Deficit Irrigation and High Planting Density Improve Nitrogen Uptake and Use Efficiency of Cotton in Drip Irrigation

Abstract

The optimization of plant density plays a crucial role in cotton production, and deficit irrigation, as a water-saving measure, has been widely adopted in arid regions. However, regulatory mechanisms governing nitrogen absorption, transportation, and nitrogen use efficiency (NUE) in cotton under deficit irrigation and high plant density remain unclear. To clarify the mechanisms of N uptake and NUE of cotton, the main plots were subjected to three irrigation amounts based on field capacity (Fc): (315 [W1, 0.5 Fc], 405 [W2, 0.75 Fc, farmers’ irrigation practice], and 495 mm [W3, 1.0 Fc]). Subplots were planted and applied at three densities: (13.5 [M1], 18.0 [M2, farmers’ planting practice], and 22.5 [M3] plants m⁻²). The results revealed that under low-irrigation conditions, the cotton yield was 5.1% lower than that under the farmer’s irrigation practice. In all plant densities and years, the nitrogen uptake of cotton increased significantly with the increase in irrigation. However, excessive irrigation resulted in nitrogen accumulation and migration, mainly concentrated in the vegetative organs of cotton, which reduced the NUE by 9.2% compared with that under farmers’ irrigation practice. Concerning the interaction between irrigation and plant density, under low irrigation, the nitrogen uptake of high-density planting was higher, and the yield of seed cotton was only 2.9% lower than that of the control (the interaction effect of farmers’ irrigation × plant density), but the NUE was increased by 10.9%. Notably, with the increase in irrigation amount, the soil nitrate nitrogen at the 0–40 cm soil layer decreased, and high irrigation amounts would lead to the transfer of soil nitrate nitrogen to deep soil. With the increase in plant density, the rate of nitrogen uptake and the amount of nitrogen uptake increased, which significantly reduced the soil nitrate nitrogen content. In conclusion, deficit irrigation and high plant density can improve cotton yield and NUE. We anticipate that these findings will facilitate optimized agricultural management in areas with limited water.

1. Introduction

Cotton (Gossypium hirsutum L.) is a major global cash crop, vital to agriculture, industrial production, and economic trade in cotton-producing countries [1]. China ranks among the top three cotton producers globally [2]. The Xinjiang region, with its unique climate, is a major contributor to China’s cotton production, accounting for more than 90% of the national output [3]. However, limited by geographical location, water shortage has seriously affected the development of the cotton industry [4]. Therefore, under resource-constrained conditions, developing agricultural technologies that improve nitrogen use efficiency (NUE) is a key pathway to achieving sustainable agriculture [5]. To address water resource challenges, the Xinjiang region has adopted measures such as drip irrigation, plastic mulch, and increased planting density to enhance cotton yield and optimize water use efficiency.

In the past, conventional flood irrigation (plastic film mulching and flood irrigation) was the predominant method for cotton production in Xinjiang. This method is labor-intensive, requires substantial water, and has poor irrigation uniformity, leading to low yield and water use efficiency [6,7]. Consequently, the development of water-saving irrigation technology is essential for agricultural advancement in Xinjiang and similar arid regions. Since the late 1990s, the extensive adoption of subsurface drip irrigation technology has substantially enhanced cotton production [8]. The uniqueness of this technology lies in its ability to deliver water and nutrients directly to plant roots, inducing alterations in soil moisture and nitrogen distribution. This thereby enhances root uptake of soil nitrogen and ultimately elevates crop yield and water–nitrogen utilization efficiency [9]. However, excessive irrigation can also intensify the vertical movement of soil moisture, which exacerbates the leaching of soil nitrate nitrogen [10], leading to a significant reduction in NUE. In water-scarce and arid regions, deficit irrigation has emerged as an innovative water-saving strategy. It sustains crop growth at normal levels by promoting nutrient movement to reproductive organs while simultaneously limiting stomatal opening and reducing transpiration [11]. Consequently, it fosters cotton yield formation and enhances water–nitrogen utilization efficiency.

