Changes in the carbon (C) stock of grassland soil in response to land use change will increase atmospheric CO2, and consequently affect the climate. In this study we investigated the effects of land use change on soil organic C (SOC) and nitrogen (N) along a cultivation chronosequence in the Xilin River Basin, China. The chronosequence consisted of an undisturbed meadow steppe, a 28-year-old cropland and a 42-year-old cropland (abbreviated as Steppe, Crop-28 Y and Crop-42Y, respectively). Crop-28Y and Crop-42Y were originally created on the meadow steppe in 1972 and 1958, respectively. The soil samples, in ten replications from three depth increments (0–10, 10–20 and 20–30 cm), were collected, respectively, in the two cropland fields and the adjacent undisturbed steppe. Bulk density, SOC, total N and 2 m KCl-extractable mineral N including ammonium and nitrate were measured. Our results showed that the greatest changes in the measurements occurred in the 0–10 cm soil depth. The SOC stock in the upper 30-cm soil decreased by 9.83 Mg C ha−1 in Crop-28Y and 21.87 Mg C ha−1 in Crop-42Y, which indicated that approximately 10 and 25% of the original SOC of the steppe had been emitted over 28 and 42 years, respectively. Similarly, the total N lost was 0.66 Mg N ha−1 and 1.18 Mg N ha−1, corresponding to approximately 9% and 16%, respectively, of the original N at the same depth and cropping duration as those noted for SOC. The mineral N concentration in the soil of both the two croplands was greater than that in the steppe soil, and the ammonium-N was less affected by cultivation than the nitrate-N. The extent of these changes depended on soil depth and cropland age. These effects of cultivation were much greater in the top 10 cm of soil than in deeper soil, and also greater in Crop-42Y than in Crop-28Y. The findings are significant for assessing the C and N sequestration potential of the land use changes associated with grassland conversion, and suggest that improved management practices are needed to sequester SOC and total N in the cropped soil in a semi-arid grassland.