Effects of Nitrogen Reduction at Different Growth Stages on Maize Water and Nitrogen Utilization under Shallow Buried Drip Fertigated Irrigation

Abstract

A field experiment of drip fertigated nitrogen reduction was set up in a typical Maize planting area in the Xiliao River Basin in 2018 and 2019. Different phased nitrogen reductions were set up under shallow buried drip irrigation during the growth period to explore ways to improve nitrogen use efficiency (NUE) by understanding the Maize regulation of nitrogen absorption and utilization. The recommended nitrogen application in the early stage (Nopt, total nitrogen 240 kg·hm⁻²) had the highest grain nitrogen uptake and total nitrogen uptake, followed by nitrogen reduction before the maximum canopy mulching (Nde-I, total nitrogen 180 kg·hm⁻²), nitrogen reduction after the maximum canopy mulching (Nde-II, total nitrogen 180 kg·hm⁻²) and no nitrogen application (N0). Without nitrogen application, the leaves were thin, green and yellow. The total nitrogen uptake was 38.54~41.31% lower than the recommended nitrogen application in the early stage. When nitrogen fertilizer was reduced in the maximum canopy mulching, grain nitrogen absorption was affected. Grain nitrogen absorption fell by 15.07% to 17.51% when nitrogen was reduced in the maximum canopy mulching compared to the recommended nitrogen application. The harvest index of nitrogen reduction before the maximum canopy coverage was 9.65~11.52% higher than that in the later stage, indicating that the nitrogen absorption between Maize grain, stem, and leaf was better regulated. Maize evapotranspiration water consumption was reduced throughout the growth cycle when nitrogen was reduced at various stages. Nitrogen reduction before maximum canopy mulching boosted water use efficiency (WUE) by 3.44% to 6.12% compared to the recommended nitrogen application in the early stage. The nitrogen fertilizer agronomic efficiency increased by 11.17% to 13.87%. The nitrogen use efficiency rose by 10.99~3.15% (5.24~6.60 percentage points). A total of 25% of nitrogen fertilizer was saved with the yield declining by only about 5%, resulting in increased NUE while maintaining the yield stability. Under shallow buried drip fertigated irrigation, the appropriate reduction in nitrogen fertilizer during the period from Maize sowing to the maximum canopy development ensured the nitrogen supply during tasseling–silking stage and filling stage, which can be used as a regulation method and a way to improve the Maize fertilizer use efficiency.

1. Introduction

Excessive application of fertilizer in farmland not only reduces the fertilizer utilization rate but also increases the leaching loss of nutrients, resulting in environmental pollution. Therefore, reducing the application of chemical fertilizer without reducing the yield or even increasing the yield is a main research goal for scholars [1]. To achieve scientific and reasonable fertilization and further reduce the amount of chemical fertilizer in farmland, the basic and primary goal is the convenience of the fertilization system. Drip fertigated irrigation precisely delivers the right amount of water and fertilizer solution to the soil in the crop root zone. The water and fertilizer solution then migrate and penetrate into the main root system, providing a good water and soil environment with adequate water and nutrition in the root zone [2]. Although water and salt regulation under subsurface drip irrigation conditions can improve soil salinity distribution, a large amount of salt accumulation still exists in the field. Due to the lack of drainage systems, there is no way to reduce the salt content in drip-irrigated farmland soil, and it is only redistributed within the soil [3]. Drip irrigation has changed the microenvironment of soil, including water, heat, salt, and nutrients, thereby altering the mode of material and energy conversion in agricultural ecosystems [4]. However, it is limited to regulating the salt content in the crop root layer and cannot fundamentally discharge it from the soil [5].

