Mechanism of combination of nitrogen fertilizer reduction and straw returning in regulating dryland nitrification intensity and keeping stable crop yield in long run

[Objectives] Low nitrogen use efficiency and environmental pollution are common in modern agricultural production due to excessive use of nitrogen fertilizer. The potential and mechanism of combination of straw returning and chemical fertilizer reduction were studied to improve the soil properties and increase nitrogen use efficiencies in condition of not affecting or even increasing the yield of crop. [Methods] Field experiments were conducted in two typical uplands of north China and south China for three years. The north site was located in Xuchang of Henan, where was fluvo-aquic soil, and the south one was in Qujing of Yunnan, where was red soil. The cropping system in both location was corn-barley/wheat rotation. The field experiments had four treatments, including no nitrogen fertilizer control (CK), inorganic NPK fertilizer alone (N), reduced inorganic nitrogen by 20% (80%N), 80%N plus crop straw returning (80%NS). The soil nutrient contents and nitrification intensity were measured during growing seasons, and the crop yields were investigated and the nitrogen use efficiency were calculated. [Results] Compared with N treatments, reducing nitrogen by 20% and combined with straw returning for three years could maintain stable or even increase crop yields, increase the content of dissolved organic C and N in dryland soils at barley/wheat heading stage and maize tasling stage, promote the accumulation of NH₄⁺-N and reduced the content of NO₃^--N. 80% NS reduced nitrification potential by 5.5%-33.9% in fluvo-aquic soil and by 7.8%-37.5% in red soil. Consecutive three-years’ 80%NS treatment maintained stable yields and even increased yield by 11.2%-20.4%, and increased nitrogen use efficiency by 6.4%-10.3%. In contrast, 80%N treatment reduced yields by 3.9%-13.4%, and reduced N fertilizer utilization efficiency by 1.8%-38.9%. [Conclusions] Adequate N reduction along with straw returning could increase soil organic carbon and nitrogen contents, slow down nitrification intensity, therefore maintain the continuous supply of nutrients to ensure crop yield and increase nitrogen utilization rate. Reducing nitrogen input without straw returning shows risk of decreasing crop yields.