Abstract: 【Objective】 In order to optimize the nitrogen nutrient management under straw returning to soil condition in the winter wheat-summer maize rotation region of Shaanxi Guanzhong Plain, a field experiment was carried out to study the N release characteristics of maize straw and the effects of returning straw to soil on loess N supply and nitrogen uptake by winter wheat. 【Method】The field experiment was initiated in October 2012 and ended in May 2013 at Zhongnan town, Zhouzhi county in Shaanxi Province. Some little nylon net bags containing the air dried maize straw were buried between the rows after local winter wheat sowing. The experiment was conducted by a combination of net bag buried in the field and ¹⁵N isotope cross labeled maize straw and N fertilizer with four replications. The changes of residual N from the straw and N content in different organs of wheat at harvest were measured for studying the N release of maize straw during the wheat growing season, the fate of straw N and fertilizer N, and their contributions to N uptake in the above-ground part of winter wheat by using two treatments, which were no nitrogen and 200 kg N·hm^-2 fertilization under returning straw to soil condition. 【Result】 The amount of total N remained in the straw was decreased from seeding to the period before the winter dormant stage of wheat, then increased until reviving stage and declined gradually later. From sowing to harvest of wheat, the amounts of net N release to soil were 9.4 and 7.2 kg·hm^-2 by the returned straw under no and 200 kg N·hm^-2 fertilization, respectively, and they were the balance of the N release of 19.7 and 18.3 kg N·hm^-2 from the straw, and the N capture of 10.4 and 7.5 kg·hm^-2 from the soil and 3.6 kg N·hm^-2 from fertilizer into the straw.At harvest, it was found that 65.1% and 67.7% of the straw N were still left in the returned straw, 31.5% and 30.4% were released into the soil or lost, and only a few, 3.4% and 1.9% were absorbed and used by winter wheat, at respectively no N and 200 kg N·ha^-1 fertilization. Also, 25.0%, 73.2% and 1.8% of fertilizer N were observed respectively to be absorbed and used by winter wheat, left in the soil or lost, and captured by the returned maize straw under straw returning condition, when the N application rate was 200 kg N·hm^-2. Under straw returning condition, soil N was shown to contribute the most to the above-ground part N uptake of wheat, and then was the fertilizer N, while the straw N contributed the least, of which they were 98.3% and 69.2%, 0 and 30.1%, 1.7% and 0.6%, at no N and 200 kg N·hm^-2 fertilization, respectively. In the above-ground part of wheat, N derived from soil, fertilizer and straw were mainly distributed in the grain of wheat, and they were respectively 98.1%, 0 and 1.9% at no N fertilization, and 68.8%, 30.5% and 0.7% as N fertilizer was applied into the soil.【Conclusion】In the winter wheat-summer maize rotation region of Shaanxi Guanzhong Plain, more than 65% in straw N was still left in the straw after one winter wheat growing season, also as high as 70% of fertilizer N was mainly immobilized by soil or lost under maize straw returning condition. And soil N contributed the most to the above-ground part N uptake of wheat, which approximated 70%.

Key words: Guanzhong Plain, straw return, N release, winter wheat, ¹⁵N tracing