The Impact of Fulvic Acids on Cotton Growth, Yield and Phosphorus Fertilizer Use Efficiency Under Different Phosphorus Fertilization Rates in Xinjiang, China

Abstract

Chemical phosphorus (P) fertilizer is often overused in arid regions with alkaline soils due to soil fixation. Fulvic acid (FA) can increase soil P availability, enhancing crop yield and P use efficiency, but its interaction with P fertilization rates and potential to reduce P fertilizer application remains unclear. A 2-year (2019–2020) field experiment was conducted in Xinjiang, China, to study the impact of FA addition (45 kg ha⁻¹) on cotton yield and P use efficiency under different P fertilization rates (0, 50, 100 and 150 kg P₂O₅ ha⁻¹). Our results showed that P fertilization significantly enhanced cotton biomass, P uptake and seed cotton yields by 17–37%, but the partial nutrient balance (PNB), agronomic efficiency (AE) and partial factor productivity (PFP) decreased with increasing P fertilization rates. FA addition did not change cotton biomass and P uptake, but significantly enhanced seed cotton yield, AE and PFP by increasing bolls per plant. No significant interactions between FA addition and P fertilization rates were observed for cotton biomass, P uptake, seed cotton yield and P use efficiencies. These findings suggest that FA can improve cotton productivity, AE and PPF of P fertilizers, helping to keep the P balance in the cotton field.

1. Introduction

Cotton is one of the most important fiber crops in the world and is vital for global textile production, rural livelihoods and economic development [1]. China is the world’s largest cotton producer (https://www.fao.org (accessed on 23 February 2025)), with the Xinjiang Uyghur Autonomous Region the primary cotton-growing region. According to data from the National Bureau of Statistics (data.stats.gov.cn (accessed on 23 February 2025)), in 2023, the planting area and the total lint yield of cotton in Xinjiang were about 2.37 million hectares and 5.11 million tons, accounting for 85% of China’s total cotton planting area and 91% of its total yield. Such extensive cultivation and high yields are mainly attributed to the widespread adoption of drip irrigation under plastic film mulching and the intensive use of chemical fertilizers, both of which have significantly increased water use efficiency and soil nutrient availability, thus promoting cotton yield [2,3]. However, to sustain the high cotton yield, excessive chemical fertilizers are often applied, especially phosphorus (P), due to its generally lower use efficiency [4,5]. Soils in Xinjiang are generally alkaline with high calcium concentrations, and this region is typically arid with limited water availability for most of the time during the growing season. These factors cause rapid P fixation through adsorption and precipitation of calcium phosphate, in addition to the low diffusion rates in soils [6,7]. According to Tang et al. [5], the seed cotton yield was about 5544 kg ha⁻¹, and the corresponding above-ground plant P uptake was 55.86 kg·ha⁻¹ in most cotton fields in Xinjiang. However, the amount of applied chemical P fertilizer was between 150 and 207 kg P₂O₅ ha⁻¹, which is significantly higher than the requirement for cotton and results in a low P use efficiency of only 15%. The overapplication of P fertilizer leads to not only a waste of P resources but also several environmental risks, such as the eutrophication of water bodies.

Humic substances, like humic acid and fulvic acid (FA), have been reported to increase soil P availability [8,9], promote crop P uptake and use efficiency [10] and enhance crop yield [11,12,13]. Several reviews have summarized the underlying mechanisms for these effects [14,15,16,17]. Firstly, humic substances can improve soil P mobility and availability by chelating cations, thus reducing their adsorption on phosphate, changing soil pH and stimulating microbial activities. Secondly, they can enhance plant nutrient uptake by increasing root growth or rhizosphere microbial activity. Thirdly, some humic substances exhibit auxin-like functions to regulate plant growth and development. Lastly, humic substances can modify plant secondary metabolism to alleviate drought or salt stresses. However, most of these studies have been conducted under controlled environmental conditions in the greenhouse, limiting their applications to field practice [18]. Additionally, some field studies have reported no significant effects or even negative outcomes from humic substance application [19,20,21], suggesting more field experiments are needed to demonstrate their effectiveness across different crops and environmental conditions [22], which is essential for the real processes of agricultural production.

