Does Nitrogen Fertilization Improve Nitrogen-Use Efficiency in Spring Wheat?

Abstract

To investigate the effects and mechanism of prolonged inorganic nitrogen (N) fertilization on the N-use efficiency of spring wheat (Triticum aestivum L.), a long-term study initiated in 2003 was conducted. The study analyzed how N fertilization affects dry matter translocation, N translocation, soil NO₃-N, and N-use efficiency. Five different N-fertilizer rate treatments were tested: N0, N52.5, N105, N157.5, and N210, corresponding to annual N fertilizer doses of 0, 52.5, 105.0, 157.5, and 210.0 kg N ha⁻¹, respectively. Results showed that increasing N-fertilizer rates significantly enhanced the two-year average dry matter accumulation amount (DMA) at maturity by 22.97–56.25% and pre-flowering crop growth rate (CGR) by 17.11–92.85%, with no significant increase beyond 105 kg N ha⁻¹. However, no significant correlation was observed between the dry matter translocation efficiency (DTE) and wheat grain yield. Both insufficient and excessive N applications resulted in an imbalanced N distribution favoring vegetative growth over reproductive growth, thus negatively impacting N-use efficiency. At maturity, the N-fertilized treatments significantly increased the two-year average N accumulation amount (NAA) by 52.04–129.98%, with no further increase beyond 105 kg N ha⁻¹. N fertilization also improved the two-year average N translocation efficiency (NTE) by 56.89–63.80% and the N contribution proportion (NCP) of wheat vegetative organs by 27.79–57.83%, peaking in the lower-N treatment (N52.5). However, high-N treatment (N210) led to an increase in NO₃-N accumulation in the 0–100 cm soil layer, with an increase of 26.27% in 2018 and 122.44% in 2019. This higher soil NO₃-N accumulation in the 0–100 cm layer decreased NHI, NUE, NAE, NPFP, and NMB. Additionally, N fertilization significantly reduced the two-year average N harvest index (NHI) by 9.89–12.85% and N utilization efficiency (NUE) by 11.14–20.79%, both decreasing with higher N application rates. The NAA followed the trend of anthesis > maturity > jointing. At the 105 kg N ha⁻¹ rate, the highest N agronomic efficiency (NAE) (9.31 kg kg⁻¹), N recovery efficiency (NRE) (38.32%), and N marginal benefit (NMB) (10.67 kg kg⁻¹) were observed. Higher dry matter translocation amount (DTA) and N translocation amount (NTA) reduced NHI and NUE, whereas higher NTE improved NHI, NUE, and N partial factor productivity (NPFP). Overall, N fertilization enhanced N-use efficiency in spring wheat by improving N translocation rather than dry matter translocation.

1. Introduction

Wheat is one of the most widely cultivated and valuable crops for global food security [1]. With the global population projected to reach to reach 7.9−10.5 billion by 2050, the increased demand for wheat may result in potential food shortages in the upcoming decades [2,3]. In China, wheat accounts for 40% of total grain consumption, with a planting area exceeding 218 million hectares [4]. Consequently, China has initiated a new “green revolution” and sustainable initiatives to bolster global food security, targeting “100 billion kilograms of grain” in the hilly regions of the Loess Plateau [5]. Thus, it is crucial to maintain crop yields and improve fertilizer use efficiency within the limited arable land in the semi-arid, rain-fed agricultural regions of the Loess Plateau.

Fertilizers, particularly nitrogen (N) fertilizers, are essential for plant growth, and are often referred to as the “food” of grain. They directly influence the accumulation of aboveground dry matter, photosynthetic characteristics, physiological metabolism, and grain yield in plants [6]. N fertilization has been shown to facilitate the translocation of assimilates from pre-flowering organs to wheat grains, likely due to the insufficiency of post-flowering photosynthetic assimilation to meet grain-filling demands [7]. Studies have indicated that increasing the N-fertilizer rate can enhance the transfer and contribution rates of stored dry matter from wheat leaves and stems to grains pre-flowering [8]. However, other research presents conflicting findings [9]. Therefore, it is essential to investigate further the impact mechanism of N-fertilizer rate on the transport volume, transport rate, and contribution rate of dry matter stored in nutrient organs post-flowering to optimize N fertilization and enhance its efficiency.

In recent years, excessive N fertilization in pursuit of maximum crop yields has become increasingly prevalent [10]. Despite increased usage, crop yields and the efficiency of N fertilizer utilization have not significantly improved, indicating a global trend of declining N-use efficiency [5]. Plants obtain N primarily through two mechanisms: firstly, by directly absorbing N from the soil through their roots; and secondly, by activating and redistributing nutrients stored in aging or previously-used tissues, a process known as N translocation and redistribution within the plant [5]. Understanding the patterns of N absorption, accumulation, and transport in wheat can aid in regulating crop growth, tailoring fertilizer application to the plant’s developmental needs, and enhancing N-use efficiency while boosting yields.

The grain-filling stage is essential for N supply in wheat, during which canopy N levels typically decline as the plant matures [11]. During this stage, the movement of N from leaves and stems to the ear becomes predominant [12]. N in crop grains primarily originates from two sources: post-flowering assimilation and N absorption by senescent organs [13]. The N uptake in various tissues during the vegetative growth period significantly affect the N supply in grains, as plants transport a substantial portion of accumulated N in their nutrient tissues to the grains in later stages [5]. During the grain filling period, N transport occurs mainly via redistribution. Studies have shown that leaves and stems are the primary N sources for grains, while the contributions of glumes (around 15%) and roots (around 10%) are relatively modest [14]. In the early flowering stage, the N content in leaves and stems increases through net nutrient absorption. During flowering, N is transferred to the developing grains, transforming these organs from a “reservoir” to a “source” [5]. Developing grains efficiently absorb N from aging parts, storage sources, and root absorption [5]. By harvest time, approximately 80% of aboveground tissue N has been transferred to the grain [15], accounting for 50–100% of the grain N content at harvest [16].

In wheat, 70 to 95% of grain N is derived from N redistribution stored in vegetative tissues during the early flowering stage [17], with only a minor portion absorbed post-flowering [18]. The N-fertilizer rate influences N absorption by plants, with high N > low N > no N application [19]. Nonetheless, excessive N fertilization can hinder N transport to grains. Cox et al. [20] showed that high-N fertilization pre-flowering increases N uptake post-flowering but reduces N transport capacity, whereas low-N conditions can enhance it [21]. Therefore, the ability to allocate N to stems and leaves during vegetative growth and subsequently transfer it to grains is crucial for determining crop N-use efficiency.

