Effective Physiological parameters and some physio-chemical parameters on soil organic carbon storage in Gonbad rangelands

Abstract Background and objectives: Access to real information and data on the impact of watershed management projects and their technical and economic evaluation is possible through representative and paired catchments. Since management operations and topographic characteristics have a great impact on carbon retention in ecosystems, this study aimed to determine the amount of soil organic carbon due to changes in some topographic factors including altitude, direction, and different percentages of slope and measurement and zoning of soil organic carbon before biological and mechanical watershed management operation. Methodology: For this purpose, 27 land units in a part of the rangelands of sample sub-basin with an area of 25 hectares were selected as the study area. soil samples were collected from plots with dimensions of 10 * 10 m2 per unit. Composite samples consisting of 5 soil samples were collected from the center and four corners of the mentioned plots from a depth of 0-10, Soil samples were analyzed for some soil properties; including, organic carbon, organic matter, texture, bulk density, electrical conductivity, and pH.. SAS v9.1 software was used for statistical analysis. After performing normality tests, homogeneity of variance was obtained by ANOVA analysis test. Then, Pearson correlation coefficient and linear regression analysis were used to evaluate the correlation and determine the relationship between independent and dependent variables (organic carbon) then, the evaluation of spatial changes of soil organic carbon and zoning map were investigated using the geostatistical method after testing the normality of the data, interpolation methods were used and the effective parameters were optimized. Finally, using the statistical criteria of Mean Absolute Error (MAE) and Root Mean Square Error (RMSE) the amount of deviation of the actual data was measured with the estimated data and the best method for interpolation of soil organic carbon was determined. Results:The results showed no significant changes of soil organic carbon in response to physiographic parameters. However, soil organic matter and organic carbon was significantly influenced by different aspects (p≤0.05). The maximum (8.46 and 55/87 t/ha) and minimum (0.9 and 22/77t/ha) organic carbon and soil organic matter were observed in north-facing and south-facing slope, respectively. Furthermore, there was a positive correlation between soil OC and organic matter and negative correlation with Ph. In order to interpolate soil OC, geostatistical and Classical statistics methods were compared considering RMSE and MAE values following by optimization procedure. The results showed geostatistical methods have lower error and higher accuracy comparing to Classical statistics methods. In summary, simple Co-kriging with organic carbon as auxiliary variable is the most appropriate one for soil organic carbon interpolation in the study area. The results indicate poor spatial distribution and low carbon sequestration in the region due to the degradation of rangelands. The exploitation and overgrazing of livestock is one of the main factors of low carbon storage potential in this basin Conclusion: The results of statistical analysis in this study show that the soil organic carbon of the region is not affected by the physiographic parameters of the basin, but changes in the amount of organic carbon and soil organic matter were significant compared to changes in slope direction.. Changes in the organic carbon of the area in the slope directions are affected by changes in vegetation and soil depth. Decreased vegetation on the southern slope means the reduced entry of organic carbon into the soil. However, the highest level of organic carbon in the topsoil in the northern slope of the region is estimated at 8.46 tons/hectare, which can be said that the Gonbad watershed is generally poor in terms of organic carbon.