Effects of combined maize straw and N application on soil nitrogen surplus amount and yield of winter wheat

【Objective】 In this paper, influence of straw incorporation and N application rates on wheat yield, soil NO-3-N accumulation, nitrogen surplus amount and nitrogen use efficiencies was systematically studied to explicit the optimum nitrogen application rate under straw incorporation conditions in lime concretion black soil based on data from a four year experiment in Mengcheng City, Anhui Province. 【Methods】 A split field trial with two factors of straw and N fertilizer was carried out. The main plot was straw treatment, designed as straw incorporation and straw cleaning up.Each main treatment was composed of 6 levels of N application amount: 0,162.0, 202.5, 243.0, 283.5 and 324.0 kg/hm2. The soil NO-3-N contents, winter wheat yields and N contents in wheat grains were determined, the apparent surplus of nitrogen in soil during winter wheat growth period, nitrogen fertilizer use efficiency and the relationship between nitrogen surplus and soil nitrate nitrogen accumulation were analyzed, and the appropriate amount of nitrogen fertilizer in winter wheat growth season was explored under the straw returned to soils. 【Results】 Compared to the straw cleaning up, the wheat yields in the four years under the straw incorporation were increased by 365-844 kg/hm2, with the increased rates of 4.2%-9.3%, especially for the N application rate of 243.0 kg/hm2, its yield was as high as 9858 kg/hm2. The N treatments under the straw incorporation could significantly increase soil NO-3-N accumulation at 0-60 cm soil depth in the whole growth period of wheat, while under the no straw incorporation, the NO-3-N accumulation was related to N fertilizer application rates, the soil NO-3-N accumulation was increased by 19.8%-28.6% when the N fertilizer application rates exceed 243.0 kg/hm2. The wheat plant N accumulation from sowing to jointing stage was increased by N fertilizations, and the increases were positively dependent on the ratio of basal N fertilization. The nitrogen applications significantly increase nitrogen surplus in the soil profiles of 0-60 cm. The soil NO-3-N accumulations under straw incorporation and cleaning up will increase respectively 74.2 and 91.4 kg/hm2 for every 100 kg/hm2 of soil N surplus. At the same N fertilizer application levels, there are no obvious differences in the stage N absorption and accumulation of wheat plant, soil NO-3-N accumulation and soil N surplus with straw incorporation or cleaning up. A combination of straw returning and N fertilizer applications increases N agronomic efficiency, plant N recovery efficiency and grain recovery efficiency, especially the basal and jointing fertilizer N use efficiencies at the high N application rates are significantly higher than the single nitrogen treatments, but the results were on the contrary under the single low N application rates. Significant differences in the N harvest indices do not exist among the N treatments and between the straw incorporation and cleaning up at the same N fertilizer application rates, which indicates that effects of the straw returning and N fertilizer application levels on N absorption and transportation of wheat plant are not significantly different. The apparent N recovery is decreased with the increase of the N fertilizer application rate, and is significantly higher in the basal fertilizer than that in the jointing fertilizer.【Conclusions】 The main effect of combined straw incorporation with N application in increasing yield and N fertilizer efficiencies is due to the high soil NO-3-N accumulations in soil depth of 0-60 cm, which provides a stable application of N nutrition during the whole growth stage of wheat. Comprehensively, the optimum N application rates for winter wheat are from N 202.5 to 243.0 kg/hm2 under the total maize straw incorporation