Objectives We studied the effects of N nutrition on summer maize across the growth period under one-time basal application and 30% reduction of N input of the two slow-release urea products.

Methods The release properties of urea coated with waterborne polymers (WF) and polyethylene terephthalate (PF) were studied by field release method, and the theoretical optimal blended ratio of the two coating urea products was calculated. Field experiments were carried out from 2018 to 2020 in Qingzhou and Huantai, Shandong Province. Conventional N rate (N225 kg/hm², level 1) and 30% reduction (N157.5 kg/hm², level 2) were setup, and ordinary urea (BF), WF, PF, and WF to PF in 1∶0.52 (MF) were used as N resource treatments under each N level, with no N fertilizer as control (CK). Common urea BF1 and BF2 treatment were applied twice (basal and topdressing), while the coated urea products were all basal-applied once. Soil N contents in the key growing stages of maize were determined. The yields, N uptake and urea use efficiency of summer maize were investigated at harvest.

Results The total N release rates of WF (>90%) was higher than that of PF (80%) during the whole maize growing period in the two sites, and the release characteristics of WF was more consistent with the nitrogen demand of summer maize at the early growth stage. In Qingzhou and Huantai, the average maize yields of three seasons among the fertilizer treatments, except BF2, were not significantly different. The maize yield in BF2 was significantly lower than that in BF1, and only WF2 produced higher (P<0.05) yield than BF2. Compared with BF1, all the six coated urea treatments increased the average apparent nitrogen utilization of three seasons in the two experimental sites. WF2 and PF2 in Qingzhou increased the average agronomic efficiency by 49.0% and 32.5%, while only WF2 in Huantai increased that by 40.9%. At tasseling and maturing stages of maize in Qingzhou, the soil NH₄⁺-N contents in WF1, WF2, PF1 and PF2 treatments were significantly higher than that in BF1; the soil NO₃⁻-N contents in WF1 and PF1 were higher than BF1, and those in WF2 and PF2 were similar with BF1. In Huantai, the soil NH₄⁺-N contents were similar among all the treatments at tasseling and maturing, while the soil NO₃⁻-N contents in the PF and WF treatments were significantly higher than in BF1.

Conclusions Compared with resin coated urea, the new waterborne-polymer coated urea has higher N release rate, and more consistent release characteristics with the N requirements of summer maize. Under the condition of reducing 30% conventional N input and once basal application, both kinds of the coated urea can provide higher N supply at the tasseling and maturing stage of maize, thereby maintaining a stable yield and N uptake of summer maize, and improving the nitrogen use efficiency and agronomic efficiency. The new waterborne-polymer coated urea showed more stable effect on yield and agronomic efficiency than polyethylene terephthalate resin coated urea.