Variations of soil aggregates and soil organic carbon mineralization across forest types on the northern slope of Changbai Mountain

Soil aggregation and soil organic carbon (SOC) mineralization are affected by soil properties, microbes and litter input, all of which vary with changes in temperature, precipitation and vegetations. In order to explore SOC dynamics in natural mature forest ecosystems, this study analyzed the variation of soil aggregates and SOC mineralization along an altitudinal gradient on Changbai Mountain in northeast China. Soil samples collected at four altitudinal sites were fractionated into different size of aggregates. SOC mineralization was measured by incubating soils at constant temperature. Results revealed that proportions of macro-aggregates (5–2 mm, 2–1 mm) increased with increasing altitude, while proportions of micro-aggregates (0.25–0.053 mm, <0.053 mm) decreased, which suggest that colder and wetter condition at higher altitude benefits the formation and stability of macro-aggregates. SOC mineralization rates displayed the following order: Erman's birch forest > Korean pine and broad-leaved mixed forest > Erman's birch mixed Spruce-fir forest > Spruce-fir mixed forest. Correlation and linear regression analysis revealed that potential mineralized carbon (C0) positively correlated with microbial biomass carbon, readily oxidizable carbon and SOC. Turnover coefficient k, which represents the turnover rates of C0, was positively correlated with proportions of macro-aggregates and negatively correlated with proportions of aggregates 1–0.25 and 0.25–0.053 mm. In conclusion, SOC mineralization was controlled by microbial activity, quality and quantity of SOC and soil structures. Greater proportions of macro-aggregates at higher altitude suggest that SOC might turnover more quickly than that at lower altitudes. Hence more attention should be paid to soils at higher altitude in the future.