Subsoiling Combine with Layered Nitrogen Application Optimizes Root Distribution and Improve Grain Yield and N Efficiency of Summer Maize

Abstract

No-tillage of summer maize after the harvest of winter wheat is the primary agricultural practice on the North China Plain. However, prolonged no-tillage without deep tillage practices negatively impacts soil properties, which is detrimental to the growth and yield of summer maize. In this study, Xianyu 688 and Jifeng 2 were used as test maize materials, no-tillage and surface fertilizing with normal nitrogen (N) (BC240), no-tillage and surface fertilizing with N reduction (BC180), subsoiling layered fertilization with normal N (FC240) and subsoiling layered fertilization with N reduction (FC180)were designed, in order to assess root distribution, N utilization and grain yield of summer maize. In the two maize cultivars, the FC240 and FC180 treatments significantly reduced soil bulk density in the 10–50 cm depth soil layer compared to the other two treatments, and also increased total N content in the 20–50 cm depth soil layer. Compare BC 240 treatment, the FC240 treatment significantly increased root length in the 20–40 cm soil layer and root rap bleeding. Additionally, the FC240 and FC180 treatments enhanced dry matter and N accumulation, grain yield, N uptake efficiency and N fertilizer partial factor productivity. In various treatment, Xianyu 688 exhibited increased grain yield, N uptake efficiency and N fertilizer partial factor productivity compare with Jifeng 2. When employing a total N application level of 180–240 kg N/hm², the synergistic improvement of summer maize grain yield and N efficiency can be achieved by incorporating subsoiling combined with layered nitrogen application.

1. Introduction

The North China Plain farming region is recognized as one of the most significant agricultural regions in China [1]. The prevalent cropping practice in this region is the winter wheat–summer maize double cropping system, which involves rotary tillage before sowing winter wheat and direct stubble sowing of summer maize [2,3]. In summer maize production, a one-time application of seed and fertilizer is commonly practiced. The main fertilizer used is quick-acting fertilizer, which primarily concentrates on the surface layer of soil [4]. This concentration of fertilizer in the surface layer can negatively impact the growth of the root system in the deeper layers of the soil, leading to premature senescence of the root system. Consequently, the nutrient acquisition ability during the grain filling stage and ultimately influences grain yield formation [5]. As the youth labor force in agriculture declines, the middle-aged and elderly labor force becomes the primary labor force in agricultural production. To address this, the basal application of urea N and slow-release N can be employed to break the bottom layer of plough, increase the depth of N application, improve N distribution in the soil, and promote the growth of deep roots [6].

Root morphology encompasses various characteristics such as root number, root length, lateral root branching, root occurrence, three-dimensional distribution in space and root growth angle [7,8,9]. The root structure of N-efficient maize varieties is characterized by a high root volume, deep root system, appropriate spatial distribution, active root activity, and prolonged duration [10,11]. The underground nodal roots, is a key component of the maize root system, play a vital role in acquiring soil resources. Maize varieties with fewer nodal roots can enhance the number of lateral roots due to their own plasticity, leading to the development of a well-established lateral root system [12,13,14,15]. This, in turn, increases grain yield and nitrogen use efficiency [16,17]. Furthermore, optimized subsoiling tillage promotes root growth and enhances maize kernel weight, ultimately improving water use efficiency and summer maize yield [18].

The distribution of maize roots in different soil layers can be affected by the application of N fertilizer [19]. Deep application of N fertilizer can lead to a more reasonable spatial distribution of N in various soil layers, particularly in the deep soil layer below the plough layer. This can fulfill the nutrient demand during the late growth stage [20,21]. Additionally, deep placement of N or P fertilizer can enhance the uptake of N and P, thereby improving fertilizer use efficiency [22,23]. Compared to farmers’ broadcast prilled urea, deep placement of fertilizer significantly increased grain yields and net economic return across different rice-growing seasons and years [24]. Nitrogen deep placement demonstrated higher fertilizer N utilization efficiency and resulted in higher grain yield compared to N deep placement [25]. Furthermore, a fertilization depth of 25 cm can delay maize leaf senescence, enhance the grain-filling capability, and improve the antioxidant defense system and photosynthetic capacity of the leaves after silking [26]. Optimized nitrogen improved the root length and root surface area through regulating auxin and jasmonic acid levels and affected N uptake and grain yield of an N-efficient spring maize variety [27]. Nutrient availability in the soil influenced both root growth and the pattern of root development [28].

Subsoiling combined with layered N application is a newly implemented tillage and fertilization practice in China. This innovative tillage method offers a new approach for application device of a layered fertilizer. By combining subsoiling and fertilization techniques, this machine applies the required fertilizer for the crop’s growth cycle to different depths of the plough layer simultaneously. This ensures that the fertilizer is distributed within a range that is beneficial for crop root absorption. While this tillage method has been applied to various economic crops in China, such as maize and wheat, there have been no reports on its impact on the yield production of summer maize in the HHH Plain. In order to address the diversity of subsoiling combined with layered N application and meet the sustainable development requirements of agriculture in this region, we conducted this study with the objective of assessing the impact of subsoiling combined with layered N application on (1) soil bulk density and N content, (2) root dry matter accumulation and morphology, and (3) grain yield and its components.

