Spatial variation of soil organic carbon density in the black soil region of Northeast China under the influence of erosion and deposition

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Soil erosion of the Songnen typical Black Soil Region (BSR) in Northeast China where contains higher soil organic carbon is serious, resulting in soil organic carbon spatial redistribution. To study the spatial redistribution pattern of cropland SOC caused by erosion, 115 cropland sample points were selected in the Songnen typical BSR, and soil samples were collected along the entire soil profile. The soil organic carbon density (SOCD) of the soil layer and profile were calculated, and the carbon storage (CS) was calculated for the erosion and deposition areas. The average soil organic carbon density (SOCDav) of the 115 cropland soil profiles (1 m) ranged from 1.5 to 4.5 kg/m2, and the total soil organic carbon density (SOCDt) of the soil profile (1 m) ranged from 10 to 20 kg/m2. The SOCD of each soil layer (20 cm) decreased vertically in a logarithmical manner from the top, and was divided into four groups with thickness through systematic clustering, which were 30–60 cm, 60–100 cm, less than 300 cm and greater than 350 cm. The SOCDt in the area north of the Songhua River and the area west of Nen River (The Greater Khingan Mountains area) ranged from 2.5 to 59.3 kg/m2, with an average is 14.1 kg/m2, and the first sample point group (30–60 cm) was mainly located in this area which had an average slope of 3.2° and an average black soil thickness of 24.6 cm. The SOCDt in the area north of the Songhua River and the area east of the Nen River ranged from 11 to 90 kg/m2, with an average of 28.6 kg/m2, the third and the fourth sample point group (greater than 100 cm) were both mainly located in this area which had an average slope of 2° and an average black soil thickness of 61 cm. The SOCDt in the area south of the Songhua River ranged from 1.6 to 28.7 kg/m2, with an average of 13.8 kg/m2, and the second sample point group (60–100 cm) was mainly located in this area which had an average slope of 2.4° and an average black soil thickness of 58.7 cm. The results showed that soil erosion occurred on steeper slopes, resulting in thinner soil layers, whereas soil deposition occurred on lower grade slopes, resulting in thicker soil layers. Kriging interpolation in ArcGIS showed that the Cs in the erosion and deposition areas of the Songnen typical BSR was 459.7 and 1234.7 Tg, respectively, and the total Cs was 1.6944 Tg (1 Tg = 1012 g).