Soil is one of the largest pools of terrestrial organic carbon. Land-use types affect not only the storage but also the depth distribution pattern of soil organic carbon (SOC), which plays important roles in global climatic change. The SOC depth distribution and its influencing factors in Lanlingxi watershed in Three Gorges Reservoir were analyzed. The results showed that landuse types significantly affected the SOC depth distribution. The SOC densities in 0-20 cm layer averaged at 8.47, 5.90, 4.65, and 2.64
kg·m-2 for forests, grasslands, shrub lands, and croplands, respectively, and the SOC densities in the 0-100 cm were 8.47, 5.90, 4.65, and 2.64 kg·m-2 respectively. The percentage of SOC in 0-20 cm layer (relative to the 0-100 cm) averaged at 69.8%, 57.6%, 50.8% and 36.2% for forests, grasslands, shrub lands and croplands, respectively. With increasing soil depth, SOC density decreased rapidly, and the decline rate was fastest in forests. The forests had a shallower SOC depth distribution. On the contrary, grasslands and shrub lands decreased slowly with soil depth, and they had a deeper SOC depth distribution. Land-use type and altitude significantly affected SOC densities in top layer (0-20 cm), but had no obvious effect on that in the deeper layers (>40 cm). Soil mechanical composition had a little effect on SOC density of surface layer (0-20 cm), but had a significant effect in deeper layers (>40 cm). The soil carbon storage was underestimated when SOC in 0-100 cm layer was considered. SOC density would be increased by 6.2%-16.5% in the study watershed with SOC storage in 0-150 cm layer being considered.
