Factors Controlling the Decomposition of Ectomycorrhizal Fungal Tissue and the Formation of Soil Organic Matter

Abstract

The turnover of ectomycorrhizal (ECM) fungi accounts for up to half of the organic carbon found in forest soils and therefore represents an important pathway for the removal of carbon from the atmosphere to be stored belowground as long-lived soil organic matter (SOM). Understanding the flux of fungal necromass inputs to SOM, and their subsequent stabilization potential in forest soils, requires an understanding of the chemical changes that occur during the degradation of fungal tissue. Additionally, it is hypothesized that degradation of fungal necromass is slowed by high melanin content and accelerated by high nitrogen content. A field degradation study was carried out at the Cedar Creek Ecosystem Science Reserve in East Bethel, Minnesota. Necromass from four species of ECM fungi with varying degrees of melanization was buried in litter bags in a Pinus-dominated forest below the soil litter layer, allowed to degrade naturally, and harvested nine times over a period of 90 days. Harvest was more frequent during the first week to gain insight into the dynamic early decomposition period. Elemental analysis (EA), Fourier-transform infrared spectroscopy (FTIR), and thermochemolysis-gas chromatography-mass spectrometry (pyGCMS), including novel methods of quantifying the contribution from various types of biopolymers to the total remaining tissue, supplement mass loss data to provide an overview of the chemical changes that occur as fungal necromass decomposes. Each of the four species lost a significant amount of mass in the first seven days of incubation but, at the end of the three-month degradation sequence, a significant fraction of fungal necromass remained. This necromass was chemically distinct from undegraded necromass, containing more aromatic compounds, suggesting that the relative abundance of melanin, which is highly aromatic, increased as other cellular components degraded away. Although melanin content was hypothesized to slow degradation, a high-melanin species degraded at effectively the same rate as the two low-melanin species. Differences in degradation rates across species can be attributed to initial nitrogen content, while melanin content could explain differences in degradation rate within a species.