Amino sugars reflect microbial residues as affected by clay mineral composition of artificial soils

Microbial residues are important contributors to soil organic matter (SOM). However, the effect of mineral composition on microbial activity and thereby SOM development is not well understood. Here, we used artificial soils composed of four different mixtures containing illite, montmorillonite, ferrihydrite and charcoal to study the effect of mineral composition on SOM development. In addition, silt- and sand-sized quartz were used to provide texture, sterilised manure was added as OM source and the mixtures were inoculated with a microbial community extracted from a natural soil. The mixtures were incubated in the dark for 3-18 months and amino sugar and muramic acid content of the bulk soil and < 20 μm fraction were determined. Amino sugar content was generally low and changed only slightly in the < 20 μm fraction with incubation time, which shows that microbial residues in the form of amino sugars did not preferentially accumulate with time. The montmorillonite-containing mixture had a higher galactosamine and glucosamine content than the others, pointing to greater fungal residue formation. The effect was most pronounced for the bulk soil, indicating that fungal residues accumulated on coarse particulate OM and macroaggregates rather than in the fine fraction. The results illustrate that amino sugars can be a biomarker for studying the effect of mineral composition on microbial residues and SOM formation. However, it is important to consider the entire soil structure in order to gain understanding of the interaction between microorganisms, minerals and SOM.