Reducing nitrogen application may be one of the most effective ways to reduce nitrogen loss and improve NUE [6,7,8]. Excessive nitrogen application does not improve crop NUE, and it can even reduce the Maize yield and cause environmental pollution to a certain extent [9]. Hu et al. [10] pointed out that reducing nitrogen application not only reduced the residue of nitrogen in soil but also effectively reduced the gaseous loss of nitrogen. According to Liu et al. [11], crop nutrient requirements are often low in the early stages and increase later. Traditionally, basic fertilization involves the application of at least half of the total nitrogen fertilizer, resulting in nutrient waste in the early stages and insufficient nutrients in the later stages. In the research of Pu Wei et al. [12], nitrogen reduction combined with synergistic application enhanced the supply capacity of soil available nitrogen, promoted Maize dry matter accumulation, improved the yield composition, and increased the Maize yield. As a result, Maize nitrogen application is reduced with no yield loss. Wu Chuanfa [13] and others reduced the application amount of conventional nitrogen by 20% for three consecutive years along with straw returning to the field and increased the content of soil organic carbon and nitrogen as well as the soil ammonium nitrogen accumulation. Crops received a consistent supply of nutrients. NUE was improved as well. To increase the yield or maintain a stable yield while reducing nitrogen fertilizer application and improving efficiency in crops, other control measures are often required, such as increasing the fertilizer level or different fertilizer blends [14,15], increasing drip irrigation water fertilizer integration to reduce nitrogen application [16], the synergy of water nitrogen coupling to reduce nitrogen application [17,18,19], and straw returning to the field with nitrogen reduction regulation [20,21]. In order to improve the soil ecological environment and increase the corn yield, some scholars have also used micro-particle fertilizers, plant-growth-promoting microorganisms, mycorrhizal fungi, and biostimulants to replace diammonium phosphate or starting fertilizers [22,23,24].

Fertilization of corn in two stages, the jointing stage and the big bell mouth stage, can delay leaf senescence, increase post-flowering material accumulation, and reduce the pre-flowering dry matter transport rate, thus increasing corn yield [25]. Applying nitrogen four times before sowing, during the jointing stage, during the big mouth stage, and 10 days after silk emergence effectively prolonged the grain filling time and leaf photosynthesis time and improved maize yield and nitrogen utilization efficiency [26]. Corn has two large nitrogen absorption peaks, from the jointing stage to the big horn stage, and from the silking stage to 15 days after silking. These two absorption peaks account for two-thirds of the total nitrogen absorption [27]. With the increase in planting density, the proportion of nitrogen fertilizer demand in the later stage of corn silk emergence gradually increases [28]. The management of applying 81, 27, 54, 54, 27, and 27 kg/hm² of nitrogen fertilizer in six stages before sowing, six-leaf stage, ten-leaf stage, silk emergence stage, ten days after the silk emergence stage, and milk maturity stage can improve the maize yield and nitrogen fertilizer utilization efficiency [29].

Farmland nitrogen reduction based on the requirements of sustainable development of agricultural ecological environment is a long-term goal. Scholars have discovered many ways to achieve this to various degrees. It is also the requirement of national agricultural development policy [30] and a main research focus. The drip-fertigated irrigation system is an efficient irrigation and fertilization technology. It has been widely used in the Xiliao River Basin, a dominant area for Maize production. The relevant research, on the other hand, lags behind the production practice [31]. As a new drip fertigated technology for regional Maize planting, shallow buried drip irrigation system lacks research on nitrogen reduction regulatory mechanism and scientific fertilizer management approaches. As a result, in this study, a nitrogen fertilizer reduction field experiment was conducted and the regulation mechanism of corn-phased nitrogen reduction was investigated, providing theoretical support for Maize field fertilization management in the Xiliao River Basin, and ways of high-efficiency utilization of nitrogen were discussed.

2. Materials and Methodology

2.1. Location of the Field

The experiment was carried out in the town of Qianjiadian, Keerqin district, Tongliao. It is the field site of Tongliao Academy of Agricultural Sciences, covering an area of 13 hectares. The site is located in the Xiliao River Basin, with a drainage area of 136,000 km². It is an important grain production base in Inner Mongolia with an average annual temperature of 5.0~6.5 °C and sunshine between 2800 and 3100 h per year. The relative humidity is 45% to 58%, with annual rainfall ranging from 300 to 400 mm and evaporation from 1199 to 2200 mm. Most areas are in semi-arid monsoon climate zones. The Xiliaohe River Basin is an ideal location for planting spring corn. Its terrain is mostly flat, making it ideal for large-scale crop farming. The common agrometeorological characteristics are dry and windy in spring and humid, and hot and rainy in late summer. During the crop growth season, there are significant changes in rainfall time and distribution, as well as an uneven spatial distribution of rainfall. Agricultural production is mainly irrigated by groundwater (Figure 1).

After retrieving soil samples from the experimental site, the soil is naturally air-dried and sieved. After processing the soil samples according to the specifications, the soil particle size composition was tested using a HELOS + OASIS laser particle size analyzer (produced in Klaustal, Germany). The classification of soil texture and pH in the experimental site is shown in

Table 1. Soil Texture and pH of the experimental field.

