Effects of Nutrient Deficiency on Crop Yield and Soil Nutrients Under Winter Wheat–Summer Maize Rotation System in the North China Plain

Abstract

The wheat–maize rotation system in the North China Plain (NCP) has a large amount of crop straw. However, improper crop straw management and blind fertilization lead to nutrient imbalance and accelerated nutrient loss from the soil, ultimately leading to nutrient deficiency affecting the wheat–maize rotation system. In order to explore the effects of nutrient deficiency on the yield and nutrient use efficiency of wheat and maize, the experiment was conducted in a randomized complete block design consisting of five treatments with three replicates for each treatment: (1) a potassium fertilizer deficiency and appropriate nitrogen and phosphate fertilizer treatment (NP); (2) a phosphate fertilizer deficiency and appropriate nitrogen and potassium fertilizer treatment (NK); (3) a nitrogen fertilizer deficiency and appropriate phosphate and potassium fertilizer treatment (PK); (4) an adequate nitrogen, phosphorus, and potassium fertilizer treatment (NPK); and (5) a no-fertilizer treatment (CK). The results showed that, compared with CK, the yields of wheat and maize treated with NPK were increased by 21.5% and 27.5%, respectively, and the accumulation of the dry matter of the wheat and maize was increased by 42.5% and 57.3%. In all the deficiency treatments, the NK treatment performed better in terms of yield compared to the NP and PK treatments, while the NP treatment demonstrated a greater increase in dry matter accumulation. The NPK treatment significantly improved the nitrogen use efficiency (NUE) and nitrogen harvest index (NHI) of the wheat and maize, which resulted in higher nitrogen accumulation in the NPK treatment, and the NP treatment was the best among the other nutrient deficiency treatments. The inorganic nitrogen content showed a similar trend. In conclusion, nutrient deficiency can severely restrict crop growth. Nitrogen deficiency can significantly reduce crop yields. Phosphorus deficiency had a greater impact than potassium deficiency in terms of nutrient absorption and accumulation. Therefore, nitrogen fertilizer application should be emphasized in crop rotation systems, with moderate increases in phosphorus fertilizer application. This practice can effectively improve the nutrient deficiency under the wheat and maize rotation system in the NCP and complete a rational fertilization system.

1. Introduction

With the increase in the population and demand for food, the intensive production of agriculture has accelerated the negative impact on the ecological environment [1]. Agricultural production is the main source of global greenhouse gas emissions, and improper nutrient management directly leads to the deterioration of the soil environment and nutrient imbalance [2]. As China is one of the most important agricultural countries in the world, the negative impact of agricultural production cannot be ignored.

The North China Plain (NCP) is a crucial region for grain production in China; it accounts for approximately 50% of China’s wheat planting area and production, and the maize planting area accounts for 30% and the production accounts for about 50% in China [3]. The stabilization of crop yields in the NCP region is important for securing China’s food production. In the rotation system, a large amount of crop straw burning has caused serious environmental pollution. At the same time, the pursuit of high yields by farmers has led to an increased prevalence of blind fertilizer application. Improper crop residue management and fertilizer application have attracted much attention from policy makers and scholars [4]. In order to reduce the air pollution and greenhouse gas emissions caused by straw burning, the government and related departments require farmers to remove crop residues from their fields after harvest. The ineffective use of straw resources will lead to a mismatch between the crop nutrient requirements and soil nutrient supply. This reduces the soil’s nutrient storage and supply capacity and negatively impacts the soil fertility in the region [5]. A study [6] showed that the long-term single fertilization pattern has a detrimental impact on the crop rotation system, accelerates soil nutrient loss, and leads to reduced sustainable soil productivity. Under the combined influence of stubble removal and long-term single fertilization, soil nutrient deficiency occurred frequently; nevertheless, the response of the grain yield and nutrient use efficiency to the nutrient deficiency in a wheat and maize rotation system remains ambiguous.

Nitrogen, phosphorus, and potassium are three essential nutrients for the growth and development of crops. Nutrient deficiency restricts the absorption of nutrients by plants, resulting in unmet nutrient demand of crops, thus seriously affecting the nutrient utilization efficiency [7]. Nitrogen deficiency reduces crop dry matter accumulation and affects the improvement in the annual yield [8]. Phosphorus deficiency reduces the soil phosphorus levels and causes soil nutrient imbalances [9]. A synergistic interaction exists between nitrogen and phosphorus fertilizers during the yield formation process. If crops cannot absorb sufficient phosphorus fertilizer during their growth period, the efficiency of the nutrient use utilization will be significantly restricted, thereby impeding the formation of the grain yield [10]. Potassium deficiency reduces the osmotic pressure of plant cells, limits stomatal activity and evaporation, restricts photosynthetic performance, and leads to a reduction in the total photosynthetic products [11,12].