Additionally, to maximize the utilization of limited land and water resources, Xinjiang employs a substantially higher planting density compared with many other regions in China, thereby augmenting cotton yield and optimizing fertilizer utilization efficiency [12]. Especially under limited irrigation conditions, increasing planting density effectively regulates the competition among plant populations and individuals, enhancing light interception in the lower part of the plant canopy [13]. Consequently, under deficit irrigation, high-density planting results in 9.1–17% and 9.3–16.8% increases in seed cotton yield and irrigation water productivity, respectively, when compared with those of low or medium planting densities [14].

Nutrient absorption serves as the foundation for dry matter accumulation and storage while constituting a prerequisite for yield formation [15]. The transport capacity of nutrients between different plant organs affects the efficiency of nutrient utilization in crops. In this respect, prior research has demonstrated that by selecting suitable cultivars [16] and implementing sound agronomic practices [17,18], nitrogen fertilizer losses can be reduced, aiding cotton’s nitrogen uptake, promoting dry matter accumulation and translocation, and thereby enhancing cotton yield and NUE [19]. However, under film drip irrigation conditions, there has been relatively limited research on how to enhance the nitrogen uptake and NUE of cotton by adjusting the irrigation amount and planting density. Particularly in deficit irrigation scenarios, further investigation is needed to determine whether increasing planting density can improve NUE in cotton fields while maintaining yield amounts.

Therefore, the main objectives of this study are the following: (a) quantitatively investigate the influence of different irrigation rates and planting densities under subsurface drip irrigation conditions on plant nitrogen uptake, translocation, and utilization efficiency; and (b) based on the nitrogen uptake and yield of cotton, propose more rational cotton-planting strategies to assist in formulating more effective field management policies and improving water–fertilizer utilization efficiency.

2. Materials and Methods

2.1. Experimental Site

During the 2019 and 2020 cotton growing seasons, two field experiments were carried out at the Cotton Comprehensive Experimental Station of the Xinjiang Academy of Agricultural Sciences, situated in the arid Awati region of Northwest China (N 41°06′, E 80°44′). The region’s air temperature from 1991 to 2020 averaged 10.4 °C annually; each year, the average accumulated temperature sum was 3988 °C. It receives around 2679 h of sunlight annually, has an average yearly rainfall of 46.7 mm, and experiences a frost-free period of about 211 days. Due to high annual evaporation (2900 mm), irrigation is vital for local agriculture.

The soil of the experimental site was sandy loam, with a bulk density of 1.48 g cm⁻³ in the 0–40 cm soil layer and a field capacity of 28.9%. The soil contained 10.6 g kg⁻¹ of organic matter and 1.8 g kg⁻¹ of total nitrogen. The maximum and minimum air temperatures and precipitation during the April to October growing seasons for both 2019 and 2020. In 2019, the average air temperature was lower than in 2020, whereas rainfall was higher (Figure 1).

2.2. Experimental Design and Field Management

The experiment was organized in a split-plot design with four replications. It included three irrigation amounts as the main plots: 315 [W1, 0.5Fc], 405 [W2, 0.75Fc, farmers’ irrigation practice], and 495 mm [W3, 1.0 Fc, Fc was the field capacity], as well as three plant densities as the subplots: 13.5 [M1], 18.0 [M2, farmers’ planting practice], and 22.5 plants m⁻² [M3]. Each subplot was an area of 39 m² (6.5 m long, 6.0 m wide). In these plots, drip irrigation is carried out through a tube placed under a plastic film. Use solenoid valves and flowmeters to monitor irrigation. To prevent water and fertilizer from flowing between different plots, plastic film (70 cm wide and 1.6 mm thick) was completely buried between the experimental fields as a barrier before sowing. In both crop seasons, irrigation was carried out using different irrigation amounts, while other measures were in line with local field practices.

High-yield, upland cotton (cv. Xinluzhong 88) was sown in early April, and harvest occurred in early October. Pre-sowing fertilization included 150 kg ha⁻¹ of urea (N, 46.4%), 450 kg ha⁻¹ of diammonium phosphate (N, 21.2%; P₂O₅, 53.8%), and 225 kg ha⁻¹ of potassium sulfate (K₂O, 51%). During growth, “one water, one fertilizer” drip fertilization was implemented, and 600 kg ha⁻¹ of urea was applied. Other practices followed local standards.

