Abstract:Carbon sequestration in plantation soils was studied in tropical region of south China. Barren land without vegetation cover had a low level of TOC (2.79 g kg-1). Establishment of eucalyptus plantation on barren land significantly increased TOC into a level of 14.16g kg-1, but only 6.12 g kg-1 if litter was frequently removed by villagers. Replacement of eucalyptus with mixed broad-leaf forest further increased TOC to a level higher than 16.5 g kg-1, which, however, was only 76% of TOC of nearby natural forest. TOC content of barren land did not change across 1m profile and kept at a level of 2-3 g kg-1. Unchanged soil TOC content across 1m profile was also found in eucalyptus plantation (at 4-5 g kg-1). Litterfall exerted influenced TOC only within the uppermost 0.5 m of soil. A mixed plantation dominated by Litsea rotundifolia, Cinnamomum burmanni, Litsea glutinosa, Lindera chuni had much higher TOC in deep soil than other plantations, similar to that of nearby secondary natural forest. Soil aggregates played an important role in carbon sequestration. Water-unstable aggregates contained highest carbon in the smallest diameter class (<0.25 mm). Other aggregates had similar levels of carbon content. For mature natural forest, carbon content of all diameter classes was similar to each other. Total carbon sequestration was highest in the largest class (>10 mm) due to well developed water-unstable aggregates. In contrast, highest carbon sequestration in water stable aggregates was found in two smallest classes (0.25-0.5 mm and 0.5-1.0 mm). The results suggested that the establishment of targeted plantation would significantly enhance soil carbon sequestration in a short time. We considered that the high soil carbon sequestration of mature natural forest might be attributed to the well developed water unstable big aggregates and water stable small aggregates.