1 Introduction

Maize (Zea mays), also known as corn, is one of the most significant staple crops globally, playing a crucial role in human nutrition, animal feed, and industrial applications. It is cultivated extensively across various regions, including the United States, China, and Brazil, which are the top maize-producing countries (Ranum et al., 2014). Maize is a vital source of essential nutrients such as starch, protein, and fats, contributing significantly to the energy intake of populations, especially in regions like sub-Saharan Africa, Southeast Asia, and Latin America, where it serves as a primary food source (Nuss and Tanumihardjo, 2010). The crop's versatility extends beyond food, as it is also used in the production of ethanol, sweeteners, and other industrial products, making it economically valuable (Ranum et al., 2014).

Optimizing planting density and fertilization practices is critical for enhancing maize yield and quality. High-density planting has been shown to increase maize yield by improving resource utilization, although it can also trigger shade avoidance responses that may negatively impact plant architecture and stability (Jafari et al., 2023). Fertilization, particularly nitrogen application, is another essential factor influencing maize productivity. Various studies have demonstrated that different fertilization strategies, including the use of organic, bio, and mineral fertilizers, can significantly affect the nutrient composition and yield of maize (Dragičević et al., 2022; Alves et al., 2023). For instance, deep nitrogen fertilization combined with optimized planting patterns, such as zigzag planting, has been found to enhance root distribution, canopy structure, and overall yield (Zheng et al., 2023). Additionally, the integration of drip irrigation and fertigation methods has shown promise in stabilizing and increasing maize yields, particularly in regions with variable rainfall patterns (Żarski and Kuśmierek-Tomaszewska, 2023).

This study explores and synthesizes current research on optimizing planting density and fertilization strategies for high-yield maize, examining various methods and their impacts on maize growth, nutrient composition, and yield, providing insights into effective agronomic practices for sustainably enhancing productivity, while delving into genetic and agronomic advancements that support high-density planting tolerance and efficient nutrient management, ultimately aiming to improve food security and economic stability in regions reliant on maize.

2 Growth and Nutrient Requirements of Maize

2.1 Key nutrient needs (nitrogen, phosphorus, potassium) at different growth stages

Maize requires a balanced supply of nitrogen (N), phosphorus (P), and potassium (K) for optimal growth and development. Nitrogen is crucial for vegetative growth, particularly in the early stages, as it supports leaf and stem development. Phosphorus is essential for energy transfer and root development, while potassium plays a significant role in water regulation and enzyme activation.

During the vegetative growth period, maize plants exhibit high nitrogen absorption, which is critical for the formation of chlorophyll and amino acids. Phosphorus and potassium uptake are also significant during this stage, contributing to root establishment and overall plant vigor. As the plant transitions to the reproductive stage, the demand for phosphorus and potassium increases to support flowering, grain filling, and maturation processes (Liu et al., 2019; 2022).

In high-density planting scenarios, the application of nitrogen at optimal levels (e.g., N200) combined with plant growth regulators has been shown to enhance nutrient uptake and translocation, leading to improved grain yield and quality. This approach maintains high enzymatic activities in leaves, which are vital for photosynthesis and nutrient assimilation (Liu et al., 2019; Huang, 2024).

2.2 Role of micronutrients in maize health and stress tolerance (e.g., Zinc, Boron)

Micronutrients such as zinc (Zn) and boron (B) play critical roles in maize health and stress tolerance. Zinc is involved in various physiological functions, including protein synthesis, gene expression, and enzyme activation. It also contributes to the structural integrity of cell membranes and the synthesis of auxins, which are essential for growth regulation (Saboor et al., 2021; Suganya et al., 2021).

Zinc deficiency can lead to stunted growth, delayed maturity, and reduced grain quality. The use of arbuscular mycorrhizal fungi (AMF) has been shown to enhance zinc uptake and mitigate zinc-induced stress, thereby improving maize growth and yield. AMF symbiosis helps in balancing zinc levels within the plant, ensuring optimal growth conditions even in zinc-deficient soils (Saboor et al., 2021; Ahmad et al., 2023).