N-use efficiency is a key indicator for assessing N absorption efficiency, impacting both the economic benefits of agricultural production and ecological environment security [22,23,24]. Thus, to advance the “second green revolution”, it is necessary to determine the N fertilizer application based on the actual N demand of crops to enhance N-use efficiency [5]. N-use efficiency is a multifaceted trait, influenced by several steps including N absorption, transport, assimilation, and migration, all regulated by multiple interacting factors [5]. Optimal fertilization practices ensure that all applied fertilizers are absorbed and utilized by crops without wastage [25]. When N fertilization exceeds crop requirements, its uptake efficiency decreases, leading to excess N remaining in the soil or being lost [26]. Conversely, if N fertilizer input falls short of crop needs, soil N will be depleted [27]. Ultimately, crop yield is restricted by N absorption, which is influenced by both genetic and environmental factors. These factors can affect the potential for crop yield improvement, disrupting the balance between N fertilization and N accumulation in crops, and ultimately impacting N-use efficiency [28]. Hence, optimizing crop N management and rational N fertilization practices is of paramount importance in agricultural research [29]. However, the N fertilizer recovery rate remains low, between 25% and 50% [30,31], primarily due to the mismatch between N fertilization and crop requirements [32]. Previous studies have predominantly evaluated N-use efficiency based on wheat grain or overall biomass yield. Nonetheless, the effects of dry matter translocation and N redistribution in various wheat organs on N-use efficiency remains unclear.

We hypothesized that N fertilization influences the translocation of both dry matter and N in wheat, ultimately affecting N-use efficiency. Optimizing N fertilization can enhance the migration of dry matter and N, thereby improving N-use efficiency. Our objectives are: (i) to analyze the patterns of dry matter translocation in wheat; (ii) to identify the characteristics of N absorption, transport, and redistribution in wheat; and (iii) to explore the mechanisms affecting N-use efficiency and determine the optimal N-fertilizer rate for spring wheat fields in the arid hilly areas of central Gansu. This research seeks to enhance the management of N in rain-fed agricultural systems and establish a theoretical foundation for sustainable and intensive wheat cultivation.

2. Materials and Methods

2.1. Experimental Site

The long-term N fertilization experiment for mono-cropping spring wheat began in 2003 in Anding District, Dingxi City, located in the hilly region of central southern Gansu Province, a typical semi-arid rain-fed agricultural area without irrigation. From 2003 to 2019, temperatures ranged from −22 °C in January to 38 °C in July. The annual mean precipitation and evaporation were 390.7 mm and 1531 mm (variation coefficient 24.3%), respectively. Solar radiation averaged 5930 MJ m⁻² annually, with 2480 h of sunshine per year. The mean annual temperature was 6.4 °C, and the accumulated temperature of ≥10 °C was 2240 °C. The frost-free period lasted 140 days. Monthly soil rainfall and temperature data for the experimental area are shown in Figure 1. In 2018, precipitation was 479 mm with an average temperature of 7.2 °C; in 2019, precipitation was 478.4 mm with an average temperature of 7.4 °C. The field had a long tradition of conventional farming, with flax (Linum usitatissimum L.) being the previous crop. The soil properties at a depth of 0–80 cm were 0.78 g kg⁻¹ total N, 5.04 mg kg⁻¹ NH₄-N, 27.60 mg kg⁻¹ NO₃-N, 8.32 pH, 1.80 g kg⁻¹ total phosphorus, 6.78 mg kg⁻¹ available potassium, and 18.40 g kg⁻¹ total potassium, 221.42 mg kg⁻¹ available potassium, 1.20 g cm⁻³ bulk density, and 12.20 g kg⁻¹ organic matter.

2.2. Experimental Design and Field Management

The experiment employed a completely randomized block design, repeated three times, with each plot encompassing a 30-square-meter area. Five different N-fertilizer rate treatments were tested: N0, N52.5, N105, N157.5, and N210, corresponding to annual N fertilizer doses of 0, 52.5, 105.0, 157.5, and 210.0 kg N ha⁻¹, respectively. Each treatment received the same amount of phosphorus fertilizer (105 kg P₂O₅ ha⁻¹) before sowing. The high-yielding spring wheat variety Dingxi 38 was sown in mid-March at a seeding rate of 187.5 kg ha⁻¹ with a row spacing of 20 cm. Harvesting occurred from late July to early August. Throughout the growing season, weeds were manually removed, and Roundup^® (glyphosate, 10%) was applied as needed during the fallow period post-harvest. Pests and diseases were monitored and controlled following conventional practices in the region.

2.3. Sampling and Analysis

Throughout the various growth stages of wheat, aboveground samples were collected in each test plot. During the jointing stage, samples were taken without differentiating between organs. At anthesis, samples were categorized into leaves, stem + sheath, and ear. At maturity, samples were divided into leaves, stem + sheath, rachilla + glume, and grain. These samples were then oven dried at 70 °C, weighed to obtain the dry matter of each organ, ground, and sieved (<2 mm) for N testing. For the maturity stage, 0.5 m of side rows were removed from each plot before harvesting the remaining plants. After natural drying and threshing, the wheat yield was measured. N concentration was analyzed using the Semimicro Kjeldahl method with a Vapodest 50s (Gerhardt, Königswinter, Germany) [33].

2.4. Calculation

Throughout the wheat reproductive growth stage, it is assumed that the decrease in dry matter and nutrient accumulation in the vegetative organs is transferred to the grains. To quantify this process, various parameters were calculated, including the dry matter translocation amount (DTA, mg stem⁻¹), dry matter translocation efficiency (DTE, %), dry matter contribution proportion (DCP, %), crop growth rate (CGR, kg ha⁻¹ d⁻¹), nitrogen (N) translocation amount (NTA, kg ha⁻¹), N translocation efficiency (NTE, %), and N contribution proportion (NCP, %). These were calculated using Equations (1)–(7).

DTA = dry matter accumulation amount at anthesis − dry matter accumulation amount in vegetative organs (leaf, stem + sheath, and glumes) at maturity

(1)

$$\mathrm{DTE}={\displaystyle \frac{\mathrm{DTA}}{\mathrm{dry} \mathrm{matter} \mathrm{accumulation} \mathrm{amount} \mathrm{at} \mathrm{anthesis}}}\times 100$$

(2)

$$\mathrm{DCP}={\displaystyle \frac{\mathrm{DTA}}{\mathrm{grain} \mathrm{yield}}}\times 100$$

(3)

$$\mathrm{CGR}={\displaystyle \frac{\mathrm{changes} \mathrm{of} \mathrm{dry} \mathrm{matter} \mathrm{accumulation}}{\mathrm{time} \mathrm{interval} \mathrm{between} \mathrm{two} \mathrm{samples}}}$$

(4)

NTA = N accumulation at anthesis − N accumulation at maturity (excluding grain)

(5)

$$\mathrm{NTE}={\displaystyle \frac{\mathrm{NTA}}{\mathrm{N} \mathrm{uptake} \mathrm{at} \mathrm{anthesis}}}\times 100$$

(6)

$$\mathrm{NCP}={\displaystyle \frac{\mathrm{NTA}}{\mathrm{grain} \mathrm{N} \mathrm{uptake}}}\times 100$$

(7)

N-use efficiency was assessed based on N harvest index (NHI), N utilization efficiency (NUE, kg kg⁻¹), N agronomic efficiency (NAE, kg kg⁻¹), N recovery efficiency (NRE, %), N partial factor productivity (NPFP, kg kg⁻¹), and N marginal benefit (NMB, kg kg⁻¹), which were calculated using Equations (8)−(13).

$$\mathrm{NHI}={\displaystyle \frac{\mathrm{grain} \mathrm{N} \mathrm{uptake}}{\mathrm{total} \mathrm{N} \mathrm{uptake} }}$$

(8)

$$\mathrm{NUE}={\displaystyle \frac{\mathrm{grain} \mathrm{yield}}{\mathrm{total} \mathrm{N} \mathrm{uptake} }}$$

