Effects of Polymer Conditioner and Nitrogen Fertilizer Application on Nitrogen Absorption and Utilization of Drip-Irrigated Wheat in Arid Areas

Abstract

Nitrogen (N), an important element for crop growth, has a great impact on dry matter weight and yield. Currently, improving N fertilizer use rate is an urgent problem to be solved in agricultural production in the world. In this field experiment, a self-developed water-soluble polymer material (PPM) with water retention and slow-release characteristics was combined with different doses of N fertilizer (N300 (100% N), PN300 (PPM + 100% N), PN240 (PPM + 80% N), PN180 (PPM + 60% N), CK (no N and PPM)) to analyze the impacts on N uptake and use efficiency of wheat plants. The results showed that the combined application of PPM and N fertilizer significantly improved yield, plant height, biomass, and N uptake and use efficiency of drip irrigated wheat, and the PN240 group had the highest N use rate. In addition, the PN300 group had the highest yield. N use efficiency in the PN240 group was 40.23% higher than that in the N300 group. Therefore, the combined application of PPM and N fertilizer, especially PN240, can reduce the N fertilizer application rate by increasing N use efficiency. This study provides technical reference for improving the N use efficiency of drip-irrigated wheat in arid areas.

1. Introduction

Nitrogen fertilizer is vital for crop yield [1]. Rational N application can increase dry matter accumulation in various organs of crops, thereby increasing crop yield [2]. Studies have shown that high basal application rate of N fertilizer could increase the number of tillers in Poaceae plants [3]. However, excessive application of N fertilizer reduces the transport of photosynthates to grains and increases the number of sterile tillers, leading to excessive vegetative growth and an increased incidence of lodging, diseases and pests, and yield reduction [4]. Especially excessive N application significantly reduces N use efficiency because most of the N applied is not absorbed by crops but lost through leaching and other ways [5]. It should be noted that the loss of large amounts of N can lead to environmental pollution [6]. Therefore, coordination of crop N demand and N supply is one of the effective ways to achieve high N use efficiency and N reduction.

Polymer soil amendments can improve soil fertility, reduce fertilizer input, and increase crop yield [7]. Wang et al. [8] found that superabsorbent polymer (SAP), as a three-dimensional crosslinked hydrophilic polymer, could be used as an excellent fertilizer carrier to improve water and fertilizer retention in soil. Kang et al. [9] reported that the height, fresh weight, and dry weight of maize were significantly increased after the application of polyacrylamide. Sun et al. [10] reported that the combination of urea humate and pyroligneous acid significantly increased wheat yield by 8.1–38.0%, achieving N reduction (20%) without affecting crop yield, and confirmed that the application of polymer materials could reduce the fertilizer application rate. Tian et al. [11] reported that the combined application of polyacrylamide and polyvinyl alcohol increased N content and dry weight of wheat leaves, ears, and grains, and the greatest N uptake occurred at the flowering period. At the same time, and the use of anionic polyacrylamide(PAM) and nitrogen fertilisers increased total soil nitrogen storage, reduced nitrogen losses and increased inorganic nitrogen accumulation [12]. The soil conditioner anionic polyacrylamide (PAM) is frequently applied to soils to reduce soil erosion and nitrogen loss; respectively, nitrification loss of nitrogen fertiliser under mineralised conditions can be enhanced by adding PAM to the soil [13]. Therefore, appropriate application of polyacrylamide can achieve significant yield increase and improve fertilizer use efficiency. However, the effect of polyacrylamide application on crop nitrogen uptake and utilization efficiency has been less studied.