Boron, another essential micronutrient, is crucial for cell wall formation and reproductive development. It aids in pollen tube growth and seed set, which are vital for successful fertilization and grain production. Adequate boron levels help maize plants withstand environmental stresses, such as drought and high salinity, by maintaining cellular integrity and metabolic functions (Gaikpa et al., 2022; Martins et al., 2023).

2.3 Impact of plant density on nutrient uptake and growth

Plant density significantly influences nutrient uptake and growth in maize. High-density planting can lead to increased competition for nutrients, water, and light, potentially reducing individual plant growth and overall yield. However, with proper nutrient management, high-density planting can be optimized to enhance productivity.

Studies have shown that the application of nitrogen and plant growth regulators in high-density planting conditions can improve root vitality and nutrient absorption. This approach enhances the concentration and delivery rates of amino acids and mineral nutrients in root-bleeding sap, leading to better nutrient translocation and higher grain yield (Liu et al., 2019).

3 Impact of Planting Density on Maize Yield

3.1 Effects of planting density on photosynthesis and competition among plants

Planting density significantly influences photosynthetic parameters and the competition among maize plants. Studies have shown that increasing planting density can enhance the leaf area index (LAI) and the amount of intercepted photosynthetically active radiation (IPAR), which promotes plant growth and crop productivity. However, this increase in density can also reduce the net photosynthetic rate (Pn), stomatal conductance (Gc), and leaf chlorophyll content, leading to a decline in crop productivity and yield stability at very high densities (Zhang et al., 2021). Additionally, while higher planting densities can increase canopy apparent photosynthesis (CAP) and biomass, they may also lead to greater competition for light, water, and nutrients, which can negatively impact yield if not managed properly (Wei et al., 2019).

Moreover, the relationship between planting density and photosynthesis is complex. For instance, while medium planting densities (e.g., 105 000 plants/ha) have been shown to significantly increase grain yield by up to 20.32% compared to lower densities, further increases in density do not necessarily lead to additional yield benefits and may even result in yield reductions for some cultivars (Yan et al., 2019). This suggests that there is an optimal planting density that maximizes photosynthetic efficiency and yield without causing excessive competition among plants.

3.2 Advantages and disadvantages of high-density planting and its impact on yield

High-density planting has several advantages, including increased IPAR and improved resource use efficiency, which can lead to higher grain yields under optimal conditions. For example, a study found that increasing planting density combined with a reduced nitrogen rate enhanced nitrogen partial factor productivity (NPFP) by 24.7% and maize grain yield by 6.6% compared to conventional practices (Figure 1) (Du et al., 2021). This indicates that high-density planting, when managed with appropriate nutrient inputs, can be beneficial for achieving high yields.

Figure 1 N uptake, N harvest index and N partial factor productivity of maize (NPFP) under different planting treatments in 2017 and 2018 (Adopted from Du et al., 2021) Image caption: Error bars indicate standard errors of replicates. Means followed by the same letter are not significantly different among different planting treatments at P < 0.05 (Adopted from Du et al., 2021)

However, high-density planting also has its disadvantages. It can trigger a shade avoidance response, leading to increased plant height and ear height, which can result in lodging and yield loss (Jafari et al., 2021). Additionally, excessive planting density can exacerbate competition for resources, reducing photosynthetic capacity and yield stability. For instance, in semiarid environments, pursuing too high planting density is not advisable as it can lead to reduced precipitation use efficiency (PUE), radiation use efficiency (RUE), and nitrogen use efficiency (NUE) (Zhang et al., 2021). Therefore, while high-density planting can be advantageous, it requires careful management to avoid negative impacts on yield.