(9)

$$\mathrm{NAE}={\displaystyle \frac{\mathrm{grain} \mathrm{yield} \mathrm{of} \text{N-treatment}-\mathrm{grain} \mathrm{yield} \mathrm{of} \text{non-N-fertilized} \mathrm{control}}{\mathrm{N} \mathrm{application} \mathrm{rate} }}$$

(10)

$$\mathrm{NRE}={\displaystyle \frac{\mathrm{total} \mathrm{N} \mathrm{uptake} \mathrm{of} \text{N-treatment}-\mathrm{total} \mathrm{N} \mathrm{uptake} \mathrm{of} \text{non-N-fertilized} \mathrm{control}}{\mathrm{N} \mathrm{application} \mathrm{rate} }}$$

(11)

$$\mathrm{NPFP}={\displaystyle \frac{\mathrm{grain} \mathrm{of} \text{N-treatment}}{\mathrm{N} \mathrm{application} \mathrm{rate} }}$$

(12)

$$\mathrm{NMB}={\displaystyle \frac{\mathrm{grain} \mathrm{yield} \mathrm{increase}}{\mathrm{N} \mathrm{application} \mathrm{rate} \mathrm{increase}}}$$

(13)

2.5. Statistical Analysis

Statistical analysis was performed using IBM SPSS version 21.0 software. To assess the differences in wheat dry matter translocation characteristics, N translocation characteristics, and N-use efficiency varied with different N-fertilizer rates, a one-way analysis of variance (ANOVA) was executed. Post hoc tests were performed to further analyze any significant differences identified by the ANOVA results, using the LSD method (p < 0.05) to assess significant differences among treatments. Additionally, Pearson correlation coefficient analysis was applied to evaluate the linear relationship among various indicators.

3. Results

3.1. Dry Matter Translocation Characteristics

3.1.1. Dry Matter Accumulation

The N-fertilized treatments (N52.5–N210) significantly increased DMA at various wheat development stages, following the trend: maturity > anthesis > jointing. At jointing, the N-fertilized treatments significantly increased the two-year average DMA by 25.91%, 30.47%, 35.02%, and 35.08%, respectively, with no significant difference among N-fertilized treatments (

Table 1. Effects of N-fertilizer rate on the dry matter accumulation amount (DMA, mg stem⁻¹), dry matter accumulation amount during specific period (DMAP, kg ha⁻¹), and crop growth rate (CGR, kg ha⁻¹ d⁻¹) ².

Year	Treatment 1	DMA			DMAP		CGR
		Jointing	Anthesis	Maturity	Jointing to Anthesis	Anthesis to Maturity	Jointing to Anthesis	Anthesis to Maturity
2018	N0	684 ± 57.2b	1184 ± 114.1c	1173 ± 100.0d	2028 ± 170.1c	1532 ± 261.4b	65.4 ± 5.5c	66.6 ± 11.4b
	N52.5	722 ± 67.5b	1361 ± 106.5bc	1572 ± 127.4cd	2478 ± 261.1c	2426 ± 33.5a	79.9 ± 8.4c	105.5 ± 1.5a
	N105	1062 ± 30.4a	2192 ± 123.0a	2634 ± 230.4a	3125 ± 145.8b	2885 ± 130.7a	100.8 ± 4.7b	125.4 ± 5.7a
	N157.5	872 ± 76.9ab	2250 ± 117.0a	2249 ± 172.2ab	4591 ± 253.5a	1572 ± 144.4b	148.1 ± 8.2a	68.4 ± 6.3b
	N5	770 ± 80.8b	1755 ± 162.8b	1827 ± 229.8bc	3678 ± 118.4b	1830 ± 138.5b	118.7 ± 3.8b	79.5 ± 6.0b
	ANOVA p-value	0.014	<0.001	0.002	<0.001	<0.001	<0.001	<0.001
2019	N0	598 ± 76.7b	1309 ± 95.4b	1783 ± 105.2c	3245 ± 90.6b	1683 ± 142.0b	124.8 ± 3.5b	51.0 ± 4.3b
	N52.5	903 ± 1.3a	2027 ± 23.1a	2100 ± 69.2b	3714 ± 313.8b	1658 ± 110.4b	142.9 ± 12.1b	50.2 ± 3.4b
	N105	909 ± 31.2a	2138 ± 57.7a	2600 ± 58.0a	5631 ± 158.3a	2449 ± 329.5a	216.6 ± 6.1a	74.2 ± 10.0a
	N157.5	877 ± 88.5a	1986 ± 109.0a	2486 ± 103.5a	5688 ± 203.7a	2576 ± 131.8a	218.8 ± 7.8a	78.1 ± 4.0a
	N5	853 ± 31.1a	2023 ± 12.5a	2396 ± 40.3a	6199 ± 147.8a	1972 ± 240.8ab	238.4 ± 5.7a	59.8 ± 7.3ab
	ANOVA p-value	0.014	<0.001	<0.001	<0.001	0.030	<0.001	0.030
Average	N0	2744 ± 14.2b	5381 ± 119.3d	6988 ± 127.8c	2637 ± 129.9d	1607 ± 83.2c	95.1 ± 4.5d	58.8 ± 5.0c
	N52.5	3456 ± 27.6a	6552 ± 102.8c	8594 ± 149.6b	3096 ± 116.8c	2042 ± 57.1b	111.4 ± 4.4c	77.9 ± 1.8b
	N105	3581 ± 24.7a	7959 ± 173.9b	10625 ± 159.6a	4378 ± 151.6b	2667 ± 224.7a	158.7 ± 5.4b	99.8 ± 7.6a
	N157.5	3706 ± 203.4a	8845 ± 121.1a	10919 ± 119.4a	5139 ± 177.7a	2074 ± 12.7b	183.4 ± 6.2a	73.2 ± 1.2b
	N5	3707 ± 129.6a	8646 ± 123.5a	10547 ± 172.2a	4939 ± 14.7a	1901 ± 50.2b	178.6 ± 0.9a	69.7 ± 2.3bc
	ANOVA p-value	<0.001	<0.001	<0.001	<0.001	0.001	<0.001	0.001

¹ N0, N52.5, N105, N157.5, and N210 represent annual N-fertilizer rates of 0, 52.5, 105.0, 157.5, and 210.0 kg N ha⁻¹, respectively. ² Columns with different letters denote statistically significant differences as determined by Duncan’s multiple range test (p ≤ 0.05).

). At anthesis, the N-fertilized treatments significantly increased the two-year average DMA by 21.76%, 47.90%, 64.38%, and 60.68%, respectively, with higher-N treatments (N157.5 and N210) significantly outperforming lower-N treatments (N52.5 and N105). At maturity, the N-fertilized treatments significantly increased the two-year average DMA by 22.97%, 52.04%, 56.25%, and 50.95%, respectively, with no significant increase beyond 105 kg N ha⁻¹.