In this study, FA was selected as the target humic substance due to its high solubility, making it suitable for the fertigation systems in arid regions. To assess the effects of FA addition on cotton fields and its potential to reduce the application rates of chemical P fertilizers, we conducted a 2-year field experiment and investigated the cotton growth, yield, P uptake and use efficiency as influenced by FA addition under different P fertilization rates. We hypothesized that FA addition would increase soil P availability and enhance cotton P uptake, thus promoting the cotton yield and P fertilizer use efficiency, especially under the lower application rates of P fertilizers.

2. Materials and Methods

2.1. Study Site

This experiment was conducted in a cotton field in Fukang City, Xinjiang, China (88°00′44.30″ E, 44°10′21.05″ N) (Figure 1). This region has a continental arid climate, with an annual mean temperature of 6.6 °C and annual precipitation of 186 mm. The frost-free period averages 174 days per year. Cotton is the primary cash crop in this region, and the soil in the region is classified as Aridisols in Chinese soil taxonomy, equivalent to Calcisol in the FAO classification. Before the field trial, surface soil (top 20 cm) core samples were collected and analyzed (the detailed methods are described in Section 2.3). The surface soil had a pH of 8.1 (water to soil = 5:1), electrical conductivity of 226 mS·cm⁻¹, organic matter content of 18.1 g·kg⁻¹, available nitrogen content of 46.2 mg·kg⁻¹ (low level), available phosphorus content of 13.6 mg·kg⁻¹ (medium level) and available potassium of 454.8 mg kg⁻¹ (extremely high level) [23].

2.2. Experimental Design

This field trial was conducted on a cotton farm with drip fertigation under plastic film. The experiment consisted of eight treatments in a factorial design of four P fertilization rates (0, 50, 100 and 150 kg P₂O₅ ha⁻¹, designated as P0, P50, P100 and P150) and two FA addition levels (45 kg fulvic acid ha⁻¹, designated as FA, and no fulvic acid addition, serving as control, CK). The applied nitrogen and potassium fertilizers were the same among the different treatments, 250 kg N ha⁻¹ and 30 kg K₂O ha⁻¹. The treatments were arranged in a randomized complete block design with four replicates, resulting in a total number of 32 plots. The area of each plot was 20 m² (5 m × 4 m).

Before seeding, the surface soil (approximately 20 cm depth) was tilled and leveled. An 80 cm wide plastic film was laid on the ground with a spacing of 20 cm between rows, and one drip irrigation tape was placed beneath the center of each film. On 8 May 2019 and 20 April 2020, cotton (Gossypium hirsutum) “XinLuZao64” was seeded on both sides of the irrigation tape (20 cm away from the tape) under the plastic film. Each plot had a water meter, valve and fertilization tank to monitor the amount of fertilizer and irrigation. Fertilization was applied via irrigation, with the total amount of fertilizers evenly distributed across six irrigation events during the growing season (

Table 1. Irrigation scheme in 2019 and 2020.

Time (Day Month Year)	Irrigation Amount m3 ha−1	Time (Day Month Year)	Irrigation Amount m3 ha−1
10 May 2019	656	28 April 2020	623
5 July 2019	578	14 June 2020	600
16 July 2019	237	26 June 2020	293
25 July 2019	320	15 July 2020	387
5 August 2019	554	2 August 2020	453
15 August 2019	492	17 August 2020	476
24 August 2019	359	26 August 2020	452
Total irrigation amount in 2019	3196	Total irrigation amount in 2020	3284

). Urea (46-0-0, Kuitun Jinjiang Chemical Co., Ltd., Kuitun, China), monoammonium phosphate (12-61-0, Yunnan Zhongzheng Chemical Industry Co., Ltd., Kunming, China), potassium sulfate (0-0-50, Laishuo Technology Co., Ltd., Hangzhou, China) and fulvic acid (powder from plant material, Xinjiang Huier Agriculture Co., Ltd., Xinjiang Uyghur Autonomous Region, China) were used as fertilizers in this study (

Table 2. Amount of fertilizers applied in different treatments every year.

