Plant hosts allocate a considerable amount of carbon to mycorrhizal fungi. The death of mycorrhizal fungal biomass, known as necromass, represents a large flux into soil carbon cycles. Fungal necromass is now recognized as a major contributor to the stable carbon held in soil organic matter (SOM). While it appears that much of the variation in the decomposition rates of mycorrhizal necromass is explained by melanin, a complex aromatic fungal cell wall polymer, and nitrogen content, the richness and composition of the decomposer communities involved in these processes have been poorly characterized. To assess the effects of melanin content on microbial decomposer communities associated with necromass substrates, we utilized the mycorrhizal fungus, Meliniomyces bicolor. The production of melanin in this fungus can be naturally manipulated in vitro, while holding other biochemical and morphological factors constant. We incubated both non-melanized and melanized Meliniomyces necromass in pine and oak forest stands at Cedar Creek LTER in Central Minnesota over a 3 month period and assessed the associated fungal and bacterial decomposer communities at 1, 2, and 3 months using with Illumina MiSeq and PacBio SMRT high-throughput sequencing.
Supporting previous studies, the melanized necromass decomposed significantly slower than the non-melanized necromass (ANOVA; P<0.0001). Fungal and bacterial decomposer community structure depended significantly on necromass melanin content (GLM; P=0.036). While there was a site effect, both of these results were largely consistent across sites, suggesting that melanin is a driver of necromass mass loss rates and microbial decomposer community structure. Incubation time also had an effect on microbial community composition. Among the dominant microbial taxa colonizing the necromass were ectomycorrhizal fungi. This result is intriguing from the standpoint that mycorrhizal fungi may have an inherent advantage accessing and processing the resources found in their own ‘tissues’ and potentially outcompete saprotrophic microbes.