N fertilization significantly enhanced DMAP across various wheat development stages. Before flowering, the N-fertilized treatments significantly increased the two-year average DMAP by 17.43%, 66.05%, 94.93%, and 87.33%, respectively, with higher-N treatments (N157.5 and N210) significantly outperforming lower-N treatments (N52.5 and N105). The DMAP from anthesis to maturity was greatest at 105 kg N ha⁻¹, exhibiting an increase of 28.58–65.90% over other treatments. The CGR responses to N-fertilizer rates mirrored those of DMAP. During the jointing to anthesis period, the N-fertilized treatments significantly increased the two-year average CGR by 17.11%, 66.84%, 92.85%, and 87.72%, respectively, with higher-N treatments (N157.5 and N210) significantly outpacing lower-N treatments (N52.5) and medium-N treatment (N105). The CGR from anthesis to maturity was greatest at 105 kg N ha⁻¹, showing an increase of 28.19–69.75% compared to other treatments.

3.1.2. Dry Matter Distribution

Table 2. Dry matter distribution proportion (DDP, %) in different wheat tissues at various development stages ².

Year	Growth Stage	Treatment 1	DDP (%)
			Leaf	Stem + Sheath	Ear	Rachilla + Glume	Grain
2018	Anthesis	N0	10.40 ± 0.34c	52.50 ± 1.12a	37.10 ± 1.46a	—	—
		N52.5	12.24 ± 0.74bc	49.76 ± 1.74ab	38.00 ± 1.94a	—	—
		N105	13.89 ± 0.11b	47.25 ± 0.56bc	38.86 ± 0.52a	—	—
		N157.5	16.16 ± 0.31a	45.68 ± 1.05c	38.17 ± 0.74a	—	—
		N210	15.86 ± 1.03a	46.10 ± 0.61c	38.04 ± 0.85a	—	—
		ANOVA p-value	<0.001	0.007	0.895	—	—
	Maturity	N0	9.58 ± 0.31a	40.23 ± 1.52a	—	19.31 ± 1.30a	30.88 ± 0.73ab
		N52.5	11.80 ± 0.88a	38.06 ± 1.39a	—	22.28 ± 2.31a	27.85 ± 1.81b
		N105	11.87 ± 0.95a	40.68 ± 1.40a	—	19.24 ± 2.10a	28.20 ± 0.56b
		N157.5	11.23 ± 0.90a	39.81 ± 1.30a	—	19.53 ± 1.79a	29.43 ± 0.63ab
		N210	9.94 ± 0.88a	37.83 ± 1.33a	—	19.90 ± 1.27a	32.32 ± 0.44a
		ANOVA p-value	0.228	0.515	—	0.738	0.041
2019	Anthesis	N0	15.01 ± 0.72b	59.20 ± 2.53a	25.80 ± 1.86a	—	—
		N52.5	17.22 ± 0.38a	56.35 ± 0.48a	26.43 ± 0.10a	—	—
		N105	17.72 ± 0.14a	57.00 ± 0.52a	25.28 ± 0.46a	—	—
		N157.5	17.27 ± 0.64a	56.44 ± 0.41a	26.28 ± 1.05a	—	—
		N210	17.16 ± 0.72a	57.57 ± 0.81a	25.27 ± 0.09a	—	—
		ANOVA p-value	0.048	0.512	0.866	—	—
	Maturity	N0	8.30 ± 0.45b	41.19 ± 0.70a	—	17.51 ± 0.48a	33.00 ± 0.28b
		N52.5	7.86 ± 0.10b	40.14 ± 0.44ab	—	17.33 ± 0.80a	34.66 ± 0.45ab
		N105	7.86 ± 0.38b	41.01 ± 1.27a	—	17.5 ± 0.22a	33.64 ± 1.27b
		N157.5	8.71 ± 0.21ab	38.10 ± 0.58b	—	16.56 ± 0.5a	36.63 ± 0.76a
		N210	9.26 ± 0.16a	39.65 ± 0.5ab	—	17.94 ± 0.25a	33.14 ± 0.56b
		ANOVA p-value	0.030	0.093	—	0.434	0.033
Average	Anthesis	N0	12.81 ± 0.55b	56.02 ± 1.29a	31.17 ± 1.25a	—	—
		N52.5	15.22 ± 0.56a	53.63 ± 0.48ab	31.15 ± 0.93a	—	—
		N105	15.79 ± 0.13a	52.07 ± 0.64bc	32.15 ± 0.64a	—	—
		N157.5	16.70 ± 0.21a	50.73 ± 0.80c	32.57 ± 0.94a	—	—
		N210	16.52 ± 0.77a	52.28 ± 0.27bc	31.20 ± 0.57a	—	—
		ANOVA p-value	0.002	0.007	0.704	—	—
	Maturity	N0	8.79 ± 0.33a	40.85 ± 0.88a	—	18.19 ± 0.61a	32.17 ± 0.21ab
		N52.5	9.51 ± 0.25a	39.31 ± 0.44a	—	19.34 ± 0.80a	31.85 ± 0.47ab
		N105	9.84 ± 0.56a	40.84 ± 1.25a	—	18.45 ± 1.07a	30.87 ± 0.72b
		N157.5	9.86 ± 0.25a	38.87 ± 0.8a	—	18.05 ± 1.03a	33.22 ± 0.64a
		N210	9.51 ± 0.4a	38.92 ± 0.81a	—	18.72 ± 0.55a	32.84 ± 0.27a
		ANOVA p-value	0.326	0.330	—	0.825	0.057

¹ N0, N52.5, N105, N157.5, and N210 represent annual N-fertilizer rates of 0, 52.5, 105.0, 157.5, and 210.0 kg N ha⁻¹, respectively. ² Columns with different letters denote statistically significant differences as determined by Duncan’s multiple range test (p ≤ 0.05).

shows the impact of varying N-fertilizer rates on the two-year and average DDP in different wheat tissues at various developmental stages. At anthesis, the two-year average DDP of stem + sheath (50.73–56.02%) was greater than other tissues, while at maturity, the ear exhibited the greatest two-year average DDP (49.32–51.57%). The smallest two-year average DDP was observed in the leaf, ranging from 12.81–16.70% at anthesis and 8.79–9.86% at maturity. The overall trend of two-year average DDP at anthesis was stem + sheath > ear > leaf, whereas at maturity, it was stem + sheath > grain > rachilla + glume > leaf. Compared to N0, N-fertilized treatments significantly increased the two-year average DDP of leaves at anthesis by 18.78%, 23.21%, 30.37%, and 28.96%, respectively. N fertilization significantly reduced the two-year average DDP of stem + sheath at anthesis, with a decrease correlating with increasing N-fertilizer rates, and had no significant impact on the DDP of ear at anthesis. Compared to higher-N treatments (N157.5 and N210), the two-year average DDP of grain at maturity was lowest at the 105 kg N ha⁻¹ rate, 6.00–7.08% lower than other treatments.

3.1.3. Dry Matter Transport

Figure 2A–C shows the impact of varying N-fertilizer rates on the DTA, DTE, and DCP in wheat. Compared to N0, the N-fertilized treatments led to significant increases in DTA by 81.51%, 46.04%, 120.12%, and 92.62%, respectively. However, the DTA for the N105 treatment was significantly lower than that of other N-fertilized treatments. Additionally, the DTE of the N105 treatment showed a significant reduction when compared to the other treatments, including N0. In contrast, other N-fertilized treatments (N52.5, N157.5, and N210) exhibited DTE values that were significantly 26.97–33.55% greater than N0. The trend of DCP mirrored that of DTE.