The wheat (Triticum aestivum L.) planting area reached 1.63 × 10⁶ hm² in Xinjiang, China in 2020, accounting for about 48% of the total grain crop planting area in Xinjiang [14]. As a consequence of unique geographic location and arid continental climate, farmers in Xinjiang always need to input large amounts of N to ensure high crop yields. However, due to the unreasonable application of N fertilizer, N loss is very prominent, which greatly increases agricultural costs and pollution risk. Therefore, some scholars have explored the slow-release effect of polymer materials on soil nutrients, but at present there is no in-depth analysis of the effects of polymer materials, especially PPM (a self-developed water-soluble polymer material) on N use and yield of crops. Therefore, in this study, a self-developed water-soluble polymer material (PPM) with water retention and slow-release properties was combined with different doses of N fertilizer to elucidate the effect on soil N release and wheat N utilization, aiming to reduce the use of N and N pollution risk. The specific objectives were to (1) clarify the effect of the combined application of PPM and N fertilizer on wheat growth parameters and yield; (2) quantify the dynamic changes in total N content in wheat organs under the combined application of PPM and N fertilizer; (3) determine the optimal ratio of PPM and N application rate that maintains high wheat yield and high N utilization rate. This study provides technical reference for improving the nitrogen fertilizer utilization rate in arid areas.

2. Materials and Methods

2.1. Experimental Site

The experiment was conducted in a two-year field trial in 2020 and 2021 (March–July) at the experimental field of Shihezi University, Xinjiang, China (44°31′ E, 86°05′ N). This region has a temperate continental climate. The average annual temperature was 2–15 °C, the average annual rainfall was 180–220 mm, the annual evaporation was 1660 mm, and the annual sunshine duration was 2721–2818 h. The soil was gray desert soil, with a texture of loam [15]. The pH (Soil:water = 1:5) was 7.51, electrical conductivity was 344 μS/cm, total N content was 24.32 g/kg, alkali hydrolyzable N was 70.24 mg/kg, available phosphorus was 18.23 mg/kg, and available potassium was 204.32 mg/kg [16].

2.2. Experimental Design

The self-developed polymer material, PPM (TRent No.: CN105801297), with high water retention and slow-release characteristics, was synthesized from polyacrylamide, polyvinyl alcohol, and manganese sulfate, with a pH of 7.46 and electrical conductivity of 1330 μS/cm. Spring wheat varieties Xinchun 38 (XC38) and Xinchun 29 (XC29) were used in this study. XC38 is a high-yielding spring wheat variety with strong gluten characteristics, jointly selected and bred by the Crop Research Institute of Xinjiang Academy of Agricultural Reclamation Sciences and Jiuhe Seed Industry Co., Ltd., Shihezi, China, Its growth period is 94 days. XC29 is a high-yielding spring wheat variety with medium-gluten characteristics selected and bred by the Institute of Food Crops of Xinjiang Academy of Agricultural Sciences. It has a high stress and disease resistance, with a growth period of 108 days. It can be planted in mildly saline soil.

This study employed a randomized complete block design with five groups (Table S1). Each group had three replicates (plots). The area of each plot was 10 m² (2 m × 5 m), and the plot spacing was 0.5 m. For all groups, wheat seeds were sown on 12 April 2020 and 8 April 2021, and 120 kg/hm² of P₂O₅ (NH₄H₂PO₄: P₂O₅ 48%, Guizhou Phosphor (Group) Co., Guiyang, China) and 90 kg/hm² of K₂O (K₂SO₄: K₂O 51%) were basally applied through drip irrigation. PPM and N fertilizer (Urea: N 46%) were dissolved in water and applied through the drip irrigation system after emergence. The potash and nitrogen fertilizer were purchased from Xinjiang Chuande Ecological Agricultural Technology Co., Shihezi, China. The total irrigation volume during the whole growth period was 4500 m³/hm². Other management factors were the same as those of local fields. Wheat was harvested on 25 July and 18 July 2021.

2.3. Determination of Plant Parameters

Twenty plants were randomly sampled in each plot at the three-leaf stage, jointing stage, flowering stage, and mature stage of wheat, to determine the wheat plant weight and height in two years. Then, the plant samples were separated into stem/sheath, leaf, glume/spike-stalk, and grain (at the mature stage only), and plant biomass was obtained by killing the plants at 105 °C for 20–30 min and then baking them in an oven at 80 °C until constant weight and weighing the dry matter. Each treatment had three replicates. At the mature stage, a subplot (1 m²) was selected from each plot to determine ear number, grain number per ear, 1000-grain weight, and grain yield. The total N content in wheat was determined according to the method of Bao (2008) [16]. Dried plant organ samples (0.20 g per organ) were moistened with water. Then, 5 mL of concentrated sulfuric acid was added. The mixture was shaken well and digested until the next day. After cooling, ten drops of 300 g L⁻¹ H₂O₂ was added; after heating, 5–10 drops of H₂O₂ was added. This procedure was repeated 2–3 times until the liquid was colorless. Finally, the liquid volume was fixed to 100 mL, followed by the determination using a nitrogen analyzer (Foss KjeltecTM2300, Hoganas, Sweden).