Burial Depth of Soil Layer (cm)	Bulk Density (g·cm−3)	Soil Particle Size Distribution (%)			Soil Texture	pH
		>0.05 mm	0.002~0.05 mm	<0.002 mm
0~20	1.44	41.89	56.78	1.33	Loam soil	8.1
20~40	1.43	13.45	85.66	0.90	Silt	7.9
40~60	1.45	52.50	47.28	0.23	Sandy loam soil	8.1
60~80	1.47	63.40	36.59	0.01	Sandy loam soil	8.2
80~100	1.48	48.26	51.47	0.28	Loam soil	7.8

.

2.2. Field Experiment

Related literature indicates that spring maize has different nutrient requirements at different growth stages. The nitrogen requirement before jointing only accounts for 2.2% of the total nitrogen requirement during the entire growth period, 51.2% from jointing to heading and flowering, and 46.6% from heading to grain filling and maturity. Overall, corn absorbs about half of the nitrogen when the canopy coverage reaches its maximum before tasseling. Therefore, the precise fertilizer conservation plan takes the maximum canopy coverage as the boundary time node. Four treatments were set up for a phased nitrogen fertilizer reduction experiment under a shallow buried drip fertigated system. For N0 treatment, no nitrogen fertilizer was applied throughout the whole growth period. For N_opt treatment, the recommended nitrogen amount was applied in the early stage (reference [19] and standard [32]). Based on N_opt treatment, two treatments with a 25% nitrogen reduction rate were set up. In N_de-I treatment, nitrogen reduction was made before the maximum Maize canopy coverage. In N_de-II treatment, reduced nitrogen was applied after the maximum Maize canopy coverage. The maximum canopy coverage occurs during the period from the big bell mouth to tasseling. Nitrogen reduction before the maximum canopy coverage is nitrogen reduction in the seedling and jointing stage. Nitrogen reduction after the maximum canopy coverage is nitrogen reduction in the tasseling and grouting stage. The specific fertilization design is outlined in the table below. The field experiment was repeated three times, with 12 plots in total. Each experimental plot was controlled by five drip irrigation belts, with an area of 6 m perpendicular to the direction of the drip irrigation belt and 10 m parallel to the direction of the drip irrigation belt. Wide and narrow rows (40 cm) are adopted for the Maize (40 cm × 80 cm) planting mode. Each drip irrigation belt is located in the middle of the narrow row, covered with soil and shallowly buried for 2 to 4 cm, and is used to irrigate 2 rows of corn. The layout spacing of the drip irrigation belt is 1.2 m. The picture depicts the arrangement mode for Maize planting and drip irrigation.

In 2018 and 2019, TK601 was used as a Maize variety in field trials, with a planting density of 75,000 plants/hm². The density was controlled by a PVC plant spacing marker rod. Manual sowing and inter-cropping were performed at the seedling stage. The amount of irrigation in the experimental field is controlled by a rotary wing water meter. The total irrigation during corn growth season in 2018 was 1990 m³/hm² and 2150 m³/hm² in 2019. Fertilizer was applied in shallow ditches during sowing. Throughout developmental stages, a TEFEN (produced in Tel Aviv, Israel) pump was connected to the drip irrigation system to deliver nitrogen fertilizer (

Table 2. Treatment details.

Treatment Identifier	Total Nutrients (kg·hm−2)			N Amount (kg·hm−2)
	Base Fertilizer with Added N	Base Fertilizer P2O5	Base Fertilizer K2O	Seedling	Jointing	Tasseling	Grouting
N0	0	135	120	0	0	0	0
Nopt	240	135	120	48	72	84	36
Nde-I	180	135	120	24	36	84	36
Nde-II	180	135	120	48	72	42	18

and Figure 2).

2.3. Data Calculation

2.3.1. Maize Above-Ground Biomass and Nitrogen Uptake

All above-ground biomass was collected when harvesting and placed in a fan-forced oven at 105 °C for 30 min, then 80 °C for 8 h. The samples were weighed every half hour until the weight became constant. The reading is the dry mass of Maize corresponding organs. The grains were separately crushed and sifted through a 0.25 mm sieve. Other organs were crushed and mixed, sifted through a 0.5 mm hole. After heat digesting in concentrated H₂SO₄-H₂O₂, the total nitrogen content is measured using the Kjeldahl method [33,34]. The calculation method is as follows:

$$\begin{array}{c}{\mathsf{\omega }}_{\mathrm{i}}=\frac{\mathrm{c}·\left(\mathrm{V}-{\mathrm{V}}_0\right)·0.014·\mathrm{D}·1000}{\mathrm{m}}\end{array}$$

