Tree species-specific effects on soil microbial residues in an upper Michigan old-growth forest system

Microbial contribution to carbon and nitrogen cycling in forestsoils is important, and may depend on tree species. The amountof amino sugars and their ratios can serve as reliable indicatorsfor bacterial and fungal contribution. We compare forest floormicrobial residues (amino sugars) beneath three canopy-treespecies (Sugar Maple (SM), Acer saccharum Marsh; Basswood (BA),Tilia americana L.; Eastern Hemlock, Tsuga canadensis L.) replicatedin five plots in an upper Michigan old-growth forest. We hypothesizedthat because individual tree species develop a unique microbialcommunity over time, they will accumulate microbial residuesto different degrees. In this study at three tree species sites,the absolute amount of fungal residue (glucosamine (GluN)) wasrelatively constant, while absolute quantities of bacterialresidues (galactosamine (GalN) and muramic acid (MurA)) wereleast in the Hemlock site. Amino sugar ratios revealed thatmicrobial residues were compositionally distinct in the threesites. The lower ratios of GluN to GalN and GluN to MurA inBA and SM sites relative to Hemlock site indicate the lowernet accumulations of GalN and MurA in Hemlock site. In termsof microbial contribution to carbon and nitrogen cycle in forestsoils, we suggest that caution may be needed when using aminosugars as a tool, especially for nitrogen pool assessment, asthe amino sugars are diluted by plant-derived litter. This studyprovides information on the microbial residues in undisturbedforest soils which may assist interpretation of data derivedfrom managed or damaged forests in the future.