The polyacrylamide (PAM) used in this experiment was linear anionic, molecular C₃xH₅xNxOx, molecular weight of 8–10 million, purchased from Shanghai McLean Biochemical Technology Co., Chongqing, China. Polyvinyl alcohol, MnSO₄, H₂SO₄ and H₂O₂ were analytically pure (AR) and purchased from Tianjin Jinbei Fine Chemical Co., Tianjin, China.

The accumulation and transport of N in wheat (in 2021) were calculated using the following formula [17]:

$$NA =DMA\times ON.$$

(1)

NA: Plant N accumulation (kg·ha⁻¹); DMA: plant dry weight; ON: plant N concentration (%), the same as below.

$$P-CR=\frac{(FNA-MNA)}{MGN}\times 100\%.$$

(2)

P-CR: Contribution of pre-anthesis organs’ N to grain N; FNA: pre-anthesis organs’ N (kg·ha⁻¹); MGN: grain N accumulation (kg·ha⁻¹).

$$A-CR=\frac{MNA-FNA}{MGN}\times 100\%.$$

(3)

A-CR: Contribution of post-anthesis organs’ N to grain N; MNA: post-anthesis organs’ N accumulation (kg·ha⁻¹); MGN: grain N accumulation.

$${NUE}_{diff}=\frac{{HN}_T-{HN}_C }{F_N}\times 100\%.$$

(4)

NUE_diff: Nitrogen use efficiency; HN_T: nitrogen accumulation in crop aboveground organs under nitrogen application; HN_c: nitrogen accumulation in crop aboveground organs without nitrogen application; F_N: nitrogen application rate. The same is below.

$$AEN (\mathrm{k}\mathrm{g}/\mathrm{k}\mathrm{g})=\frac{Y_F-Y_0}{F_N}.$$

(5)

AEN: Nitrogen agronomic efficiency.

$$PEN\left(\mathrm{P}\mathrm{h}\mathrm{y}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{l}\mathrm{o}\mathrm{g}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{l} \mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y} \mathrm{o}\mathrm{f} \mathrm{n}\mathrm{i}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{g}\mathrm{e}\mathrm{n}\right)\left(\mathrm{k}\mathrm{g}/\mathrm{k}\mathrm{g}\right)=\frac{Y_F-Y_0}{{HN}_T-{HN}_C},$$

(6)

$$NPFP (Partial factor productivity of nitrogen) (\mathrm{kg}/\mathrm{kg})= { Y}_F/F_N,$$

(7)

$$NHI\left(Nitrogen harvest index\right)=\frac{G_N}{MNA}.$$

(8)

2.4. Data Analysis

Data analysis and plotting were completed using Excel 2019 software 2312 Build 16.0.17126.20132, variance analysis was performed using SPSS 19.0 software, and multiple comparisons were performed using Duncan’s Multiple Range Test.

3. Results

3.1. Wheat Yield under Different PPM and N Combination Treatments

In 2020–2021, PPM and N combination treatments had significant effects on the yield parameters of wheat (

Table 1. Wheat yield parameters under different polymer and nitrogen combination treatments.

