Objectives Timely and effective supply of nitrogen can enhance the activity of protective enzymes in maize leaves, delay the senescence of maize leaves and increase the grain yield of maize. This study was conducted to investigate the effects of the combination of controlled - release urea and common urea on the leaf area index (LAI), relative chlorophyll content (SPAD), antioxidant enzyme activity, and yield of maize, thereby providing a theoretical basis and technical support for the high-yield and high-efficiency cultivation of maize.

Methods A randomized block experiment was conducted in Chengyang Township, Pengyang County, Ningxia, during 2017−2018 to evaluate nitrogen application strategies under a uniform nitrogen rate of 225 kg/hm². Four N application treatments were set up as follows: all nitrogen applied as common urea, with 2/3 applied as basal fertilizer + 1/3 topdressed at the small trumpet stage (T1); 2/3 of nitrogen applied as basal fertilizer (half common urea and half controlled-release urea), and 1/3 of nitrogen topdressed as common urea at the small trumpet stage (T2); all nitrogen applied as basal fertilizer, with 1/3 of nitrogen in the form of common urea and 2/3 in the form of controlled-release urea (T3); all nitrogen applied as controlled-release urea with one-time basal application (T4). Additionally, a no nitrogen fertilizer control (CK) was set up. The variation patterns of maize leaf area index (LAI), SPAD value, antioxidant enzyme activity, content of membrane lipid peroxides, content of non-enzymatic protective substances, and grain yield under different treatments were analyzed.

Results Results from the two-year experiment showed that the rational combined application of controlled-release urea and common urea (T3) effectively increased maize’s LAI, SPAD value, and antioxidant enzyme activity, decreased proline (Pro) and malondialdehyde (MDA) contents, and ultimately enhanced grain yield. At the silking stage (R1), in 2017, the LAI of the three controlled-release urea treatments (T2, T3, and T4) was 11.73%, 16.82%, and 8.63% higher than that of T1, respectively. In 2018, the LAI of T2 and T3 was 13.84% and 16.69% higher than T1, and 7.64% and 10.34% higher than T4, respectively. For the SPAD value, in 2017, T2 and T3 showed an average increase of 14.92% and 18.14% compared with T1, and T3 was 10.42% higher than T4. In 2018, T3 was 5.25% higher than T1, and T2 and T3 were 10.67% and 13.03% higher than T4 on average. For superoxide dismutase (SOD) activity, which peaked in maize leaves at R1 in both years, in 2017, T2, T3, and T4 were 3.38%, 5.53%, and 1.82% higher than T1, respectively. T3’s SOD activity was also 2.08% and 3.65% higher than the average of T2 and T4. In 2018, T3 was 6.25%, 5.03%, and 6.53% higher than T1, T2, and T4, respectively. For MDA content, T3 had the lowest MDA content in 2017, with no significant difference from T2 and T4, while in 2018, T3’s MDA content was significantly 18.56%, 18.30%, and 18.98% lower than that of T1, T2, and T4, and there was no significant difference in MDA content among T1, T2, and T4. T3 produced the highest grain yield. In 2017, it was 11.91% and 9.88% higher than T1 and T4, respectively. In 2018, it was 19.26% and 11.65% higher than T1 and T4, respectively. In terms of economic benefits, T3 achieved the highest for maize, which was 21.93% and 15.54% higher than the average benefits of CK and T1 respectively. A comprehensive evaluation using the entropy weight-TOPSIS method showed that T3 had the highest relative closeness coefficient (Ci) in both years, with an average of 0.9615, and the evaluation results from the CRITIC weight method and principal component analysis were highly consistent with this finding.

Conclusions The combined basal application of controlled-release urea and conventional urea can maintain a relatively high leaf area index and relative chlorophyll content in maize during its late growth stages. It enhances the activities of antioxidant enzymes (SOD, POD) and catalase (CAT) in maize leaves during these late stages, reduces the accumulation of membrane lipid peroxides, delays leaf senescence, and thereby improves photosynthetic efficiency, and ultimately increases the yield.