Impacts of vegetation type and climatic zone on neutral sugar distribution in natural forest soils

•Neutral sugar contents decrease in order: leaf litter > organic soil > mineral soil.•Climatic zone but not vegetation type affects soil neutral sugar pool.•C6/C5 ratio is higher in coniferous than broadleaf forest soils in both climates.•Increased C6/C5 ratios in deeper horizons depict higher dominance of microbial sugars.

Soil neutral sugars are a significant component of labile soil organic carbon (SOC) and are derived from both plant and microbial biomass. While plants synthesize both pentose and hexose neutral sugars, microbes almost exclusively produce hexoses. Hexose to pentose ratios in soil thus potentially indicate the extent to which microbes process labile SOC. In this study, we used the ratio of galactose + mannose (G + M) to arabinose + xylose (A + X) to estimate the contribution of sugars derived from microbes and plants to SOC in forest ecosystems. We explored how forest type and climatic zone influence soil neutral sugar profiles by studying coniferous and broadleaf forests located in temperate and subtropical regions in China. At each site, neutral sugars from organic (O) and top-layer mineral (A) soil horizons, as well as from freshly-fallen leaf litter, were measured. Total SOC and soil neutral sugar contents were lower in the subtropical region than in the temperate region, with lower levels in the A horizon than in the O horizon. In both climatic zones, litter (G + M)/(A + X) ratios were higher in coniferous forests (1.2 ± 0.3) than in broadleaf forests (0.4 ± 0.1). Differences in the (G + M)/(A + X) ratios between forest types (coniferous and broadleaf) persisted in the O horizon (1.4 ± 0.2 > 0.9 ± 0.0) and in the A horizon (1.8 ± 0.1 > 1.3 ± 0.0). Across climate zones and forest types, ratios increased from litter over the O horizon to the A horizon. Contrary to our expectations, climate zone did not affect soil (G + M)/(A + X) ratios. Our findings emphasize the important contribution of microbial biomass to labile SOC pools while revealing that soil neutral sugar profiles do not respond to climatic zone drivers as expected.