Variety	Treatment	Grain Yield (kg/hm2)	Ears Per Square Meter	1000-Grain Weight (g)	Grain Number Per Ear
2020
XC29	N300	6080.42 ± 77.41 b	592.98 ± 7.10 a	40.92 ± 0.72 b	27.63 ± 0.84 b
	PN300	6947.36 ± 87.14 a	598.41 ± 3.82 a	42.78 ± 0.92 a	30.57 ± 0.57 a
	PN240	6965.83 ± 65.64 a	590.79 ± 3.97 c	40.96 ± 0.15 b	27.90 ± 0.52 b
	PN180	5564.75 ± 68.58 c	542.85 ± 8.30 b	38.98 ± 0.48 c	24.13 ± 0.31 c
	CK	4530.29 ± 94.61 d	539.84 ± 3.44 b	36.81 ± 0.35 d	20.91 ± 0.56 d
XC38	N300	6360.91 ± 28.6 b	548.04 ± 10.51 a	41.88 ± 0.23 b	28.25 ± 0.05 c
	PN300	6326.72 ± 57.6 b	497.17 ± 12.41 b	42.13 ± 0.54 b	36.12 ± 0.93 a
	PN240	7252.65 ± 78.6 a	491.08 ± 9.36 bc	44.46 ± 0.65 a	30.32 ± 0.72 b
	PN180	5575.95 ± 26.6 c	471.73 ± 12.35 cd	40.11 ± 0.48 c	26.03 ± 0.43 d
	CK	4675.65 ± 24.8 d	451.71 ± 10.32 d	37.38 ± 0.53 d	26.53 ± 1.28 d
2021
XC29	N300	6000 ± 6.11 b	676.51 ± 19.50 a	48.47 ± 0.87 a	23.11 ± 0.49 cd
	PN300	6031 ± 4.84 b	668.12 ± 15.02 a	43.81 ± 0.18 b	26.77 ± 0.30 a
	PN240	6691 ± 6.38 a	539.52 ± 13.41 a	47.05 ± 0.15 a	25.36 ± 0.07 b
	PN180	5542 ± 8.58 c	528.50 ± 12.52 b	41.32 ± 0.14 c	23.80 ± 0.34 c
	CK	4532 ± 9.61 d	554.32 ± 13.12 b	30.58 ± 0.60 d	22.91 ± 0.75 d
XC38	N300	6361 ± 2.86 b	551.52 ± 10.51 a	46.12 ± 0.93 a	30.15 ± 0.33 b
	PN300	6322 ± 5.76 b	536.21 ± 12.35 b	45.32 ± 0.72 b	30.33 ± 0.27 b
	PN240	7252 ± 7.86 a	548.51 ± 12.41 a	38.25 ± 0.05 c	32.01 ± 0.48 a
	PN180	5565 ± 2.66 c	490.47 ± 9.36 c	36.03 ± 0.43 d	28.88 ± 0.46 c
	CK	4674 ± 2.48 d	476.35 ± 10.32 d	36.53 ± 1.28 d	26.91 ± 0.56 d
	V	ns	*	ns	**
	T	**	**	**	**
	Y	ns	**	**	**
	V × T	ns	ns	ns	ns
	V × Y	ns	ns	**	**
	V × T × Y	**	**	**	**

Note: V: Variety, T: Treatment, Y: Year. Different lowercase letters in the same column indicate significant difference between groups (* p < 0.05, ** p < 0.01, ns: p > 0.05).

). The grain number per ear of XC29 of the PN300 group increased by 2.99–10.64%, that of the PN240 group increased by 1.00–9.73%, respectively (p < 0.05), and the 1000-grain weight of XC29 of the PN300 and PN240 group increased by 2.99–10.64% and 1.00–9.73% (p < 0.05) compared with the values of the N300 group. The grain yield of XC29 of the PN300 and PN240 groups increased by 12.71–14.26% and 5.17–11.52% (p < 0.05), while there was no difference between PN240 and N300 groups (p > 0.05). The grain yield of XC29 of the PN300 and PN240 group increased by 5.17–14.26% and 11.52–14.56% compared with that of the N300 group (p < 0.05), while there was no difference between PN240 and N300 groups (p > 0.05). The ear number of XC38 of the PN300 and PN240 groups increased by 0.60–27.86% and 6.17–7.33%, and that of the PN180 group reduced by 4.21–7.86%, respectively (p < 0.05); the 1000-grain weight of the PN300 and PN240 group increased by 5.97–17.65% and 6.16–6.47%, respectively (p < 0.05), compared with the values of the N300 group. In addition, the grain number per ear and the 1000-grain weight of XC38 of the PN180 group reduced by 4.21–7.86% and 4.23–21.88%, respectively (p < 0.05), compared with the values of the N300 group.