(1)

$$\begin{array}{c}\mathrm{U}\mathrm{A}={\displaystyle \sum _{\mathrm{i}=1}^{\mathrm{n}}}\frac{{\mathrm{B}}_{\mathrm{i}}·{\mathsf{\omega }}_{\mathrm{i}}}{1000}\end{array}$$

(2)

${\mathsf{\omega }}_{\mathrm{i}}$ is the total nitrogen content of the sample and g·kg⁻¹. c is the concentration of sulfuric acid standard titration solution, in mol·L⁻¹. V is the volume of sulfuric acid standard titrant consumed for titrating the sample solution, in ml. V₀ is the volume of sulfuric acid standard titrant consumed for titration of blank sample solution, in ml. The molar mass of N is 0.014, in kg·mol⁻¹. D is the division multiple, that is, the ratio of titration volume to division volume. M is the sample weight, in g. UA is the total nitrogen uptake of crops per unit area, in kg·hm⁻². ${\mathrm{B}}_{\mathrm{i}}$ is the dry biomass per unit area corresponding to the aboveground sampling organs of crops, in kg·hm⁻².

2.3.2. Maize Grain Yield

To test the grain yield, the Maize ear of the second row (10 m long) in the third drip irrigation belt in each plot was harvested manually for the plot yield. The yield per hectare was calculated using the following formula:

$$\begin{array}{c}\mathrm{Y}=\mathsf{\alpha }·\frac{{\mathrm{D}\mathrm{M}}_{\mathrm{g}}}{{\mathrm{W}\mathrm{M}}_{\mathrm{s}}}{·\mathrm{W}\mathrm{M}}_{\mathrm{p}}·\mathsf{\delta }\end{array}$$

(3)

WMP is the fresh weight of the whole ear of all Maize collected after weighing in batches without threshing, representing the fresh weight of the plot, in kg. WMS is the fresh weight of 10 corn ears as representative samples, in kg. DMg is the dry weight of the 10 corn ear samples after threshing and natural air drying, in kg. α is the area calculation coefficient, which is converted by the yield of the sampling area into yield per hectare. δ is the grain moisture conversion coefficient, which is calculated using the grain moisture content. The purpose is to convert the air-dried grain yields with different moisture contents into the grain yield with 14% standard moisture.

2.3.3. Maize Evapotranspiration Water Consumption and WUE

Driver 2000 FDR (produced in Beijing, China) was used to monitor the change value $\Delta$W (mm) of soil water storage during sowing and harvesting. The calculation method is shown in Formula (4). The evapotranspiration water consumption ET (mm) of Maize in the whole growth period is obtained by the water balance method, and the calculation method is shown in Formula (5). WUE (kg·mm⁻¹·hm⁻²) is the ratio of grain yield to water consumption. The calculation method is shown in Formula (6).

$$\begin{array}{c}∆\mathrm{W}=\left({\mathsf{\theta }}_{\mathrm{i}\mathrm{n}\mathrm{i}}-{\mathsf{\theta }}_{\mathrm{e}\mathrm{n}\mathrm{d}}\right)·\mathrm{h}\end{array}$$

(4)

$$\begin{array}{c}\mathrm{E}\mathrm{T}=\mathrm{I}+\mathrm{P}+∆\mathrm{W}-\mathrm{Q}-\mathrm{R}\end{array}$$

(5)

$$\begin{array}{c}\mathrm{W}\mathrm{U}\mathrm{E}=\frac{\mathrm{Y}}{\mathrm{E}\mathrm{T}}\end{array}$$

(6)

${\mathsf{\theta }}_{\mathrm{i}\mathrm{n}\mathrm{i}} \mathrm{a}\mathrm{n}\mathrm{d} {\mathsf{\theta }}_{\mathrm{e}\mathrm{n}\mathrm{d}}$ are the soil volume moisture content during sowing and harvesting, %. h is the soil layer thickness, in mm. I and P are the irrigation amount and rainfall in the whole growth period, in mm. R is the runoff loss. Q is the soil water exchange capacity at the lower boundary, in mm. Considering that the shallow drip irrigation field is flat, the irrigation quota is small, and the buried depth of groundwater is less than 6 m, R and Q can be ignored. Y is the yield per hectare, in kg·hm⁻².