2.3. Measurements

2.3.1. Seed Cotton Yields

The final cotton yield was determined on 18 September 2019 and 24 September 2020. Within a central sampling area of 6.67 m² (length × width: 2.9 m × 2.3 m) in each plot, all plants were harvested to assess actual plant density, boll numbers per plant, and boll weight. Bolls were sun-dried with 13% water content. The seed cotton yield (including fiber and seeds) was calculated according to the boll numbers per unit ground area and single boll weight.

2.3.2. Biomass, Nitrogen Uptake, and NUE

In two planting seasons, samples were collected at 40, 65, 85, 115, and 140 days after seeding, with four replicates per sampling point. From each treatment, five plants were selected, and plant tissue samples were separated into different organs, such as stems, leaves, squares, and roots. These samples were initially blanched at 105 °C for 30 min, then oven-dried at 80 °C until a constant weight was reached; their dry matter weights were subsequently recorded. The dried cotton plant samples were pulverized, sieved through a 0.5 mm mesh, digested with H₂SO₄–H₂O₂, and made up to a known volume. The total nitrogen content of various plant organs was quantified using Nessler’s colorimetric method. The nitrogen uptake in cotton was fitted to a logistic curve [20].

$$Y={\displaystyle \frac{K}{(1+{ae}^{bt})}}$$

(1)

where t is the number of days after sowing, Y (kg ha⁻¹) is the N uptake at t, K (kg ha⁻¹) is the maximum nitrogen uptake, and a and b are the constants to be assessed.

On the basis of Equation (1), the following equations are calculated:

$$t_1={\displaystyle \frac{1}{b}}\mathrm{l}\mathrm{n}\left({\displaystyle \frac{2+\sqrt{3}}{a}}\right)$$

(2)

$$t_2={\displaystyle \frac{1}{b}}\mathrm{l}\mathrm{n}\left({\displaystyle \frac{2-\sqrt{3}}{a}}\right)$$

(3)

$$t_m=-{\displaystyle \frac{1}{b}}\mathrm{l}\mathrm{n}a$$

(4)

$$V_m=-{\displaystyle \frac{bK}{4}}$$

(5)

where V_m (kg hm⁻² d⁻¹) is the highest nitrogen uptake rate and t_m (d) is the largest nitrogen uptake period, which initiates at time t₁ and terminates at t₂.

NUE, the seed cotton yield (SY, kg ha⁻¹) produced per unit of nitrogen uptake (N_uptake, kg ha⁻¹), is calculated using Equation (6) [21].

$$NUE={\displaystyle \frac{SY}{N_{uptake}}}$$

(6)

2.3.3. Soil Nitrate Nitrogen Content

During the cotton harvesting period, soil samples at depths of 0–10, 10–20, 20–30, 30–40, 40–50, and 50–60 cm were collected for each treatment, both under and between the drip irrigation lines. Soil solution was extracted from the soil samples and 2 mol L⁻¹ KCl solution at a 1:5 ratio. The nitrate nitrogen content in the solution was subsequently measured using a UV spectrophotometer [22].

2.4. Statistical Analysis

All data were subjected to a two-way ANOVA using SPSS 22.0 software (IBM Inc., Chicago, IL, USA), with statistical differences between treatments determined by the least significant difference (LSD) at p = 0.05. All graphs were processed using OriginPro. 2019 (Origin Lab Corporation, Northampton, MA, USA).

3. Results

3.1. Seed Cotton Yield and NUE

Both the irrigation amounts and plant density significantly impacted the seed cotton yield (p < 0.05), and there was a notable interaction between treatments and years (

Table 1. Seed cotton yield, total N uptake, and NUE under different irrigation amounts and planting densities.

