The huge background pool of soil organic carbon (SOC) is likely to impede the ready detectionof SOC changes. We propose to explore SOC changes by monitoring the dynamics of soil labile organic car bon (LOC); namely if LOC could be largely retained in soils rather than respired rapidly, the SOC wouldbe ready to be sequestered. The effects of the two major functional groups of plants, that is, canopy treesand understory plants, on SOC accumulation were then illustrated with this LOC-based approach. Thecharacteristics of LOC and SOC of topsoils (0–20 cm) in a fifield manipulation experiment with 5-yr treat ments of understory removal and tree girdling in both a young and a mature Eucalyptus plantations wereexamined. The concentration and potential turnover time of soil LOC were used to indicate the state ofvegetation-induced C accumulation in soils, which were estimated by a sequential fumigation-incubationprocedure. Soil natural abundances of 13C and 15N were measured to reflflect the proportion of newlyretained LOC in soils. We found that, in the young plantation, understory removal did not signifificantlyaffect both soil LOC and SOC concentrations, but signifificantly increased the potential turnover time of soilLOC. In contrast, in the mature plantation, understory removal signifificantly decreased soil LOC and SOCconcentrations, but did not signifificantly alter the potential turnover time of soil LOC. However, tree gird ling did not signifificantly affect SOC concentration, soil LOC concentration, or potential turnover time ineither the young plantation or the mature plantation. These results demonstrated that understory plant derived C was one of the major components of LOC pool in topsoils, and it may be readily mineralized inthe young plantation but accumulated as an important fraction of SOC in the mature plantation. This studysuggests that the LOC-based approach is potentially useful in monitoring SOC changes and improves ourunderstanding of how plant functional groups and soil fertility status could jointly affect LOC and SOCdynamics. In considering the great contribution of understory plants to SOC processes, we propose thatunderstory plants should be maintained in subtropical plantation ecosystems.