2.3.4. Nitrogen Utilization Index

A typical fertilizer utilization index in field experimental research was chosen for the analysis of nitrogen utilization. The ratio of crop yield to nitrogen application refers to the partial nitrogen fertilizer productivity. The calculation method is shown in Formula (7). The agronomic efficiency of nitrogen fertilizer, also known as agronomic efficiency, is the ratio of the difference in grain yield between the nitrogen application area and non-nitrogen application area to the amount of nitrogen application. The calculation method is shown in Formula (8). The nitrogen utilization rate, also known as the nitrogen recovery and utilization rate, is the ratio of difference between the aboveground biomass nitrogen absorption in the nitrogen application area and non-nitrogen application area to the nitrogen application amount. The calculation method is shown in Formula (7).

$$\begin{array}{c}{\mathrm{P}\mathrm{F}\mathrm{P}}_{\mathrm{N}}=\frac{\mathrm{Y}}{{\mathrm{A}}_{\mathrm{N}}}\end{array}$$

(7)

PFP_N is the partial productivity of nitrogen fertilizer, in kg·kg⁻¹. Y is Maize grain yield per unit area, in kg·hm⁻². AN is nitrogen fertilizer application amount, in kg·hm⁻²

$${\mathrm{A}\mathrm{U}\mathrm{E}}_{\mathrm{N}}=\frac{{\mathrm{Y}}_{{\mathrm{N}}_{\mathrm{t}}}-{\mathrm{Y}}_{{\mathrm{N}}_0}}{{\mathrm{A}}_{\mathrm{N}}}$$

(8)

AUE_N is the agronomic efficiency of nitrogen fertilizer, in kg·kg⁻¹. ${\mathrm{Y}}_{{\mathrm{N}}_{\mathrm{t}}}$ is the corn grain yield per unit area under nitrogen treatment, in kg·hm⁻²; ${\mathrm{Y}}_{{\mathrm{N}}_0}$ is the corn grain yield per unit area without nitrogen treatment, in kg·hm⁻².

$$\begin{array}{c}{\mathrm{R}\mathrm{U}\mathrm{E}}_{\mathrm{N}}=\frac{{\mathrm{U}\mathrm{A}}_{{\mathrm{N}}_{\mathrm{t}}} - {\mathrm{U}\mathrm{A}}_{{\mathrm{N}}_0}}{{\mathrm{A}}_{\mathrm{N}}}\end{array}$$

(9)

RUE_N is nitrogen fertilizer utilization rate, %. ${\mathrm{U}\mathrm{A}}_{{\mathrm{N}}_{\mathrm{t}}}$ is the nitrogen uptake per unit area of Maize under nitrogen treatment, in kg·hm⁻². ${\mathrm{U}\mathrm{A}}_{{\mathrm{N}}_0}$ is the nitrogen uptake per unit area of Maize without nitrogen treatment, in kg·hm⁻².

3. Results

3.1. Effects of Nitrogen Reduction at Different Growth Stages under Shallow Buried Drip Fertigated Irrigation on Maize Yield

The Maize yield of the field experiment is shown in the figure below. For the yield in 2018 and 2019, although there were significant differences, the order of the yield is the same. The results were as follows: the recommended nitrogen application treatment in the early stage Nopt had the highest yield, followed by nitrogen reduction before the maximum canopy coverage Nde-I, nitrogen reduction after the maximum canopy coverage Nde-II, and without nitrogen application N0. In 2018, the N0 treatment yielded 29.19% less than that of Nopt treatment. The yield of Nde-I treatment was 4.85% lower than that of Nopt treatment. The Nde-II treatment yielded 8.36% less than the Nopt treatment. In 2019, the yield of N0 treatment was 35.06% lower than that of Nopt treatment. The yield of Nde-I treatment was 5.12% lower than that of Nopt treatment. The yield of Nde-II was 11.32% lower than that of Nopt treatment. It can be seen that without nitrogen application, the Maize yield loss is quite large. With nitrogen reduction at different growth phases under shallow buried drip fertigated irrigation, the nitrogen reduction effect before the maximum canopy coverage on Maize grain yield is the smallest. With 25% less total nitrogen application, the yield loss is only about 5%, which achieved fertilizer reduction with a stable yield to a certain extent (Figure 3).

