Abstract: Vegetation restoration affects the spatial distribution of soil organic carbon (SOC). We examined spatial variations and influencing factors of SOC in small watershed of loess hills and gullies, across four typical sub-watersheds (Nanbeiyao agricultural land watershed, Beipo artificial forest watershed, Liugou enclosed watershed, and Renzushan natural secondary forest watershed) in Caijiachuan watershed in western Shanxi Province. Through field investigation and indoor analysis, we analyzed the distribution of SOC and its relationship with environmental factors in 0-100 cm profile of watershed scale using classical statistics and geostatistics methods. SOC content varied in different sub-watersheds, with Liugou enclosed watershed > Renzushan natural secondary forest watershed > Beipo artificial forest watershed > Nanbeiyao agricultural land watershed. Compared with artificial restoration, natural restoration accumulated and stored more soil organic carbon. Spatially, SOC decreased gradually from west to east. The distribution pattern of SOC was more uniform with increasing soil depth. Vegetation type, land use pattern, and topographic characteristics affected the spatial distribution pattern of SOC at the small watershed scale. The SOC content of shrub-grassland was the highest (14.43±8.91 g·kg^-1), followed by forest land (13.11±6.98 g·kg^-1), and the agricultural land (8.48±2.98 g·kg^-1). Compared with agricultural land, the content of SOC in shrub-grassland and forest land was increased by 70.2% and 54.6%, respectively. In 0-20 cm soil layer, the maximum value of SOC occurred when the slope was 15°-25°. In 0-20 cm soil layer, SOC content in different slope directions was in order of shady slope (15.82±7.63 g·kg^-1) > half shady slope (12.94±7.53 g·kg^-1) > half sunny slope (11.48±6.28 g·kg^-1) > sunny slope (11.1±6.19 g·kg^-1). The results provide a basis for evaluating soil ecological effects of vegetation restoration in the loess watersheds. Attention should be paid to nature-approaching management of artificial forests and enclosed management of natural forests during vegetation restoration and reconstruction.

Key words: Loess Plateau, small watershed, soil organic carbon, spatial distribution, geostatistics