Treatment	Urea kg ha−1	Monoammonium Phosphate kg ha−1	Potassium Sulfate kg ha−1	Fulvic Acid kg ha−1
P0 + CK	543	0	60	0
P0 + FA	543	0	60	45
P50 + CK	522	82	60	0
P50 + FA	522	82	60	45
P100 + CK	501	164	60	0
P100 + FA	501	164	60	45
P150 + CK	479	246	60	0
P150 + FA	479	246	60	45

Note: P0, P50, P100 and P150 mean P fertilization rates of 0, 50, 100 and 150 kg P₂O₅ ha⁻¹; FA means fulvic acid addition of 45 kg fulvic acid ha⁻¹; CK means no fulvic acid addition as the control.

). Other management practices followed the local agronomic standards.

2.3. Measurement and Calculation

Soil sampling and analysis: At the boll opening stage (17 September 2019 and 7 September 2020), five soil cores of 0–20 cm depth were randomly collected from each plot and thoroughly mixed to create a composite soil sample weighing approximately 1.5 kg. Approximately 50 g of each soil sample was separated and stored at 4 °C to analyze the soil alkaline phosphatase activity. The remaining portion was air-dried and ground, then sieved through a 1 mm screen for chemical analysis. Soil pH and electrical conductivity (soil-to-water ratio of 1:5) were determined using a pH meter (PHS-3C, Shanghai Shengci Instrument Co., Ltd., Shanghai, China) and an electrical conductivity meter (DDS-307A, INASE Scientific Instrument Co., Ltd., Shanghai, China). Soil organic carbon (SOC) was measured using the Walkley and Black wet oxidation method [24]. Soil available nitrogen was measured according to Bao [24]. A total of 1 g of air-dried soil was incubated in 5 mL sodium hydroxide solution (1.2 M) in the outside ring of an airtight Conway diffusion cell, with 3 mL boric acid (0.3 M) added to the inner well at 40 °C for 24 h. With methyl red-bromocresol green as an indicator, 0.01 M hydrochloric acid was used to titrate ammonia absorbed in the boric acid. The soil available P was extracted using 0.5 mol L⁻¹ NaHCO₃ (2.5 g soil, 50 mL solution and 25 °C, shaken for 30 min) and measured using the molybdate–ascorbic acid method with a spectrophotometer (UV-1780, Shimadzu Corporation, Kyoto, Japan) [24]. The soil alkaline phosphatase activity was analyzed by the method using p-nitrophenyl phosphate (p-NPP) as substrate (1 g soil, 4 mL modified universal buffer and 1 mL p-nitrophenyl phosphate solution, incubated at 37 °C for 1 h), according to Tabatabai [25].

Cotton biomass and nutrient uptake: At the boll opening stage, 15 cotton plants were randomly sampled from each plot. Each plant was separated into different organs (shoot, leaf, shell, fiber and seed) and dried in an oven at 110 °C for 30 min, followed by drying at 80 °C until a constant weight. The dry matter weight of each organ was measured using an electronic scale with a precision of 0.01 g. The plant samples were then ground and digested with H₂SO₄–H₂O₂, and the total phosphorus concentration was determined using the molybdenum antimony colorimetric method [24]. Total plant P uptake was calculated as the sum of P uptake in each organ, determined by multiplying its dry weight by its P concentration.

Yield components: At the harvesting stage, the number of plants and bolls were counted manually based on a 1 m length film in each plot to calculate the indices of plants per hectare (plant density) and bolls per plant. Sixty bolls of seed cotton were collected from the bottom, middle and top of the plants and weighed to calculate the index of single boll weight. The theoretical yield was calculated as the product of plant density, number of bolls per plant and single boll weight.