3.2. Wheat Plant Height

The plant height of both wheat varieties for the five groups first increased and then gradually stabilized over time (Figure 1, Table S2), and the plant height was the highest at the mature stage from 2020 to 2021. The PPM and N combination treatments (N300, PN300, PN240, and PN180) significantly increased wheat plant height (p < 0.05). At the mature stage, the PN300 group had the highest plant height, followed by the PN240, PN180, N300, and CK groups. The plant height of XC29 of the PN300 and PN240 groups increased by 2.49–12.78% and 3.10–10.70% (p < 0.05), respectively, and that of XC38 increased by 2.49–11.00% and 0.3–19.54%, respectively, compared with the values of the N300 group (p < 0.05).

3.3. Wheat Biomass under Different PPM and N Combination Treatments

The PPM and N combination treatments increased the dry matter accumulation in XC29 and XC38 organs compared to the values of CK at different growth stages (p < 0.05), and the dry matter accumulation of the two varieties reached the maximum at the mature stage in two years (Figure 2, Table S3). The biomass of XC29 stems/sheaths of the PN300 group increased by 7.72–26.43%, 5.02–41.83% and 10.72–25.71% at the three-leaf stage, the jointing stage, and the flowering stage, respectively (p < 0.05), compared with that of the N300 group. In addition, at the mature stage, the biomass of XC29 stems/sheaths, leaves, ears, and grains of the PN300 group increased by 10.09–26.27%, 13.24–47.95%, 13.34–20.37%, and 10.01–11.36%, respectively (p < 0.05), and those of the PN240 group increased by 7.52–57.3%, 9.90–11.80%, 9.87–46.79%, and 3.44–50.80%, respectively (p < 0.05), compared with those of the N300 group.

The stem/sheath and leaf biomass of XC38 of the PN300 group increased by 5.01–11.39% and 5.03–30.69%, at the jointing stage, respectively (p < 0.05), the stem/sheath, leaf, and ear biomass increased by 9.35–9.62%, 9.42–30.69%, and 4.62–10.31% at the flowering stage, and the grain biomass increased by 11.00–14.14% (p < 0.05) at the mature stage compared with the values of the N300 group. However, the leaf biomass of XC38 of the PN240 and PN180 groups reduced by 2.18–29.92% and 1.01–20.75%, respectively (p < 0.05), at the maturity stage.

3.4. Plant Nitrogen Content and Utilization

3.4.1. Total Nitrogen Content of Plant Organs

N accumulation of wheat organs of the N300, PN300, and PN240 groups increased compared with that of CK, and the N accumulation of stem/sheath and leaf first increased and then decreased over time (Figure 3). The N content of XC29 stems/sheaths and leaves of the PN300 group increased by 13.66% and 15.59%, respectively (p < 0.05), at the jointing stage, that of XC29 stems/sheaths, leaves, and ears increased by 17.44%, 22.25%, and 11.11%, respectively (p < 0.05), at the flowering stage, and that of XC29 leaves, ears, and grains increased by 40.19%, 24.69%, and 34.83%, respectively (p < 0.05), at the mature stage compared with those of the N300 group. Similarly, the N content of XC29 leaves, ears, and grains of the PN240 group increased by 51.99%, 11.71%, and 28.81%, respectively (p < 0.05), at the mature stage compared with those of the N300 group.

The N content of XC38 stems/sheaths, leaves, and grains of the PN300 group increased by 17.03%, 13.94%, and 11.43%, respectively (p < 0.05), at the flowering stage compared with those of the N300 group. In addition, the N content of XC38 grains of the PN300 and PN240 groups increased by 28.75% and 17.74%, respectively (p < 0.05), at the mature stage compared with that of the N300 group.

3.4.2. Nitrogen Transport Characteristics of Plants

The PPM and N combination treatments increased the TR and A-CR values and reduced P-CR compared with CK (p < 0.05) (

Table 2. Effects of different treatments on nitrogen transport characteristics of wheat.

