Objectives Unreasonable fertilization in maize (Zea mays L.) production in the mountainous regions of southern Ningxia will cause imbalance between the N supply and demand. We studied the combination ratio and application method of conventional urea and controlled release urea, in order to propose an efficient and economically suitable nitrogen management strategy for maize production.

Methods In this research, spring maize cultivar ‘Xianyu 698’ was used as the experimental materials, the controlled release urea (CRU) and conventional urea (urea) were applied in different proportions and growing stages of maize, at the total N input rate of 225 kg/hm². The five treatments were: T1, no fertilization; T2, urea, 2/3 base applied and 1/3 top-dressed; T3, 2/3 N (half CRU and half urea) basal applied, and 1/3 N in urea top-dressed; T4, total N basal applied in one time (2/3 CRU + 1/3 urea); T5, total N basal applied in one time (100% CRU). The soil N contents and some enzyme activities, and the maize plant N contents and yield were measured.

Results In grain filling period of maize, the NH₄⁺-N contents in 0–20 and 20–40 cm soil layers of T3, T4 and T5 were all significantly higher than those of T2. In the 0–20 cm soil layer, the NO₃^–-N content of T2 was higher than those of T3, T4 and T5, respectively. In the maize filling period, those of T3, T4 and T5 were higher than that of T2, respectively. In the 20–40 cm soil layer, the soil NO₃^–-N content of T2 was higher than those of T3, T4 and T5, respectively. Those of T3, T4 and T5 were higher than that of T2 in maize ripening period. The difference among treatments in the same period reached a significant level (P < 0.05). In the jointing stage of maize, the soil inorganic nitrogen content of T2 in 0–20 and 20–40 cm soil layers was higher than those of T3, T4 and T5, respectively. In the mature stage of maize, the soil inorganic nitrogen contents of T3, T4 and T5 in 0–20 and 20–40 cm soil layers were higher than those of T2, respectively. The difference among treatments in the same period reached a significant level (P < 0.05). At the jointing stage of maize, the urease activity of T2 in 0–20 cm soil layer was 45.8% and 54.7% higher than those of T4 and T5. At the same time, the soil urease activity of T2 in 20–40 cm soil layer was 38.2% and 76.9% higher than those of T4 and T5. During the grain filling stage of maize, the urease activity of T5 in the 20–40 cm soil layer was increased by 15.6% compared with T2. With the increase of the proportion of controlled release urea, the later the activity of soil alkaline phosphatase reached peak. Catalase activity did not change significantly in controlled release urea treatment. The average yield of T4 in the two-year experiment was 6.89% higher than that of T2, and the relative economic benefit was 6.67% higher than that of T2. Principal component analysis of soil related indicators and yields showed that nitrate nitrogen, inorganic nitrogen contributed to the yield most.

Conclusion Comprehensively, compared with conventional urea, conventional urea combined with controlled release urea has more positive effects on the content of ammonium nitrogen, nitrate nitrogen and inorganic nitrogen in the plough layer soil, coordinates the activities of soil urease and alkaline phosphatase, and improves the nitrogen needed for the soil to sustain and stably supply the growth and development of maize. Conventional urea/controlled release urea is applied in the ratio of 1∶2 in the circumstances of equal nitrogen fertilization, which can fully exert the advantages of the nutrient release characteristics and the crop nutrient absorption, achieve the balance between the level of nitrogen supply in the soil and the nitrogen requirement of crops. At the same time, this pattern could achieve the goal of high yield and high efficiency cultivation of spring maize.