Phosphorus use efficiency: The apparent recovery efficiency (RE), partial nutrient balance (PNB), agronomic efficiency (AE) and partial factor productivity (PFP) of P fertilizer were calculated using the following equations:

RE = (U − Uo)/F

PNB = Uh/F

AE = (Y − Yo)/F

PFP = Y/F

where U and Uo refer to the plant total P uptake with and without P fertilization; F refers to the amount of P fertilizer applied; Uh refers to the P content of the harvested portion (in this study, the straw was removed post-harvest, so Uh = U) and Y and Yo refer to the seed cotton yield with and without P fertilization.

2.4. Statistical Analysis

A three-way analysis of variance (ANOVA) was conducted to assess the effects of P fertilization rates, FA addition, year and their interactions on soil and plant variables. The least significant difference (LSD) method was used for multiple comparisons of means where the main or interactive effect was significant. Statistical analysis and plotting were performed using SPSS 16.0 (SPSS Inc., Chicago, IL, USA)) and SigmaPlot 12.5 (Systat Software Inc., Palo Alto, CA, USA) software.

3. Results

3.1. Soil pH, Olsen P and Alkaline Phosphatase Activity

P fertilization significantly reduced the soil pH value. Soil pH decreased progressively with increasing P fertilization rates, with a significantly lower value observed under the P150 treatment compared to the P0 treatment. The P fertilization significantly increased the soil available P levels. Compared to the P0, the soil available P content was significantly higher under the P50, P100 and P150 treatments, with the highest value observed under the P50 and P150 treatments. Additionally, the FA addition significantly increased the soil available P by 4%. Neither the P fertilization nor FA addition affected the soil alkaline phosphatase activity. There was no significant interaction between the P fertilization and FA addition on soil pH, available P and alkaline phosphatase activity (

Table 3. Soil pH, available P and alkaline phosphatase activity under different P fertilization rates and FA addition treatments in 2019 and 2020.

	pH	Available P mg kg−1	Alkaline Phosphatase Activity mg kg−1 h−1
P fertilization rates
P0	8.63 ± 0.03 a	19.20 ± 0.91 c	200.73 ± 14.14
P50	8.61 ± 0.03 a	22.94 ± 0.93 a	175.65 ± 13.40
P100	8.59 ± 0.06 ab	21.43 ± 0.76 b	167.57 ± 13.82
P150	8.52 ± 0.05 b	22.07 ± 0.64 ab	172.46 ± 12.19
FA addition
CK	8.61 ± 0.03	21.06 ± 0.57 b	179.28 ± 9.46
FA	8.57 ± 0.03	21.89 ± 0.67 a	178.09 ± 9.81
Year
2019	8.71 ± 0.02 a	19.40 ± 0.36 b	140.78 ± 4.54
2020	8.46 ± 0.02 b	24.30 ± 0.49 a	227.78 ± 5.04
ANOVA (p values)
P fertilization rates	0.038	<0.001	0.022
FA addition	0.107	0.0444	0.582
Year	<0.001	<0.001	<0.001
P × FA	0.362	0.906	0.693
P × Year	0.082	0.833	0.976
FA × Year	0.945	0.878	0.966
P × FA × Year	0.499	0.946	0.926

Note: P0, P50, P100 and P150 mean P fertilization rates of 0, 50, 100 and 150 kg P₂O₅ ha⁻¹; FA means fulvic acid addition of 45 kg fulvic acid ha⁻¹; CK means no fulvic acid addition as the control. Means ± SE followed by the same letter are not significantly different at the p < 0.05 level. The same applies to the following.

).

3.2. Cotton Biomass

The cotton shoot, leaf, shell, fiber, seed and total biomasses increased with the increasing P fertilization rates, with significantly higher values observed under the P150 treatment compared to the P0 treatment. The FA addition significantly increased the cotton leaf and shell biomass (

Table 4. Cotton biomass under different P fertilization rates and FA addition treatments at the boll opening stage in 2019 and 2020.