Variety	Group	TR (/%)	P-CR (%)	A-CR (%)
XC29	N300	66.91 ± 1.24 b	71.09 ± 3.57 b	25.34 ± 1.05 d
	PN300	72.34 ± 2.39 a	63.9 ± 4.23 d	35.81 ± 2.17 b
	PN240	66.65 ± 3.31 b	67.18 ± 3.27 c	30.97 ± 2.03 a
	PN180	62.26 ± 2.33 c	65.14 ± 1.89 d	32.66 ± 1.95 c
	CK	49.32 ± 3.81 d	76.37 ± 3.51 a	18.32 ± 1.86 e
XC38	N300	67.48 ± 4.12 b	74.99 ± 4.12 b	22.45 ± 2.34 d
	PN300	74.33 ± 3.9 a	65.43 ± 2.95 c	30.63 ± 2.9 c
	PN240	68.76 ± 3.62 b	62.88 ± 3.43 d	34.59 ± 2.11 b
	PN180	64.96 ± 2.55 c	61.97 ± 4.11 e	37.31 ± 2.15 a
	CK	51.85 ± 2.52 d	79.84 ± 2.56 a	16.65 ± 2.09 e

Notes: TR, Plant N accumulation; P-CR, Contribution of pre-anthesis organs’ N to grain N; A-CR, Contribution of post-anthesis organs’ N to grain N. Different lowercase letters in the same column indicate significant difference between groups (p < 0.05).

). The TR value of XC29 of the PN300 group increased by 8.11% (p < 0.05), but that of the PN180 group decreased by 7.47% (p < 0.05) compared with that of the N300 group. In addition, the P-CR value of XC29 of the PN300, PN240, and PN180 groups reduced by 11.25%, 5.82%, and 9.13%, respectively (p < 0.05), and the A-CR value increased by 41.32%, 22.22%, and 28.87%, respectively (p < 0.05), compared with those of the N300 group.

The TR value of XC38 of the PN300 group increased by 10.15% (p < 0.05), while that of the PN180 group decreased by 3.87% (p < 0.05) compared with that of the N300 group. In addition, the P-CR value of XC38 of the PN300, PN240, and PN180 group decreased by 12.75%, 16.15%, and 17.36%, respectively (p < 0.05), and the A-CR value increased by 36.44%, 54.08%, and 66.19%, respectively (p < 0.05) compared with the values of the N300 group.

3.4.3. Plant Nitrogen Use Efficiency

The NUE_diff value of XC29 and XC38 of the PN180, PN240, and PN300 groups increased (p < 0.05) compared with the values of the N300 group, and the values of the PN240 group were the highest (

Table 3. Effects of different treatments on nitrogen use efficiency of wheat.

Variety	Group	NUEdiff	AEN (kg/kg)	PEN (kg/kg)	NPFP (kg/kg)	NHI
XC29	N300	26.24 ± 1.34 c	4.91 ± 0.19 c	11.39 ± 0.26 b	20.03 ± 1.01 d	54.19 ± 2.34 d
	PN300	35.32 ± 2.01 b	7.19 ± 0.45 a	14.91 ± 0.43 a	22.31 ± 1.05 c	60.49 ± 3.16 c
	PN240	38.06 ± 1.89 a	6.27 ± 0.23 b	11.16 ± 0.39 b	25.17 ± 1.23 b	63.67 ± 2.78 b
	PN180	35.65 ± 1.99 b	5.56 ± 0.39 c	6.62 ± 0.25 c	30.77 ± 1.27 a	74.58 ± 3.21 a
	CK	\	\	\	\	\
XC38	N300	31.42 ± 1.68 c	5.61 ± 0.41 c	17.87 ± 0.99 b	21.2 ± 1.13 d	64.47 ± 3.11 c
	PN300	43.9 ± 0.92 a	8.59 ± 0.53 a	19.57 ± 1.02 a	24.17 ± 1.21 c	73.24 ± 3.79 a
	PN240	44.06 ± 1.71 a	6.88 ± 0.47 b	15.61 ± 1.11 c	26.36 ± 1.09 b	73.09 ± 2.48 a
	PN180	39.74 ± 1.23 b	5 ± 0.46 d	12.59 ± 0.78 d	30.97 ± 1.07 a	70.07 ± 2.56 b
	CK	\	\	\	\	\

Notes: NUE, nitrogen use efficiency; AEN, nitrogen agronomic efficiency; PEN, physiological efficiency of nitrogen; PFPN, Partial factor productivity of nitrogen; NHI, nitrogen harvest index. Different lowercase letters indicate significant difference between groups (p < 0.05).

