Three-Dimensional Estimation of Soil Organic Carbon Storagein The Dry Farming Region of China

Soil is recognized as the largest carbon reservoir in the terrestrial ecosystem.China is one of the most important agricultural countries with an important source of CO₂ emission in the world. The dry farming region, as a crucial grain production area in China, had a larger change of organic carbon recently. Understanding spatial variation of soil organic carbon density (SOCD) in three-dimensional direction is helpful for estimating soil organic carbon storage. At present, the spatial variation of soil organic carbon was mainly focused on the two-dimensional scale, and considered only horizontal direction or vertical direction without concerned on the interrelation between upper and lower soil layers. Due to the effect of profile depths and soil texture on vertical distribution of SOCD, the stationary assumption for SOCD cannot be met in the vertical direction.Therefore, the 3D ordinary kriging technique cannot be directly used to map the distribution of SOCD at a regional scale, it is necessary to find a simple and reasonable way to improve the applicability of 3D simulation on SOCD. In this paper, a total of 1608 samples were collected from 402 soil profiles (0.0 to 0.4 m, 0.1m increment) in dry farming region of China and SOCD contents were determined for each soil sample. The different methods and parameters effected on the 3D simulation accuracy of SOCD was compared, then the gradient plane nodal functions (GPNF) method was selected to estimate the SOCD of 0-40 cm soil layers, and to finally calculate its organic carbon storage.The results showed that the spatial distribution of SOCD was more accurate simulated by GPNF method without amplifying the vertical multiple, while the shepard(SPD)method need to increase the vertical magnification to achieve more precise results. When the vertical magnification multiple and search point were 3.0×10⁶ and 25, the spatial distribution of SOCD was the best simulated by GPNF method. The SOCD in the dry farming region increased from south to north, and it was decreasing with the increase of soil depth.The soil organic carbon reserves of 0 to 40 cm and 0 to 20 cm soil in dry farming region are 22.28×10¹¹kg and 13.12×10¹¹ kg respectively.