2. Material and Methods

2.1. Site Description

The experiment was conducted in 2020 year, located in the Xinji Mazhuang Experimental Station (37°54′ N, 115°12′ E, Xinji), affiliated with the Agricultural University of Hebei, and the Agricultural Experimental Station of Hebei Normal University of Science and Technology (39°07′ N, 119°17′ E, Changli). At the Xinji site, the 0–20 cm soil layer had an organic matter content of 16.80 g/kg, total nitrogen (N) of 1.17 g/kg, available N of 64.9 mg/kg, available phosphorus (P) of 23.8 mg/kg, and available potassium (K) of 120.6 mg/kg. Similarly, in the 0–20 cm soil layer at the Changli site, the soil contained 12.40 g/kg organic matter, 1.36 g/kg total N, 64.08 mg/kg available N, 11.69 mg/kg available P, and 54.50 mg/kg available K.

2.2. Experimental Design

Xianyu 688 and Jifeng 2 were used as test materials in this study. Two tillage and fertilizer application practices were designed. The first practice involved no-tillage and surface fertilizing, while the second practice involved subsoiling combined with two-fertilizer ectopic layered fertilizing. Each practice had two N fertilizer rates: normal N (240 kg N hm⁻²) and reduced N (180 kg N hm⁻²). Therefore, four treatments were designed: no-tillage and surface fertilizing combined with normal N (BC240), no-tillage and surface fertilizing combined with N reduction (BC180), subsoiling layered fertilization combined with normal N (FC240), and subsoiling layered fertilization combined with N reduction (FC180). The machines used for the two tillage practices are shown in Figure 1, and the fertilization method and amount under different treatments are presented in

Table 1. Fertilization method and amount under different treatments.

Cultivars	Treatment	Tillage Practices	Fertilization Method	Urea N (kg/hm2)	Slow Release Urea N (kg/hm2)	P2O5 (kg/hm2)	K2O (kg/hm2)
XY688	BC240	No-tillage	Surface	168	72	75	90
	FC240	subsoiling	Layered	168	72	75	90
	BC180	No-tillage	Surface	126	54	75	90
	FC180	subsoiling	Layered	126	54	75	90
JF-2	BC240	No-tillage	Surface	168	72	75	90
	FC240	subsoiling	Layered	168	72	75	90
	BC180	No-tillage	Surface	126	54	75	90
	FC180	subsoiling	Layered	126	54	75	90

. The no-tillage and surface fertilizing practice utilized an integrated machine for single-grain precision seeding and fertilization. The subsoiling and layered fertilization practice utilized a subsoiling and two-fertilizer ectopic layered fertilizing machine. Two deep placements were made, with the top layer receiving urea N, calcium superphosphate, and potassium sulfate at a rate of 30% of the total N, and the bottom layer receiving slow-release N fertilizer at a rate of 70% of the total N. Additionally, P₂O₅ and K₂O were applied at rates of 75 kg hm⁻² and 90 kg hm⁻², respectively. Common urea contained 46% N, controlled-release urea contained 44% N, calcium superphosphate contained 18% P₂O₅, and potassium sulfate contained 50% K₂O. The planting density was 64,500 plants hm⁻².

2.3. Sampling and Measurement

2.3.1. Soil Bulk Density

Soil bulk density was measured at silking. It was determined by using a steel cylinder (6 cm diameter, 30 cm height, total volume of 847.8 cm³) for soil depths of 0–10, 10–20, 20–30, 30–40 and 40–50 cm. The bulk density was calculated by dividing the weight of the dried soil by the volume of the soil [29].

2.3.2. Soil Total N Content

Soil total N content was measured at silking. It was determined by using an earth drill for soil depths of 0–10, 10–20, 20–30, 30–40 and 40–50 cm. After the soil was dried, it was sifted through a 5 mm mesh, and 1 g of soil sample and 3 g of catalyst were mixed in a Kjeldahl digestion tube. The Kjeldahl method was used to determine the total N content.

