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Background: Soil organic carbon (SOC) is a large reservoir of terrestrial carbon (C); it consists of different fractions of varying complexity and stability. Partitioning SOC into different pools of decomposability help better predict the trend of changes in SOC dynamics under climate change. Information on how physical fractions and chemical structures of SOC are related to climate and vegetation types is essential for spatial modelling of SOC processes and responses to global change factors.

Method: Soil samples were collected from multiple representative forest sites of three contrasting climatic zones (i.e. cool temperate, warm temperate, and subtropical) in eastern China. Measurements were made on SOC contents and physical fractions of the 0-20 cm soil layer, and the chemical composition of SOC of the 0-5 cm soil layer, along with measurements and compilation of the basic site and forest stand variables. The long-term effects of temperature, litter inputs, soil characteristics and vegetation type on the SOC contents and factions were examined by means of "space for time substitution" approach and statistical analysis.

Result: Mean annual temperature (MAT) varied from 2.1 degrees C at the cool temperate sites to 20.8 degrees C at the subtropical sites. Total SOC of the 0-20 cm soil layer decreased with increasing MAT, ranging from 89.2 g.kg(-1) in cool temperate forests to 57.7 g.kg(-1) in subtropical forests, at an average rate of 1.87% reduction in SOC with a 1 degrees C increase in MAT. With increasing MAT, the proportions of aromatic C and phenolic C displayed a tendency of decreases, whereas the proportion of alkyl C and A/O-A value (the ratio of alkyl C to the sum of O alkyl C and acetal C) displayed a tendency of increases. Overall, there were no significant changes with MAT and forest type in either the physical fractions or the chemical composition. Based on the relationship between the SOC content and MAT, we estimate that SOC in the top 20 soil layer of forests potentially contribute 6.58-26.3 Pg C globally to the atmosphere if global MAT increases by 1 degrees C-4 degrees C by the end of the twenty-first century, with nearly half of which (cf. 2.87-11.5 Pg C) occurring in the 0-5 cm mineral soils.

Conclusion: Forest topsoil SOC content decreased and became chemically more recalcitrant with increasing MAT, without apparent changes in the physical fractions of SOC.