In a wheat and maize rotation system, we analyzed the difference between a nutrient sufficiency treatment and nutrient deficiency treatment by simulating the nutrient deficiencies on an annual basis and explored the mechanism of nutrient deficiency on the synergistic effect of the plant aboveground and underground parts, crop nutrient use efficiency, and soil nutrient change degree. In this experiment, winter wheat–summer maize double cropping in southern Shanxi Province was used as the carrier, and the experiment started in the winter wheat season in 2019. The residual straw of the previous season’s crop was removed before sowing the following season’s crop to avoid additional nutrient input due to stubble decomposition. We have conducted research on the effects of nutrient deficiency on the root morphology and distribution characteristics in the early stage and initially explored the effects of individual nutrient deficiency on root characteristics, providing a theoretical basis for improving root distribution through rational fertilization. The main objectives of this study are to (i) determine the effect of nutrient deficiency on the grain yield of rotational crops; (ii) investigate the influence of different nutrient deficiency treatments on nitrogen utilization efficiency; and (iii) clarify the extent of the change in the inorganic nitrogen content under nutrient deficiency conditions. It is our goal to provide an effective nutrient management strategy for rotational cropping in the NCP to reduce the occurrence of nutrient imbalance caused by indiscriminate fertilization.

2. Materials and Methods

2.1. Experimental Site and Design

This study was conducted in two consecutive sowing seasons in 2021–2023 at Bai Village (35°67′ N, 111°18′ E), Xinjiang County, Yuncheng City, Shanxi Province. The experimental area is characterized by a temperate continental monsoon climate. The soil is classified as a Calcic Cambisol according to the FAO-UNESCO soil map [13], with a sandy loam texture. The meteorological data of the growing season during the experiment are shown in Figure 1. The main agricultural production method in this region is wheat maize rotation.

Table 1. Topsoil properties at experimental sites in 2021–2022 and 2022–2023.

Year	pH	SOM (g/kg)	TN (g/kg)	TP (g/kg)	TK (g/kg)	AP (g/kg)	AK (g/kg)	HN (g/kg)
2021	8.57	12.94	0.96	0.86	15.74	11.3	113	54.2
2022	8.49	13.06	0.93	0.75	16.08	10.9	109	63.5
2023	8.41	13.18	0.90	0.70	16.42	10.5	105	66.2

SOM: soil organic matter; TN: total nitrogen; TP: total phosphorus; TK: total potassium; AP: available phosphorus; AK: rapidly available potassium; HN: alkaline hydrolysis nitrogen.

shows the basic properties of the plow layer soil before sowing two seasons of crops.

This experiment commenced at the winter wheat season in 2019. The experiment was conducted in a completely randomized block design with 3 replications and a total of 15 plots; each experimental area was 30 m² (6 m in length and 5 m in width); the treatments were set up as potassium fertilizer deficiency and appropriate nitrogen and phosphate fertilizer treatment (NP); phosphate fertilizer deficiency and appropriate nitrogen and potassium fertilizer treatment (NK); nitrogen fertilizer deficiency and appropriate phosphate and potassium fertilizer treatment (PK); adequate nitrogen, phosphorus and potassium fertilizer treatment (NPK); and no-fertilizer treatment (CK). The experimental plot was compacted by rotary tillage and then manually sown. The summer maize variety was Denghai 605, and the winter wheat variety was Malan No. 1. Urea with 46% nitrogen (N), superphosphate (P₂O₅ 15%), and potassium sulfate (K₂O 50%) were used in this experiment, all of which were applied as a one-time basal fertilizer before sowing. Winter wheat for all plots in 2021–2023 was sown on 15–20 October and harvested on 10–15 June; summer maize for all plots in 2022–2023 was sown on 15–20 June and harvested on 10–15 October. The experimental fields were irrigated, weeded, and disease and insect controlled according to local standard farmland. Specific nutrient allocation and cultivation management measures are shown in

Table 2. Cultivation and management practices of different nutrient allocations.

