The objectives of this study were to determine the response of soil organic carbon (SOC) and nitrogen (N) in different density fractions and the response of delta C-13 and delta N-15 signatures of soil to land-use change in a semiarid grassland. We measured soil delta C-13 and delta N-15 and the concentrations of SOC and N in whole soil and in the light and heavy fractions in adjacent tracts of native grassland, cropland, and shrubland in a semiarid region of China. The cropland and shrubland were established on existing grassland 27 years ago. The conversion of grassland to cropland or to shrubland significantly decreased SOC and N concentrations and stocks in both whole soil and density fractions in the 0-80 cm soil layer. The decreases in SOC and N stocks associated with the heavy fraction accounted for > 90 % of the total decrease in the 0-80 cm soil layer after grassland cultivation or afforestation. The conversion of grassland to cropland or to shrubland significantly enriched soil delta C-13 but depleted delta N-15. Our results suggested that the losses of SOC and N in whole soil after land-use changes in this semiarid grassland were primarily due to losses of SOC and N associated with the heavy fraction.