3.2. Effects of Nitrogen Reduction at Different Growth Stages under Shallow Buried Drip Fertigated Irrigation on Maize Nitrogen Uptake

The Maize nitrogen uptake in the field is shown in the table below. The nitrogen uptake of Maize grains, stems and leaves without nitrogen fertilizer was far less than that of different nitrogen fertilizer treatments. In the experiment, in N0 treatment, without nitrogen fertilizer, the Maize leaves were thin and yellow. The total nitrogen uptake in N0 treatment was 38.54% and 41.31% lower than Nopt in 2018 and 2019, respectively. Shallow buried drip fertigated irrigation improved the problem in leaf development. In the two experimental years, the recommended nitrogen application in the early stage Nopt had the highest grain nitrogen uptake, followed by nitrogen reduction before the maximum canopy coverage Nde-I, nitrogen reduction after the maximum canopy coverage Nde-II and no nitrogen application treatment N0. The nitrogen uptake of stems and leaves of different treatments had a slight difference in the two experimental years. The nitrogen uptake of Nopt and Nde-II treatments was significantly higher than that of other treatments. The result revealed that reducing nitrogen fertilizer after the maximum canopy coverage, i.e., entering the reproductive stage of Maize, did not affect the nitrogen uptake of stems and leaves. However, both nitrogen reduction before and after the maximum canopy coverage reduced the nitrogen uptake of grains. Reduction after the maximum coverage reduced the grain nitrogen by 15.07~17.51%. Nopt had the highest overall Maize nitrogen intake in both experiment years, followed by Nde-I, Nde-II, and N0, which was consistent with grain nitrogen uptake. Compared with other treatments, Nde-I treatment had the highest nitrogen harvest index HI and was significantly higher than that of Nde-II, and N0. The difference with Nopt treatment was significant. In the Maize stem and leaf development stage, nitrogen reduction before the maximum canopy coverage regulated the nitrogen absorption between Maize grain, stem and leaf. More nitrogen was transferred to the grain. The lack of nitrogen supply in the reproductive stage and no nitrogen application in the whole growth period are not conducive to the transport and accumulation of nitrogen to economic organs. The nitrogen harvest index HI is the percentage of grain nitrogen uptake to total nitrogen uptake (

Table 3. Nitrogen uptake of Maize under different experimental treatments.

Year	Treatment	Grain N Uptake (kg·hm−2)	Stem and Leaf N Uptake (kg·hm−2)	Total N Uptake (kg·hm−2)	HI (%)
2018	N0	100.70 ± 2.63 d	81.67 ± 4.31 c	182.38 ± 4.12 d	55.23 ± 1.68 b
	Nde-I	171.21 ± 3.84 b	106.37 ± 1.36 b	277.58 ± 3.62 b	61.67 ± 0.69 a
	Nde-II	152.52 ± 5.69 c	118.57 ± 2.41 a	271.09 ± 3.40 c	56.25 ± 1.42 b
	Nopt	179.59 ± 6.05 a	117.15 ± 5.44 a	296.74 ± 3.24 a	60.52 ± 1.85 a
2019	N0	98.75 ± 3.42 d	72.50 ± 3.72 d	171.24 ± 4.14 d	57.67 ± 1.71 b
	Nde-I	180.05 ± 3.86 b	93.48 ± 2.34 c	273.54 ± 3.01 b	65.82 ± 0.95 a
	Nde-II	155.31 ± 6.04 c	107.80 ± 4.53 a	263.11 ± 4.86 c	59.02 ± 1.74 b
	Nopt	188.29 ± 6.92 a	103.50 ± 5.37 b	291.78 ± 9.85 a	64.53 ± 1.25 a

N: Nitrogen fertilizer; HI: nitrogen harvest index. Different lowercase letters (a,b,c,d) in the same column mean significant differences at 5% level, the same below.

).

3.3. Effects of Nitrogen Reduction at Different Stages under Shallow Buried Drip Fertigated Irrigation on Maize Evapotranspiration, Water Consumption and WUE

Figure 4 depicts the effect of nitrogen reduction at different stages under shallow buried drip fertigated irrigation on evapotranspiration and water consumption. The evapotranspiration and water consumption ET in Maize during the entire growth period decreased to varying degrees after nitrogen reduction. When no nitrogen is applied, the water consumption is the lowest. Without nitrogen application, the overall canopy development is poor. The transpiration capacity is much weaker than other treatments, i.e., 76.25 mm lower compared with Nopt treatment. The ET values in Nde-I were slightly lower than Nde-II but the difference was not statistically significant. Both treatments were lower than Nopt. Nitrogen reduction at various growth periods lowered water consumption throughout the whole development period. Nitrogen reduction plays a role in water conservation.

The effect of phased nitrogen reduction on Maize evapotranspiration and water consumption under shallow drip irrigation is shown in Figure 5. There was no significant difference in WUE between Nopt, Nde-I and Nde-II, but all were significantly higher than N0. Despite the fact that ET in N0 treatment was the lowest, the drop in Maize yield was more than the reduction in ET. Therefore, the ratio, WUE, was still significantly lower. Although there was no significant variation in WUE amongst nitrogen application treatments, Nde-I had the highest WUE value, which is 3.44% higher than Nopt in 2018 and 6.12% higher in 2019. Since the yield loss in Nde-I treatment was low and Nde-I treatment reduced water consumption compared with Nopt, the WUE of Nde-I was slightly higher than Nopt. WUE was improved on some level. Since Nde-I treatment used less water than Nopt with a modest yield loss; it had higher WUE than Nopt. The efficiency of water consumption improved to some extent.