Treatment		NP		NK		PK		NPK		CK
		Wheat	Maize	Wheat	Maize	Wheat	Maize	Wheat	Maize	Wheat	Maize
Tillage method		Straw removal, manual rotary tillage
Seeding rate (×104 seeds ha−1)		300	6.75	300	6.75	300	6.75	300	6.75	300	6.75
Fertilizer application rates (kg ha−1)	N	276	276	276	276	0	0	276	276	0	0
	P	67.5	67.5	0	0	67.5	67.5	67.5	67.5	0	0
	K	0	0	150	150	150	150	150	150	0	0

.

2.2. Sampling and Measurements

2.2.1. Yield

At the physiological maturity stage of summer maize, three 6 m² (10 m × 2 rows) areas were selected in the central area of each plot, and 30 consecutive plants were harvested to measure the yield of summer maize. After natural air drying, indoor seed test was conducted, including spike length, spike thickness, the number of grains per ear, the number of ears per hectare, and the thousand-kernel weight. Maize kernel yield was measured when the moisture content was 14%.

During the winter wheat maturity stage, three 1 m² (1 m × 4 rows) areas of uniform growth were demarcated for the determination of winter wheat grain yield. The number of ears per hectare, the number of grains per ear, and the thousand-kernel weight were examined indoors. After natural air drying, the wheat yield was measured when the water content was 14%.

2.2.2. The Accumulation of Dry Matter

The sampling period of maize samples refers to Wang et al. [14]; three representative plants with uniform growth were selected from each plot at the six-leaf (V6), twelve-leaf (V12), tasseling (VT), milky (R3), and physiological (R6) stages of summer maize. Plant samples were packed according to stem and leaf (before tasseling) or stem, leaf, grain, and axis (after tasseling). The samples were placed in the oven at a temperature of 110 °C for 30 min and then adjusted to 80 °C to dry to a constant weight and weighed.

Ten representative wheat samples with consistent growth were selected in each plot at the jointing, flowering, and maturity stages of winter wheat, packed according to stem, leaf (before flowering), and stem, leaf, and grain (after flowering), placed in the oven at 110 °C for 30 min, dried to constant weight at 80 °C, and weighed.

2.2.3. Plant Nitrogen Accumulation and Nitrogen Use Efficiency Index

The maize and wheat samples at each growth stage after classification were ground into powder with a pulverizer, filtered through a 60-mesh sieve, and digested by H₂SO₄-H₂O₂. Automatic Kjeldahl nitrogen analyzer was used to determine the aboveground nitrogen content of plants at various growth stages [15]. The nitrogen use efficiency index was calculated using the methodology by Rubin et al. [16] and Habbib et al. [17]:

N use efficiency (NUE kg kg⁻¹) = grain yield (kg ha⁻¹)/N fertilizer application rate (kg ha⁻¹);

N uptake efficiency (UPE%) = aboveground N accumulation at maturity (kg ha⁻¹)/N fertilizer application rate (kg ha⁻¹) × 100;

N harvest index (NHI) = N accumulation in grains at maturity (kg ha⁻¹)/aboveground N accumulation at maturity (kg ha⁻¹);

Nitrogen partial factor productivity (PFP_N kg kg⁻¹) = grain yield of nitrogen fertilizer application treatment/N fertilizer application rate (kg ha⁻¹);

Agronomic efficiency of nitrogen (AE_N kg kg⁻¹) = (grain yield of nitrogen fertilizer application treatment − grain yield without nitrogen fertilizer application) (kg ha⁻¹)/N fertilizer application rate (kg ha⁻¹).

2.2.4. NO₃⁻-N and NH₄⁺-N Content

The sampling period of soil samples in this experiment refers to Gao et al. [18]. Three representative soil samples were collected from each treatment at the six-leaf (V6), twelve-leaf (V12), tasseling (VT), milky (R3), and physiological (R6) stages of summer maize. The sampling depth ranged from 0 to 60 cm, with each 20 cm layer placed in a polyethylene sealed bag. Inorganic nitrogen was extracted from the soil using 1 M KCl, followed by filtration of insoluble particles through a 10 μm qualitative filter paper. Finally, nitrate and ammonium nitrogen concentrations were quantified using an automatic flow analyzer (AutoAnalyzer3-AA3, SEAL Analytical, Norderstedt, Germany).