The DTA of various wheat tissues followed the order: rachilla + glume > stem + sheath > leaf (excluding N105) (Figure 2D). Compared to N0, the N-fertilized treatments significantly increased the DTA of leaves by 180.37%, 188.42%, 308.85%, and 276.95%, respectively. Relative to N0, the N52.5, N157.5, and N210 treatments significantly increased the DTA of stem + sheath by 97.87%, 66.03%, and 74.82%, respectively, while the N105 treatment did not show significant improvement. Similarly, N-fertilized treatments significantly increased the DTA of rachilla + glume by 44.60%, 75.10%, 116.17%, and 61.45%, respectively.

The DTE of various wheat tissues followed the order: rachilla + glume > leaf > stem + sheath (Figure 2E). Relative to N0, the N52.5, N157.5, and N210 treatments significantly increased the DTE of leaves by 70.79%, 82.66%, and 86.89%, respectively, with no significant difference between N105 and N0. The DTE of stem + sheath at 105 kg N ha⁻¹ was significantly lower than other treatments, including N0.

The DCP followed the order: rachilla + glume > stem + sheath > leaf (excluding N105) (Figure 2F). Relative to N0, the N-fertilized treatments significantly increased the DCP of leaves by 94.37%, 64.95%, 144.03%, and 149.80%, respectively. Additionally, higher-N-fertilized treatments (N157.5 and N210) significantly increased the DCP of leaves. Conversely, the DCP of stem + sheath and rachilla + glume at 105 kg N ha⁻¹ was significantly reduced.

3.1.4. Correlation between Yield and Dry Matter Translocation Characteristics

Increases in DMA at various developmental stages, DMAP, and CGR from jointing to anthesis improved wheat grain yield (

Table 3. Correlation coefficients between dry matter accumulation amount (DMA, mg stem⁻¹), dry matter accumulation amount in period (DMAP, kg ha⁻¹), crop growth rate (CGR, kg ha⁻¹ d⁻¹), dry matter distribution proportion (DDP, %), dry matter translocation amount (DTA, mg stem⁻¹), dry matter translocation efficiency (DTE, %), and grain yield (kg ha⁻¹).

Item			Grain Yield
DMA		Jointing	r = 0.554 *, p = 0.032
		Anthesis	r = 0.752 **, p = 0.001
		Maturity	r = 0.726 **, p = 0.002
DMAP		Jointing to anthesis	r = 0.863 **, p < 0.001
		Anthesis to maturity	r = 0.132, p = 0.638
CGR		Jointing to anthesis	r = 0.863 **, p < 0.001
		Anthesis to maturity	r = 0.132, p = 0.638
DDP	Anthesis	Leaf	r = 0.847 **, p < 0.001
		Stem + sheath	r = −0.867 **, p < 0.001
		Ear	r = 0.412, p = 0.127
	Maturity	Leaf	r = 0.422, p = 0.117
		Stem + sheath	r = −0.564 *, p = 0.029
		Rachilla + glume	r = −0.026, p = 0.926
		Grain	r = 0.333, p = 0.225
DTA		Leaf	r = 0.718 **, p = 0.003
		Stem + sheath	r = 0.184, p = 0.512
		Rachilla + glume	r = 0.595 *, p = 0.019
DTE		Leaf	r = 0.442, p = 0.099
		Stem + sheath	r = −0.149, p = 0.597
		Rachilla + glume	r = 0.233, p = 0.404

Correlation coefficients marked with * and ** are considered statistically significant at p-values less than 0.05 and 0.01, respectively.

). Additionally, a higher DDP in leaves at anthesis, as well as DTA in leaves and rachilla + glumes, also increased wheat grain yield. Conversely, a higher DDP in stem + sheath at anthesis and maturity decreased grain yield.

3.2. Nitrogen Translocation Characteristics

3.2.1. Nitrogen Accumulation

Over a 17-year period, annual N-fertilizer rates significantly influenced the aboveground NAA in wheat (Figure 3). N fertilization significantly increased NAA at various wheat development stages. At anthesis, the N-fertilized treatments increased the two-year average NAA by 96.94%, 167.68%, 169.61%, and 134.60%, respectively. At maturity, the N-fertilized treatments significantly increased the two-year average NAA by 52.04%, 129.98%, 126.05%, and 104.78%, respectively, with no significant increase beyond 105 kg N ha⁻¹.

The NAA of ear (5.27–12.95 mg stem⁻¹) was higher than in other tissues at anthesis. However, at maturity, the NAA in the grain (9.92–18.43 mg stem⁻¹) was higher (Figure 3). At anthesis, the NAA followed the trend: ear >stem + sheath > leaf, while at maturity, the trend was: grain > stem + sheath > glume > leaf. Notably, at anthesis, the N-fertilized treatments increased the two-year average NAA of leaves by 161.61%, 267.26%, 295.88%, and 234.26%, respectively, with no significant increase beyond 157.5 kg N ha⁻¹. The N-fertilized treatments also increased the two-year average NAA of stem + sheath by 82.38%, 125.41%, 141.89%, and 106.48%, respectively. Additionally, the N-fertilized treatments increased the two-year average NAA of ear by 72.05%, 145.75%, 135.48%, and 107.73%, respectively. At maturity, the N-fertilized treatments increased the two-year average NAA of leaves by 122.28%, 226.40%, 246.83%, and 200.88%, respectively. The N-fertilized treatments also increased the two-year average NAA of stem + sheath by 71.20%, 193.34%, 175.20%, and 140.00%, respectively. Furthermore, the N-fertilized treatments increased the two-year average NAA of glume by 111.83%, 181.61%, 201.66%, and 196.74%, respectively. Lastly, the N-fertilized treatments increased the two-year average NAA of grain at maturity by 23.58%, 85.78%, 82.25%, and 62.40%, respectively.

3.2.2. Nitrogen Distribution

Table 4. Nitrogen distribution ratio of various wheat organs (%) ².

