Abstract: The importance of soil organic carbon(SOC) in forest soils to the global carbon cycle depends on stability of this soil carbon.However,little is yet known about changes in stability of SOC during the development of fir plantations.To scientifically evaluate the carbon pool function and carbon sequestration potential of the soils under Chinese fir plantations,investigations were done of biochemical characteristics and degree of physical protection of the soil organic carbon in the soils under Chinese fir plantations different in growth stage.Results show that the contents of total SOC,microbial biomass carbon(MBC),labile pool I(LP I),labile pool II(LP II),heavy organic carbon(HOC),and microbial quotient(MQ) fell to the bottom at the middle-age stage,while the contents of light organic carbon(LOC) and recalcitrant fraction(RF) increased with the age of the stands,but the proportion of recalcitrant fraction in SOC increased first and decreased afterwards,which means that it peaked at the middle-age stage,and ranged from 41.9% to 57.6% under the stand at the mature stage,and from 38.7% to 43.0% under the stand at the young stage.The proportion of HOC in SOC ranged from 86.4% to 87.5% under the stand at the young stage and from 82.5% to 83.9% under the stand at the mature stage.On the whole,the development of a Chinese fir plantation is a process of SOC accumulation in the soil,and during the process SOC per se increased in recalcitrance while decreased in the degree of physical protection.Besides,among all the fractions researched in this study,LP I is the highest in availability to soil microbial biomass.

Key words: Chinese fir plantation, soil organic carbon, stability, growth stage, physical protection