Abstract: [Objective] The patterns of spatial variation of the density of soil organic carbon (SOC) in forest land was studied on the southern slope of Qinling Mountains in Shaanxi Province, which provided a theoretical basis for the scientific management of SOC in Qinling Mountains.[Method]Sample plots were set up in different forest regions (Changqing of Yang County, Longcaoping of Foping County, Taibai Mountain of Taibai County, Ningdong of Ningshan County and Ningxi of Ningshan County) and different site conditions (elevation, slope aspect, slope position and slope) on the southern slope of the Qinling Mountains. Investigation, sampling and determination were carried out. Difference tests were used to analyze the effects of different site factors on SOC density (tC·hm^-2). Stepwise regression analysis was employed to quantify the relative importance of each factor on the variation of SOC density.[Result]The average SOC density was 125.41 tC·hm^-2(52.60-307.36 tC·hm^-2)on the southern slope of Qinling Mountains, which were 59.04, 41.65, and 24.73 tC·hm^-2 in 0-10, 10-30, and 30-60 cm soil layers respectively, accounting for 47.07%, 33.21% and 19.72% of the total SOC density, respectively. The SOC density varied greatly among different regions on the southern slope of Qinling Mountains, showing Longcaoping of Foping County (143.55 tC·hm^-2), Ningdong of Ningshan County (138.37 tC·hm^-2), Ningxi of Ningshan County (134.09 tC·hm^-2), Taibai Mountain of Taibai County (109.25 tC·hm^-2), and Changqing of Yang County (90.22 tC·hm^-2). The SOC density firstly increased and then decreased with increasing altitude. The SOC density (90.24 tC·hm^-2) was the lowest at 800-1 200 m above sea level, and the SOC density (166.43 tC·hm^-2) was the highest at the 2 000-2 400 m above sea level. When the altitude was higher than 2 400 m, the SOC density decreased, with an average of 132.51 tC·hm^-2. The SOC density (127.23 tC·hm^-2) of shady slope was slightly higher than that (123.25 tC·hm^-2) of sunny slope. The SOC density decreased from 147.52 to 87.06 tC·hm^-2 with the increase of slope. The SOC density in the down slope (166.36 tC·hm^-2) was higher than those in the middle (129.43 tC·hm^-2) and up-slope (77.14 tC·hm^-2).[Conclusion] The variations of SOC density among different forest regions were significant. The trend was increased first and then decreased with the increase of elevation. Significant differences (P<0.05) or extremely significant differences (P<0.01)of SOC density appeared in elevation ranges and different slope position. However, the difference of SOC density was not significant between shady slope and sunny slope. Stepwise regression analysis showed that the slope position and elevation were the dominant topographic factors that affect the SOC density. Compared with other forest areas in China, the SOC density in forest land on the southern slope of Qinling Mountains was at a higher level in the country. The SOC on the southern slope of Qinling Mountains plays an important role in the soil carbon pool of forest ecosystem in China and the management should be strengthened.

Key words: southern slope of Qinling Mountains, soil organic carbon density, spatial characterization, site conditions, analysis of stepwise regression