Abstract: Long-term application of nitrogen fertilizer is approved to cause low nitrogen use efficiency in crops and environmental pollution by NO₃^--N accumulation in soil. Straw returning to field takes the advantages of improving physical and chemical characteristics and nutrition status of soil, resulting in yield promotion of crops. The nitrogen managements for straw mulching cultivation have been studied well in single cropping system. The wheat (Triticum aestivum L.)-maize (Zea mays L.) double cropping system is typical in North China Plain, however, nitrogen fertilizer application regimes in straw-mulching double-cropping system extended rapidly in the region are poorly studied. In the current fixed-plot field experiments, we aimed at the changes of nitrogen use efficiency and grain yield under straw mulching plus a series of amount of nitrogen fertilizer in winter wheat, and provided some advices on nitrogen application regime in mulch farming. A wheat cultivar, Zhoumai 14, was used in a two-factor randomized block design with 3 replicates. A total of 10 treatments were coded as N0, N90, N180, N270, and N360 for only nitrogen fertilizer applied (0, 90, 180, 270, and 360 kg ha^-1, respectively) and S+N0, S+N90, S+N180, S+N270, and S+N360 for straw mulching plus nitrogen applications (nitrogen rates as above), respectively. About 7 500 kg ha^-1 straw residue from previous cropping season was ploughed into soil. Compared with no straw mulching treatments, S+N treatments increased dry matter accumulation during wheat growth. The S+N treatments showed the amount and proportion of dry matter accumulation lower before heading than N treatments, but significantly higher at the period of heading to maturity. Under the same nitrogen fertilizer rates, grain yields of S+N90, S+N180, S+N270, and S+N360 were higher than those of corresponding N treatments by 7.1%, 8.4%, 11.1%, and 10.2%, respectively. The S+N270 treatment had the largest amount and increasing ratio of grain yield with significant differences from other S+N and those of N treatments, indicating that S+N270 might be the proper treatment in wheat production. Nitrogen fertilizer use efficiency, nitrogen use efficiency, and agronomic nitrogen use efficiency in S+N treatments were higher than those of N treatments by 3.3–9.2%, 0.7–4.0, and 2.7–7.3 kg kg^-1, respectively. Our results indicate that returning straw to field has positive effects on winter wheat under the experimental conditions, and straw mulching plus nitrogen fertilizer has larger productivity than only applying nitrogen fertilizer.

Key words: Returning straws to soil, Nitrogen fertilizer, Winter wheat, Nitrogen efficiency, Grain yield