Abstract:Land use change and tillage management is an important aspect for influencing carbon process in terrestrial ecosystem by human. The objective of this study is to identify effects of conversion of degraded cropland to alfalfa forage land on soil carbon sequestration and related soil properties in Zhangye oasis in the middle of Hexi corridor region. We selected 49 sites along altitude gradient as sampling sites where alfalfa were planted for 5 years, and taken paired alfalfa soil and adjacent crop soil in the 0-15cm depth. The results showed that no changes in soil particle size and bulk density were found following land use change. Compared to adjacent cropland, soil pH value increased in average 0.11 unit but electrical conductivity (EC) decreased by 34.8% in alfalfa field, and the averaged concentrations of SOC, total N and Labile C increased by 18.5%, 9.3% and 53.3%, respectively. SOC concentration was influenced by altitude and soil silt and clay content. The sandy loam soils had a greater increase percentage of SOC and total N but a smaller increase percentage of labile C compared to the silt loam soils after land use conversion. SOC and total N storage in the 0-15 cm depth increased 2.84 Mg hm-2 and 0.21 Mg hm-2, respectively, and the average rates of C and N sequestration were 0.57 Mg hm-2a-1 and 0.04 Mg hm-2a-1 following the retirement of degraded croplands into alfalfa forage land over 5 years. Our results suggest that conversion of annual crops to perennial alfalfa had significant carbon sequestration effect and great potential in the region. Labile C showed a more remarkable increase than SOC following crops to alfalfa conversion, suggesting that it is a more sensitive index of C dynamics under the short-term land use change.