Estimation of Organic Carbon Content and its Changes in Soybean-Wheat Cropping Rotation

The aim of this study was to investigate the effect of crop type on soil carbon accumulation in soybean (Glycin max L.) and wheat (Triticum aestivum L.) rotation, in 150 fields of soybean and 89 wheat fields in Gorgan county, in the cropping years of 2016-2017 and 2017-2018, respectively. In order to determine the amount of organic carbon and its changes, soil samples from two depths of 0-15 cm (first depth) and 15-30 cm (second depth) were prepared and then soil carbon was measured by Walkley-Black method. Spatial distributions of soil organic carbon and carbon accumulation potential at different depths and also, in the pre-sowing and post-harvest stages were performance using different types of interpolation methods in ArcGIS enviroment. The results of interpolation methods showed that Kriging was the best model for interpolation of organic carbon distribution and carbon accumulation in agricultural fields of Gorgan. Average amounts of soil organic carbon in the first depth of soybean fields were as 14.84 mg ha-1and 13.41 mg ha-1for the second depth in the pre-sowing stage. Also, it was estimated as 16.85 mg ha-1 in the first depth and 15.52 mg ha-1 for second depth, in the post-harvest stage. The amounts of soil organic carbon in the wheat fields were determined about 16.78 and 15.25 mg ha-1for first and second depths in the pre-sowing stage, respectively. Those were 13.68 and 12.30 mg ha-1for of first and second depths in the post-harvest stage sampling, respectively. The results showed that the eastern, northern, northeastern, southern, southeastern and central parts of the county had the highest carbon accumulation due to access to modern irrigation systems, more water resources, better crop management, suitable return of crop residue to the soil, develop of minimum tillage, minimizing the burning of residue and using manure. Results showed that the western and southwestern parts of the county had the lowest carbon accumulation.