Simulating soil organic carbon dynamics as affected by different water erosion scenarios and grazing management in semi-arid rangelands of Bajgah using the Century model

Most studies have reported a strong relationship between water erosion and soil organic carbon (SOC) losses, so that carbon displacement caused by water erosion stated as an important factor affecting the concentration of CO2 in the atmosphere. Therefore, understanding water erosion processes effect on SOC within an ecosystem is essential. Since evaluating the impacts of water erosion on SOC stock dynamic in long-term is difficult without modeling tool, studying of SOC dynamic through use of simulation models in combination with measured data is recommended. Considering that rangelands of Iran have taken a large part of the country, modeling the effect of water erosion on SOC dynamics in rangelands can provide appropriate management solutions for related organizations. The SOC modeling has not been conducted under the influence of water erosion, and this study could be basis for the future of SOC modeling. Therefore, the present study aims to: 1) improve the validation of the Century model as the most widely used model in the soil carbon studies, by using the output of three water erosion models of GLEAMS, WEPP, and ANSWERS, and 2) simulating the SOC stock dynamic under the influence of two erosion scenarios in the semi-arid rangelands of Bajgah.

To conduct this research, 60 soil samples were taken by randomized sampling methodfrom a depth of 0-20 cm of soil in the semi-arid rangelands of Bajgah. To determine the soil bulk density, two cylinders were collected beside each soil puddle (120 samples). After determining the volume of vegetation sampling (number of plots); aboveground biomass, litter and belowground biomass were collected in 17 plots then transferred to the laboratory. Parameterization and calibrating the Century model was performed by using 43 years of weather data, site specific data, as well as physiological and ecological parameters of rangeland. Then, the long-term average of water erosion rate simulated by WEPP, GLEAMS and ANSWERS erosion models entered as input in the Century model. Finally, a water erosion model, that its simulated erosion rate improves the validation results of the Century model, was selected then illustrated the effect of two water erosion scenarios on the changes of SOC stock for two periods of grazing management including period of nationalization of rangelands with moderate grazing management.

The simulation results showed that use of the GLEAMS model output made the Century model more accurately predict SOC stock. Therefore, the GLEAMS erosion model output was used for simulate the SOC stock variations under the influence of water erosion. The simulation results of the Century model showed that the SOC stock in the Bajgah rangelands during the II period with moderate grazing management, decreased from 3496 to 3260.93 (g cm-2) and 3496 to 3243.90 (g cm-2) in the no-erosion and erosion scenarios, respectively and there was no significant difference between them (p<0.05). Also during the III period with no grazing management, SOC stock decreased from 3245.30 to 3356.04 (g cm-2) and 3227.37 to 3350.42 (g cm-2) in the no-erosion and erosion scenarios, respectively and there was no significant difference between them (p <0.05). Generally, water erosion decreased the SOC stock in the semi-arid rangelands of Bajgah compared with the non-erosion scenario by 0.52% and 0.16% in the end of periods II and III, respectively. The erosion resulted in a slight reduction in SOC stock in the semi-arid rangelands of Bajgah compared to the non-erosion scenario. Based on the results of this study, it can be state that the role of grazing management than the erosion in the changes and losses of SOC stock was more evident in the semi-arid rangelands of Bajgah.