3.3 Regional variations and cultivar adaptability to optimal planting density

The optimal planting density for maize can vary significantly depending on regional conditions and the specific cultivars used. In semiarid regions, moderate planting densities are recommended to stabilize grain yield and ensure sustainable agricultural practices. Density-tolerant cultivars, such as Zhengdan958 and Xianyu335, have shown better canopy structure, photosynthetic capacity, and yield stability under varying planting densities (Zhang et al., 2021). This highlights the importance of selecting cultivars that are well-adapted to the specific environmental conditions of the region.

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^-1 yr^-1, 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 (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).

5 Rational Application of Phosphorus and Potassium

5.1 Supportive role of phosphorus in root development and early growth

Phosphorus (P) plays a crucial role in the early stages of maize growth by enhancing root development and overall plant vigor. Studies have shown that optimal P fertilization significantly increases root weight, root number, and biomass, which are essential for nutrient uptake and plant stability. For instance, a field experiment demonstrated that positioning P fertilizer closer to the root zone (5 cm) improved root growth and maize yield compared to a 10 cm distance, highlighting the importance of strategic P placement (Wang et al., 2023). Additionally, P application has been found to maximize the leaf area index (LAI) and photosynthetic rate, which are critical for achieving high grain yields in summer maize (Zhang et al., 2018).

Moreover, the synchronization of P application methods and rates can further enhance root development. Research indicates that foliar application of P at critical growth stages, such as knee height and tasseling, significantly boosts grain yield and phosphorus use efficiency (PUE) in maize (Rafiullah et al., 2020). This method ensures that P is readily available during key developmental phases, thereby supporting robust root growth and early plant development.

5.2 Contribution of potassium in enhancing stress resistance and improving yield quality

Potassium (K) is vital for improving maize's stress resistance and yield quality, particularly under challenging environmental conditions. K management has been shown to enhance growth and yield components of maize, especially under moisture stress conditions. Field experiments revealed that foliar K application at rates of 1%~3% during the vegetative stage resulted in better growth and higher yields compared to late-stage applications (Amanullah et al., 2016). This suggests that timely K application can significantly mitigate the adverse effects of moisture stress.

Furthermore, the combined use of organic amendments like biochar with inorganic K fertilizers has been found to improve maize's defensive systems and nutrient uptake. This combination not only enhances yield quantity and quality but also increases the plant's resilience to drought conditions by improving nitrogen use efficiency and reducing oxidative stress (El-Syed et al., 2023). These findings underscore the importance of K in bolstering maize's ability to withstand environmental stresses and produce high-quality yields.

5.3 Best Application Rates and Timing for Phosphorus and Potassium Fertilizers

Determining the optimal application rates and timing for P and K fertilizers is essential for maximizing maize productivity. Research indicates that a P application rate of 90 kg/ha~135 kg/ha is optimal for maize grown in saline-alkali soils, as it maximizes grain yield and nutrient uptake without causing excessive nutrient accumulation (Ma et al., 2023). Additionally, localized application of P combined with ammonium has been shown to significantly improve nutrient uptake and plant growth by stimulating root proliferation and rhizosphere acidification (Jing et al., 2010).

For K, studies suggest that soil application of K at a rate of up to 90 kg/ha in two equal splits (50% at sowing and 50% at knee height) is effective in improving maize growth and productivity under semiarid climates (Amanullah et al., 2016). Early foliar application of K during the vegetative stage is also recommended to enhance growth and yield under moisture stress conditions. These strategies ensure that P and K are available to the plants at critical growth stages, thereby optimizing nutrient use efficiency and crop performance.

6 Interaction between Planting Density and Fertilization

6.1 Influence of planting density on nutrient competition and fertilization needs

High planting density in maize cultivation can significantly influence nutrient competition among plants. As plant density increases, the competition for essential nutrients such as nitrogen (N), phosphorus (P), and potassium (K) intensifies, which can negatively impact plant growth and productivity. For instance, high-density planting has been shown to aggravate competition among maize plants, leading to reduced nutrient uptake and lower grain yields (Liu et al., 2016). Additionally, nutrient spatial heterogeneity can exacerbate root competition, further decreasing maize yield at high planting densities (Li et al., 2018).

