Maize Genomics and Genetics 2025, Vol.16, No.1, 20-33 http://cropscipublisher.com/index.php/mgg 23 Furthermore, long-term studies have indicated that the agronomic optimum plant density (AOPD) has increased over the years, contributing to yield gains. For instance, the AOPD has increased at a rate of 700 plants/ha/yr, with higher rates of increase observed in very high yielding environments compared to low yielding ones (Assefa et al., 2018). This suggests that breeding efforts and agronomic practices have progressively optimized planting densities to enhance yield potential. However, it is crucial to consider regional variations and cultivar-specific responses to planting density to achieve the best outcomes in maize production. 4 Nitrogen Management and its Role in High Maize Yield 4.1 Importance of nitrogen in maize growth and grain formation Nitrogen (N) is a critical nutrient for maize growth, playing a vital role in various physiological and biochemical processes. It is a key component of chlorophyll, which is essential for photosynthesis, and is also involved in the synthesis of amino acids, proteins, and nucleic acids. Adequate nitrogen availability enhances the vegetative growth of maize, leading to increased leaf area and higher photosynthetic rates, which are crucial for biomass accumulation and grain yield (Asibi et al., 2019; Wang et al., 2021). Moreover, nitrogen is essential for the development of reproductive structures, influencing kernel number and grain filling, which directly impacts the final grain yield (Su et al., 2020; Deng et al., 2023). However, the efficiency of nitrogen use in maize is often low, with less than half of the applied nitrogen being recovered by the crop. This inefficiency is due to various factors, including the timing and method of nitrogen application, soil properties, and environmental conditions. Understanding the mechanisms of nitrogen uptake, assimilation, and remobilization during different growth stages is crucial for optimizing nitrogen use efficiency (NUE) and achieving high maize yields (Asibi et al., 2019; Wang et al., 2021). 4.2 Nitrogen application methods and split-application effects The method and timing of nitrogen application significantly influence maize yield and NUE. Traditional practices often involve a single application of nitrogen at sowing, which can lead to low NUE and environmental risks due to nitrogen losses through leaching and volatilization. In contrast, split-application methods, where nitrogen is applied in multiple doses throughout the growing season, have been shown to improve NUE and grain yield (Abbasi et al., 2013; Deng et al., 2023). Split applications can be tailored to match the nitrogen demand of maize at different growth stages. For instance, applying nitrogen at sowing and then at critical stages such as V6 (six-leaf stage), V12 (twelve-leaf stage), and R1 (silking stage) can enhance nitrogen availability during periods of high demand. This approach has been demonstrated to increase photosynthetic efficiency, promote kernel development, and improve grain filling, leading to higher yields compared to single applications (Figure 2) (Davies et al., 2020; Deng et al., 2023). Additionally, split applications can reduce nitrogen losses and environmental impacts, making them a more sustainable practice (Abbasi et al., 2013; Quan et al., 2021; Dong and Li, 2024). 4.3 Environmental impact of nitrogen overuse and optimization strategies Excessive nitrogen application in maize production can lead to significant environmental issues, including soil acidification, water eutrophication, and greenhouse gas emissions. Overuse of nitrogen fertilizers results in nitrogen leaching into groundwater and runoff into surface waters, causing pollution and contributing to the formation of hypoxic zones in aquatic ecosystems. Additionally, the volatilization of nitrogen as ammonia and the emission of nitrous oxide, a potent greenhouse gas, contribute to air pollution and climate change (Quan et al., 2021; Zhang et al., 2023). To mitigate these environmental impacts, optimizing nitrogen management is essential. Strategies such as reducing the overall nitrogen application rate, using slow-release fertilizers, and incorporating nitrification inhibitors can enhance NUE and reduce nitrogen losses. Long-term field studies have shown that optimized nitrogen management practices, including the use of lower nitrogen rates combined with advanced agronomic practices, can maintain high maize yields while significantly reducing environmental and health impacts (Yan et al., 2021; Zhang et al., 2023). Implementing these strategies can promote sustainable maize production and minimize the ecological footprint of nitrogen fertilization (Quan et al., 2021; Zhang et al., 2023).
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