Year	Treatment 1	Anthesis			Maturity
		Leaf	Stem + Sheath	Ear	Leaf	Stem + Sheath	Rachilla + Glume	Grains
2018	N0	9.78 ± 0.57c	27.91 ± 2.13a	56.68 ± 1.76a	6.23 ± 0.63b	12.17 ± 0.99ab	11.64 ± 0.76b	75.14 ± 1.99a
	N52.5	13.73 ± 1.91bc	26.37 ± 2.00a	51.01 ± 1.58b	11.46 ± 1.30a	10.99 ± 0.67b	15.82 ± 0.69a	55.98 ± 1.22c
	N105	16.74 ± 1.11ab	24.43 ± 0.98a	49.38 ± 1.56b	9.81 ± 0.87a	14.03 ± 0.59a	13.61 ± 1.14ab	60.38 ± 1.19bc
	N157.5	19.42 ± 1.06a	24.17 ± 0.86a	47.94 ± 1.06b	10.15 ± 1.20a	11.77 ± 0.53ab	15.46 ± 0.35a	58.91 ± 2.33c
	N210	17.76 ± 2.51ab	23.28 ± 1.85a	50.79 ± 2.00b	8.47 ± 1.18ab	12.74 ± 0.70ab	15.80 ± 1.44a	64.92 ± 1.07b
	ANOVA p-value	0.012	0.339	0.030	0.051	0.104	0.044	< 0.001
2019	N0	23.39 ± 1.17b	26.07 ± 1.67a	25.64 ± 1.51a	6.20 ± 0.23b	13.20 ± 1.75a	8.54 ± 1.09b	80.50 ± 0.91a
	N52.5	27.42 ± 0.25ab	24.44 ± 2.16a	25.72 ± 1.53a	6.51 ± 0.70b	18.24 ± 2.41a	11.57 ± 0.77ab	70.79 ± 5.76ab
	N105	28.84 ± 1.95a	21.34 ± 2.14a	25.32 ± 2.39a	7.37 ± 0.43b	19.64 ± 2.80a	10.55 ± 0.96ab	66.81 ± 4.28b
	N157.5	29.14 ± 1.42a	24.87 ± 2.01a	22.97 ± 1.66a	8.84 ± 0.48a	19.63 ± 1.71a	10.68 ± 1.19ab	67.10 ± 2.34b
	N210	28.62 ± 1.77a	24.55 ± 1.32a	22.95 ± 0.88a	9.63 ± 0.15a	17.08 ± 3.42a	13.16 ± 1.23a	59.42 ± 0.63b
	ANOVA p-value	0.090	0.519	0.588	0.001	0.395	0.108	0.017
Average	N0	16.60 ± 0.87c	26.94 ± 1.80a	41.16 ± 1.86a	6.20 ± 0.18b	12.76 ± 1.42a	9.90 ± 0.61b	78.18 ± 1.33a
	N52.5	21.90 ± 0.33b	25.08 ± 1.80a	36.12 ± 0.59b	8.96 ± 1.03a	14.55 ± 0.82a	13.58 ± 0.60a	63.95 ± 4.17b
	N105	22.52 ± 0.75ab	23.04 ± 1.04a	37.82 ± 0.80ab	8.73 ± 0.77a	16.76 ± 1.09a	12.17 ± 0.89ab	62.82 ± 1.04b
	N157.5	24.32 ± 0.75a	24.40 ± 1.28a	35.80 ± 1.73b	9.44 ± 0.76a	16.04 ± 1.59a	13.06 ± 0.23a	62.94 ± 1.66b
	N210	23.44 ± 0.72ab	23.91 ± 0.55a	36.49 ± 1.10b	9.16 ± 0.49a	15.43 ± 2.15a	14.2 ± 1.07a	61.78 ± 0.35b
	ANOVA p-value	<0.001	0.393	0.085	0.052	0.417	0.016	0.001

¹ N0, N52.5, N105, N157.5, and N210 represent annual N-fertilizer rates of 0, 52.5, 105.0, 157.5, and 210.0 kg N ha⁻¹, respectively. ² Columns with different letters denote statistically significant differences as determined by Duncan’s multiple range test (p ≤ 0.05).

showed the impact of N-fertilizer rates on NDP across various wheat tissues at different development stages. At anthesis, the NDP of the ear (36.12–41.16%) was higher than that of other tissues, while at maturity, the NDP of the grain (61.78–78.18%) was the highest. The leaf exhibited the smallest NDP (16.60–24.32% at anthesis and 6.20–9.44% at maturity). The overall NDP trend was ear > stem + sheath > leaf at anthesis, and grain > stem + sheath > glume > leaf at maturity. Compared to N0, N-fertilized treatments significantly increased the two-year average leaf NDP by 31.93%, 35.70%, 46.50%, and 41.21%, respectively. Additionally, N52.5, N157.5, and N210 treatments significantly reduced the NDP of ear at anthesis by 12.25%, 13.02%, and 11.35%, respectively. N fertilization did not affect the NDP of stem + sheath at maturity. At maturity, N-fertilized treatments increased the two-year average NDP of leaves by 44.46%, 40.69%, 52.22%, and 47.65%, respectively. N52.5, N157.5, and N210 treatments also significantly increased the NDP of glumes by 13.58%, 13.06%, and 14.20%, respectively. Conversely, N-fertilized treatments significantly reduced the two-year average NDP of grain by 18.20%, 19.64%, 19.49%, and 20.97%, respectively. Finally, N fertilization had no effect on the NDP of stem + sheath.

3.2.3. Nitrogen Transport

Compared to N0, N-fertilized treatments increased the NTA of wheat vegetative organs by 95.00%, 143.82%, 146.61%, and 108.46%, respectively (Figure 4A). The N-fertilized treatments also increased the NCP of wheat vegetative organs by 57.83%, 31.13%, 35.19%, and 27.79%, respectively. The N52.5 treatments showed a significantly greater NCP than other treatments (Figure 4C). However, N-fertilizer rate did not significantly affect the NTE of wheat vegetative organs (Figure 4B).

The NTA of different wheat tissues followed the order: glume > leaf > stem + sheath (excluding N0) (Figure 4D). Compared to N0, N-fertilized treatments increased the NTA of leaves by 185.50%, 292.07%, 325.69%, and 254.54%, respectively, with no significant increase beyond 105 kg N ha⁻¹. N105-N210 treatments increased the NTA of glume by 134.59%, 114.90%, and 80.07%, respectively. The NTE of different wheat tissues followed the order: glume > leaf > stem + sheath (Figure 4E). Higher N-fertilized treatments (N157.5 and N210) significantly reduced the NTE of glume by 8.86% and 13.00%, respectively. The NCP followed the order: glume > leaf > stem + sheath (Figure 4F). Compared to N0, N-fertilized treatments significantly increased the leaf NCP by 128.31%, 108.72%, 132.60%, and 113.79%, respectively. Only the N52.5 treatment significantly reduced the NCP of stem + sheath by 12.23% compared to N0. Similarly, the N52.5 and N105 treatments significantly increased the glume NCP by 29.84% and 25.02%, respectively.

3.2.4. Correlation between Yield and Nitrogen Translocation Characteristics

Table 3 showed the correlation between yield and N accumulation, distribution, and transportation (

Table 5. Correlation coefficients between nitrogen (N) accumulation amount (NAA, mg stem⁻¹), N distribution proportion (NDP, %), N translocation amount (NTA, mg stem⁻¹), N translocation efficiency (NTE, %), and grain yield (kg ha⁻¹).

Item			Grain Yield
NAA		Jointing	r = 0.838 **, p < 0.001
		Anthesis	r = 0.851 **, p < 0.001
		Maturity	r = 0.859 **, p < 0.001
NDP	Anthesis	Leaf	r = 0.820 **, p < 0.001
		Stem + sheath	r = −0.448, p = 0.094
		Ear	r = −0.490, p = 0.064
	Maturity	Leaf	r = 0.645 **, p = 0.009
		Stem + sheath	r = 0.459, p = 0.085
		Rachilla + glume	r = 0.542 *, p = 0.037
		Grain	r = −0.711 **, p = 0.003
NTA		Leaf	r = 0.858 **, p < 0.001
		Stem + sheath	r = 0.511, p = 0.051
		Rachilla + glume	r = 0.716 **, p = 0.003
NTE		Leaf	r = 0.164, p = 0.559
		Stem + sheath	r = −0.450, p = 0.093
		Rachilla + glume	r = −0.567 *, p = 0.028

Correlation coefficients marked with * and ** are considered statistically significant at p-values less than 0.05 and 0.01, respectively.

