In order to understand the effects of vegetation restoration and ecological restoration process on soil carbon cycling

the profile distribution of soil organic and inorganic carbon under different slopes and vegetation types along two toposequences of Liudaogou watershed on the Loess Plateau (China) were investigated

and the profile distribution characteristics and the influencing factors of soil organic carbon and inorganic carbon among different slopes and among different vegetation types on the same slope were analyzed. The typical toposequences were located in the northeast slope (NE sequence) and west slope (W sequence) of the Liudaogou watershed

respectively. The results showed that soil organic carbon content decreased significantly with increasing of soil depth within 0 - 50 cm soil layer

and then remained relatively constant in the soil layer deeper than 50 cm

and the organic carbon content of the 0 - 50 cm soil layer was significantly higher than those of the 50 - 200 cm soil layers (p < 0.05). However

in the same soil depth (0 - 50

50 - 200 or 0 - 200 cm)

there was no significant difference on the mean organic carbon content under either forestlands or grasslands among different slopes (p < 0.05). Compared with organic carbon

soil inorganic carbon content was relatively high

and mainly enriched in the different soil layers deeper than 50 cm. In NE sequence

the mean inorganic carbon contents were comparable in the forestland and the grassland (p > 0.05). In contrast

in the W sequence the mean inorganic carbon content of the forestland was significantly higher than that of the grassland (p < 0.05); there was no significant difference on the mean inorganic carbon content of the grasslands among different slopes (p > 0.05)

while the mean inorganic carbon content of the forestland in W sequence was higher than that in NE sequence (p < 0.05). Organic carbon content in the 0 - 50 cm soil layer was significantly negatively correlated with pH

bulk density and soil water content

but was significantly positively correlated with soil total porosity; inorganic carbon content in the 50 - 150 cm soil layer was significantly negatively correlated with pH and total soil porosity

but was significantly positively correlated with bulk density

clay content and soil water content. Soil carbon densities within 0 - 2 m profile were 15.3 ~ 47.4 kg/m2 and 18.3 - 51.3 kg/m2 in the NE sequence and W sequence

respectively

in which inorganic carbon density accounted for 78% ~ 94%. Total carbon density of 1 - 2 m soil layer were 35% ！ 74% of the total carbon density of 0 - 2 m soil profile. Our study indicated that neglecting inorganic carbon or neglecting deep soil carbon (1 - 2 m) would lead to underestimation of soil carbon stock by 88% and 51%

respectively.