2.3. Statistical Analysis

The data were analyzed using Microsoft Excel 2019 (Microsoft Corp., Redmond, WA, USA) and SPSS 26.0 (SPSS Inc., Chicago, IL, USA). Analysis of variance was used to verify the significant differences among the means, followed by Duncan’s multiple range test.

3. Results

3.1. Grain Yield

The grain yields of the NPK, NK, and NP treatments in the wheat season were increased by 21.5%, 13.5%, and 5.5%, respectively, compared with CK, and those of the NPK, NK, and NP treatments in the maize season were increased by 27.5%, 25.3%, and 17.9%, respectively. There was no significant difference in grain yield between the wheat and maize in the PK treatment and CK treatment. The same trend occurred in the wheat and maize seasons. Compared with CK, the annual yields of the NPK, NK, and NP treatments increased by 21.1%, 18.2%, and 10.5%, respectively. In terms of the yield components, except for the NPK treatment, the NK treatment had the highest number of spikes and 1000-grain weight among the other three deficiency treatments. Compared with CK, the number of grains per spike in the wheat season was increased by 23.1%, and the thousand-kernel weight exhibited no notable discrepancy between the NK treatment and the CK, and the number of grains per spike and thousand-kernel weight in the maize season were increased by 6.6% and 8.7%, respectively (

Table 3. Effect of nutrient deficiency on yield component and annual yield of wheat and maize in 2022 and 2023.

Year	Treatment	Winter Wheat				Summer Maize				Annual Yield (kg·ha−1)
		EN (106 Ears·ha−1)	KN	TKW (g)	Yield (kg·ha−1)	EN (104 Ears·ha−1)	KN	TKW (g)	Yield (kg·ha−1)
2022	CK	7.47 ab	35.08 d	45.02 b	11,799 c	4.30 b	614.10 b	287.40 b	7653 c	19,452 d
	NPK	7.66 a	40.91 a	42.56 d	13,329 b	5.11 a	618.40 ab	323.40 a	10,202 a	23,531 a
	PK	7.56 ab	39.46 b	43.41 c	12,936 b	4.47 ab	598.00 b	267.50 b	7221 c	20,157 c
	NK	7.45 b	40.96 a	45.43 b	13,798 a	4.77 ab	649.40 a	319.60 a	9925 ab	23,723 a
	NP	6.86 c	37.73 c	46.64 a	12,079 c	4.80 ab	578.20 c	316.00 a	8957 b	21,036 b
2023	CK	7.64 a	31.70 c	46.87 a	11,346 c	44,200 b	599.77 ab	320.61 b	8492 b	19,839 c
	NPK	7.50 b	38.55 b	47.41 a	13,727 a	48,100 a	623.53 ab	343.87 a	10,309 a	24,036 a
	PK	6.89 c	37.15 c	39.98 b	10,230 d	45,400 ab	589.70 b	307.56 b	8065 b	18,294 d
	NK	6.49 d	41.02 a	46.72 a	12,448 b	45,700 ab	644.70 a	340.48 a	10,256 a	22,704 b
	NP	6.80 c	38.40 b	47.15 a	12,309 b	46,000 ab	633.06 ab	345.14 a	10,081 a	22,390 b
Treatment		***	***	***	**	***	NS	**	***	***
Year		**	**	NS	*	NS	NS	**	***	***
Treatment × Year		*	NS	*	*	NS	NS	NS	***	***

EN: ear number; KN: kernel number per ear; TKW: thousand-kernel weight. Different letters in each column indicate significant differences at p < 0.05 (Duncan’s multiple range test). NS: not significant; * significant at the 0.05 probability level; ** significant at the 0.01 probability level. *** significant at the 0.001 probability level.

).

3.2. Dry Matter Accumulation

The deficiency of the different nutrients had a considerable effect on the dry matter accumulation in the wheat and maize. The aboveground dry matter accumulation was higher in all the deficiency treatments than that in CK. Compared with CK, the dry matter accumulation in the wheat season under the NPK, NK, and NP treatments increased by 42.5%, 11.6%, and 15.1%, respectively, and the results demonstrated no statistically significant difference between the PK and CK treatments. In the maize season, the dry matter accumulation under the NPK, PK, NK, and NP treatments increased by 57.3%, 33.2%, 44.1%, and 61.3%, respectively (Figure 2).