	Shoot kg ha−1	Leaf kg ha−1	Shell kg ha−1	Fiber kg ha−1	Seed kg ha−1	Total kg ha−1
P fertilization rates
P0	1968 ± 104 c	1408 ± 142 c	1925 ± 158 b	1667 ± 74 b	2501 ± 107 c	9469 ± 484 c
P50	2443 ± 147 b	1613 ± 150 bc	2320 ± 155 ab	1901 ± 91 ab	2802 ± 129 bc	11,079 ± 541 b
P100	2470 ± 142 b	1784 ± 159 b	2530 ± 141 a	2049 ± 106 a	3074 ± 159 ab	11,907 ± 583 ab
P150	2951 ± 112 a	2074 ± 114 a	2606 ± 134 a	2164 ± 77 a	3191 ± 120 a	12,985 ± 381 a
FA addition
CK	2391 ± 118	1604 ± 88 b	2184 ± 112 b	1927 ± 77	2890 ± 114	10,996 ± 432
FA	2525 ± 97	1836 ± 122 a	2507 ± 107 a	1963 ± 61	2893 ± 89	11,724 ± 394
Year
2019	2444 ± 110	2110 ± 96 a	2229 ± 126	1942 ± 74	2885 ± 109	11,608 ± 469
2020	2473 ± 107	1330 ± 67 b	2462 ± 93	1945 ± 65	2898 ± 95	11,111 ± 356
ANOVA (p values)
P fertilization rates	<0.001	<0.001	0.009	0.003	0.003	<0.001
FA addition	0.295	0.023	0.032	0.695	0.985	0.173
Year	0.822	<0.001	0.118	0.939	0.920	0.349
P × FA	0.338	0.758	0.949	0.495	0.462	0.808
P × Year	0.905	0.703	0.821	0.746	0.717	0.619
FA × Year	0.689	0.020	0.071	0.946	0.927	0.311
P × FA × Year	0.846	0.749	0.834	0.572	0.307	0.985

Note: P0, P50, P100 and P150 mean P fertilization rates of 0, 50, 100 and 150 kg P₂O₅ ha⁻¹; FA means fulvic acid addition of 45 kg fulvic acid ha⁻¹; CK means no fulvic acid addition as the control. Means ± SE followed by the same letter are not significantly different at the p < 0.05 level.

).

There was a significant interaction of FA addition treatments and year on leaf biomass. In 2019, the FA addition significantly increased leaf biomass, but in 2020, there was no significant difference in the leaf biomass between the FA and CK treatments (Figure 2).

3.3. Cotton P Concentration

Neither P fertilization nor FA addition affected the P concentrations of different cotton organs (

Table 5. Cotton P concentration under different P fertilization rates and FA addition treatments in 2019 and 2020.

	Shoot g kg−1	Leaf g kg−1	Shell g kg−1	Fiber g kg−1	Seed g kg−1
P fertilization rates
P0	1.34 ± 0.11	1.94 ± 0.12	2.22 ± 0.15	1.28 ± 0.06	9.80 ± 0.40
P50	1.27 ± 0.11	1.71 ± 0.11	2.04 ± 0.25	1.15 ± 0.05	10.11 ± 0.29
P100	1.24 ± 0.09	1.84 ± 0.13	1.80 ± 0.13	1.10 ± 0.05	10.72 ± 0.36
P150	1.22 ± 0.10	1.82 ± 0.15	2.13 ± 0.22	1.08 ± 0.05	10.10 ± 0.40
FA addition
CK	1.31 ± 0.08	1.87 ± 0.09	1.99 ± 0.12	1.15 ± 0.04	10.16 ± 0.24
FA	1.23 ± 0.07	1.78 ± 0.09	2.10 ± 0.15	1.16 ± 0.04	10.21 ± 0.28
Year
2019	0.95 ± 0.03 b	1.45 ± 0.05 b	1.46 ± 0.07 b	1.12 ± 0.04	9.46 ± 0.20 b
2020	1.59 ± 0.06 a	2.20 ± 0.07 a	2.63 ± 0.10 a	1.19 ± 0.04	10.91 ± 0.24 a
ANOVA (p values)
P fertilization rates	0.508	0.342	0.063	0.058	0.224
FA addition	0.197	0.314	0.290	0.839	0.854
Year	<0.001	<0.001	<0.001	0.244	<0.001
P × FA	0.145	0.287	0.116	0.685	0.199
P × Year	0.579	0.510	0.012	0.264	0.370
FA × Year	0.197	0.369	0.022	0.710	0.916
P × FA × Year	0.380	0.539	0.744	0.662	0.514

Note: P0, P50, P100 and P150 mean P fertilization rates of 0, 50, 100 and 150 kg P₂O₅ ha⁻¹; FA means fulvic acid addition of 45 kg fulvic acid ha⁻¹; CK means no fulvic acid addition as the control. Means ± SE followed by the same letter are not significantly different at the p < 0.05 level.