Year	Irrigation Amount	Plant Density	Seed Cotton Yield	Total Nitrogen Uptake	N Use Efficiency
	(mm)	(Plants m−2)	(kg ha−1)	kg ha−1	kg kg−1
2019	W1	M1	4602 b	181.8 c	25.4 a
		M2	4702 b	202.7 b	23.2 a
		M3	5303 a	230.4 a	23.0 a
		SE	79	4.65	0.35
	W2	M1	4818 c	224.8 b	21.5 a
		M2	5501 a	281.2 a	19.6 a
		M3	5146 b	283.4 a	18.2 b
		SE	81	7.57	0.45
	W3	M1	5274 a	251.1 b	21.0 a
		M2	5501 a	275.2 ab	18.7 ab
		M3	5146 b	290.8 a	16.7 b
		SE	59	6.21	0.55
2020	W1	M1	4499 c	183.3 c	24.6 a
		M2	5396 b	230.7 b	23.3 a
		M3	5878 a	251.5 a	23.3 a
		SE	147	7.08	0.45
	W2	M1	4972 c	206.5 c	24.1 a
		M2	6007 a	277.2 b	21.7 a
		M3	5573 b	290.5 a	19.2 b
		SE	118	8.43	0.45
	W3	M1	5391 a	259.7 b	20.7 a
		M2	5280 b	274.2 b	19.3 b
		M3	5004 ab	309.8 a	16.1 b
		SE	65	7.22	0.65
p-value
Year			0.006	0.000	0.711
Irrigation			0.012	0.007	0.008
Density			0.004	0.005	0.005
Irrigation × Density			0.009	0.008	0.004
Year × Irrigation × Density			0.006	0.075	0.679

Note: Matching lowercase letters indicate no significant differences within the same year and irrigation treatment at the p = 0.05 level. SE shows that there is a marginal standard error for all density treatments repeated under irrigation conditions in the same year.

). Considering both measurement years, the yield under W1 for all three plant densities was 5063 kg ha⁻¹, which was 5.1% lower than the yield of W2; the yield under W3 was 5266 kg ha⁻¹, representing a 1.3% reduction in yield. Under W1M3 conditions, the yield was higher at 5590 kg ha⁻¹, whereas the yield at W3M1 (5332 kg ha⁻¹) was slightly lower than that of W2M2.

NUE was markedly (p < 0.05) affected by the main effects of irrigation amount, planting density, and its interactions (Table 1). Increasing irrigation amounts and plant density enhanced the total N uptake in cotton plants, but there was a decreasing trend in NUE. Compared with W1, the total N uptake increased by 18.1–22.8% in W2 and W3 scenarios, whereas NUE decreased by 13.1–21.1%. The NUE underW1M3 was highest in both years, averaging 23.2 kg kg⁻¹, a 10.9% increase in W2M2.

3.2. Biomass

The irrigation amount and planting density exerted significant effects on biomass distribution and accumulation (p < 0.05). The biomass increased with the augmentation of irrigation amount and planting density over the two-year experiment (

Table 2. Effects of irrigation amounts and planting density on the biomass of cotton.

Year	Irrigation	Density	Vegetative Organ	Reproductive Organ	Total Dry Matter Accumulation
	(mm)	(Plants m−2)	kg hm−2
2019	W1	M1	4900 b	5501 b	10,401 b
		M2	5230 b	5628 b	10,859 b
		M3	6242 a	6152 a	12,395 a
		SE	216	110	290
	W2	M1	5608 b	5473 b	11,082 b
		M2	6853 a	6129 a	12,983 a
		M3	7045 a	6096 a	13,141 a
		SE	228	128	347
	W3	M1	6933 b	6021 b	12,954 b
		M2	7333 b	5980 b	13,314 b
		M3	8401 a	5713 a	14,114 a
		SE	234	62	163
2020	W1	M1	4795 c	5220 c	10,015 c
		M2	5929 b	6262 b	12,192 b
		M3	7060 a	6809 a	13,869 a
		SE	296	204	497
	W2	M1	5714 b	5985 b	11,700 b
		M2	7282 a	6979 a	14,261 a
		M3	8022 a	6792 a	14,815 a
		SE	308	154	432
	W3	M1	6480 b	6441 b	12,921 b
		M2	8022 a	6246 a	14,268 a
		M3	8457 a	6156 a	14,614 a
		SE	272	43	301
p-value
Year			0.014	0.055	0.118
Irrigation			0.013	0.008	0.023
Density			0.005	0.009	0.006
Irrigation × Density			0.008	0.017	0.004
Year × Irrigation × Density			0.055	0.045	0.998

Note: Matching lowercase letters indicate no significant differences within the same year and irrigation treatment at the p = 0.05 level. SE shows that there is a marginal standard error for all density treatments repeated under irrigation conditions in the same year.

