Vertical distribution and pools of microbial residues in tropical forest soils formed from distinct parent materials

The contribution of soil microbial residues to stable carbon pools may be of particular importance in the tropics where carbon residence times are short and any available carbon is rapidly utilized. In this study we investigated the vertical distribution of microbially-derived amino sugars in two tropical forests on contrasting meta-sedimentary and serpentinite parent materials in the lowlands of Mt. Kinabalu, Borneo. Despite their similar climate, vegetative cover, and general microbial community structure, the two soils were chemically and physically distinct. We found that both parent material and depth significantly influenced the pool sizes of microbial residues in the two soils. In particular, the soil derived from sedimentary parent material had greater amino sugar contents, glucosamine to galactosamine ratios, and percentage of total soil carbon that is amino sugar derived, than the soil derived from serpentinite substrate. We speculate that residue stabilization was linked to soil iron oxide content, with significant difference in amino sugars contribution to total soil carbon at depth in the serpentinite-derived soil versus that derived from sedimentary parent material. Based on observed patterns of amino sugar content and relative abundance we suggest that near the surface of both soils vegetation and litter input determines the composition and quantity of microbial residues. With increasing depth the influence of vegetation declines and production and stabilization of microbial amino sugars becomes driven by soil matrix characteristics. These differences in stabilization mechanism and carbon dynamics with depth may be particularly critical in deep weathered tropical soils.