypothesis conflicts with earlier microcosm experiments that examined the decomposability of pure chitin, in which it was found that chitin was actually more decomposable than cellulose. However, chitin does not exist by itself in the cell wall so it is important to examine its decomposition in the presence of the complete cell wall. The aim of this study was to:

1. explicitly examine the change in chitin concentrations in EMF tissue during decomposition
2. determine what influences that chitin might have on the decomposability of ectomycorrhizal fungal tissue

This was done by examining the decomposition of chitin in EMF mycelia over the course of time in a litter bag study. 

Results/Conclusions

The results from these experiments show that chitin concentrations do not increase as decomposition progresses but decrease significantly in several species of ectomycorrhizal fungi, suggesting that chitin is not recalcitrant relative to other compounds found in fungal cell walls. A positive relationship was found between initial chitin concentrations and decomposition (% biomass loss) of fungi suggesting that chitin may actually increase the decomposability of fungal litter.