2.3.3. Dry Weight and Total N Content

Three maize plants were taken from each treatment, and the plants were divided into root, leaf and stem at jointing, trumpet and silking stage. The plant was divided into root, leaf, stem (including sheaths), bract, grain and cob at milk and mature stage. All fresh samples were placed in an oven and dried at 70 °C to a constant weight after 30 min of 105 °C chlorination [30]. After weighing the dry weight, the samples were crushed with a grinder, and 0.200 g samples were taken in a Kjeldahl digestion tube and cooked by H₂SO₄-H₂O₂ method. Additionally, N uptake efficiency (NUPE) (Equation (1)), N use efficiency (NUE) (Equation (2)), N partial factor productivity (PFPN) (Equation (3)) and N harvest index (NHI) (Equation (4))were computed to explore the performance of agricultural management practices [30].

$$\mathrm{N}\mathrm{u}\mathrm{p}\mathrm{E}=\frac{\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l} \mathrm{N} \mathrm{u}\mathrm{p}\mathrm{t}\mathrm{a}\mathrm{k}\mathrm{e} \mathrm{b}\mathrm{y} \mathrm{p}\mathrm{l}\mathrm{a}\mathrm{n}\mathrm{t}}{\mathrm{N} \mathrm{a}\mathrm{p}\mathrm{p}\mathrm{l}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{a}\mathrm{m}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{t}}$$

(1)

$$\mathrm{N}\mathrm{U}\mathrm{E}=\frac{\mathrm{G}\mathrm{r}\mathrm{a}\mathrm{i}\mathrm{n} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}}{\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l} \mathrm{N} \mathrm{u}\mathrm{p}\mathrm{t}\mathrm{a}\mathrm{k}\mathrm{e} \mathrm{b}\mathrm{y} \mathrm{p}\mathrm{l}\mathrm{a}\mathrm{n}\mathrm{t}}$$

(2)

$$\mathrm{P}\mathrm{F}\mathrm{P}\mathrm{N}=\frac{\mathrm{G}\mathrm{r}\mathrm{a}\mathrm{i}\mathrm{n} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}}{\mathrm{N} \mathrm{a}\mathrm{p}\mathrm{p}\mathrm{l}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{a}\mathrm{m}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{t}}$$

(3)

$$\mathrm{N}\mathrm{H}\mathrm{I}=\frac{\mathrm{G}\mathrm{r}\mathrm{a}\mathrm{i}\mathrm{n} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}}{\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l} \mathrm{N} \mathrm{u}\mathrm{p}\mathrm{t}\mathrm{a}\mathrm{k}\mathrm{e} \mathrm{b}\mathrm{y} \mathrm{p}\mathrm{l}\mathrm{a}\mathrm{n}\mathrm{t}}$$

(4)

2.3.4. Root Bleeding Sap

Root bleeding sap was collected at the stage of trumpet, silking, milk and maturity stage. Three plants from each treatment were cut at 18:00, to a stem height of 10 cm, put on pre-weighed absorbent cotton, cover with a rubber balloon, and tie with a rubber band, at 6 am the next day, the bleeding fluid will be taken back to the laboratory and weighed. The volume of the bleeding fluid will be measured and stored in a −20 °C refrigerator.

2.3.5. Root Morphological and Dry Weight

At silking, the sampling were divided from the top to down into five levels, i.e., 0–10 cm, 10–20 cm, 20–30 cm, 30–40 cm, and 40–50 cm. For the soil samples, the “3D Monolith” root spatial sampling method [31] was adopted with a small cube in situ root-soil sampler, and the roots of three maize plants with the same growth and continuous position (planting row direction) were sampled. The soil volume 30 cm (intra-row spacing) in width × 50 (inter-row spacing) in length × 50 cm in depth surrounding each plant was excavated with a shovel. Each soil block was put into a net bag and brought back to the laboratory. After gently patting the soil block, all the maize roots in the soil block were carefully picked out with tweezers and stored under 20 °C in a sealed bag. The root samples were scanned and the root-related indexes were measured by the software WinRHIZO version 5.0 (Regent Instruments Inc., Quebec, QC, Canada).

At silking, the maize nodal root system consists of brace roots above the soil, crown roots in the soil, and fine lateral roots of small diameter encompassing the nodal roots. Crown and brace roots are initiated from consecutive underground and above-ground nodes of the stem. Nodal whorls number refers to the whorls of roots that arise from shoot nodes. In this study, we investigated the number, length, and whorl number of nodal roots. First, we randomly selected five plants from the central two rows of each subplot and cut them with a sickle. Then, we excavated the soil volume surrounding each plant, which measured 30 cm in width (intra-row spacing) × 50 cm in length (inter-row spacing) × 30 cm in depth, using a shovel. After shaking off the excess soil, we excised the nodal roots with a knife. We investigated various root traits, including crown root number, aerial root number, root length, volume, surface area, and dry weight of the nodal roots. The root samples were scanned and the root-related indexes were measured by root analysis software.

2.3.6. Grain Yield and Ear Characteristics

At harvest, plants from a 5 m × 2.4 m site in the middle rows of each plot were collected to determine the grain yield at a 14% moisture content. Out of the harvested ears, 20 were selected to measure various ear characteristics after 20 days of natural air drying. These characteristics included ear length, bare top length, ear perimeter, row number, kernels per row, and kernels per ear. Once the measurements were obtained, the kernels were threshed using a grain thresher, and the weight of 500 kernels was measured and converted to 1000 kernel weight.