Moreover, the form of nitrogen supplied can also affect nutrient uptake efficiency under different planting densities. A mixed supply of nitrate (NO₃^-) and ammonium (NH₄⁺) has been found to improve plant growth and nutrient uptake efficiency, particularly under high planting densities. This mixed nitrogen form increases energy use efficiency, thereby enhancing biomass production and nutrient absorption (Figure 3) (Wang et al., 2019). Therefore, understanding the interaction between planting density and nutrient competition is crucial for optimizing fertilization strategies in high-density maize cultivation.

6.2 Optimization Strategies for Appropriate Fertilization at High Planting Densities

To optimize fertilization at high planting densities, it is essential to adjust the type and timing of fertilizer application. Slow-released fertilizers (SF) have been shown to be more effective than conventional fertilizers (CF) in high-density maize planting. SF can increase post-silking dry matter accumulation and promote nutrient uptake, resulting in higher grain yields and nutrient use efficiencies (Li et al., 2021). Additionally, the application of plant growth regulators in combination with nitrogen fertilization can enhance nutrient absorption and translocation, further improving grain yield and quality in high-density planting (Liu et al., 2016).

Another effective strategy is to reduce the nitrogen application rate while increasing planting density. Studies have demonstrated that a 30% increase in planting density combined with a 15% reduction in nitrogen rate can enhance nitrogen partial factor productivity and maize grain yield (Du et al., 2021). This approach not only improves yield but also promotes sustainable agricultural practices by reducing excessive nitrogen use. Therefore, optimizing fertilization strategies at high planting densities involves a combination of slow-released fertilizers, plant growth regulators, and adjusted nitrogen application rates.

6.3 Adjustments in Fertilization Techniques Based on Density Conditions

Adjusting fertilization techniques based on planting density conditions is crucial for achieving high maize yields and efficient nutrient use. At lower planting densities, a higher nitrogen application rate may be required to maximize yield. For example, under a planting density of 7.5 plants m², the recommended nitrogen application rate is 340 kg/ha, with a distribution ratio of 61.2% before silking and 38.8% after silking (Zhai et al., 2022). In contrast, at higher planting densities, the nitrogen application rate should be increased to 380 kg/ha, with a distribution ratio of 65.8% before flowering and 34.2% after flowering (Zhai et al., 2022).

Furthermore, the timing of nitrogen application is critical. Sidedress nitrogen fertilization at different growth stages can significantly impact nitrogen uptake and grain yield. Higher plant densities have been shown to increase pre-silking nitrogen uptake, which is crucial for achieving high yields (Ciampitti and Vyn, 2011). Therefore, adjusting the timing and rate of nitrogen application based on planting density can optimize nutrient use efficiency and improve maize productivity.

7 Application of New Fertilization Technologies

7.1 Effects of controlled-release and slow-release fertilizers in maize cultivation

Controlled-release and slow-release fertilizers have shown significant promise in enhancing maize yield and nutrient use efficiency. A study conducted over two years demonstrated that the application of slow-release fertilizers (SF) at an optimal planting density of 7.5 plants per square meter resulted in the highest grain yields and nutrient use efficiencies for nitrogen (N), phosphorus (P), and potassium (K). This method increased post-silking dry matter accumulation and promoted nutrient uptake at both pre- and post-silking stages, leading to higher grain nutrient concentrations and improved nutrient use efficiencies compared to conventional fertilizers (CF) (Li et al., 2021). Additionally, the long-term application of controlled-release urea (CRU) mixed with conventional urea fertilizer (CUF) significantly improved soil aggregate stability, humic acid content, and maize nitrogen uptake, resulting in a 9.4%~14.0% increase in average yield over three growing seasons (Gao et al., 2021).

