Labile C dynamics reflect soil organic carbon sequestration capacity: Understory plants drive topsoil C process in subtropical forests

The huge background pool of soil organic carbon (SOC) is likely to impede the ready detection of SOC changes. We propose to explore SOC changes by monitoring the dynamics of soil labile organic carbon (LOC); namely if LOC could be largely retained in soils rather than respired rapidly, the SOC would be ready to be sequestered. The effects of the two major functional groups of plants, that is, canopy trees and understory plants, on SOC accumulation were then illustrated with this LOC‐based approach. The characteristics of LOC and SOC of topsoils (0–20 cm) in a field manipulation experiment with 5‐yr treatments of understory removal and tree girdling in both a young and a mature Eucalyptus plantations were examined. The concentration and potential turnover time of soil LOC were used to indicate the state of vegetation‐induced C accumulation in soils, which were estimated by a sequential fumigation‐incubation procedure. Soil natural abundances of ¹³C and ¹⁵N were measured to reflect the proportion of newly retained LOC in soils. We found that, in the young plantation, understory removal did not significantly affect both soil LOC and SOC concentrations, but significantly increased the potential turnover time of soil LOC. In contrast, in the mature plantation, understory removal significantly decreased soil LOC and SOC concentrations, but did not significantly alter the potential turnover time of soil LOC. However, tree girdling did not significantly affect SOC concentration, soil LOC concentration, or potential turnover time in either 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 in the young plantation but accumulated as an important fraction of SOC in the mature plantation. This study suggests that the LOC‐based approach is potentially useful in monitoring SOC changes and improves our understanding of how plant functional groups and soil fertility status could jointly affect LOC and SOC dynamics. In considering the great contribution of understory plants to SOC processes, we propose that understory plants should be maintained in subtropical plantation ecosystems.