Topography and land use effects on the spatial variation of soil organic carbon: A case study in a typical small watershed of the black soil region in northeast China

Soil organic carbon is a soil property of central importance for soil quality and the global carbon cycle. Studies specifically aimed at the relationship between the spatial variation of soil organic carbon and environmental factors are few. In this paper, a typical small watershed named Tongshuang in the black soil region of northeast China, which was subjected to drastic erosion before 1980 and was managed subtly after 1980, was chosen as a study area. Classical statistic and geostatistic analysis methods, in combination with a geographic information system (GIS), were used to quantitatively research the spatial variation characteristics of the soil organic carbon and their relation to the topographic factors and land use. The data on the soil organic carbon, topographic factors, and land use were obtained by soil sampling and measurements derived from DEM, remote sensing images, and field investigations, respectively. The classical statistics analysis results indicated that the variability of the soil organic carbon was moderate (Cv = 0.30). The slope position and land use types were the most discriminating factors. The soil organic carbon content was the highest in the grassland and lowest in the coniferous forest (P < 0.01). It increased gradually along the slope position gradients from the interfluve to the toe slope. The geostatistics analysis showed that the soil organic carbon had a strong spatial correlation. The C0/(C0 + C) was 0.1608, which was mainly induced by structural factors. The mean soil organic content is 2.27% in this watershed. It is on a very low level in the northern black soil of northeast China. In this small watershed of the eroded black soil region, the present soil and water conservation measures play an important role in controlling the soil loss. However, the soil organic carbon’s restoration is unsatisfactory. Nearly three-quarters of the land has worrisome productivity. How to improve the soil organic carbon content while targeting the soil fertility is a pressing need.