3.3. Nitrogen Use Efficiency

The nutrient treatment had a significant effect on the nitrogen use efficiency (NUE), nitrogen uptake efficiency (UPE), nitrogen partial factor productivity (PFP_N), agronomic efficiency of nitrogen (AE_N), and nitrogen harvest index (NHI). In general, the NPK treatment was the best for each nitrogen use efficiency index in both the single-season and annual aspects of the two crops, and significant differences were observed between the NPK treatment and the other treatments, and the trend was basically the same in the two years. In 2023, except for the highest NUE under CK, the highest single-season and annual nitrogen use indexes were all under the NPK treatment. In 2022, the trends of each nitrogen use index of the maize season, the NHI, NUE, and UPE of the wheat season, and two seasons were the same as those of 2023; the NK treatment had the highest PFP_N and AE_N of the wheat season and two seasons. In addition, among the three nutrient deficiency treatments, the PFP_N, AE_N, NHI, NUE, and UPE of the NP treatment were significantly lower than those of the other two treatments, and the two-year trends were basically the same, thus indicating that the potassium deficiency had a higher impact on the nitrogen use efficiency of the crop rotation (

Table 4. Effect of nutrient deficiency on nitrogen use efficiency of wheat and maize at maturity stage in 2022 and 2023.

Year	Treatment	Winter Wheat					Summer Maize					Annual Dimension
		PFPN (kg· kg−1)	AEN (kg· kg−1)	NHI (%)	UPE (%)	NUE (%)	PFPN (kg· kg−1)	AEN (kg· kg−1)	NHI (%)	UPE (%)	NUE (%)	PFPN (kg· kg−1)	AEN (kg· kg−1)	NHI (%)	UPE (%)	NUE (%)
2022	CK	/	/	51.1 c	0.6 bc	70.2 c	/	/	34.9 d	0.9 c	31.1 a	/	/	86.1 d	1.5 c	101.2 c
	NPK	48.3 b	5.6 b	62.1 a	0.7 a	73.9 a	36.9 a	9.24 a	40.5 a	1.2 a	30.6 a	85.3 a	14.8 a	102.6 a	2.0 a	104.3 a
	PK	/	/	51.9 c	0.6 c	73.6 a	/	/	36.9 c	0.9 c	29.2 b	/	/	88.9 c	1.5 c	102.8 b
	NK	49.9 a	7.2 a	55.9 b	0.6 b	72.4 b	35.9 a	8.23 b	39.9 a	1.1 b	31.5 a	86.0 a	15.5 a	95.8 b	1.8 b	103.9 ab
	NP	43.8 c	1.0 c	48.6 d	0.7 ab	64.3 e	32.5 b	4.72 c	38.9 b	1.2 b	27.8 c	76.2 b	5.7 b	87.5 cd	1.8 b	92.1 d
2023	CK	/	/	54.9 b	0.7 c	62.3 a	/	/	29.3 b	0.7 c	43.2 a	/	/	84.2 b	1.4 c	105.5 a
	NPK	49.7 a	8.6 a	57.9 a	0.9 a	52.9 c	37.4 a	6.58 a	38.3 a	1.1 a	32.9 d	87.1 a	15.2 a	96.3 a	2.1 a	85.7 d
	PK	/	/	20.5 d	0.7 c	57.5 b	/	/	29.6 b	0.7 c	40.1 c	/	/	50.1 c	1.4 c	97.6 b
	NK	45.1 b	3.9 b	56.7 a	0.8 b	56.8 b	37.2 a	6.39 a	26.9 c	0.9 b	41.4 b	82.3 b	10.4 b	83.7 b	1.7 b	98.1 b
	NP	44.6 b	3.5 c	47.3 c	0.8 b	52.8 c	36.5 a	5.76 b	38.3 a	0.9 b	42.0 ab	81.1 b	9.2 c	85.6 b	1.7 b	94.8 c
Treatment		***	***	***	***	*	***	***	***	***	NS	***	***	***	***	*
Year		NS	NS	***	***	***	NS	NS	**	***	***	NS	NS	***	***	*
Treatment × Year		NS	NS	***	***	NS	NS	NS	***	***	NS	NS	NS	***	NS	*

PFPN: nitrogen partial factor productivity; AEN: agronomic efficiency of nitrogen; NHI: nitrogen harvest index; UPE: nitrogen uptake efficiency; NUE: nitrogen use efficiency. Different letters in each column indicate significant differences at p < 0.05 (Duncan’s multiple range test). NS: not significant; * significant at the 0.05 probability level; ** significant at the 0.01 probability level. *** significant at the 0.001 probability level.