Moreover, the combination of controlled-release urea with optimal irrigation practices has been shown to further enhance maize growth and yield. Under adequate water conditions, the application of CRU at 210 kg N ha^-1 was sufficient to meet maize growth requirements, while under mild water stress, increasing the CRU rate to 315 kg N ha^-1 effectively alleviated the adverse effects of drought by improving photosynthetic performance and delaying leaf senescence (Li et al., 2020). These findings highlight the potential of controlled-release and slow-release fertilizers to optimize nutrient management and improve maize productivity.

7.2 Foliar application as a supplementary method during critical growth stages

Foliar application of fertilizers has emerged as an effective supplementary method to enhance maize growth and yield, particularly during critical growth stages. Foliar-applied mixed mineral fertilizers and organic biostimulants have been shown to significantly improve root growth, seed yield, and quality in maize. For instance, treatments with NPK+hydrolyzed animal epithelium and micronutrients (NPK+Hae+micro) and PK+Ascophyllum nodosum extracts (PK+An) increased root dry biomass and volumetric root length density, leading to a higher number of commercial seeds produced and reduced seed disposal rates (Boscaro et al., 2023).

Furthermore, the foliar application of molybdenum (Mo) has been found to enhance photosynthetic metabolism and grain yields in maize. Mo application increased leaf nitrate reductase activity, nitrogen and protein content, Rubisco activity, net photosynthesis, and grain yield, indicating its efficiency in improving nitrogen metabolism and carbon fixation (Oliveira et al., 2022). Similarly, the combined foliar application of iron (Fe) and Mo with nitrogen fertilization improved the shade tolerance of soybean in maize-soybean intercropping systems by enhancing chlorophyll content, photosynthetic activities, and associated enzymatic activities, ultimately leading to better growth and yield (Nasar et al., 2022). These studies underscore the benefits of foliar fertilization as a supplementary strategy to boost maize productivity.

7.3 Promotion of fertigation (combined fertilization and irrigation) and its advantages in high-density planting

Fertigation, the combined application of fertilizers and irrigation, offers several advantages in high-density planting of maize. This method ensures the efficient delivery of nutrients directly to the root zone, enhancing nutrient uptake and utilization. A study on the interaction of irrigation management and nitrogen fertilization revealed that controlled-release urea (CRU) combined with optimal irrigation significantly improved maize yield, nitrogen uptake, and growth. Under conventional irrigation, CRU application at 210 and 315 kg N ha^-1 resulted in similar yields, which were significantly higher than those obtained with common urea. In areas with mild water stress, increasing the CRU rate to 315 kg N ha^-1 further improved yield by counteracting the adverse effects of drought (Li et al., 2020).

Additionally, the application of nitrogen fertilizer and plant growth regulators in high-density planting has been shown to enhance root-bleeding sap rate, nutrient absorption, and grain yield. The combination of nitrogen application at 200 kg/ha with chemical control improved the delivery rates of amino acids and mineral nutrients in root-bleeding sap, leading to higher phosphorus and potassium uptake and translocation. This treatment also maintained high levels of key enzymatic activities in leaves and grains, ultimately increasing maize yield and quality (Liu et al., 2022). These findings highlight the potential of fertigation to optimize nutrient and water management, thereby improving maize productivity in high-density planting systems.

8 Environmental and Economic Considerations

8.1 Potential environmental impacts of over-fertilization and sustainable management practices

Over-fertilization in maize cultivation has significant environmental repercussions, including soil degradation, water pollution, and increased greenhouse gas emissions. Excessive nitrogen (N) fertilization, in particular, has been linked to severe pollution issues, such as nitrate leaching into groundwater and the emission of nitrous oxide, a potent greenhouse gas (Li et al., 2020; Nasar et al., 2021). Studies have shown that the fertilization phase has the most detrimental influence on ecosystems, followed by the harvesting period (Kumar et al., 2022). Sustainable management practices, such as optimizing N application rates and employing N-transformation inhibitors, have been developed to mitigate these impacts. These practices not only improve crop yield and nitrogen use efficiency (NUE) but also reduce N losses, thereby minimizing environmental pollution (Quan et al., 2021).