2.3.7. Statistical Analysis

An ANOVA was conducted using SPSS software (ver. 22.0; SPSS Inc., Chicago, IL, USA). The data from each experimental site were analyzed separately. Graphs were plotted using either Sigmaplot (ver.14.0; Systat Software Inc., San Jose, CA, USA) or Excel 2017 (MicrosoftCorp., Redmond, WA, USA). To identify significant differences at the 0.05 probability level, treatment were compared by computing least significant differences (LSDs).

3. Results

3.1. Soil Bulk Density

The soil bulk density in the 0–50 cm soil layer at the two experimental sites, are shown in Figure 2. There was no significant difference in the bulk density of the 0–10 cm surface soil between XY688 and JF2. At both two experimental site, there was no significant difference in the bulk density of the 0–10 cm surface soil between various treatments, the FC240 and FC180 treatments resulted in a significant reduction in the soil bulk density of the 10–50 cm soil layer compared to the BC240 and BC180 treatments. Overall, the subsoiling combined with layered N application treatment of FC240 and FC180 reduce soil bulk density of the 10–50 cm surface soil across two cultivars and two experimental sites, which provide better climate conditions for root growth.

3.2. Soil Total N Content

The total N content in the 0–50 cm soil layer at the two experimental sites, are shown in Figure 3. The soil total N content in the 0–50 cm soil layer of XY688 was higher than that of JF2 at all treatments. At both two experimental site, FC240 and FC180 treatments decrease in total nitrogen content in the 0–20 cm soil layer, and increase in the 20–50 cm soil layer. Overall, the subsoiling combined with layered N application treatment of FC240 and FC180 increase total nitrogen content in the 20–50 cm across two cultivars and two experimental sites.

3.3. Root Morphology

The Root morphology traits at the two experimental sites, are shown in

Table 2. Total number of root, root length and root surface area of maize under different treatments.

Site	Cultivar	Treatment	Number of Aerial Roots (plant)	Number of Crown Roots (plant)	Total Number of Root (plant)	Root Length (cm/plant)	Specific Root Length (cm/g)	Root Surface Area (cm2/plant)	Specific Root Surface Area (cm2/g)
Xinji	XY688	BC240	23.6 b	31.0 ab	54.6 ab	10,810.6 c	435.25 c	4357.0 a	175.4 b
		FC240	26.3 a	36.0 a	60.6 a	11,166.3 c	481.31 c	4317.4 a	186.1 ab
		BC180	21.0 c	28.0 b	49.0 c	12,981.3 b	558.98 b	4308.4 a	185.5 ab
		FC180	24.0 ab	29.0 b	53.0 ab	13,861.9 a	605.06 a	4389.9 a	191.6 a
	JF2	BC240	25.0 b	36.6 b	61.6 c	9981.6 c	439.54 c	4075.6 a	179.5 ab
		FC240	35.0 a	33.0 c	68.0 ab	10,981.5 b	483.48 b	4017.9 a	176.9 b
		BC180	31.3 a	43.0 a	74.3 a	11,671.2 a	521.35 a	4133.8 a	184.6 a
		FC180	25.3 b	39.3 ab	64.6 bc	11,833.1 a	533.74 a	4103.3 a	185.1 a
Changli	XY688	BC240	14.2 b	40.1 ab	54.6 b	5892.4 c	424.4 b	1823.9 c	123.7 bc
		FC240	14.1 b	43.2 a	57.2 ab	6093.1 c	494.6 b	2523.9 b	173.2 b
		BC180	16.3 b	34.3 a	50.4 ab	12,954.8 b	1008.9 a	2810.4 b	218.9 a
		FC180	17.3 a	43.2 a	60.2 a	15,842.0 a	1083.6 a	3378.9 a	231.1 a
	JF2	BC240	14.4 b	32.5 b	46.6 b	7221.4 b	301.0 b	926.3 bc	108.5 b
		FC240	16.2 b	34.4 b	50.2 b	7633.9 b	321.0 b	1016.4 b	119.8 b
		BC180	25.3 a	33.7 a	57.2 ab	7067.2 a	566.2 a	994.5 b	130.4 a
		FC180	24.2 a	34.5 b	68.1 a	8335.7 ab	623.9 a	2043.3 a	152.9 a

Note: Lowercase letters in the same column of data indicate that the same variety is compared between different treatments, and the difference reaches a significant level of 5% (p < 0.05).

. At the Xinji experimental site, FC240 and FC180 treatments increase total number of root comparing the BC240 and BC180 treatment for both two cultivars, contribute to the higher number of aerial or crown root. Similarly, the FC240 and FC180 treatments lead to an increase in root length, specific root length and surface area for both varieties. At the Changli experimental site, FC180 treatments increase total number of root comparing the other treatment for both two cultivars, contribute to the higher number of aerial or crown root. Similarly, the FC180 treatments lead to an increase in root length, specific root length and root surface area for both varieties. Overall, the subsoiling combined with layered N application treatment of FC240 and FC180 increase the numbers of crown root, root length and specific root length across two cultivars and two experimental sites.