). The shoot, leaf, shell and seed P concentrations in 2019 were significantly higher than those measured in 2020.

The significant interaction between the year and P fertilization or FA addition was detected for the shell P concentration. In 2019, the highest shell P concentration was found in the P0 treatment, but in 2020, the highest value was found in the P50 and P150 treatments. FA addition significantly increased the shell P concentration in 2020, but not in 2019 (Figure 3).

3.4. Cotton P Uptake

The P fertilization significantly increased the cotton P uptake (

Table 6. Cotton P uptake under different P fertilization rates and FA addition treatments in 2019 and 2020.

	Shoot kg ha−1	Leaf kg ha−1	Shell kg ha−1	Fiber kg ha−1	Seed kg ha−1	Total kg ha−1
P fertilization rates
P0	2.62 ± 0.25 b	2.60 ± 0.23 b	4.40 ± 0.55 b	2.15 ± 0.16	24.35 ± 1.26 c	36.12 ± 1.99 c
P50	3.14 ± 0.32 ab	2.65 ± 0.20 b	4.75 ± 0.70 ab	2.16 ± 0.11	28.18 ± 1.26 bc	40.88 ± 2.12 bc
P100	2.98 ± 0.21 b	3.10 ± 0.23 ab	4.53 ± 0.39 ab	2.24 ± 0.15	33.09 ± 2.11 a	45.94 ± 12.68 ab
P150	3.68 ± 0.39 a	3.64 ± 0.27 a	5.55 ± 0.68 a	2.34 ± 0.13	31.94 ± 1.42 ab	47.28 ± 2.20 a
FA addition
CK	3.11 ± 0.24	2.91 ± 0.17	4.46 ± 0.42	2.20 ± 0.11	29.28 ± 1.30	41.95 ± 1.80
FA	3.10 ± 0.20	3.08 ± 0.19	5.16 ± 0.41	2.25 ± 0.09	29.50 ± 1.19	43.16 ± 1.71
Year
2019	2.33 ± 0.14 b	3.04 ± 0.16	3.14 ± 0.18 b	2.15 ± 0.10	27.23 ± 1.16 b	37.8 ± 1.49 b
2020	3.88 ± 0.20 a	2.95 ± 0.20	6.48 ± 0.37 a	2.30 ± 0.09	31.56 ± 1.20 a	47.23 ± 1.61 a
ANOVA (p values)
P fertilization rates	0.010	0.014	0.162	0.765	0.001	0.002
FA addition	0.984	0.475	0.078	0.687	0.890	0.569
Year	<0.001	0.727	<0.001	0.318	0.009	<0.001
P × FA	0.065	0.446	0.171	0.328	0.791	0.993
P × Year	0.284	0.365	0.057	0.623	0.971	0.782
FA × Year	0.232	0.410	0.487	0.816	0.970	0.940
P × FA × Year	0.240	0.992	0.735	0.397	0.892	0.952

Note: P0, P50, P100 and P150 mean P fertilization rates of 0, 50, 100 and 150 kg P₂O₅ ha⁻¹; FA means fulvic acid addition of 45 kg fulvic acid ha⁻¹; CK means no fulvic acid addition as the control. Means ± SE followed by the same letter are not significantly different at the p < 0.05 level.

). With the increasing P fertilization rates, the shoot, leaf, shell, seed and total P uptake showed an increasing trend, with significantly higher values observed under the P100 or P150 treatments. The shoot, shell, seed and total P uptake were significantly higher in 2020 than those in 2019. No significant effects of FA addition and its interaction with P fertilization were detected for cotton P uptake.