Sustainable management practices also include the use of organic-inorganic fertilizer combinations, which have been shown to significantly increase water use efficiency (WUE) and reduce the environmental footprint of maize production (Shi et al., 2023). For instance, the Nutrient Expert (NE) management system, which combines optimized nutrient management with improved plant density, has demonstrated the potential to sustain maize yields while reducing reactive nitrogen losses and greenhouse gas emissions (Huang et al., 2021). These practices are crucial for achieving climate-smart agricultural production with minimal environmental damage.

8.2 Cost-benefit analysis of different planting density and fertilization combinations

The economic benefits of different planting density and fertilization combinations in maize cultivation can vary significantly. For example, a study conducted in northwest China found that the optimal combination of irrigation and fertilization could achieve high grain yield and economic benefits while minimizing soil nitrate residue (Yan et al., 2021). Specifically, an irrigation amount of 447~452 mm and a fertilization rate of 290 kg/ha~303 kg/ha resulted in the highest grain yield and economic benefit, reaching a 95% confidence interval of their maximum values simultaneously (Yan et al., 2021).

In another study, optimizing nitrogen management (Opt. N) at an average rate of 160 kg N ha^-1 over a 12-year period resulted in the highest average grain yield and grain protein yield among five different N treatments. This optimized approach also reduced various environmental impacts and health risks, while enhancing economic benefits (Zhang et al., 2023). Similarly, the Nutrient Expert (NE) management system not only increased grain yields but also reduced nitrogen and carbon footprints, demonstrating its economic viability and environmental sustainability (Huang et al., 2021).

8.3 Recommendations for agricultural practices balancing yield with environmental protection

To balance high maize yields with environmental protection, it is essential to adopt integrated nutrient management strategies. One effective approach is the use of split fertilization and deep placement of fertilizers, which have been shown to increase grain yield and reduce fertilizer-N loss consistently (Quan et al., 2021). Additionally, combining organic and inorganic fertilizers can significantly enhance water use efficiency and reduce the environmental footprint of maize production (Shi et al., 2023).

Farmers should also consider adopting advanced agronomic practices such as ridge-furrow planting and mulching film, which have been found to optimize water and nutrient use efficiency (Shi et al., 2023). Moreover, the Nutrient Expert (NE) management system, which integrates optimized nutrient management with improved plant density, offers a sustainable solution for maintaining high yields while minimizing environmental impacts (Huang et al., 2021). By implementing these practices, farmers can achieve sustainable maize production that balances economic benefits with environmental protection.

9 Future Research Directions and Technological Innovations

9.1 Precision density and fertilization strategies based on data and AI

The integration of data analytics and artificial intelligence (AI) in agriculture holds significant promise for optimizing planting density and fertilization strategies in maize cultivation. Precision agriculture technologies can analyze vast amounts of data from field sensors, satellite imagery, and historical crop performance to determine the optimal planting density and fertilization schedules. For instance, studies have shown that specific planting densities and fertilization modes can significantly impact maize yield and nutrient use efficiency. By leveraging AI, farmers can dynamically adjust these parameters to maximize yield and minimize resource use (Xu et al., 2017; Ren et al., 2020a; Li et al., 2021).

Moreover, AI-driven models can predict the outcomes of different planting and fertilization strategies under varying environmental conditions. This predictive capability is crucial for adapting to climate change and ensuring sustainable agricultural practices. For example, the Decision Support System for Agrotechnology Transfer (DSSAT) has been used to simulate the effects of different nitrogen fertilizer inputs and planting densities, demonstrating the potential for AI to enhance decision-making in maize cultivation (Ren et al., 2020b). Future research should focus on developing more sophisticated AI models that can integrate real-time data and provide actionable insights for farmers.

