Impact of Land Use Change on Soil Carbon Storage in topsoil of the tableland of the Loess Plateau in Shaanxi in Recent Thirty Years

The impact of land use change on soil organic carbon storages at the regional scale is crucial to the ecological effects of the local environment. According to soil samples and land use data, the effect of land use change on soil organic carbon density and reserves in the surface layer (0~20 cm) of the Loess Plateau, Shaanxi, from 1985 to 2015, was studied. The results show: ⑴The policy of returning farmland to forest has a great impact on the transformation of land use type in the tableland of Shaanxi Loess Plateau. The change of land type mainly focused on the area reduction of cultivated land and grassland, respectively reduced by 6.02% and 15.14%.And the areas of forest land and residential land increased, with an increase of 281.07% and 24.09% respectively. ⑵ In the past three decades, the spatial variation trends of soil organic carbon density in surface soils of the study area were quite different. The increase was 11.43% in 2006 compared with 1985, and 89.74% in 2015 compared with 2006. ⑶ From 1985 to 2006, the area of maintaining land use type was 111997.18 km², and the surface soil carbon storage increased by 1404.31×10⁶kg. The area of changing land use type was 776.15 km², and the carbon storage increased by 14.49×10⁶kg. From 2006 to 2015, the area of maintaining land use type was 9359.68 km², and the surface soil carbon storage increased by 3829.98×10⁶kg. The area of changing land use type was 776.15 km², and the carbon storage increased by 1318.13×10⁶kg. ⑷ In the past 30 years, the area of maintaining land use type decreased, and the area of changing land use type increased. The conversion of land use type to forest land and grassland was conducive to the formation of surface soil carbon sink, and the conversion to cultivated land and residential land resulted in the release of surface organic carbon. The comparative analysis between the two periods showed that the conversion of land use type was more intense and the soil carbon storage increased significantly after the policy of returning farmland to forests was implemented. Therefore, the policy of returning farmland to forests in the tableland of Shaanxi Loess Plateau can increase the carbon fixation of ecosystem.