3.4. Root Length Spatial Distribution

The root length spatial distribution in the 0–50 cm soil layer at the two experimental sites, are shown in Figure 4. At both two experimental site, FC240 and FC180 treatments increased the root length of Xianyu 688 in the 0–50 cm soil layer compared to the BC240 treatment, the impact of the FC180 treatment on the root length in the 0–50 cm soil layer was significantly higher than that of the FC240 treatment. In the 0–50 cm soil layer, XY688 exhibited a longer root length than JF2. Furthermore, the FC240 and FC180 treatments enhanced the root length of Xianyu 688 in the 0–30 cm soil layer compared to the BC240 treatment, but there was no significant difference in the effect on root length in the 30–50 cm layer. This indicates that subsoiling combine with layered nitrogen application has a regulatory impact on the root length distribution for XY688 across each layer of 0–50 cm soil. In contrast, this method only regulated root length distribution in the 0–30 cm soil layer for JF2, and had minimal impact on the root length in the 30–50 cm soil layer. Overall, The subsoiling combined with layered N application treatment of FC240 and FC180 optimize root length spatial distribution across two cultivars and two experimental sites.

3.5. Root Bleeding Sap

The root bleeding sap of maize at the two experimental sites, are shown in Figure 5. At both two experimental site, the FC240 and FC180 treatments significantly increased the root bleeding sap compared to the BC240 treatment, and also increase the root bleeding sap of two cultivars during silking and milk stages. Overall, Subsoiling combined with layered N application treatment of FC240 and FC180 improve root bleeding sap across two cultivars and two experimental sites.

3.6. Shoot Dry Matter Accumulation

The shoot dry matter accumulation (DMA) of maize at the two experimental sites, are shown in

Table 3. Shoot dry matter weight, grain dry matter weight and harvest index of maize under different treatments.

Site	Cultivars	Treatment	Pre-Silking Shoot DMA (g plant−1)	Post-Silking Shoot DMA (g plant−1)	Maturity Shoot DMA (g plant−1)	Grain DMA (g plant−1)	HI
Xinji	XY688	BC240	143.3 c	186.2 b	329.5 a	173.2 b	0.51 a
		FC240	146.4 c	200.2 a	346.5 b	180.6 ab	0.52 a
		BC180	153.3 b	210.3 a	363.6 a	186.4 ab	0.51 a
		FC180	151.8 a	208.1 a	359.9 a	184.5 a	0.51 a
	JF2	BC240	128.0 c	177.5 bc	305.6 b	163.7 a	0.52 a
		FC240	147.3 a	181.1 b	328.3 ab	173.0 b	0.53 a
		BC180	135.4 b	186.3 b	321.6 a	168.0 ab	0.52 a
		FC180	136.1 b	195.6 a	341.6 a	171.1 a	0.53 a
Changli	XY688	BC240	139.5 b	145.3 c	284.7 b	149.0 c	0.52 b
		FC240	147.7 a	198.2 a	355.8 a	201.7 a	0.56 a
		BC180	138.7 b	186.6 b	325.3 b	181.9 c	0.56 a
		FC180	153.0 a	182.0 b	335.0 a	190.3 ab	0.57 a
	JF2	BC240	129.0 bc	140.3 c	269.4 b	132.5 c	0.49 b
		FC240	142.0 a	173.4 a	315.4 a	172.9 a	0.55 a
		BC180	124.5 bc	161.5 b	286.0 b	152.6 c	0.53 a
		FC180	135.4 ab	158.2 b	293.6 a	158.5 b	0.54 a

Note: Lowercase letters in the same column of data indicate that the same variety is compared between different treatments, and the difference reaches a significant level of 5% (p < 0.05).

. At both two experimental site, the FC240 and FC180 treatments increased the post-silking DMA and grain DMA, resulting in an improvement in total plant DMA. However, there was no significant difference in the impact on HI. The post-silking DMA, maturity total plant DMA, and grain DMA of XY688 were higher than that of JF2. Overall, the subsoiling combined with layered N application treatment of FC240 and FC180 significantly improved post-silking DMA, grain DMA, and maturity total plant DMA across two cultivars and two experimental sites.

3.7. Characteristics of the Maize Ear

The Characteristics of the maize ear at the two experimental sites, are shown in

Table 4. Effective panicle number, kernels per ear and 1000-grain weights of maize under different treatments.