).

3.4. Plant Nitrogen Accumulation

The nitrogen uptake and accumulation of the wheat and maize increased gradually with the growth stage and were significantly affected by the fertilizer application. The nitrogen accumulation in the NPK treatment was the highest, and the difference was the most significant. Compared with CK, the nitrogen accumulation in the NPK treatment was increased by 78.3%, 67.8%, and 24.9% at the jointing, flowering, and maturing stages of the wheat season, and by 71.6%, 107.2%, and 47.5% at the V6, VT, and R6 stages of maize, respectively. Compared with the other single-nutrient deficiency treatments, the NP treatment had the highest nitrogen accumulation. The NP treatment increased 53.7%, 54.3%, and 16.9% in the jointing stage, flowering stage, and maturity stage of winter compared with CK and 30.1%, 59.8%, and 26.4% in the V6, VT, and R6 stages of summer maize, respectively; the NK treatment increased 43.2%, 41.9%, and 14.2% in the three growth stages of wheat compared with CK, and increased 29.1%, 52.2%, and 24.6% in the three growth stages of summer maize, respectively. Compared with CK, the PK treatment increased the nitrogen accumulation at the jointing stage and flowering stage of wheat by 5.2% and 6.8%, and the nitrogen accumulation at the maturity stage was lower than CK and increased by 7.5% and 1.4% at the V6 and R6 stages of summer maize, respectively. The nitrogen accumulation in the VT period was lower than that in CK. In general, the nitrogen accumulation of the rotation crops increased significantly in the early to middle growth period and decreased in the late growth period (Figure 3).

3.5. Soil Inorganic Nitrogen Content

The concentration of inorganic nitrogen in the soil was found to be the highest in the topsoil layer, with a gradual decline observed with increasing soil depth (Figure 4). In all the soil layers of the wheat season, the NPK treatment had the highest inorganic content, and the difference was the most significant. Compared with CK, the NO₃⁻-N content in the 0–60 cm soil layers was increased by 24.9%, 34.4%, and 46.2%; the PK treatment and CK treatment were not significantly different, while the NK treatment increased by 17.3%, 20.7%, and 30.2% in each soil layer, respectively; the NP treatment increased by 19.9%, 24.7%, and 36.3%, respectively. The difference in the NO₃⁻-N content within the 0–20 cm and 40–60 cm soil layers between the NK and NP treatments was not significant. The NH₄⁺-N content of each nutrient deficiency treatment differed significantly compared to the CK and NPK treatments, and the NH₄⁺-N content increased by 22.9%, 21.5%, and 22.0% in the 0–60 cm soil layers; the PK treatment demonstrated smaller increases, increasing by 4.0%, 4.9%, and 5.9%, respectively. The NK treatment increased by 16.0%, 12.9%, and 13.3% in each soil layer, respectively. The NP treatment increased by 19.5%, 12.9%, and 16.9%, respectively. There was no significant difference in the NH₄⁺-N content between the NK treatment and NP treatment in the 20–40 cm soil layer.

During the maize season, compared with CK, the NO₃⁻-N content of the NPK treatment was increased by 26.7%, 32.3%, and 44.3% in the 0–60 cm soil layers. The content of NH₄⁺-N in the PK treatment was the same as the trend for the wheat season. The NK treatment increased by 11.7%, 14.7%, and 22.4% in the three soil layers, respectively. The NP treatment increased by 14.8%, 18.6%, and 22.2%, respectively. Compared with CK, the content of NH₄⁺-N in the different nutrient deficiency treatments increased significantly in all the soil layers. The NPK treatment increased by 27.5%, 31.1%, and 32.5% in the 0–60 cm soil layers. The NK treatment increased by 21.1%, 25.2%, and 24.64% in the three soil layers, respectively. The NP treatment increased by 24.2%, 28.1%, and 28.0%, respectively.

3.6. Correlation Analysis

The plant nitrogen accumulation, the accumulation of dry matter, NUE, inorganic nitrogen content, and grain yield in the wheat season were significantly positively correlated, and the nitrogen use efficiency was non-significantly and positively correlated with the yield, but the nitrogen use efficiency and all the other indicators were non-significantly and negatively correlated. The correlation trend of the maize season indexes was basically the same as that of the wheat season, but the NUE was significantly negatively correlated with the UPE in the maize season (Figure 5).