). The NUE_diff value of XC29 of the PN300, PN240, and PN180 groups increased by 34.60%, 45.04% and 35.75%, respectively (p < 0.05), the PFPN value increased by 11.38%, 25.66%, and 53.62%, respectively (p < 0.05), and the NHI value increased by 11.63%, 17.49% and 37.63%, respectively (p < 0.05), compared with the values of the N300 group. The AEN value of XC29 of the PN300 and PN240 groups increased by 76.25% and 29.43%, respectively (p < 0.05), and the PEN value of XC29 of the PN300 group increased by 30.90% (p < 0.05) compared with the values of the N300 group.

The NUE_diff value of XC38 of the PN300, PN240, and PN180 groups increased by 39.72%, 40.23%, and 26.48%, respectively (p < 0.05), the PFPN value increased by 14.01%, 24.34%, and 46.08%, respectively (p < 0.05), and the NHI value increased by 13.60%, 13.37%, and 8.69%, respectively (p < 0.05), compared with the values of the N300 group. The AEN value of XC38 of the PN300 and PN240 groups increased by 53.12% and 22.64%, respectively (p < 0.05), and the PEN value of XC38 of the PN300 group increased by 9.51% (p < 0.05) compared with the values of the N300 group.

3.5. Redundancy (RDA) Analysis

For XC29, PN300 and N300 treatments had a positive effect on ear number (Ears) and yield (Yield), and PN300, PN240, PN180, and N300 treatments had a positive effect on the dry weight (W-Plants, W-Stem, W-leaf, W-Grain, W-Ear) and N content of wheat organs (N-Plants, N-Stem, N-Leaf, N-Grain, N-Ear). In addition, PN240 and PN180 treatments had the greatest positive effect on N use (NUE, NHI, NPFP), and CK treatment had a great effect on 1000-grain weight (TKW). For XC38, PN300 and N300 treatments had a great positive effect on grain number per ear (Grain number), Yield, N-leaf, and grain dry weight (W-Grain), and PN240 treatment had a positive effect on Ears, W-Stem, and N use (NUE, NHI, and NPFP).

4. Discussion

Nitrogen is an important factor influencing dry matter accumulation and yield formation in wheat [18]. In this study, the combination of PPM and different doses of N fertilizer (PN300, PN240, PN180) promoted dry matter accumulation in wheat and increased wheat yield, and a similar trend was shown in two different genotypes of wheat (Figure 4), which indicates that PPM and different doses of N fertiliser are not restricted by varieties and are universal. At the jointing, flowering, and mature stages, P-CR gradually decreased with the decrease in N application rate. This indicates that blind reduction in N fertilizer application rate is not conducive to the vegetative growth of wheat [19]. In this study, there was no difference in dry matter accumulation in wheat plants between PN240 and N300 groups. This indicates that although PN240 treatment reduces N application rate by 20%, it can improve N use efficiency to promote dry matter accumulation in wheat plants. It was also found that compared with CK, PPM and N treatments (N300, PN180, PN240, and PN300) increased wheat plant height and dry weight, and PN180, PN240, and PN300 treatments promoted dry matter accumulation and N use efficiency of wheat. This is consistent with previous findings [20]. For example, Larroque et al. [21] also found that the combination of PPM and N fertilizer significantly increased wheat yields. The nutritional status of plants affects the grain number per ear and yield of wheat [22]. In this study, PN300 and PN240 treatments significantly increased the grain number per ear compared to N300 treatment. This may be due to the fact that PPM can slowly release nitrogen into the soil so that wheat can obtain a continuous supply of nutrients especially in the reproductive stage [23]. In addition, it was also found that the PN300 group had the highest grain yield, followed by the PN240, N300, and PN180 groups, but there was no difference between the PN240 and the N300 groups. The lowest grain yield in the PN180 group may be due to the insufficient N supply under PN180 treatment. The insignificant difference between the PN240 and the N300 groups indicates that although the N application rate was reduced by 20%, PPM could slowly release N fertilizer to provide wheat with a continuous supply of nutrients [24], thus avoiding N pollution and reducing the costs of crop production [25]. This may be due to the fact that anionic PAM carries the same negative charge as the soil surface, but it is able to bind to the soil through the action of cation bridges [26], which can occupy the anionic sites in the soil salinity ions, allowing for the anions to enter into the soil solution and enhancing the exchange ions in the soil colloid [27]. The nitrogen exchanged into the soil solution can be rapidly absorbed and utilized by plants to promote nutrient growth of crops. The highest grain yield in the PN300 group may be due to the fact that (1) sufficient N supply provides sufficient nutrients for wheat growth and reproduction; (2) PPM can slowly release N fertilizer to provide wheat with a continuous supply of nutrients; (3) PPM has the water retention function.

