Land use and environmental factors are considered significant in regulating the spatial distribution of surface soil organic carbon (SOC), but are not equally important at different scales. Accurate knowledge of SOC stocks and the effects of environmental factors on SOC are crucial, both from the perspective of regional carbon budgets and appropriate landscape management of SOC. The Muyu small watershed and the Ansai catchment in loess hilly-gully area were taken as study area in the paper. On the basis of soil sampling in the field, 35 and 79 samples were taken separately in Muyu small watershed and Ansai catchment. Classical statistics and RDA were applied to study on the relationship of the distribution of SOC, soil organic carbon density (SOCD) with land use and environmental factors at two different scales. The research results were as follows. The effects of land use type on SOC and SOCD had significant difference at two scales. SOC varied with land use in the order: grass land > woodland > shrubland > farmland at the Muyu small watershed. And the order was woodland > shrubland > farmland > grassland at the Ansai catchment. Meanwhile, the SOCD was highest in the grassland and declined as follows: grassland > woodland > farmland > shrubland at the Muyu small watershed, and it was highest in the woodland and declined as: woodland > farmland > grassland > shrubland at the Ansai catchment. SOC and SOCD showed scale effects under different influence of land use. The main land use at the Muyu watershed was grassland. The soil physical and chemical properties were good enough to protect the soil and water. Thus the SOC and SOCD were both higher than other land use, which was different from that at the Ansai catchment. There were a lot of woodland and shrubland at the Ansai catchment. Since woodland was the optimal land use type to keep soil and water, and the SOC and SOCD were much higher in woodland at the catchment. Also since changes in land use had influences on physical, chemical and biological soil attributes, they changed SOC and SOCD by disturbing the equilibrium between the formation and mineralization of soil organic matter. The land use change played different roles in affecting SOC and SOCD at different scales.
Then we chose slope, slope position, slope aspect, altitude, vegetational cover and land use as environmental factors. Among them, aspect, slope and vegetational cover were positively related to the SOC and SOCD at both the Muyu watershed and the Ansai catchment. Altitude, slope position and land use were negatively related to SOC at the Muyu watershed while they were positively related to SOC at the Ansai catchment. Altitude and land use were negatively related to SOCD at two scales. Slope position was negatively related to SOCD at the Muyu watershed, and positively at the Ansai catchment. The influences of environmental factors were varies: slope aspect, slope and slope position decided the migration direction and speed of material, altitude affected the vertical distribution of the hydrothermal conditions, and vegetational coverage, with land use, reflected the input and output of soil organic carbon. But since different geographical conditions, the influences of those environmental factors were complex.