3.5. Yield and Components

The seed cotton yield increased with increasing P fertilization rates, with the highest value observed under the P100 and P150 treatments (

Table 7. Seed cotton yields and components under different P fertilization rates and FA addition treatments in 2019 and 2020.

	Density ×103 Plant ha−1	Bolls per Plant	Single Boll Weight g	Seed Cotton Yield kg ha−1
P fertilization rates
P0	122.90 ± 1.38 c	6.20 ± 0.27 c	5.29 ± 0.08 b	4019 ± 176 c
P50	130.66 ± 2.07 b	6.65 ± 0.21 b	5.61 ± 0.05 a	4851 ± 130 b
P100	130.65 ± 1.92 b	6.99 ± 0.24 a	5.65 ± 0.09 a	5117 ± 115 a
P150	135.68 ± 5.07 a	6.63 ± 0.24 b	5.79 ± 0.09 a	5161 ± 123 a
FA addition
CK	129.10 ± 1.81	6.39 ± 0.18 b	5.59 ± 0.07	4592 ± 120 b
FA	130.84 ± 1.87	6.84 ± 0.16 a	5.58 ± 0.05	4982 ± 123 a
Year
2019	127.13 ± 1.80 b	7.44 ± 0.09 a	5.43 ± 0.05 b	5143 ± 94 a
2020	132.82 ± 1.64 a	5.79 ± 0.10 b	5.74 ± 0.07 a	4431 ± 123 b
ANOVA (p values)
P fertilization rates	<0.001	<0.001	<0.001	<0.001
FA addition	0.264	<0.001	0.829	<0.001
Year	0.001	<0.001	<0.001	<0.001
P × FA	0.376	0.323	0.582	0.990
P × Year	0.896	0.484	0.002	0.068
FA × Year	0.371	0.390	0.498	0.332
P × FA × Year	0.199	0.059	0.350	0.208

Note: P0, P50, P100 and P150 mean P fertilization rates of 0, 50, 100 and 150 kg P₂O₅ ha⁻¹; FA means fulvic acid addition of 45 kg fulvic acid ha⁻¹; CK means no fulvic acid addition as the control. Means ± SE followed by the same letter are not significantly different at the p < 0.05 level.

). The FA addition significantly increased the seed cotton yield by 9%. The seed cotton yield was significantly higher in 2019 than in 2020. No significant interaction between the P fertilization and FA addition was detected in the seed cotton yield.

Analyzing yield components provides insights into the factors driving yield change. These results showed that P fertilization significantly increased plant density and single boll weight, with the highest value observed under the P150 treatment, and increased bolls per plant, with the highest value under the P100 treatment. FA addition significantly increased bolls per plant but did not affect the density and single boll weight. The density and single boll weight were significantly higher in 2020, but the bolls per plant were significantly higher in 2019.

There was a significant interaction between year and P fertilization on single boll weight. In 2019, there were no significant differences among the P fertilization rate treatments; however, in 2020, the single boll weight increased with increasing P fertilization rates, with the highest value observed under the P150 treatment (Figure 4).

3.6. Phosphorus Use Efficiency

PNB, AE and PFP decreased with increasing P fertilization rates and were significantly higher in 2020 than in 2019 (

Table 8. P fertilizer use efficiencies under different P fertilization rates and FA addition treatments, 2019.

