Abstract: Aims The present study aimed to quantify the characteristics of soil organic carbon (SOC) fractions and the carbon (C) pool management index (CPMI) in the topsoil (0?20 cm) of the four different natural forest types in the temperate climate-transition area in Pangquangou National Nature Reserve, Shanxi Province. The study’s findings may offer a potential valuable reference and significant insight into enhancing soil C storage and the forestland quality, as well as the sustainable forest management within native woodlands in the future. Methods A total of four forest types, which were characterized by similar site conditions, were selected to in-vestigate the variations and influencing factors of soil C fractions and the CPMI in the topsoil layer. Here, soil samples were collected from representative forestlands, including Betula platyphylla forest (BP), Picea wilsonii + Larix principis-rupprechtii + Betula conifer-broadleaf mixed forests (PLB), P. meyeri + P.wilsonii conifer mixed forest (PP), L. principis?rupprechtii forest (LP) and scrub-grass land (CK), respectively. The contents of SOC and its fractions contents were measured, and CPMI was calculated. Pearson correlation and redundancy analysis (RDA) were used to examine the relationships between soil environmental factors and carbon pool characteristics. Random forest analysis (RFA) was used to identify the soil properties significantly affecting the soil C fractions and CPMI. Important findings The buildup of SOC and its fractions, as well as soil C pool index (CPI) and CPMI, was influenced by the distinct species composition and forest structure of different forest types. First, the contents of SOC were as follows: PP ? PLB ? BP ? LP, which increased by 74.22%、41.62%、39.05% and 3.01% respectively in the topsoil layer compared to CK. For soil C components, dissolved organic carbon (DOC), easily oxidizable organic carbon (EOC), microbial biomass carbon (MBC) and recalcitrant organic carbon (ROC) contents from the different forest types followed similar trends to the concentration of SOC at the surface layer where PP recorded the maximum concentration. Here we show that MBC and DOC contents did not vary significantly among dif-ferent forest types. Second, our data clearly evidenced that the DOC/SOC and ROC/SOC ratios were significantly higher and lower in LP and BP, respectively, in comparison to the other three forest types. Conversely, the EOC/SOC, MBC/SOC and AOC/SOC rations remained consistent, and no significant differences were observed among the four forest types, with the PP exhibiting the lowest values. Third, the variation tendency of CPMI fol-lowed the same trend as SOC, while the CPMI in LP was significantly lower than that observed in other forest types. RDA revealed that TN and NO3--N play a prominent role in the variation characteristics of soil C pools in the natural forests in the temperate climate?transition zone. Furthermore, we observed that soil water content and available potassium had a significant impact on soil C fractions, while the CPMI was significantly affected by soil water, pH and TP. Taken together, our findings demonstrate that both temperate mixed forests, especially conif-erous mixed forests promoted SOC stock by increasing C pool fractions contents, which in turn promoted soil fertility and quality of the forest land in a temperate transition zone. The results of this study emphasize the key role of the interplay between soil nitrogen, soil water and soil pH in predicting soil C pool in temperate forests. Consequently, these results should be considered by the forestry sectors, and suggest that the forest ecological restoration should promote biodiversity especially conifer species in the context of improving C stock and soil quality in temperate forests. Thus, it is predicted that increasing soil nitrogen and species diversity may be an ef-fective measure for improving soil C sequestration in natural forest ecosystems in the temperate climate?transition zone.

Key words: forest types, soil organic carbon, soil organic carbon fractions, soil carbon pool management index, temperate climate-transition zone