【Objective】Nitrogen is one of the most important and essential elements for growth of crops. Reasonable application of nitrogen fertilizer may promote crop growth and increase its yield, while excessive N fertilization may not only inhibit crop growth, but also leave a large amount of fertilizer nitrogen (N) in the soil as residue, which disturbs balance of soil nutrients and poses a potential threat to the ecological environment. So, it is necessary to study dynamics and availability to crops of the residual N to provide a theoretical basis for rationalizing N fertilization, improving N use efficiency and reducing fertilizer N loss. 【Method】To that end, a 4-year stationary field experiment was carried out on a winter wheat and summer maize rotation system, using¹⁵N-tracing technique. During the first cycle of the winter wheat and summer maize rotation, 240 kg hm^-2 of nitrogen was applied in the form of ¹⁵N-laballed urea in all the plots before winter wheat was sown, in order to make a background of high N residual, and in the following summer maize season, the plots were divided into two groups applied with 0 and 120 kg hm^-2 of ordinary urea, separately, at the prolonging stage of the crop to explore effect of N fertilizer application on fertilizer N residue in the soil. During the following three cycles of the rotation, no N fertilizer was applied for analysis of dynamic and availability to the crops of the residual fertilizer N in the soil. 【Result】Results show that residue of the ¹⁵N-laballed N fertilizer that was applied before sowing of winter wheat during the first cycle of the rotation, was found throughout the whole soil profiles of 0~200 cm, after the crop was harvested, and most of that accumulated in the 0~40 cm soil layers. The residue amounted to 200.9 kg hm^-2 in total, accounting for 83.7% of the total N fertilizer applied. In the subsequent summer maize growth season, the amount of residual fertilizer N first dropped rapidly, and then declined slowly with the season going on, and eventually leveled off. After for 4 years of rotation, still a considerable amount of ¹⁵N was found in the 0~300 cm soil profile, reaching 47.1 kg hm^-2 and 54.0 kg hm^-2, respectively, in the plots where no or 120 kg hm^-2 N fertilizer was applied to summer maize during the first circle of the rotation. Obviously a part of the fertilizer N was fixed by organic matter in the soil. The recovery rate of residual N by the crop decreased gradually year by years, but the rate varied differed between the two treatments. The total N use efficiency of four years was 46.9% and 50.4%, respectively, in the plots with or without N sidedressing for maize. The total N use efficiency of the winter wheat and summer maize was 41.6% and 42.0%, respectively, in the first year of the rotation, and only 5.3% and 8.4% in the remaining 3 years. During the four years, about 38.1% and 29.7% of the residual fertilizer N was lost, respectively, in the two treatments, with or without N sidedressing, and the loss occurred mainly in the first summer maize growing season. 【Conclusion】It indicates that some of the fertilizer N applied in upland wound inevitably remain in the soil as residue, and this part of residual fertilizer N is low in availability to crops, and has a limited portion being gradually taken up by crops in the late 3 years, a portion remaining in the form of organic N, and the other turning unavailable and lost. Sidedressing of N fertilizer in the summer maize season may promote crop uptake of residual fertilizer N in the soil, while retaining more residual fertilizer, and consequently it reduces the loss of residual N. However, all the effects are attained at the cost of high loss of the N fertilizer per se. Therefore, it is advisable to take into account fertilizer N in the soil when making plans for N fertilizer application.