Site	Cultivars	Treatment	Ear Length (cm)	Length of Bare Top (cm)	Perimeter (cm)	Row Number	Kernels per Row	Effective Panicle Number (104 Ear ha−1)	Kernels per Ear	1000-Grain Weights (g)
Xinji	XY688	BC240	20.8 ab	1.1 a	15.9 ab	15.7 a	33.7 c	5.71 a	529.1 b	356.2 b
		FC240	20.7 ab	0.7 b	15.9 ab	15.4 a	34.2 ab	5.17 b	526.7 a	362.2 a
		BC180	16.9 b	0.7 b	15.4 b	15.6 a	34.2 ab	5.76 a	533.5 b	367.7 a
		FC180	22.0 a	0.9 ab	16.4 a	15.7 a	36.2 a	5.19 b	549.5 ab	368.4 a
	JF2	BC240	17.4 b	0.7 a	15.7 a	14.5 ab	30.4 b	5.58 a	440.8 b	357.4 ab
		FC240	16.3 b	0.6 ab	15.7 a	14.5 b	31.1 b	5.69 a	449.5 b	356.3 ab
		BC180	20.5 a	0.4 b	15.6 a	14.6 ab	34.0 a	5.34 b	496.4 b	362.0 a
		FC180	16.6 b	0.5 ab	15.7 a	15.3 a	33.3 a	5.32 b	509.5 a	361.1 a
Changli	XY688	BC240	18.7	1.8 a	15.9 ab	15.1 ab	29.9 b	5.14 b	451.3 bc	364.2 a
		FC240	20.0	1.6 ab	15.9 ab	16.5 a	29.2 b	5.21 ab	481.8 b	372.1 a
		BC180	21.9	1.4 b	15.4 b	16.3 a	30.7 b	5.33 a	500.4 a	363.9 a
		FC180	18.7	1.6 a	16.4 a	15.1 ab	31.8 a	5.25 ab	512.0 a	362.3 a
	JF2	BC240	20.0	1.6 a	15.7 a	16.5 a	23.0 b	5.08 b	379.5 c	362.1 a
		FC240	21.9	1.4 ab	15.7 a	16.3 a	28.2 a	5.24 a	459.7 b	372.1 a
		BC180	18.7	1.5 a	15.6 a	15.1 ab	29.6 a	5.11 ab	476.6 b	374.8 a
		FC180	20.0	1.6 a	15.7 a	16.5 a	29.7 a	5.24 a	490.1 a	366.1 a

Note: Lowercase letters in the same column of data indicate that the same variety is compared between different treatments, and the difference reaches a significant level of 5% (p < 0.05).

. At the Xinji experimental site, compared with the BC240 treatment, the FC240 and FC180 treatment increased the thousand grain weight, grain number per ear and number of ears per unit area of XY688, and decreased the number of grains per ear of JF2, increased the number of ears per unit area. At the Changli experimental site, compared with the BC240 treatment, the FC240 and FC180 treatment only increased the spike number of XY688, and the spike number and spike number per unit area of JF2. Overall, Analysis shows that the increase in grain number per spike is mainly due to the increase in grain number per row, with no significant difference in grain number per spike.

3.8. Grain Yield

The grain yield of maize at the two experimental sites, are shown in Figure 6. At the Xinji experimental site, the FC240 and FC180 treatment significantly improve grain yield than that of other treatments. compared with BC240 treatment, the grain yield in FC240 and FC180 treatments of XY688 increased by 9.8% and 5.7%, respectively, and JF2 increased by 23.88% and 15.93%, respectively. At the Changli site, compared with BC240treatment, the gain yield in FC240 and FC180 treatments of XY688 increased by 12.3% and 15.38%, and JF2 increased by 20.12% and 29.23%, respectively. Overall, the subsoiling combined with layered N application treatment of FC240 and FC180 significantly improved grain yield across two cultivars and two experimental sites.

3.9. N Efficiency

The N absorption efficiency, N physiological utilization efficiency, N fertilizer partial productivity of maize at the two experimental sites, are shown in

Table 5. Plant N contents, NupE, NPE, PFPN and NHI under different treatments at the two experimental sites.

Site	Cultivars	Treatment	Plant N Contents (g plant−1)	Grain N Contents (g plant−1)	NupE (kgkg−1)	NUE (kgkg−1)	PFPN (kgkg−1)	NHI (%)
Xinji	XY688	BC240	4.08 a	2.26 b	0.92 b	49.54 a	45.37 c	55.39 b
		FC240	4.25 a	2.53 a	0.92 b	54.43 a	49.81 c	59.53 b
		BC180	3.63 b	2.23 b	1.16 a	50.30 a	58.41 b	61.43 ab
		FC180	4.11 a	2.62 a	1.19 a	53.86 a	63.94 a	63.75 a
	JF2	BC240	3.67 b	2.02 a	0.76 c	45.71 c	34.68 c	54.50 ab
		FC240	3.83 a	2.28 a	0.91 b	47.33 bc	42.96 b	59.53 a
		BC180	3.32 c	1.82 b	0.92 b	50.26 ab	46.48 b	59.82 a
		FC180	3.59 b	2.13 a	1.06 a	50.93 a	54.11 a	59.33 a
Changli	XY688	BC240	3.85 b	2.69 b	0.87 b	47.20 b	32.91 c	69.80 b
		FC240	4.25 a	3.13 a	0.84 b	49.60 ab	38.88 c	73.57 a
		BC180	3.63 b	2.62 b	0.92 a	50.09 ab	48.72 ab	72.27 a
		FC180	4.11 a	3.16 a	0.95 a	52.16 a	51.04 a	76.79 a
	JF2	BC240	3.26 b	2.15 c	0.76 bc	46.11 bc	29.35 c	65.9 b
		FC240	3.83 a	2.81 a	0.80 b	47.76 ab	37.41 b	73.33 a
		BC180	3.12 b	2.24 c	0.87 b	47.67 ab	47.58 a	71.72 a
		FC180	3.59 ab	2.53 b	0.91 a	50.68 a	48.47 a	70.50 a

Note: Lowercase letters in the same column of data indicate that the same variety is compared between different treatments, and the difference reaches a significant level of 5% (p < 0.05).