4. Discussion

Excessive fertilization leads to more frequent soil nutrient imbalance, and the final result of accelerating nutrient loss will inevitably lead to soil nutrient deficiency. The decrease in nutrient content and availability has become the most limiting factor restricting the increase in yield in NCP [19]. However, at the time scale, the response of crop rotation to nutrient deficiency in NCP is poorly understood. By analyzing the gap in yield and nutrient accumulation and utilization between nutrient-sufficient and nutrient-deficient areas, it is possible to better determine crop responses to nutrient deficiency, soil nutrient supply capacity, and the ability to maintain soil fertility in the long term, which is critical for the development of improved nutrient management practices.

In this study, nutrient deficiencies significantly reduced the grain yield of crops and led to different degrees of reduction in the yield component. Although there was no significant difference between the NK treatment and NP treatment in terms of the overall yield, the impact of the potassium fertilizer on the yield components is more pronounced than that of the phosphate fertilizer, a finding that aligns with the results of previous studies [20]. Moreover, another study [21] found that potassium application significantly increased the number of spikes, spike length, and 100-grain weight. Only when the amount of nitrogen and potassium applied reached the right amount could an increased phosphorus fertilizer application affect the yield components and grain quality. This is because potassium can activate more than 60 enzymes during the yield formation, which directly or indirectly affects the key processes of plant growth and development. However, the main role of phosphorus in this process is reflected in the utilization of starch and sugar and the transfer of energy. The dry matter accumulation during the growth of crops directly determines the final grain yield of the crops [22]. The accumulation of dry matter under a nitrogen deficiency treatment was basically the same as CK (Figure 2). The accumulation of dry matter under the NK treatment was observed to be consistently lower than that under the NP treatment, and, with the advancement in the growth process, the limiting effect of the different nutrient deficiencies on the dry matter accumulation appeared, occurring due to the different growth and development directions of each nutrient element. Nitrogen deficiency results in plant dwarfing and leaf withering, making the crop unable to photosynthesize adequately above ground, leading to lower dry matter mass accumulation. Phosphorus fertilizer promotes the growth and proliferation of crop cells, increases plant height and leaf area [23], and allows the crop canopy to prolong the duration of photosynthesis in the period of sufficient light and heat, produce a sufficient amount of photosynthates, and promote an increase in total dry matter. The effect of phosphorus fertilizer on dry matter accumulation was mainly from the silking stage to the milking stage, and the proportion of dry matter accumulated in this period accounted for the largest proportion of total dry matter at maturity. The effect of the potassium fertilizer on dry matter quality was mainly in the late growth period. Timlin et al. [24] found that the total aboveground biomass and dry matter accumulation of maize were significantly lower in phosphorus-deficient plots than in potassium-deficient plots.

Nutrient deficiency will cause an interaction effect between nutrients that cannot be fully released, affecting the absorption and utilization of the applied nutrients [25]. In this experiment, the nitrogen efficiency index can be divided into two parts for explanation purposes. Firstly, compared with the nitrogen application treatment (i.e., NPK, NP, and NK), the nitrogen utilization indexes of the nitrogen deficiency treatment (i.e., CK and PK) were significantly higher than those of the nitrogen application treatment. This may be attributed to the lack of exogenous nitrogen application; the plant population is always in the state of “nitrogen starvation” when the soil nitrogen fixation is performed, and the nitrogen compounds in the soil can be efficiently absorbed and utilized, thus promoting an improvement in the nitrogen efficiency index. Secondly, in the nitrogen application treatments, the nitrogen use efficiency indexes of a single season and two seasons of NPK treatment were significantly improved, and they were higher than those of the NP and NK treatments on the whole. Compared with the NK treatment, the nitrogen efficiency indexes of the NP treatment were significantly lower than in the NK treatment, and they were more prominent in the three indexes of AE_N, NHI, and NUE. Potassium deficiency significantly affected the nitrogen uptake and utilization and reduced the nitrogen effectiveness [26] (Table 3), and the reduction in nitrogen utilization simultaneously produced the same trend regarding the grain yield (Table 2), which is the same as the research results of [27,28].

