【Objective】Land use and fertilization are the two important types of human activities that can influence agricultural production and soil carbon (C) stock, especially in farming-grazing interlacing zone, like Wuchuan County, Inner Mongolia of China. 【Method】In this study soil samples were collected from a long-term field experiment on land-use and fertilization located in Wuchuan County for analysis of soil organic carbon (SOC), soil inorganic carbon (SIC), total nitrogen (TN), δ¹³C_SOC, δ¹³C_SIC and δ¹⁵N to investigate impacts of land-use (land planted with Horqin peashrub (Caragana korshinskii), land planted with ryegrass and deserted land) and fertilization (chemical fertilizer, organic manure and no fertilization) on C and nitrogen (N) transformations and C stock in the soil. 【Result】Results show that SOC stock in the land under Horqin peashrub (Caragana korshinskii) or ryegrass was 0.60~0.98 Mg hm^-2 a^-1 higher than that in the crop land, and similarly, SIC was slightly higher than or on the same level, except in the land under Horqin peashrub. Compared with the deserted land, the lands under “grain for green”(planted with Horqin peashrub or ryegrass) were lower in content of primary carbonate and almost unchanged in soil TN level. Compared with CK (no fertilization), application of organic manure significantly increased SOC stock (1.08~1.19 Mg hm^-2 a^-1), while application of chemical fertilizer decreased SIC stock, mainly of secondary carbonate, at the rate of 0.06~0.16 Mg hm^-2 a^-1. Compared with application of chemical fertilizer alone, application of organic manure and application of organic manure in combination with chemical fertilizer increased SOC and SIC stock at the rate of 0.6 and 0.1 Mg hm^-2 a^-1, respectively. Changes in land-use did not have much impact on δ¹³C of SOC and SIC, but the crop land was significantly higher than the deserted land or the lands under “grain for green” in soil δ¹⁵N, due to application of chemical fertilizer. Application of organic manure significantly decreased δ¹³C value of SOC, but application of chemical fertilizer had a reverse effect, which was further enhanced by soil acidification caused by chemical fertilization. The discrepancies between fertilization treatments in SIC loss maybe attributed to leaching of SIC into deeper soil layers, which indicates that it is necessary to do in-depth study on SOC and SIC in deeper soil layers. 【Conclusion】The impact of fertilization on soil C and N transformation is much higher than that of changes in land-use in this farming-grazing interlacing zone. As regards the soil in the arid and semi-arid Inner Mongolia, the effects of land use and fertilization on SOC and SIC should be taken into full consideration in the management of regional C sequestration.