3.4. Effects of Nitrogen Reduction at Different Stages under Shallow Buried Drip Fertigated Irrigation on Maize NUE

3.4.1. Effect of Nitrogen Reduction at Different Stages under Shallow Buried Drip Fertigated Irrigation on Partial Productivity of Nitrogen Fertilizer

Nde-I had the highest partial nitrogen fertilizer productivity, followed by Nde-II and Nopt. In 2018, the partial productivity of nitrogen fertilizer of Nde-I and Nde-II was 26.86% and 22.19% higher than Nopt treatment. In 2019, Nde-I and Nde-II treatment was 26.51% and 18.25% higher than Nopt. All the nitrogen reduction treatments enhanced Maize nitrogen partial productivity, with Nde-I having the greatest improvement. The ratio of grain output to fertilizer application amount is known as the fertilizer partial productivity. The results of the yield comparison and analysis showed that nitrogen reduction reduced the grain yield significantly after the maximum canopy coverage, which was dominated by reproduction. For the same amount of nitrogen reduction, the nitrogen partial productivity was higher in Nde-I than Nde-II (Figure 6).

3.4.2. Effect of Staged Nitrogen Reduction under Shallow Buried Drip Fertigated Irrigation on Agronomic Efficiency of Nitrogen Fertilizer

The agronomic efficiency of Nde-I was much higher than the other treatments in the field experiment. There was no significant difference in the agronomic efficiency of nitrogen fertilizer between Nde-II and Nopt. The agronomic efficiency, also known as the agronomic utilization rate, reflected the contribution of each unit nitrogen application rate to the difference in grain output between nitrogen and non-nitrogen application areas. In the comparison of field experiments, an appropriate amount of nitrogen reduction in the canopy development stage significantly improved the agronomic efficiency of nitrogen fertilizer. In 2018 and 2019, the agronomic efficiency of Nopt treatment increased by 11.17% and 13.87%. The agronomic efficiency in nitrogen application reduction at the reproductive stage was slightly lower than the recommended nitrogen application in the early stage (Figure 7).

3.4.3. Effects of Nitrogen Reduction at Different Stage under Shallow Buried Drip Fertigated Irrigation on Maize NUE

Nde-I treatment had the highest NUE, followed by Nde-II and Nopt, but the difference between Nde-II and Nopt was not significant. In 2018 and 2019, the NUE of Nde-I treatment was 5.24 and 6.60 percentage points higher than Nopt. The relative improvement rates were 10.99% and 13.15%. In the comparison of plot experiments, nitrogen reduction before the maximum canopy coverage improved the NUE of that season. In general, limiting the application of nitrogen nutrients in the early stage of Maize development would be more beneficial to the absorption and utilization of nitrogen than limiting nitrogen delivery in the reproductive stage. Limiting the nitrogen supply during the period from sowing to maximum canopy development, and allocating more nitrogen fertilizer during the tasseling, silking, and grain filling stages could be a way to improve the absorption and utilization of nitrogen fertilizer input per unit in the context of reducing nitrogen fertilizer application for the sake of farmland ecological environment. In this study, Nde-I treatment improved the utilization rate of nitrogen fertilizer. In recent years, scholars [35,36,37] demonstrated that reducing nitrogen fertilizer in the early stages and shifting back nitrogen supply had a positive regulatory effect on crop absorption and utilization rate of nitrogen (Figure 8).

4. Discussion

(1)

The effect of nitrogen reduction on maize yield and nitrogen uptake

Nitrogen plays a crucial role in crop yield formation, and rational fertilization is a key measure to achieve higher target yields in crop production [9]. However, excessive use of nitrogen fertilizer can lead to nitrogen fertilizer waste as well as nitrogen fertilizer loss and environmental pollution. The results of this experiment indicate that under the condition of saving 25% of total nitrogen application, the loss of yield is only about 5%, which can achieve a certain degree of weight loss and stable yield. In addition, reducing nitrogen fertilizer application by 25% before maximum canopy coverage can regulate the nitrogen absorption relationship between maize grains and stems and leaves, allowing more nitrogen to be transferred to the grains. However, the lack of nitrogen supply in the reproductive stage or the absence of nitrogen application throughout the entire growth period are not conducive to the transportation and accumulation of nitrogen to economic organs. This result is consistent with the research results of Dong et al. [38].

