Objectives The study aimed to investigate the effects of basal application rates of controlled-release urea (CRU) on the post-flowering photosynthetic performance and grain filling characteristics of spring maize, thereby providing a theoretical basis for high-yield, high-efficiency, and simplified fertilization practices 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 225 kg/hm², four fertilization treatments were set up: no nitrogen fertilizer control (CK); conventional urea N 150 kg/hm² for basal application + 75 kg/hm² for topdressing (T1); conventional urea N 75 kg/hm² for basal application+75 kg/hm² for topdressing + CRU N 75 kg/hm² for basal application (T2); conventional urea N 75 kg/hm² for basal application + CRU N 150 kg/hm² for basal application (T3); and CRU N 225 kg/hm² for basal application (T4). The treatment without urea application serves as the control (CK). 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 were 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 Across the two study years, the LAI at the R1 stage showed a consistent trend of T3>T2>T4>T1>CK, with T3 significantly increasing LAI by 16.82% and 7.53% compared with T1 and T4 in 2017, and by 16.69% and 10.34% compared with T1 and T4 in 2018. For Pn at the R1 stage across the two years, T3 significantly increased this parameter by 30.51%−89.80% (in 2017) and 26.69%−74.55% (in 2018) compared with other treatments, while the instantaneous carboxylation rate (ICR) under T3 was the highest. Specifically, in 2017, T3 significantly increased ICR by 21.26%, 28.62%, and 34.10% compared with T2, T4, and T1, respectively, and by 20.37%, 29.33%, and 30.79% in 2018. At 50 days after pollination (DAP), the 100-grain dry weight of maize under T3 was the highest, with significant increases of 6.60%−16.38% (in 2017) and 5.60%−21.24% (in 2018) compared with other treatments. Regarding the maximum dry matter accumulation (Wmax), T3 significantly increased this index by 8.35%, 6.00%, and 11.89% compared with T1, T2, and T4 in 2017, and by 13.30%, 7.16%, and 18.12% in 2018. Across the two years, the grain yield of maize showed a consistent overall trend of T3>T2>T4>T1>CK. In 2017, T3 significantly increased grain yield by 11.91% and 9.88% compared with T1 and T4, respectively, while in 2018, T3 significantly increased grain yield by 19.26% and 11.65% compared with T1 and T4 (with no significant difference from T2). Additionally, T3 exhibited the highest increase rate in economic benefits, being 12.07, 2.95, and 16.57 percentage points higher than T1, T2, and T4 in 2017, and 20.36, 5.00, and 18.19 percentage points higher in 2018. The comprehensive evaluation scores of all treatments followed the order of T3>T2>T4>T1>CK, leading to the conclusion that the rational combined application of controlled-release urea and conventional urea (T3) could significantly increase the membership function eigenvalues and comprehensive score, as well as effectively improve the leaf photosynthetic performance and grain yield of spring maize.

Conclusions Basal application of 75 kg/hm² conventional urea combined with 150 kg/hm² controlled-release urea enhanced leaf source quantity and quality (LAI, chlorophyll content, and photosynthetic activity), accelerated grain filling, increased the duration of the maximum filling rate, and ultimately improved grain yield and economic benefits. Therefore, T3 is recommended as the optimal nitrogen management strategy for achieving simplified fertilization and yield improvement in spring maize production in the southern mountainous area of Ningxia.