). In comparison with that of W2, nutrient organs and reproductive organs under W1 experienced respective average reductions of 15.7% and 5.1%. In contrast, nutritive organs increased by 11.1% under W3 compared with that of W2, but reproductive organs decreased by 2.4%. Under the joint regulation of irrigation amount and planting density, the biomass of W1M3 was 3.6% lower than that of W2M2, but the accumulation of reproductive organs reached 6481 kg hm⁻².

3.3. Nitrogen Uptake Dynamics

The nitrogen uptake in cotton was markedly affected by irrigation amounts, plant densities, and their interactions (p < 0.05). Over the two years and across the three plant densities, the nitrogen uptake under W1 exhibited an 18.1% reduction relative to that of W2, whereas W3 led to a 5.9% increase (Table 1). However, within the 2-year period, the combination of W1M3 resulted in a nitrogen uptake of 241.1 kg hm⁻², which was 13.6% lower than that of W2M2.

As the number of days after emergence increased, the nitrogen accumulation in cotton exhibited a typical “slow–fast–slow” accumulation pattern, with rapid accumulation from early flowering to peak boll stage, followed by reaching a steady state in the later stages (Figure 2). Simulation results for nitrogen accumulation in cotton plants (

Table 3. Eigen values of cotton nitrogen accumulation dynamics under different irrigation amounts and planting density treatments.

Year	Irrigation	Density	Ym	Vm	tm	t1	t2
	(mm)	(Plants m−2)	(kg ha−1)	(kg ha−1 d−1)	(d)	(d)	(d)
2019	W1	M1	109.07 b	2.80 c	96.16 a	77.32 a	116.20 a
		M2	113.18 ab	3.10 b	93.07 ab	75.66 a	112.08 a
		M3	119.81 a	3.54 a	89.32 b	71.57 b	105.50 b
		SE	1.74	0.09	1.05	0.83	1.51
	W2	M1	113.71 b	3.11 c	94.54 a	76.52 a	113.11 a
		M2	120.30 b	3.46 b	90.35 ab	73.62 a	107.66 a
		M3	128.74 a	3.92 a	88.48 b	69.44 b	102.34 b
		SE	2.21	0.11	1.03	1.01	1.33
	W3	M1	116.84 b	3.48 b	91.76 a	74.66 a	108.29 a
		M2	134.76 a	3.81 ab	88.09 b	70.63 ab	105.95 ab
		M3	140.18 a	4.16 a	86.41 b	67.23 b	100.70 b
		SE	3.07	0.12	0.76	1.11	1.35
2020	W1	M1	108.69 b	3.05 b	94.77 a	78.28 a	113.50 a
		M2	114.95 a	3.36 ab	92.66 ab	75.66 b	110.16 ab
		M3	120.76 a	3.67 a	90.90 b	73.11 c	106.76 b
		SE	2.02	0.1	0.68	0.59	1.29
	W2	M1	111.79 b	3.23 b	93.05 a	75.08 a	109.62 a
		M2	121.79 ab	3.67 a	90.18 ab	72.21 ab	104.05 ab
		M3	130.29 a	3.91 a	87.90 b	70.26 b	102.14 b
		SE	2.82	0.11	0.67	0.86	1.36
	W3	M1	122.42 b	3.42 b	90.62 a	74.31 a	110.02 a
		M2	128.62 b	3.79 ab	87.75 ab	70.14 b	104.46 ab
		M3	138.26 a	4.17 a	85.59 b	67.33 b	100.65 b
		SE	2.23	0.12	0.64	1.04	1.45
p-value
Year			0.206	0.023	0.097	0.142	0.035
Irrigation			0.000	0.000	0.000	0.000	0.000
Density			0.000	0.000	0.000	0.000	0.000
Irrigation × Density			0.038	0.967	0.941	0.023	0.729
Year × Irrigation × Density			0.546	0.819	0.628	0.130	0.337

Note: Matching lowercase letters indicate no significant differences within the same year and irrigation treatment at the p = 0.05 level. SE shows that there is a marginal standard error for all density treatments repeated under irrigation conditions in the same year.