). Higher NAA at various wheat development stages (jointing, anthesis, and maturity), NDP of leaves at anthesis and maturity, and NDP of rachilla + glume at maturity all increased wheat grain yield. Additionally, higher NTA of leaves and rachilla + glume also contributed to increased wheat grain yield. However, higher NDP of grain at maturity and NTE of rachilla + glume decreased grain yield.

3.3. Soil NO₃-N

The N-fertilizer rate significantly affected NO₃-N accumulation in the 0–100 cm soil layer at maturity (Figure 5). Compared to N0, the higher-N treatment (N210) resulted in a 26.27% increase in NO₃-N accumulation in the 0–100 cm soil layer in 2018. In 2019, the higher-N treatments (N157.5 and N210) led to increases of 81.58% and 122.44%, respectively, with no significant differences observed among other treatments (including N0). Over the two years, the N-fertilizer rate had no significant impact on NO3-N accumulation in the 0–40 cm soil layer, whereas the N210 treatment showed the highest NO₃-N accumulation in the 40–100 cm soil layer.

3.4. Nitrogen-Use Efficiency Indicators

This study evaluated various N-use efficiency indicators, including NHI, NUE, NAE, NRE, NPFP, and NMB (Figure 6). The N-fertilized treatments significantly decreased the two-year average NHI of wheat by 11.08%, 9.89%, 10.06%, and 12.85%, respectively (Figure 6A). Additionally, the N-fertilized treatments led to a significant reduction in the two-year average NUE of wheat by 11.14%, 15.14%, 17.42%, and 20.79%, respectively, decreasing with increasing N-fertilizer rates (Figure 6B). NAE, NRE, NPFP, and NMB also significantly differed among the N-fertilized treatments (Figure 6C–F).

The highest NAE (9.31 kg kg⁻¹) was observed at N105, followed by 8.62 kg kg⁻¹ at N157.5 and 7.95 kg kg⁻¹ at N52.5. The lowest NAE, 5.35 kg kg⁻¹, was observed at N5. Both NRE and NPFP exhibited a declining trend with increased N-fertilizer rates over the years. The highest NRE of 38.32% was observed at N105, while the highest NPFP (47.66 kg kg⁻¹) was noted at N52.5. The NMB trend paralleled that of NAE, with the highest NMB of 10.67 kg kg⁻¹ at N105, followed by 7.95 kg kg⁻¹ at N52.5 and 7.23 kg kg⁻¹at N157.5. The lowest NMB, −4.48 kg kg⁻¹, was recorded at N5, suggesting an over-application of N beyond the crop’s needs. Higher DTA and NTA decreased NHI and NUE, while higher NTE increased NHI, NUE, and NPFP (Figure 7). However, DTE had no significant impact on N-use efficiency indicators. Additionally, higher soil NO₃-N accumulation in the 0–100 cm layer decreased NHI, NUE, NAE, NPFP, and NMB, and had no significant effect on NRE.

4. Discussion

4.1. Effect of N-Fertilizer Rate on Dry Matter Characteristics

The final grain yield is closely related to the DMA [34,35]. Research indicates that dry matter accumulation exhibits a “slow–fast–slow” pattern across growth stages: slow before jointing, fastest from jointing to anthesis, and slow again post-flowering [36], aligning with the findings of this study. Increasing the N-fertilizer rate can elevate DMA at each growth stage and ultimately increase crop yield [37]. N fertilization significantly increased DMA at various wheat development stages, following the trend: maturity > anthesis > jointing (Table 1). Si et al. [38] reported that DMA was highest at 300 kg·ha⁻¹. We found that N-fertilized treatments significantly increased the two-year average DMA by 22.97–56.25% at maturity, with no significant increase beyond 105 kg N ha⁻¹. Regional differences contribute to the variation in N-fertilizer rates, with lower N demand in semi-arid rain-fed agricultural areas on the Loess Plateau due to water and variety constraints [39]. Furthermore, consistent with Si et al. [38], increasing N fertilization enhanced the intensity of dry matter accumulation, but a threshold effect was observed, with no significant difference between higher N105-N210 doses (Table 1). This underscores the importance of appropriate N fertilization for DMA [40].

N-fertilizer rate significantly influenced wheat crop growth [41]. Throughout this study, N fertilization significantly increased the DMAP at various wheat development stages, showing an overall trend of pre-flowering > post-flowering, consistent with Ye et al. [38]. During jointing to anthesis, N fertilization significantly increased the two-year average DMAP by 17.43–94.93%, with high-N treatment significantly greater than low-N treatment. N fertilization also significantly increased the two-year average CGR pre-flowering by 17.11–92.85%, with higher averages under higher-N treatments (N157.5 and N210). Nevertheless, the CGR post-flowering was highest at 105 kg N ha⁻¹, increasing by 28.19–69.75% compared to other treatments, further proving the threshold effect of N fertilization on DMA at various growth stages [40]. Additionally, the DDP of stem + sheath (50.73–56.02%) was greater than other tissues at anthesis, while the DDP of ear (49.32–51.57%) was greater at maturity, consistent with Dai et al. [41].

Grain yield can be attributed to two components: the transport of compounds stored in vegetative organs pre-flowering and the accumulation of photosynthetic products post-flowering. Zheng et al. [42] found that approximately 30% of wheat grain yield comes from pre-flowering vegetative organs, while around 60% is derived from post-flowering leaf photosynthesis. Ma et al. [37] found that increasing the N-fertilizer rate boosts the DTA and the DCP to grains post-flowering. Yan et al. [43] and Zhang et al. [44] noted that higher N-fertilizer rates reduce the DCP of pre-flowering, while increasing both the DTA and DCP post-flowering. This phenomenon may be associated with fertilization shortening the nutritional growth period of wheat, advancing the reproductive growth period, initiating spike differentiation earlier, and leading to premature grain filling. In this study, higher-N treatments (N157.5 and N210) were at excess levels, and a moderate reduction of N fertilization proved advantageous in sustaining the growth of wheat and enhancing the role of assimilate accumulation in grain production post-flowering. The N105 treatment resulted in the lowest DTA, DTE, and DCP, with no significant difference observed among N52.5, N157.5 and N210. A too-low N-fertilizer rate fails to meet the N demand during the production and growth stages, often resulting in premature senescence of wheat leaves during grain filling. Conversely, an excessively high N-fertilizer rate can delay crop maturity and is detrimental to dry matter accumulation in grains [45]. Yang et al. [46] also indicated that excessive N fertilization delays wheat aging, leading to a significant amount of non-structural carbohydrates remaining in the straw and reducing grain yield. Appropriate N reduction can improve the transport and filling characteristics of assimilates to grains post-flowering [35]. Additionally, the fitting results between pre-flowering and post-flowering DMA and yield indicate that DMA is fundamental for wheat grain yield (Table 3), which contradicts the conclusion by Yan et al. [43]. This discrepancy indicates certain limitations in using only dry matter characteristics to evaluate grain yield formation.

