Soil organic carbon sequestration rate and its influencing factors in farmlands of semi-arid regions—A case study in Zhuanglang County, Gansu Province

To estimate the soil carbon sequestration rate and its influencing factors at a county level, we selected Zhuanglang County in southern Gansu Province as the study area. Soil organic carbon sequestration rates were calculated by using some of the results of the second national soil survey and as well as the data of farmland quality evaluation in Zhuanglang County. Farmland soil samples were collected and analyzed in July 2011 The results show that the soil organic carbon contents of the countys farmlands in the valley terraces are the highest, followed by the lands located in valleys and mountain ridge slopes, and the organic carbon contents in flat lands in the valleys are the lowest. Significant differences are found in organic carbon contents in different terrains (P0.05). The soil organic carbon contents in 80% of the sampling sites are in the range of 6.0-11.0 g/kg, and the organic carbon contents in 0-20 cm soils present a normal distribution. The organic carbon contents in soil profiles show a decrease trend from top to bottom. The organic carbon density in 0-20 cm soils of the study area is 20.02 t/ha, which is below the average level of the country (33.45 t/ha). The carbon sequestration rate in 0-20 cm soils in the past 30 years is C 63 kg/(haa), while it is C 26 kg/(haa) in the past five years. At the county level in the semiarid region, the topographic position can explain 43% of organic carbon variability, the organic fertilization, aspect and field surface slope can explain 47.1% of the organic carbon variability, and the soil type can explain 9.9% of organic carbon variability. The results of the comprehensive analysis show that the farmland soil organic carbon density of Zhuanglang County in the past 30 years is increased, and this probably relies on the terrace construction and soil and water conservation. Further study should be conducted on the impact of the terrace construction and fertilizer inputs on soil organic carbon density and carbon sequestration rate in the farmlands.