) indicated that Y_m and V_m were significantly influenced by the irrigation amount and planting density, as well as their interaction (p < 0.05). Under W1, Y_m and V_m decreased by 4.6% and 8.3%, respectively, compared with that of W2. Greater planting density led to significant increases in Y_m and V_m. The collective effect of W1M3 resulted in no significant difference in the rapid accumulation period and V_m compared with W2M2, leading to a 0.6% decrease in Y_m.

3.4. Cotton Plant N Distribution and N Economic Coefficient

The irrigation amounts and planting density significantly influenced the nitrogen allocation in cotton plants (p < 0.05). Compared with that of W2, the nitrogen uptake of cotton reproductive organs with W1 and W3 decreased by 3.4% and 7.7%, respectively (

Table 4. Effects of irrigation amounts and planting density on N distribution and NEC in cotton.

Year	Irrigation	Density	Bud Stage			Flower and Boll Stage			Open Boll Stage
			DVO	DRO	NEC	DVO	DRO	NEC	DVO	DRO	NEC
	(mm)	(Plants m−2)	kg ha−1	kg ha−1		kg ha−1	kg ha−1		kg ha−1	kg ha−1
2019	W1	M1	26.2 c	1.9 b	0.067 b	93.1 c	74.4 b	0.442 a	85.1 c	96.7 c	0.532 a
		M2	33.8 b	3.3 b	0.089 b	107.4 b	82.6 a	0.435 ab	97.8 b	104.b	0.517 ab
		M3	41.8 a	5.6 a	0.116 a	124.4 a	87.6 a	0.413 b	112.9 a	117.4 a	0.509 b
		SE	1.89	0.51	0.01	3.47	1.75	0.01	3.32	2.66	0.003
	W2	M1	27.2 c	1.9 b	0.067 a	118.8 c	78.2 b	0.396 a	125.0 b	99.7 b	0.443 a
		M2	34.2 b	3.4 ab	0.085 a	151.4 b	89.2 a	0.370 b	162.4 a	118.a	0.422 ab
		M3	43.5 a	5.6 a	0.113 a	180.9 a	85.6 a	0.321 c	173.0 a	110.3 a	0.389 b
		SE	1.92	0.56	0.01	6.80	1.45	0.01	6.22	3.21	0.008
	W3	M1	27.1 c	1.9 c	0.067 a	140.4 c	84.4 a	0.376 a	138.3 c	112.8 c	0.448 a
		M2	33.8 b	3.7 b	0.101 a	169.7 b	80.8 b	0.322 b	173.5 b	101.7 b	0.369 b
		M3	42.6 a	5.8 a	0.119 a	193.0 a	77.0 c	0.285 c	198.3 a	92.5 a	0.318 c
		SE	1.90	0.51	0.01	6.09	0.99	0.01	6.78	3.9	0.016
2020	W1	M1	16.1 c	1.2 c	0.067 c	96.6 c	77.4 c	0.444 a	86.1 c	97.1 c	0.530 a
		M2	25.3 b	2.6 b	0.093 b	120.8 b	87.6 b	0.420 b	111.1 b	119.6 a	0.518 ab
		M3	36.6 a	4.5 a	0.110 a	138.3 a	93.3 a	0.403 b	122.2 a	117.3 a	0.494 b
		SE	2.31	0.38	0.01	4.71	1.93	0.01	4.2	4.96	0.005
	W2	M1	16.5 c	1.3 c	0.074 b	114.2 c	78.2 b	0.406 a	108.0 c	98.5 c	0.501 a
		M2	25.1 b	2.8 b	0.106 ab	150.9 b	89.2 a	0.371 b	150.7 b	126.5 b	0.474 a
		M3	35.3 a	4.6 a	0.116 a	196.3 a	85.6 a	0.303 c	176.3 a	114.2 a	0.410 b
		SE	2.35	0.41	0.01	9.03	1.45	0.01	8.34	2.10	0.012
	W3	M1	15.9 c	1.1 c	0.063 b	141.4 c	84.4 a	0.374 a	154.4 c	105.2 c	0.43 a
		M2	24.6 b	2.9 b	0.107 a	186.4 b	80.8 b	0.302 b	174.7 b	99.5 b	0.399 b
		M3	35.8 a	4.7 a	0.115 a	211.0 a	77.0 c	0.267 c	218.8 a	90.9 a	0.319 c
		SE	2.27	0.44	0.01	7.87	0.99	0.01	5.09	2.74	0.011
p-value
Year			0.000	0.001	0.602	0.001	0.002	0.183	0.882	0.000	0.000
Irrigation			0.927	0.798	0.776	0.000	0.000	0.000	0.000	0.000	0.000
Density			0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
Irrigation × Density			0.995	0.986	0.916	0.020	0.029	0.044	0.015	0.032	0.013
Year × Irrigation × Density			0.976	0.998	0.926	0.851	0.915	0.564	0.999	0.197	0.121