(2)

The effect of nitrogen reduction on nitrogen fertilizer utilization in spring maize

Reducing nitrogen application may be one of the most effective methods to improve nitrogen fertilizer utilization efficiency [6,7,8]. The results of two years of shallow buried drip irrigation experiments on corn with soil cover showed that reducing nitrogen usage by 25% before reaching maximum canopy coverage can increase nitrogen fertilizer productivity by 26.51% to 26.86%, increase nitrogen fertilizer agronomic efficiency by 11.17% to 13.87%, and increase nitrogen fertilizer utilization efficiency by 10.99% to 13.15%. This result is similar to the research results of Wang et al. on nitrogen absorption and utilization of spring maize in the Yellow River irrigation area [39], and also to the experimental study of water and fertilizer utilization efficiency of maize drip irrigation conducted by Liu et al. [40] in Zhongjiang County, Deyang City, Sichuan Province, which belongs to a subtropical monsoon climate. Guo et al. [41] found that under water-limited conditions, reducing nitrogen can improve the nitrogen utilization efficiency, nitrogen harvest index, nitrogen fertilizer apparent utilization efficiency, and nitrogen fertilizer agronomic efficiency, which is similar to the results obtained in this study. Compared to the previous studies on nitrogen fertilizer reduction by scholars, the innovation of this experimental study lies in the use of regional emerging shallow buried drip irrigation technology to carry out integrated nitrogen reduction regulation of water and fertilizer as well as dividing the nitrogen reduction regulation into stages to explore and optimize the regulation mechanism. In future research in the region, it will be necessary to add other measures on the basis of reducing nitrogen [42,43,44,45] in order to further improve nitrogen fertilizer utilization efficiency.

5. Conclusions

(1) Although there was no significant difference in water use efficiency among different nitrogen application treatments, N_de-Ⅰ had the highest water use efficiency, which was 3.44% and 6.12% higher than that of N_opt in 2018 and 2019, respectively. Due to the smaller yield loss of N_de-Ⅰ treatment and the lower water consumption of N_de-II treatment compared to N_opt, the WUE of N_de-Ⅰ is slightly higher than that of Nopt. Due to the lower water consumption and moderate yield loss of N_de-Ⅰ treatment compared to Nopt, the WUE is higher than that of N_opt.

(2) The suggested nitrogen application in the early stage N_opt resulted in the highest Maize grain production, followed by nitrogen reduction before maximum canopy coverage N_de-Ⅰ, nitrogen reduction after maximum canopy coverage N_de-II, and no nitrogen application N₀. When compared to recommended nitrogen treatment in the early stage, nitrogen reduction before maximum canopy coverage saved 25% of total nitrogen while the yield loss was around 5%. Nitrogen reduction was achieved with a stable yield.

(3) Without nitrogen application N₀, Maize leaves were thin, green and yellow. The total nitrogen uptake was 38.54~41.31% lower than the recommended nitrogen application in the early stage N_opt. Shallow buried drip fertigated irrigation could significantly improve leaf development. Under different nitrogen regulation methods, the nitrogen uptake and total nitrogen uptake of Maize seeds were highest in recommended nitrogen application in the early stage (N_opt 240 kg·hm⁻²) followed by maximum nitrogen reduction before canopy coverage (N_de-Ⅰ 180 kg·hm⁻²), maximum nitrogen reduction after canopy coverage (N_de-II 180 kg·hm⁻²) and no nitrogen application N.

(4) Nitrogen reduction after the maximum canopy coverage had a great effect on grain nitrogen. Compared with the recommended nitrogen application, the grain nitrogen uptake decreased by 15.07~17.51%. The nitrogen harvest index in nitrogen reduction before the maximum canopy coverage was 9.65~11.52% higher than reduction after the maximum canopy coverage. Reduction before the maximum canopy coverage adjusted the nitrogen absorption between grain, stem and leaf. More nitrogen was transferred to the grain.

(5) Compared with the recommended nitrogen application in the early stage, nitrogen reduction before the maximum canopy coverage under shallow buried drip fertigated irrigation increased the WUE by 3.44~6.12%. The partial productivity of nitrogen fertilizer increased by 26.51~26.86%. The agronomic efficiency of nitrogen fertilizer increased by 11.17~13.87%. NUE also increased by 10.99~13.15% (5.24~6.60 percentage points).