4.2. Effect of N-Fertilizer Rate on N Translocation Characteristics

N uptake, a critical measure of N limitation on plant growth, requires both N sources and reservoirs [47]. Beyond the available N conditions in the external soil, plant N absorption also influences the N content [48]. N fertilization can alter the effectiveness of soil N [49], thereby affecting plant N absorption and translocation [50]. During crop growth, N accumulates in the form of photosynthetic compounds, and the N in plants is continuously utilized and recycled. While N fertilization can increase the N absorption of crops, it may reduce N absorption efficiency [51]. Studies have shown that the pre-flowering period is the main phase for N absorption in wheat, accounting for about 80% of total N uptake [52]. Liu et al. [53] found that N absorption accumulates more significantly after the jointing stage, accounting for more than 60% of total N absorption, with the highest N absorption rate occurring during the jointing to flowering stage. Additionally, research indicates that N loss occurs in plants post-flowering through various pathways, leading to a decrease in N accumulation during the mature stage compared to the flowering stage [54]. In our study, the NAA exhibited an overall trend of anthesis > maturity > jointing. N105-N210 treatments notably boosted N content in wheat at various developmental stages (Figure 3), in line with Huang et al. [55]. N fertilization significantly increased NAA at various wheat development stages. At anthesis, N fertilization increased the two-year average NAA by 96.94–169.61%, with no further increase observed beyond 157.5 kg N ha⁻¹ (N157.5). At maturity, N fertilization significantly increased the two-year average NAA by 52.04–129.98%, with no significant increase beyond 105 kg N ha⁻¹, consistent with previous research [51]. However, N52.5, N157.5, and N210 treatments significantly reduced the NDP of the ear at anthesis by 12.25%, 13.02%, and 11.35%, respectively. This suggests that both insufficient and excessive N application can lead to plants allocating more N to vegetative growth, with some of the N not being effectively transported to reproductive growth, thereby significantly impacting N-use efficiency.

From flowering to maturity, N in various organs of wheat is continuously redistributed, primarily transported from vegetative organs to grains. Grain N is sourced from two main pathways: first, direct absorption and assimilation post-flowering; second, the retranslation of N accumulated by pre-flowering plants. Notably, the majority of N in grains originates from N stored in vegetative organs pre-flowering [56]. In our study, N fertilization increased the NTA of wheat vegetative organs by 95.00–146.61% (Figure 3A). Additionally, the NTA of different wheat tissues followed the order: glume > leaf > stem + sheath (Figure 4D), with the highest NTA of glume observed at 105 kg N ha⁻¹. The two-year average NTE ranged from 56.89–63.80%, with the highest value observed in the low-N treatment (N52.5), which was lower than the global meta-analysis estimates of 62.1% [57]. However, N fertilization did not significantly affect the NTE of wheat vegetative organs, which was inconsistent with results in perennial plants [58,59]. N fertilization increased the NCP of wheat vegetative organs by 27.79–57.83%, with the highest value observed in the low-N treatment (N52.5).

Efficient storage and movement of N in plant tissues are crucial for growth and reproduction, ultimately affecting N uptake and distribution in N-deficient environments, which are typically linked to increased plant productivity [60]. In many N-deficient ecosystems, plant N uptake is usually positively correlated with productivity [61]. This research revealed similar findings, demonstrating significant positive correlations between grain yield and NAA, as well as between grain yield and the NDP of leaves at the thesis and maturity stages, and the NDP of rachilla and glume at maturity. Conversely, a significant negative relationship was observed between grain yield and the NDP of grain, as well as the NTE of rachilla + glume at maturity. These findings align with the conclusions drawn by Millard and Grelet [60] in their study on trees, suggesting that increased N transfer and leaf N reabsorption can enhance crop yield. Therefore, increasing N accumulation positively impacts grain yield improvement. During the later stage of wheat growth, senescence can promote N retranslocation in leaves, thereby increasing distribution to grains. Promoting the reutilization and recycling of N in aging plant organs is also a critical strategy for achieving high wheat yield, consistent with Johannes et al. [62,63].

4.3. Nitrogen-Use Efficiency

The primary objective of N fertilizer is to maximize crop growth by ensuring an adequate N supply to crops [64]. Investigating methods to enhance N absorption and utilization in crop farming is crucial for the advancement of sustainable agriculture [65]. N-use efficiency is a common measure of N absorption efficiency [66]. Over-application of N fertilizer can reduce its utilization efficiency and result in economic losses [22,23]. The accumulation of NO₃–N in most Chinese farmlands is primarily due to the prolonged and unchecked application of N fertilizers [67]. In this study, compared to N0, the higher-N treatment (N210) resulted in a 26.27% increase in NO₃-N accumulation in the 0–100 cm soil layer in 2018 and a 122.44% increase in 2019 (Figure 5). Additionally, higher soil NO₃-N accumulation decreased NHI, NUE, NAE, NPFP, and NMB. Thus, optimizing N nutrition for crops and using N fertilizers judiciously is essential [29]. The N uptake efficiency is relatively low, typically ranging from 25% to 50% [30]. A global fertilization experiment at 306 locations found that N uptake efficiency declined as N-fertilizer rates increased [59]. In the northwest Loess Plateau region, N fertilization reduced the NHI of wheat by 9.89–12.85% and the NUE by 11.14–20.79% (Figure 6). The NRE (38.32%) and NMB (10.67 kg kg⁻¹) were highest at 105 kg N ha⁻¹ and lowest at 210 kg N ha⁻¹. Additionally, NMB was negative at 210 kg N ha⁻¹, indicating an over-application of N beyond the crop’s needs, consistent with Li et al. [68].

The decline in N fertilizer utilization efficiency indicates an increase in N availability that crops do not convert into grain yield [69]. Therefore, reducing the N fertilization used by farmers in the region is necessary to enhance N fertilizer utilization efficiency [70]. Ladha et al. [71] also emphasized the importance of N management to enhance absorption efficiency. Consequently, improving N-use efficiency through minimizing N fertilization and enhancing N uptake by crops is essential for sustainable crop yield. Our study found that DTA and NTA showed significant negative correlations with NHI and NUE, whereas NTE demonstrated significant positive correlations with NHI, NUE, and NPFP. However, DTE had no significant impact on N-use efficiency indicators. This indicates that N fertilization improved the N-use efficiency of spring wheat by enhancing N translocation rather than dry matter translocation.

5. Conclusions

This long-term experiment on N fertilization in mono-cropped spring wheat revealed that increasing the rate of N fertilizer significantly enhanced the DMA, NAA, NTE, and NCP of wheat, with no significant increase beyond 105 kg N ha⁻¹. Additionally, the high-N treatment (N210) led to an increase in NO₃-N accumulation in the 0–100 cm soil layer, ranging from 26.27% to 122.44%. N fertilization also significantly reduced the two-year average NHI by 9.89–12.85% and NUE by 11.14–20.79%, both decreasing with higher N application rates. However, both insufficient and excessive N applications led to more N being allocated to vegetative growth rather than reproductive growth, thereby impacting N-use efficiency. A rate of 105 kg N ha⁻¹ was identified as optimal for improving wheat DMA, CGR, and NAA, achieving the highest NAE of 9.31 kg kg⁻¹, NRE of 38.32%, and NMB of 10.67 kg kg⁻¹. This N rate improved N-use efficiency by enhancing N translocation rather than dry matter translocation.