Note: DVO: distribution in vegetative organs; DRO: distribution in reproductive organs; NEC: N economic coefficient. Matching lowercase letters indicate no significant differences within the same year and irrigation treatment at the p = 0.05 level. SE shows that there is a marginal standard error for all density treatments repeated under irrigation conditions in the same year.

). Under the combined regulation of irrigation amounts and planting density, the reproductive organ nitrogen accumulation of W1M3 reached 120.9 kg ha⁻¹, which was slightly reduced by 3.38% compared with that of W2M2.

The irrigation amounts and planting density significantly impacted the nitrogen economic coefficient (NEC) (p < 0.05). Compared with W1, W2 and W3 decreased NEC by 14.8% and 25.8%, respectively (Table 4). With the increase in planting density, NEC showed a decreasing trend. The NEC of W1M3 reached 0.50 in the open boll stage, which was 10.67% higher than that of W2M2.

3.5. Spatial Distribution of Soil Nitrate Nitrogen Content

In all treatments and years, the soil nitrate nitrogen was more pronounced in the 0–40 cm soil layer. In comparison with W1, both W2 and W3 led to an increase in soil nitrate nitrogen content by 15.58–22.61% in the 0–40 cm soil layer and a decrease by 18.12–24.99% in the 40–60 cm soil layer (Figure 3). Increasing the plant density significantly reduced the soil nitrate nitrogen in all soil layers. Under the combined regulation of irrigation amounts and planting density, in the 0–40 cm soil layer, the soil nitrate nitrogen content increased by 5.94% in W1M3 compared with that of W2M2. However, the soil nitrate nitrogen decreased by 26.22% in the 40–60 cm soil layer.

4. Discussion

In the cotton field with film drip irrigation, the combination of reduced irrigation amounts and high planting density resulted in greater nitrogen uptake by plants, with only a 2.9% decrease in seed cotton yield compared with that of the farmers’ practice. However, NUE increased by 10.9%. Decreasing irrigation amounts restrained plant nitrogen absorption and utilization while reducing the leaching of soil nitrate to deeper layers. Increasing plant density contributed to higher nitrogen uptake in the plant population and reduced the residual nitrate in the soil. The losses in yield and NUE caused by lower irrigation amounts were more than compensated for by the benefits of increased planting density. We anticipate that these findings will aid farmers in optimizing agricultural management strategies in regions with limited water resources.

4.1. Effects of Irrigation Amount and Planting Density on Biomass and Yield

Cotton biomass is the material basis of yield formation [23]. Previous research has indicated that insufficient irrigation fails to maintain the desired soil moisture content, potentially inhibiting cell elongation and restricting photosynthesis and carbohydrate synthesis in crops [24]. These are detrimental to cotton yield. However, a greater allocation of assimilates to reproductive organs significantly enhances the harvest index [25]. Therefore, increasing the biological yield is essential for maintaining the stability of the yield. Increasing the plant density is one of the key strategies in this regard [26]. Under low-irrigation conditions, increasing the plant density can make up for the decrease in biomass caused by insufficient irrigation, making cotton yield comparable with that of conventional management measures [14]. Similar results were obtained in this study under 76 cm equal row spacing planting mode. This may be because, in the case of water deficit, increasing plant density can establish an appropriate population structure, maintain relatively strong population photosynthesis capacity [13], and slow leaf aging during late growth and development, thus preserving dry matter supply to increase yield. However, although this study achieved positive results under certain conditions, further research is needed under different climatic conditions to determine the broad applicability and optimization of these strategies. In particular, the optimal combination of irrigation and planting density according to the specific climatic characteristics of different regions must be explored to achieve the best yield and resource utilization efficiency.

