1. Introduction
Maize is the largest crop in the world, ranking first among global cereal grain crop species in total production and unit yield [
1,
2,
3]. As a C4 plant, maize has great potential for increasing yield due to its high light and efficiency. At the same time, corn is widely planted. More than 100 countries around the world produce corn, and a large number of them are planted from 58° N to 40° S in latitude [
4,
5]. In addition, corn kernels are rich in nutrients and rich in protein, fat, and starch, making them an excellent food [
6]. Therefore, improving the yield and quality of corn is of great significance to global food security and human health. With the continuous increase in the world’s population, the annual reduction of per capita arable land, and the demand for corn from animal husbandry and industrial development, the demand for corn may further increase in the future.
Therefore, improving the yield of corn while reducing the yield difference on the limited corn arable land is a key scientific problem that needs to be solved urgently in current corn production [
7,
8,
9,
10]. Many previous studies have confirmed the effect of dense planting and N application on maize yield [
11,
12,
13,
14,
15]. As one of the important mineral elements affecting the yield and quality of maize, N contributes about 45% to the yield per unit of maize [
16]. Therefore, the rational application of N fertilizer is one of the effective measures to improve corn yield and quality. However, with the continuous increase in N fertilizer application rates in corn farmland in recent years, its utilization level has dropped to 40–60% [
17]. This not only causes a huge waste of fertilizer resources but also increases the cost of agricultural production and seriously affects the quality of agricultural products. At the same time, differences in climatic factors and soil conditions in different maize production areas lead to differences in optimal N application rates for maize [
18,
19,
20]. Taking China as an example, the average recommended dosage of N fertilizer is 181 kg hm
−2 in east China, 219 kg hm
−2 in northwest China, 150 kg hm
−2 in the Northwest spring corn region, and 178 kg hm
−2 in the southwest plateau region [
21]. Therefore, coordinating the relationship between maize growth and N use efficiency (NUE) is the key to high-efficiency and high-yield maize while reducing environmental pollution.
Corn planting density also affects yield and quality. It is generally believed that low planting densities lead to a decrease in dry matter accumulation per unit area, which in turn reduces maize yield and agronomic traits [
22,
23,
24,
25]. On the contrary, high planting density will lead to an increase in the canopy area of maize colonies, and poor colony ventilation conditions will lead to intense competition for light resources, which will cause maize stem nodes to be thin and prone to lodging [
26]. Studies have shown that high-density planting increases the plant height and ear position of maize plants [
27]. However, some studies have found that the effect of density on plant height is not significant under high-density planting conditions [
28]. A moderate increase in planting density can help increase maize yield by increasing dry matter accumulation in the population. With the increase in planting density, the dry matter weight per plant decreased gradually, but the dry matter accumulation of the population increased instead [
29]. Therefore, in actual production, we should improve corn yield while improving corn quality through reasonable dense planting according to different variety characteristics and production conditions.
There is also an interaction between planting density and N utilization. Planting density affects N accumulation and transport mainly by affecting dry matter production and transport [
30]. Increasing planting density increases the N requirement of plant populations, but also reduces plant tolerance to high levels of N. Studies have shown that corn planting density is proportional to corn NUE, absorption efficiency, and harvest index, but after a certain density, corn NUE, absorption efficiency, and harvest index will decline [
31,
32]. Under high-density conditions, N metabolism is more vigorous and excess photosynthetic products are consumed, limiting the growth of yield [
33]. The effects of planting density and N fertilizer application rates on corn light utilization efficiency and yield are very critical. These factors affect the yield change of corn throughout the cycle and are also key ways to improve corn yield. How to control the planting density of corn and arrange the amount of N fertilizer reasonably, so that corn can quickly convert light energy, thereby increasing corn yield, has become a prominent issue.
The main purpose of this study is to examine the effects of planting density and N fertilizer application on corn light energy utilization and yield through field experiments. It aims to provide a scientific basis for corn production management.
