Spatial Prediction of Soil Organic Matter Using Geostatistics and Topographic Unit Zoning Integrated in GIS: A Case Study

The spatial distribution of soil organic matter (SOM) has a close connection with topography. To understand the effects of topographic synergy effects in traditional geostatistic methods, the influence of topography is considered in SOM geostatistic studies by combining geographic unit zoning and spatial prediction. We explored the changes in the SOM distribution between that obtained using spatial interpolation integrated with 13 different classical topographic units and determined using global interpolation with 6485 random soil samples obtained from Zhongxiang City, Hubei Province, China. The steps are as follows. At first, the terrain factors were calculated from the digital elevation data (DEM) and the topographic units were precisely divided into 13 different classical types more subtly by integrating the terrain factors. The regions were divided, which was based on terrain classification rules formed by the distribution of terrain factors in different landforms. Secondly, soil samples were collected in different topographic types, and the distribution of SOM for each sample set in different topographic units was generated by ordinary Kriging. Then, the corresponding results of interpolation for each sample set were segmented based on topographic unit region, and combining the result in each region, the spatial distribution of SOM based on topographic unit was obtained. Finally, verification and comparison with the accuracy of each SOM distributions were performed, which were obtained by using topography based geostatistics and traditional global geostatistics, respectively. Our results indicated that more accurate SOM spatial distributions can be obtained using the proposed method, especially in regions with gentle topography, such as ridge, shoulder, summit, toe slope (north/northeast side), and low-lying terrain units.