Abstract: 【Objective】 Increased organic carbon (C) sequestration, especially the accumulation of organic C in soil, and decreased carbon losses from soil are of great importance for improving the dryland soil fertility and mitigating the greenhouse effect. Soils in the Loess Plateau areas are known typically lower in contents of organic C, and increasing application of nitrogen (N) fertilizer is proved to be the important tactical measure to increase crop yields, but information about how the N fertilizer input can affect the soil C is still lacking. 【Method】 A 23 years long-term experiment was conducted with winter wheat as test crop growing at five N rates of 0, 45, 90, 135, 180 kg N•hm-2 per year on a basal fertilization of 39 kg P2O5•hm-2 per year in dryland area of Loess Plateau. Soil samples were collected from 0 to 40 cm soil layers for each plots at harvest of winter wheat, in order to study the effects of long-term application of different nitrogen fertilizer rates on soil total carbon (TC), total organic carbon (TOC), light fraction organic carbon (LFOC) and inorganic carbon (IC), estimate the changes of TOC, LFOC and IC accumulation, and quantify the effects of N fertilizer application rates on different kinds of C in dryland soil. 【Result】 Obtained results showed that, with the increase of N fertilizer rates, the TC showed no significant change, while the TOC contents in 0 to 30 cm soil layers were increased by 7% to 28%, and the LFOC contents in 0-40 cm soil were increased by 31% to 106 %, but over application of N fertilizer showed no advantages on the organic carbon accumulation. Analysis of regression with the accumulation of different soil C fractions to N fertilizer application rates showed that, TOC accumulation in 0 to 30 cm soil layers reached the maximum of 36.6 Mg at 120 kg N•hm-2 and LFOC in 0 to 40 soil layers reached the maximum of 2.69 Mg at 161 kg N•hm-2, with the application of 1.0 kg fertilizer N per year led to an increase of TOC by 1.34 kg•hm-2 and LFOC by 0.31 kg•hm-2. In addition, the ratio of LFOC to TOC in 0 to 20 cm soil layers was found also increased with the increase of N fertilizer rate. In contrast, the IC content decreased significantly with the increase of N fertilizer application rates, especially in the 5 to 20 cm soil layers, where accumulation of IC was found decreased by 2.8 Mg at 180 kg N•hm-2, with the application of 1.0 kg fertilizer N per year led to a decrease of IC by 0.67 kg•hm-2. 【Conclusion】 Long term application of different rates of N fertilizer showed no significant effects on the amount of TC, but the composition of different C fraction was remarkably changed in the dryland soil of the Loess Plateau, with the accumulation of TOC increased due to increase of LFOC sequestration in soil, but the IC accumulation decreased. Therefore, rational application of N fertilizer is not only the key measure to increase the crop yield, but is also of great importance to increase the organic carbon sequestration and then the soil fertility. However, the loss of inorganic carbon from soil caused by application of N fertilizer should not be overlooked, and sufficient attentions and concerns should be paid to the effects from the losses of soil IC on the sustainability of agriculture, ecology and environment in this area.

Key words: soil , N fertilizer rate , total carbon , organic carbon , inorganic carbon , light fraction of organic carbon