4. Discussion
Our results show that maize yield is related to planting density and N application. Different planting densities and N application rates affect the number of rows per ear, the number of grains per ear, etc., thus affecting the yield of maize. The number of rows per ear and the number of grains per ear play a key role by affecting the photosynthesis and light energy utilization of maize plant types under different planting densities and N fertilizer conditions, such as the distribution of photosynthetic products. This study shows that, within a certain range of N application rates, with the increase in N fertilizer amount, the yield of corn stalk can be significantly improved, and the accumulation of corn dry matter can be improved, which is consistent with previous studies [
38,
39,
40,
41,
42]. During the growth period of maize, the rational application of N fertilizer can coordinate the growth of vegetative organs and reproductive organs, thus affecting the yield of maize. A large number of studies have shown that the use of N fertilizer can not only increase grain yield, promoting the accumulation of dry matter in maize, but also increase the accumulation of N [
43]. Hou Yunpeng et al. showed that the application of N fertilizer can significantly increase the dry matter accumulation rate and the maximum N absorption rate of corn. When the N application rate is between N60–180 kg hm
−1, increasing the N fertilizer amount can significantly improve the corn yield and the maximum dry matter, the accumulation rate, the N maximum absorption rate, and other indicators [
44]. In this study, the amount of N uptake by maize was significantly correlated with the amount of N applied, which was consistent with the results of previous studies. Zhao et al. showed that when the N application rate was lower than 270 kg m
−1, the application of N fertilizer could increase the dry matter weight of maize per plant. The dry matter weight decreased [
45,
46,
47,
48,
49].
The total N accumulation in maize plants is caused mainly by increasing the accumulation of N in leaves and grains, improving NUE, reducing N distribution in stems, increasing N distribution in leaves and grains, and promoting N movement from stems to seeds to improve grain yield [
50]. Under the conditions of this study, the grain yield of grain maize reached the maximum when the N application rate was 270 kg hm
−2, and the grain yield began to decline when the N application rate exceeded this N rate. Studies have shown that the amount of N that is too low or too high will reduce the grain weight, and the amount of N in the appropriate range can increase the grain weight [
51,
52]. Some studies have also shown that the yield of summer maize increases first and then decreases with the increase in the N application rate. Reasonable N application can significantly improve grain yield, while excessive N application will reduce N utilization efficiency and partial N fertilizer productivity [
49]. The results of this study are consistent with the above, the application of N fertilizer has a significant effect on the increase in maize yield, and the increase is 184.9%–193.7% under the optimal N application rate of grain maize. However, the yield began to decrease when the N fertilizer dosage exceeded 270 kg hm
−2. The optimal N application rate is higher than the optimal N application rate of previous studies. This is mainly because of the special geographical environment in Northwest China, with many hills and mountains. The seasonal rainfall is large and the soil nutrient loss is relatively fast, resulting in low soil fertility. To obtain high yield, it is necessary to increase the input of N fertilizer.
Reasonable dense planting is to determine the appropriate density according to the changes of varieties and environmental conditions. Under the same conditions in the same area, the plant type, plant height, and yield of each variety are very different, so the appropriate density in the same area varies from variety to variety. The increased tolerance of maize to high densities allows maize to expand planting densities which will increase yields. Under the conditions of this experiment, the planting density of maize varieties was 55,500 plants/hm to obtain higher grain yield, and the yield began to decrease after exceeding this density. A large number of studies have shown that the effect of planting density on corn yield is related to the quadratic curve [
51]. The yield of maize increased with the increase in planting density, but after a certain planting density, the yield began to decrease, and the determination of density was different for different varieties. Studies have shown that the best density estimate for the 1990 cultivar in Brazil is 85,000 plants hm
−2 [
52,
53,
54,
55], and the 1990 Corn Belt planting density in the United States reaches 80,000 plants hm
−2 [
56]. It can be seen that, compared with European and American countries, the density tolerance of spring maize varieties in Northwest China is much lower than the national base level, which shows that there is a lot of room for improvement in the density tolerance of spring maize varieties in Northwest China. Previous studies conducted in central China showed that with the increase in density, the 1000-grain weight and the number of grains per ear showed a downward trend, but the population yield increased, and the dry matter accumulation and yield of the maize population increased with the increase in planting density. Under the condition of 9.0 and 10,000 plants hm
−1, the dry matter accumulation and yield of maize increased by 8.3%, 5.2%, 27.7%, and 32.9%, respectively, compared with 6.75 and 45,000 plants hm
−1 [
57]. The results of this experiment showed that with the increase of planting density, the yield of corn stalks showed an increasing trend, which indicated that higher planting density was beneficial to the production of more corn stalk feed.