Nitrogen–phosphorus interaction can promote nitrogen utilization efficiency, and nitrogen–phosphorus interaction can significantly affect the nitrogen uptake rate, and the change in uptake rate is directly related to plant nitrogen accumulation. In this experiment, compared with other single-nutrient deficiency treatments, the NP treatment had a more obvious gain in nitrogen absorption, so the accumulation of nitrogen in the NP treatment was found to be significantly superior to that observed in the NK and PK treatments. Meanwhile, the result of the correlation analysis demonstrated a significant positive correlation between crop yield and various other factors, including dry matter accumulation, plant nitrogen accumulation, and nitrogen efficiency. These results indicated that nutrient deficiency affected grain yield by affecting nutrient accumulation and plant nitrogen uptake and utilization efficiency.

Soil inorganic nitrogen content is closely related to nutrient application pairs during crop growth [29]. In this experiment, the NPK treatment had the highest inorganic nitrogen content in the 0–60 cm soil layer in two seasons, and the content of ammonium and nitrate nitrogen of the NP treatment and NK treatment was lower than that of the NPK treatment. However, the soil inorganic nitrogen content was slightly lower in the NK treatment than that in the NP treatment at maturity. This is due to the coupling effect between various nutrients, and phosphorus participates in the metabolic process of nitrogen, affecting its metabolic process and nitrogen availability. The phosphorus fertilizer accelerated the dry matter synthesis and nitrogen transport in the middle growth stage, the plants absorbed nitrogen from the soil, and the residual inorganic nitrogen in the soil decreased, and then the decline trend was gradually slow. However, this process regarding the potassium fertilizer appears in the late growth stage, which leads to the residual amount of inorganic nitrogen in the NK treatment being lower than that in the NP treatment at the maturity stage. This result is the same as that of a previous study [30]. In addition, the content of ammonium nitrogen in this experiment was always lower than that of nitrate nitrogen; this is due to a series of nitrification and ammoniation processes of ammonium nitrogen, which eventually lead to transformation and dissipation, and the content decreases, which is similar to the research results of Wang et al. [31]. At the same time, combined with the climatic factors and experimental results, it was found that the treatments of nutrient deficiency in this study affected the NO₃⁻-N and NH₄⁺-N content in the 40–60 cm soil layer, which was different from the results of a previous study [32]. In conjunction with the meteorological data from this experiment, this may be due to the gradual increase in precipitation with the growth process during the experimental period. Rainwater entered the topsoil along the furrow, and the infiltration of water caused the transfer of inorganic nitrogen from the topsoil to the deeper soil during the leaching process, which ultimately affected the inorganic nitrogen content of the deeper soil.

In summary, nutrient deficiency can significantly affect the nutrient circulation system of the soil–crop–fertilizer. This result is the same as the results of our previous study on nutrient deficiency regarding crop root conformation [11]. The nutrient deficiency treatment significantly reduced the root length, root diameter, and root surface area in the 0–40 cm soil layer and also reduced the RLD (root length density), RSAD (root surface area density), and RDWD (root dry weight density), resulting in decreased root activity. Nutrient deficiency can also affect the composition and activity of soil microbial communities and impede the normal operation of rhizosphere ecosystems [33], and the root tip sensing mechanism could not respond to the changes in the soil nutrients in time, resulting in the interaction between the aboveground part and the underground part of the plant not being timely, which affected the growth and development of the aboveground organs [34]. At the same time, the same amount of fertilizer input for wheat and maize in this study would not be fully absorbed by the crops, resulting in a fertilizer residual effect in the soil, and eventually resulting in continuous accumulation of the fertilizers in the soil, which cannot be effectively utilized by the crops, thus reducing the efficiency of nutrient utilization. Combined with the above results, this provides a new idea for us to explore the changes in the rhizosphere microbial communities and the interaction between the aboveground and the underground part of the plant caused by nutrient deficiency and residual effects caused by blind fertilization in the future.

5. Conclusions

The deficiency of nitrogen can result in unmet nutrient requirements during the nutritive growth stage of crops, which can subsequently affect the formation of the yields at maturity. Phosphorus deficiency has more significant restraints on crop nutrient accumulation and dry matter accumulation. Potassium deficiency leads to significant reductions in nutrient use efficiency. Therefore, it is imperative that greater attention be paid to the application of nitrogen fertilizer in wheat and maize rotation systems, and, meanwhile, long-term positioning tests should be conducted to verify the fertilizer situation of the soil, and the application amount of phosphate and potassium fertilizer should be determined according to the requirements of the crops and the supporting capacity of the soil so as to avoid the nutrient imbalance caused by the improper application of fertilizer and the reduction in nitrogen use efficiency. This approach is absolutely necessary to promote sustainable productivity and green agriculture development in the NCP.