In the field, photosynthetically active radiation (PAR) is the main source of photosynthesis for crops, which is crucial for crop growth (Fan et al., 2023). Leaves are the main part in intercepting PAR and performing photosynthesis (Drouet and Bonhomme, 2004). Leaf area formation and senescence affect PAR capture and vertical distribution (Usandivaras et al., 2018). This study revealed a notable trend in LAI, which initially increased and then reached a stable level with increasing nitrogen application. These findings suggest that excessive nitrogen application may not further enhance LAI and may even have adverse effects. Plow tillage with straw return (SP) significantly increased the LAI at the same nitrogen application level compared with no tillage with straw mulch (SM), especially in stage R3 (Table 2, Fig. 6). Maintaining a high LAI during the growth period can intercept more PAR (Plénet and Pellerin, 2000; Liu et al., 2017; Hou et al., 2019). In the present study, we found that SP LAI was significantly higher at stage R3 than SM LAI, especially in the upper and middle leaves, which of the canopy are the main nutrient organs that promote grain development (Yin and Struik, 2015; Yin et al., 2019). Plants can reasonably adjust the canopy structure through various environmental factors to maximize canopy photosynthesis (He et al., 2023). Due to the higher leaf area of the middle and upper layers of the canopy in SP during stage R3, more PAR was retained in the later stage of growth. In addition, it was observed that the radiation interception rate under SP conditions with N150 could be maintained at levels comparable to or even higher than those under SM with N210 (Table 2). On the basis of these findings, it can be concluded that SP combined with nitrogen application is an effective method for improving radiation interception and reducing light loss.
Furthermore, high PAR means higher radiation use efficiency, which can produce more dry matter (Monteith, 1977; Yu et al., 2022). The post–flowering dry matter accumulation is crucial for high maize grain yield, as grain dry matter mainly comes from post–flowering photosynthetic products (He et al., 2004; Liu et al., 2023). In this study, the post–flowering dry matter accumulation was 8.87% − 69.26% higher in ST than in SM (Fig. 3), which was probably because the dry matter accumulated by photosynthesis in the canopy in the late growth period provided sufficient assimilates for grain dry matter (Liu et al., 2023). As nitrogen application increased, there was an initial increase followed by stabilization in the proportion of stem dry weight and grain (Table 2, Fig. 3). This phenomenon can be attributed to the dependence of nitrogen absorption on a continuous supply of carbohydrates from the aboveground parts to the roots. Additionally, prolonged persistence of the green leaves during the later stages of plant growth promotes nitrogen uptake at post–flowering, thus enhancing the photosynthetic activity of the canopy and increasing the proportion of grain dry matter weight (Winterhalter et al., 2011; Liu et al., 2023). In this study, when the NUE was moderate, the relative accumulation of dry matter and nitrogen in the grains reached a maximum (Fig. 3, Fig. 4 and Fig. 5), indicating that low or extremely high NUE was not conducive to the redistribution of dry matter or nitrogen from nutrient organs to grains. The higher nitrogen content of grain in SP than in SM can also be explained (Fig. 4). Under the same nitrogen supply, SP can delay leaf senescence, increase leaf area, extend photosynthesis duration, and improve canopy photosynthetic capacity. This leads to an enhanced proportion of dry matter and nitrogen to grains during the post–flowering period. Additionally, at different nitrogen applications, the proportion of grain dry matter and nitrogen in SP was higher than that in SM (Fig. 3), indicating that SP was conducive to the transport of dry matter or nitrogen from nutrient organs to grain.
The NUE of maize was determined by coordinating the N uptake efficiency (NUpE) and the N utilization efficiency (NUtE) (Duan et al., 2023). In the present study, we found that the NUE and NUpE of SP were significantly higher than those of SM at the same nitrogen application level (Fig. 6). This can be explained by the fact that SP maintains a higher LAI during the reproductive stage, allowing it to intercept more PAR and absorb more nitrogen from the soil. Additionally, the root nitrogen uptake capacity of SP is superior to that of SM, especially under low nitrogen conditions (Mi et al., 2016; Ajala et al., 2018). The high NUpE means that more nitrogen necessary for grain growth can be absorbed from the soil, reducing the need for N redistribution. This led to the retention of more nitrogen in the leaves (Boomsma et al., 2009; Li et al., 2019), thus prolonging the useful life of the leaves. Therefore, we believe that the nitrogen proportion plays an important role in regulating NUpE under different straw return methods. SP enhanced the relative N content of the grains, resulting in improved NUpE and NUE.
In this study, the changes in the number of grains in spikes were smaller than the changes in 1000-grain weight under the two straw returning methods, and the interaction effects of straw returning methods and nitrogen application had no effect on the number of grains per ear (Table 1). The proportion of dry weight and nitrogen partitioning of grain was significantly higher in SP than in SM. Therefore, the 1000-grain weight of SP was on average 10.35% − 16.04% higher than that of SM, resulting in the mean yield of SP being 11.66% − 24.34% higher than that of SM when N was below N150. We believe that this phenomenon may be the result of coordinated interactions between aboveground and belowground portions. The aboveground portion, by prolonging the green leaf area during the reproductive period, produces more assimilates through photosynthesis to provide energy for nitrogen uptake by the belowground portion. At the same time, the efficient nitrogen uptake capacity of the belowground portion provides sufficient nitrogen for the growth of the aboveground portion (Mi et al., 2016), which achieves high yield and efficiency between maize yield and nitrogen fertilizer. Furthermore, previous studies showed that no tillage increased soil organic carbon compared to plow tillage (Ajala et al., 2018; Kan et al., 2020). This explains why there were no significant differences in yield between SM and SP at high nitrogen application levels, while SP had a significantly higher yield than SM at middle nitrogen application levels (Table 1). We believe that the carbon nitrogen ratio in the soil of SM is too high at the middle nitrogen application rate, resulting in slow decomposition and mineralization of microbes, which require the consumption of available nitrogen in the soil (Bastida, 2021), leading to competition between crops and microorganisms. Reducing the carbon nitrogen ratio of soil is beneficial at higher nitrogen application rates for microbial growth and nitrogen release (Shen et al., 2021), promoting plant uptake of nitrogen nutrition. This result provides a promising direction for the future development of conservation agriculture.