In a certain nitrogen application rate range, the nitrogen accumulation of wheat increased with the increase in nitrogen application rate. However, beyond the range, it does not increase or decrease [28]. This study found that high nitrogen application rates (N300 and PN300) significantly increased the nitrogen accumulation and contribution of pre-anthesis organs’ N to grain (PCR) in wheat organs, but the contribution of post-anthesis organs’ N to grain N (ACR) decreased. This may be due to the fact that excessive application of nitrogen fertilizer leads to excessive vegetative growth in the late growth of wheat and inhibited reproductive growth [29], which reduces the transport of wheat dry matter to the ears and grains after flowering [30]. However, the PN240 group had an opposite result, that is, the transfer rate of nitrogen to the grain before flowering and the contribution rate of nitrogen transfer to the grain showed a decreasing trend, and the ACR showed an upward trend after flowering. This is consistent with the results of Ding [31]. This indicates that the redistribution of nitrogen in vegetative organs in the later growth stage could directly affect the accumulation of nitrogen in grain. Combination of nitrogen fertilizer and PPM promoted dry matter distribution in wheat organs and increased the contribution of post-flower assimilation to the grain. In this study, AEN and PFPN decreased with the increase in nitrogen application rate. This is consistent with the results of Yang [32]. This indicates that the addition of nitrogen reduces the ability of wheat to absorb nitrogen and convert it into grains. The NUE of the PN300 and N300 groups reduced compared with that of the PN240 group. This may be due to that fact that a high nitrogen application rate leads to excessive nitrogen uptake by wheat and increased nitrogen content in vegetative organs [33]. This reduces the proportion of nitrogen in the grain, which ultimately leads to a decrease in NUE [34]. It was also found that the NUE values of the PN180 and N60 groups were low. This may be due to the fact that excessive nitrogen reduction leads to a decrease in soil nitrogen storage [29], causing N deficit for plant growth [29]. This study also found that the NUE, PFPN, and yield values of the PN240 group were the highest. This may be due to the fact that PN240 can reduce nitrogen loss, improve crop yield, and achieve nitrogen reduction and NUE improvement. It can be seen that under the premise of high N accumulation, PPM combined with 80% nitrogen fertilizer can be used as an optimal nitrogen application strategy to achieve high yield.

5. Conclusions

The combined application of PPM and N fertilizer (except PN180) significantly increased the yield, biomass, N transport indexes (TR, P-CR, and A-CR), and N use efficiency (NUE, AEN, and PEN) of wheat varieties “XC29” and “XC38” under drip irrigation. Especially PN240 treatment slowed down the release of N fertilizer, increased the uptake of soil N by crops, and improved N use efficiency. In addition, the yield of wheat under PN240 treatment reached that of single N fertilizer (N300) treatment. Therefore, PN240 (PPM + 80% of traditional nitrogen application rate) treatment can achieve the purpose of fertilizer reduction without reducing wheat yield. The research results will provide theoretical support for N fertilizer reduction and improve N use efficiency.