Objectives To study the effects of basal application rates of controlled-release urea (CRU) on post-flowering photosynthetic and grain filling characteristics of spring maize, aiming to provide a theoretical basis for high-yield, high-efficiency, and simplified cultivation of spring maize in the southern mountainous areas of Ningxia.

Methods A randomized block experiment was conducted in Pengyang County, Ningxia from 2017 to 2018. At a uniform nitrogen application rate of N 225 kg/hm², five treatments were set up: no nitrogen fertilizer control (CK); urea N 150 kg/hm²for basal application + 75 kg/hm²for topdressing (T1); urea N 75 kg/hm²for basal application+75 kg/hm²for topdressing + CRU N 75 kg/hm²for basal application (T2); urea N 75 kg/hm² for basal application + CRU N 150 kg/hm²for basal application (T3); and CRU N 225 kg/hm²for basal application (T4). At the silking stage (R1), grain filling stage (R3), and maturity stage (R6) of spring maize, the leaf area index (LAI), chlorophyll content, net photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO₂ concentration (Ci), transpiration rate (Tr), grain dry weight was determined every 10 days after maize flowering, with grain filling parameters calculated accordingly, and the plant samples were collected at the same stages, and grain yield and economic benefits were analyzed at harvesting stage. Additionally, a comprehensive evaluation of each treatment was conducted using the membership function method.

Results Treatment T2, T3, and T4 increased the LAI, chlorophyll content, Pn, Gs, Ci, and Tr of maize at R1 stage, optimized grain filling characteristics, and ultimately improved yield. Over the two years, post-flowering LAI of spring maize showed the order: T3>T2>T4>T1>CK. Compared with T1, the post-flowering (R1) LAI of T2, T3, and T4 increased by an average of 12.80%, 16.75%, and 7.18%, respectively; the LAI of T3 was 3.51% and 8.93% higher than that of T2 and T4 on average. T3 treatment had the highest Pn, which increased by an average of 44.73%, 28.60%, and 38.15% over the two years compared with T1, T2, and T4, respectively. T3 also showed the highest instantaneous water use efficiency (IWUE) and instantaneous carboxylation rate (ICR). Compared with T1, T2, and T4, the IWUE of T3 increased by an average of 19.59%, 19.43%, and 14.36%, respectively; the ICR of T3 increased by an average of 32.45%, 20.82%, and 28.98%, respectively, compared with the same treatments. At 50 days after pollination, the grain dry weight under T2 and T3 was 3.36% and 9.65% higher than that under T1 on average, while T4 showed a 2.68% decrease. Over the two years, compared with T1, the maximum growth at peak grain filling rate (W_max) of T2 and T3 increased by an average of 3.83% and 10.69%, respectively, while T4 decreased by 4.27%. T3 achieved the highest grain yield, reaching 11,968 kg/hm² and 12,741 kg/hm² in the two years, respectively, which was 3.18%−31.32% and 4.62%−29.75% higher than the average of other treatments. T3 also showed the largest increase in economic benefits, with an average increase of 2.47%−22.47% compared with other treatments. Comprehensive evaluation scores of all treatments showed: T3>T2>T4>T1>CK. In other words, the rational combined application of controlled-release urea and conventional urea (T3) could significantly improve the membership function characteristic values and comprehensive scores, effectively enhancing the photosynthetic performance of spring maize leaves and grain yield.

Conclusions The treatment of 75 kg/hm² conventional urea as basal application + 150 kg/hm² controlled-release urea resulted in the highest leaf source quantity and quality (LAI, chlorophyll content, and photosynthetic parameters) of spring maize, with the most active grain filling, the highest grain yield and economic benefits, making it the optimal treatment under the conditions of this experiment.