Relationship between soil organic carbon density and topography in three-dimensional space in a small watershed of loess hilly region in China

Abstract

In order to display the three-dimensional spatial distribution of soil organic carbon density (SOCD) and analyze the relationship between SOCD and topography, this study took 290 soil samples at the depth of 0 to 100 cm in the Nangou watershed in the loess hilly region of China, simulated the spatial distribution of SOCD by the three-dimensional inverse distance weighting method, and compared the SOCD of several soil sections of different slope shapes and different slope aspects when the land use were all forests. The results showed that the SOCD tended to decrease with increasing soil depth; the mean varied from 0.52 kg m^–2 (60 to 100 cm) to 1.03 kg m^–2 (0 to 10 cm), and there was obvious stratification in the vertical direction. The variation coefficients of the SOCD were between 23.43% (60 to 100 cm) and 41.15% (10 to 20 cm), which were relatively close in the topsoil of the depth of 0 to 30 cm and gradually decreased with soil depth. The SOCD in the watershed was higher in the northeast, followed by the southwest and lower in the middle. The three-dimensional spatial distribution of the SOCD contained more information, and its prediction accuracy was higher than that of the corresponding two-dimensional method. In slopes where the land use were all forests, the SOCD was ranked as concave slope > straight slope > convex slope and shady slope > sunny slope, and its complexity was ranked as concave slope > convex slope > straight slope and shady slope > sunny slope. Our results highlighted the carbon (C) sequestration ability of concave slopes on the shady side and provided scientific reference for the development of slope C sequestration policies in the loess hilly region of China.