4.2. Effects of Irrigation Amount and Planting Density on Nitrogen Uptake and Utilization in Cotton

The nitrogen uptake capacity of plants is closely related to nitrogen transport and utilization efficiency, which lays the foundation for efficient nitrogen uptake by crops [27,28]. The nitrogen in reproductive organs usually comes from nitrogen transport and redistribution in vegetative organs [29]. In low-irrigation conditions, the nitrogen uptake of plants is constrained, resulting in the reduction in transport from nutrient organs to reproductive organs and the reduction in NUE [30]. However, high-density planting effectively enhances nutrient utilization, resulting in increased N uptake during the early reproductive stage and elevated nitrogen transport to reproductive organs in the later stages [31]. With the increase in the planting density, the number of bolls per unit area increases to a certain extent, which indirectly increases the nitrogen uptake [32]. It may be that an increased planting density helps enhance root activity and root exudates, increase microbial abundance and diversity index, promote nutrient cycling and crop absorption, and thus indirectly promote nitrogen absorption [33]. However, when the planting density is too high, although the light interception of the canopy can be increased, the light interception rate at the bottom of the canopy is significantly reduced, thus affecting nitrogen absorption [34,35]. In this study, under the 76 cm equal row spacing planting mode, compared with the local conventional density, high-density planting can increase nitrogen uptake by 8.68%. Under low-water irrigation and high-density planting conditions, the nitrogen uptake was only 3.74% lower than that of the control, but the NUE increased by 9.31%. Therefore, this study shows that, under deficit irrigation in arid areas, appropriately increasing the planting density may be a feasible measure to improve NUE.

4.3. Effects of Irrigation Amount and Planting Density on Soil Nitrate Nitrogen

Nitrate nitrogen is a key type of nitrogen absorption by plants. It is easily soluble in water because it exists in water as an ion, which is easily absorbed by root systems [36]. In drip-irrigated cotton fields, cotton roots are mainly distributed in the depth range of 0–40 cm in the soil, which is closely related to the nitrogen absorption of the roots [7]. The growth of these roots is affected by the nitrogen content in the soil because the root system must find enough nitrogen in the soil to support plant growth [37]. When the irrigation amount is low, nitrogen usually accumulates in the topsoil layer, and increasing the irrigation amount will cause nitrogen in the soil to be washed into the deep soil [10], which is not conducive to nitrogen uptake by the roots. In addition, N uptake by plants is affected by the planting density. Increasing the planting density can promote the absorption of nitrogen by cotton plants, resulting in an overall decrease in soil nitrate nitrogen accumulation in different soil layers with an increase in the planting density [12]. However, in this study, under the 76 cm equal row spacing planting condition, the nitrate nitrogen content of the combination of low irrigation and high planting density in the 0–40 cm soil layer increased by 5.94% compared with the control group. Thus, low irrigation may lead to the lateral migration of nitrogen [38], but increasing planting density intensifies the competition among plants for soil water and nutrients, stimulates the expansion of roots to all sides, improves the uptake of lateral nitrogen by root groups, and improves the efficiency of N uptake and utilization. Although low-irrigation and high-density planting combinations have complex effects on nitrate nitrogen, further investigations are required to fully elucidate the impacts of reduced irrigation and increased planting density on soil nitrogen uniformity.

5. Conclusions

Under the conditions of drip irrigation under film, the optimal irrigation amount was 315 mm, and the planting density was about 22.5 plants m⁻² in the oasis cotton area of Xinjiang. By optimizing the effects of N uptake, utilization, and transport in plant populations, the yield and NUE were improved under optimal irrigation and planting density strategies. The findings of this study provide useful information for farmers in the oasis cotton region of Xinjiang and other cotton-growing areas in similar dry and water-scarce regions.