. At both two experimental site, the FC240 and FC180 treatments increase plant N accumulation and grain N content compared to the BC240 treatment. Furthermore, these treatments showed improvements in N harvest index, N absorption efficiency, N physiological utilization efficiency, and N fertilizer partial productivity. Overall, the subsoiling combined with layered N application treatment of FC240 and FC180 significantly improved N absorption efficiency, N physiological utilization efficiency, N fertilizer partial productivity across two cultivars and two experimental sites.

3.10. Correlation Analysis

To determine the correlation between various factors (Figure 7 and Figure 8), we observed a highly significant positive correlation between soil bulk density (SBD20-50), RSC, and PRCS. Additionally, there was a significant positive correlation between soil total N content (STD 10-20), RL and PRCS. A highly significant positive correlation between grain yield and N absorption efficiency, N utilization efficiency, N partial factor productivity, plant total dry matter weight, grain dry matter weight. Additionally, there was a significant positive correlation between N absorption efficiency, N utilization efficiency, N partial factor productivity and total dry matter weight, post dry matter weight.

4. Discussion

4.1. Soil Physical and Chemical Properties and Maize Root Traits

Tillage practices, particularly deep tillage practices, have been shown to enhance soil properties [32] and create a more favorable soil environment for root growth and development compared to no-tillage or shallow tillage practices. A significant reduction in soil hardness and a notable decrease in soil bulk density at depths ranging from 20–30 cm to 30–40 cm as a result of deep tillage [33]. Similarly, the total N content in the surface soil was higher than in deeper soil layers. Furthermore, during the late growth stage, the deep tillage treatment exhibited higher total N content in deeper soil layers compared to conventional tillage [34], suggesting that deep tillage altered the distribution of N in the soil. Deep vertical rotary tillage, such as the method used by Zhai et al. [35], can be employed as a deep tillage practice to improve soil properties and enhance grain yield of summer maize. It was found that deep tillage mainly increased the proportion of roots in the shallow soil layer (0–40 cm). In our current study, we observed a significant reduction of 4.5–5.29% in soil bulk density at depths of 20–40 cm when deep tillage and stratified N application were used compared to conventional tillage methods. During the silking stage, the total N content in the surface soil (0–20 cm) decreased, while it notably increased in the deeper soil layer (20–50 cm). These results suggest that stratified N application in deep tillage primarily increased the total N content in the shallow soil layer (20–50 cm). This may be attributed to the distribution of N, where 30% of the total N was applied in the upper layer and 70% in the lower layer, as implemented in this experiment. In future experiments, the inclusion of conventional N fertilizer with stratified application would further enhance the significance of this study.

Topical application of chemical fertilizers has a decades-long history in China especially during the initial stages of plant growth, particularly in early spring seasons with lower temperatures, applying fertilizers near seeds or in bands was an effective strategy to promote root development, establish an optimal root system structure, and enhance yield. Root distribution has been suggested to play an important role in nutrient uptake and yield formation [36,37]. Deep N fertilization significantly enhanced the root proportion in soil depths of 40–100 cm, inducing deeper rooting, increasing distribution of deep root biomass, and promoting the postponement of root senescence, maintaining substantial root biomass in the late growth stages. Deep N application in rice resulted in an expansion of both root absorption area and active absorption area in the deeper soil layers [33], promoting an effective contact area between roots and soil, while improving the transport efficiency of substances from roots to the above-ground components. Wang et al. [19] reported a significant impact of nitrogen (N) application depth on root surface area and root volume in the 0–100 cm soil layers, with a noticeable decrease as the N depth increased. In addition, Niedziński et al. [38] indicated that deep N fertilization conditions led to an increase in crop root biomass and nutrient accumulation.

Further analysis revealed that combining deep tillage with N application could regulate the total number of maize roots and adventitious roots, resulting in an increase in both for the maize varieties ‘Xianyu 688’ and ‘Jifeng 2’. Specifically, deep tillage and stratified fertilization significantly influenced the root length in the 20–40 cm soil depth of ‘Xianyu 688’ and ‘Jifeng 2’, indicating that an expanded deep root system benefited in delaying root aging in summer maize. Expanding nutrient acquisition areas to enhance the spatial distribution of maize roots can improve their antioxidative capacity, delay root aging, increase late growth stage root vitality, and ensure sufficient nutrient supply for grain filling, ultimately leading to higher yields [39]. These findings suggest that subsoiling combined with layered N application has a regulatory effect on the root length, specific root length, total root number, and aerial root number of both maize varieties.