9.2 Breeding high-efficiency nutrient use varieties for high-density planting conditions

Breeding maize varieties that are efficient in nutrient use and tolerant to high-density planting is essential for improving productivity and sustainability. High-density planting often triggers a shade avoidance response in maize, leading to increased plant height and lodging, which can reduce yield. Recent advances in understanding the genetic and molecular mechanisms underlying these responses have paved the way for breeding maize with traits such as reduced plant height, more erect leaf angles, and stronger root systems (Jafari et al., 2023).

For instance, breeding programs have successfully developed maize hybrids that perform well under high-density and low-nitrogen conditions, demonstrating the potential for genetic improvements to enhance nutrient use efficiency and yield stability (Al-Naggar et al., 2015; Zhang et al., 2021). Future research should continue to explore the genetic basis of these traits and develop new varieties that can thrive under high-density planting conditions while maintaining high nutrient use efficiency. This approach will be crucial for meeting the growing global demand for maize without expanding agricultural land.

9.3 Areas for further research to integrate planting and fertilization optimization

While significant progress has been made in optimizing planting density and fertilization strategies, several areas require further research to fully integrate these practices. One critical area is the interaction between planting density, fertilization, and environmental factors such as soil type, water availability, and climate. Understanding these interactions can help develop more tailored and effective cultivation practices. For example, studies have shown that the optimal planting density and fertilization rates can vary significantly depending on local conditions, highlighting the need for site-specific recommendations (Piao et al., 2022; Zheng et al., 2023).

Another area for further research is the long-term impact of optimized planting and fertilization practices on soil health and environmental sustainability. While increasing planting density and optimizing fertilization can improve yield and nutrient use efficiency, it is essential to ensure that these practices do not lead to soil degradation or increased greenhouse gas emissions. Research should focus on developing sustainable practices that balance high productivity with environmental conservation (Xu et al., 2017; Zhai et al., 2022). Additionally, exploring the potential of integrating organic fertilizers and cover crops into high-density planting systems could provide a more holistic approach to sustainable maize cultivation.

10 Concluding Remarks

Optimizing planting density and fertilization strategies has a profound impact on achieving high-yield and sustainable maize production. The integration of increased planting density with reduced nitrogen (N) rates has been shown to enhance maize yield and resource use efficiency significantly. For instance, a study conducted in the Huanghuaihai Plain region of China demonstrated that a 30% increase in planting density combined with a 15% reduction in N rate improved maize grain yield by 6.6% and N partial factor productivity by 24.7%. Similarly, zigzag planting combined with deep nitrogen fertilization has been found to optimize root-canopy structures, thereby increasing maize yield and resource utilization.

The balance between production efficiency and environmental responsibility is crucial. Excessive N fertilizer application not only leads to diminishing returns in yield but also poses severe environmental risks, such as increased greenhouse gas emissions and nutrient runoff. Research indicates that reducing N application rates while increasing planting density can maintain or even enhance grain yield and significantly improve N use efficiency, thereby reducing environmental impacts. For example, a study in the North China Plain showed that a higher planting density with a reduced N rate decreased N2O emissions and greenhouse gas intensity by 7.3% and 4.3%, respectively.

Further research is essential to support the development of precision and sustainable agriculture. Future studies should focus on the long-term impacts of optimized planting density and fertilization on soil health, biodiversity, and overall ecosystem services. Additionally, the development of advanced agronomic practices and technologies, such as precision farming tools and decision support systems, will be critical in fine-tuning these strategies to local conditions and crop varieties. By continuing to explore and refine these approaches, we can ensure that maize production remains both highly productive and environmentally sustainable.

Acknowledgments

This article was completed under the guidance of professor Renxiang Cai and professor Binrong Xu. We are deeply grateful to Professor Cai for his multiple reviews of this paper and for his constructive revision suggestions. We would also like to thank the two anonymous peer reviewers for their valuable comments and recommendations.

Funding

This work was supported by the Major Scientific and Technological Project for New Agricultural Varieties Breeding of Zhejiang (2021C02064-4-4), Regional Demonstration Project of the Municipal Academy of Agricultural Sciences (2023SLM04).

Conflict of Interest Disclosure

The authors affirm that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

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