A quantitative assessment of amino sugars in soil profiles

Recent evidence suggests that a significant proportion of stable soil organic matter is derived from microbial necromass, which can be evaluated by measuring biomarker amino sugars. However, our understanding of the vertical distribution and control of amino sugars in soil profiles at the global scale is still limited. Here, we synthesized the concentrations of amino sugars relative to soil mass (mg/g soil) and to soil organic carbon (C) concentration (mg/g C) in various soil horizons (litter layer, O horizon soil and mineral soil at 0–20, 20–50, 50–100 and > 100 cm depths) from 131 forest, grassland and cropland sites. We found that the concentration of soil amino sugars declined with soil depth, ranging from 5.90 mg/g soil in O horizon soil and 1.56 mg/g soil at 0–20 cm depth to 0.24 mg/g soil at >100 cm depth. However, the proportion of amino sugars in soil organic C significantly increased from 18.1 mg/g C in O horizon soil to 61.9–83.3 mg/g C in mineral soil. The concentration of amino sugars was positively correlated with soil organic C and microbial biomass C concentrations. The proportion of amino sugars in soil organic C was significantly higher in microaggregates (<2 μm) and in soil with low carbon to nitrogen ratios. Microbial biomass carbon and soil pH had positive and negative effects, respectively, on the concentration of amino sugars in topsoil, and together explained 58.8% of the total variation in the concentration of amino sugars. These results suggest that soil biogeochemical properties are more important than climatic variables and latitude in determining the long-term persistence of microbial residues in soils.