4.2. Dry Matter Accumulation, N Absorption and Utilization, and Grain Yield

Tillage practices have a significant impact on the shoot DMA of crops, particularly affecting dry matter translocation after silking and the contribution ratio of post-silking dry matter to grain in maize [40]. Deep tillage beyond 30 cm increased the total biomass of summer maize, both before and after flowering [22]. Deep vertical rotary tillage, as a form of deep tillage practice, can be employed to enhance soil properties and grain yield of summer maize, primarily by significantly increasing post-silking DMA [35]. In the present study, the FC240 and FC180 treatments exhibited higher post-silking DMA and grain DMA in both varieties, leading to an overall improvement in total plant DMA. However, there was no significant difference in the impact on harvest index (HI). Moreover, XY688 demonstrated higher post-silking DMA, total plant DMA, and seed DMA compared to JF2. Additionally, subsoiling and layered fertilization were found to significantly enhance post-silking DMA, seed DMA, and ripening DMA. Deep tillage, in comparison to other tillage methods, considerably improved the soil’s water retention capacity, increased dry matter accumulation, reduced the proportion of dry matter transferred to stems (including leaf sheaths), elevated the proportion of dry matter allocated to the ear, and consequently promoted maize yield [41].

The impact of deep placement of N fertilizer on plant grain yield and NUE in rice has been studied by Wang et al. [19]. They found that deep placement affects soil N distribution and plant N uptake. Additionally, deep placement of fertilizer and the use of slow/controlled-release fertilizer can greatly improve NUE in rice [42,43]. In a recent study, integrated deep tillage sowing resulted in a significant increase in the number of grains per maize ear and the thousand-grain weight compared to conventional sowing [18]. In our current study, we found that the grain yield of FC240 and FC180 treatments increased the number of grains per ear and the hundred grain weight, the increase in grain number per ear was mainly due to the increase in grain number per row.

Previous studies have demonstrated that fertilization with nitrogen (N), phosphorus, and potassium at depths of 10 cm and 15 cm resulted in a significant increase in rapeseed yield compared to fertilization at depths of 0 cm and 5 cm [44,45]. Other studies have suggested that applying conventional urea and compound fertilizers at a depth of 7–10 cm in paddy fields notably enhanced rice yield and N recovery efficiency compared to surface application of ordinary urea. Similarly, applying controlled-release urea at a depth of 15 cm significantly promoted grain yield in summer maize compared to applications at depths of 5 cm and 10 cm. Guo et al. [14] found that deep tillage and one-year tillage led to a significant increase in soil organic carbon and N content, resulting in substantial improvements in winter wheat yield (5.6% and 10.5%) and spring maize yield (8.4% and 12.7%). In the current study, the FC240 and FC180 treatments increased plant N accumulation and grain N content compared to the BC240 treatment. These treatments also improved N harvest index and significantly enhanced N absorption efficiency, N physiological utilization efficiency, N harvest index, and N fertilizer partial productivity.

Optimal fertilization depth has been found to have a positive impact on the growth and development of maize, leading to increased maize yield and improved fertilizer utilization efficiency. Numerous studies have shown that increasing fertilization depth enhances the leaf area index and chlorophyll content of maize, which promotes photosynthesis in leaves, leading to organic matter accumulation, improved nitrogen absorption efficiency, and the establishment of a foundation for higher yields in later stages. Xia et al. [46] demonstrated that deep N fertilization resulted in the highest nitrogen use efficiency of 46.23%, leading to a substantial increase in maize yield. Similarly, Wu et al. [47] observed that stratified band fertilization improved the efficiency of nitrogen transfer and utilization in the soil-plant-atmosphere system, effectively minimizing nitrogen losses from fertilizers and resulting in a significant 9.6–16.7% increase in nitrogen use efficiency.

This study aimed to investigate the root morphology, yield, and nitrogen utilization characteristics of nitrogen-efficient maize varieties under deep tillage and layered fertilization conditions. The findings of this study revealed that subsoiling and layered fertilization can be considered as a novel deep tillage practice in the HHH Plain to enhance soil properties and crop yield. These results may provide valuable guidance to farmers and agricultural managers in the region in the future.

5. Conclusions

Our study findings demonstrate that the subsoiling combine with layered nitrogen application had a significant impact on soil capacity and the total nitrogen content in the 20–50 cm soil layer of summer maize. This combination also leads to increased root length, promoting dry matter and nitrogen accumulation, and ultimately enhancing the number of grains in the ears and improving grain yield. Furthermore, when employing a total N application level of 180–240 kg N/hm², the synergistic improvement of summer maize grain yield and N efficiency can be achieved by incorporating subsoiling combined with layered nitrogen application.