Abstract:Soil organic matter (SOM) is a stable and long-lasting carbon source and contains nearly all nutrients for plant growth, therefore is a meaningful index for evaluating soil fertility. The quantity and quality of soil organic carbon influence soil potential productivity and reflect the spatial distribution and succession of plant community. However, the measurement of total SOM is not sensitive enough to detect short and medium term changes, and thus techniques that measure meaningful fractions of SOM should be used. The carbon management index (CMI) calculated from changes in labile and total carbon fractions relative to a reference soil was show to be a useful technique for describing soil fertility. An investigation was carried out to study the changes of soil carbon fractions and CMI, and their relationships with fertility under natural evergreen broad-leaved forest and its artificial regeneration forests of Sassafras tzumu, Cryptomeria fortunei and Metasequoia glyptostroboides in southern Sichuan Province. Soil samples were collected from each forest to determine soil carbon fractions, available nutrients, enzyme activity and CMI. Regression analysis was used to determine the relationship between soil carbon fractions, CMI and fertility. The results showed that the contents of soil organic carbon, water-soluble carbon, microbial biomass carbon, labile carbon, non-labile carbon, hydrolysis-N, available-P and available-K, the activity of invertase, phosphatase and catalase, and CMI were ranked with different seasons and followed the order: natural evergreen broad-leaved forest ＞ Sassafras tzumu plantation ＞Metasequoia glyptostroboides plantation ＞ Cryptomeria fortunei plantation. The soil carbon fractions and CMI were positively and significantly (P＜0.05) correlated with available nutrients and enzyme activity. Artificial regeneration of natural evergreen broad-leaved forest would result in decrease in all soil carbon fractions, CMI and fertility, but the size of change varies with regenerated forest types. The results indicate that soil carbon fractions and CMI could be used to evaluate the soil fertility for natural evergreen broad-leaved forest and its artificial regeneration forests, which will lay a solid foundation for protecting natural evergreen broad-leaved forest, tree selection for its artificial regeneration, and soil management, moreover, tree selection during the process of converting farmland to forestland.