	RE %	PNB %	AE kg kg−1	PFP kg kg−1
P fertilization rates
P0	-	-	-	-
P50	26.73 ± 6.97	186 ± 10 a	20.36 ± 1.77 a	97.02 ± 2.61 a
P100	24.86 ± 5.72	104 ± 6 b	12.84 ± 1.23 b	51.17 ± 1.15 b
P150	18.42 ± 2.10	71 ± 3 c	8.85 ± 0.71 c	34.40 ± 0.82 c
FA addition
CK	22.25 ± 4.27	119 ± 11	11.56 ± 1.10 b	58.41 ± 5.42 b
FA	23.43 ± 4.47	122 ± 12	16.48 ± 1.57 a	63.32 ± 5.91 a
Year
2019	21.60 ± 4.49	106 ± 10 b	12.14 ± 1.43 b	64.76 ± 6.14 a
2020	25.08 ± 3.77	135 ± 12 a	15.89 ± 1.37 a	56.97 ± 5.06 b
ANOVA (p values)
P fertilization rates	0.581	<0.001	<0.001	<0.001
FA addition	0.749	0.749	<0.001	<0.001
Year	0.611	<0.001	0.004	<0.001
P × FA	0.984	0.984	0.162	0.162
P × Year	0.642	0.052	0.593	<0.001
FA × Year	0.895	0.895	0.386	0.386
P × FA × Year	0.999	0.999	0.310	0.310

Note: P0, P50, P100 and P150 mean P fertilization rates of 0, 50, 100 and 150 kg P₂O₅ ha⁻¹; FA means fulvic acid addition of 45 kg fulvic acid ha⁻¹; CK means no fulvic acid addition as the control. Means ± SE followed by the same letter are not significantly different at the p < 0.05 level.

). FA addition significantly increased AE and PFP. No significant interaction between P fertilization and FA addition was detected for RE, PNB, AE and PFP.

There was a significant interaction between year and P fertilization rates on PFP. The PFP under the P100 and P150 treatments was no different between 2019 and 2020, but that under the P50 treatment was significantly higher in 2019 than in 2020 (Figure 5).

4. Discussion

Soil available P is an important indicator of P that can be absorbed by plants. In this study, the P fertilization significantly increased soil available P, which is consistent with previous studies [26,27]. The increased soil available P might be caused by added P to the soil through fertilization. Additionally, soil pH decreased with increasing application of monoammonium phosphate (acidic) in this study, suggesting soil acidification caused by fertilization might be the other mechanism promoting soil P availability [7].

It has been widely reported that FA can increase soil P availability through mechanisms like acidification, chelating cations, or increasing microbial activity [9,10,28,29,30]. In this study, there was a significant increase in soil available P caused by FA addition. Concurrently, there was a slight decrease in soil pH under the FA addition treatment, although it was not significant. The changes in soil available P and pH suggest that FA addition might enhance soil P availability by promoting acidification.

Lots of studies have reported that FA could stimulate plant root growth and rhizosphere microbial activity, increase plant nutrient uptake and promote plant growth [9,15,16,31,32]. However, in this study, we did not find a significant effect of FA on cotton P uptake and growth, except for increased leaf and shell biomass. It might be caused by the complicated impact factors in the field condition, which overwhelmed the effects of FA acid addition [18]. Additionally, the dosage effect might influence the response of plant growth to FA addition. The appropriate amount of FA can stimulate seed germination, root growth and nutrient uptake, but lower or higher amounts of FA have no positive effect or may even have negative effects [31,32].

In this study, P fertilization significantly promoted seed cotton yield, which was consistent with previous studies [26,33,34]. The increasing harvest plant density, bolls per plant or single boll weight might explain the higher seed cotton yield under P fertilization treatment (Table 7). P is a crucial nutrient for seedling emergence rate, because at that stage, plants grow fast and cell division happens frequently, which requires more P for cell membranes and the energy transfer system [35]. Additionally, P has the functions of stimulating crop development and enhancing carbohydrate transportation from the leaves to the bolls [36], which results in more bolls per plant and higher single boll weight.

The seed cotton yield and bolls per plant were significantly higher under the FA addition treatment (Table 7), suggesting that FA addition might have promoted seed cotton yield through increasing bolls per plant in this study. It is common in arid regions that extreme drought and heat cause the shedding of cotton bolls during the flowering boll stage and significantly reduce cotton yield [37]. FA has the function of increasing plant resistance to drought and other environmental stresses [16,17,38], which might reduce boll shedding and result in more bolls per plant and higher seed cotton yield.

In this study, PNB, AE and PFP decreased with the increasing P fertilization rates, which was consistent with most previous studies [39,40,41,42] and can be explained by the law of diminishing returns to fertilizer [43]. P fertilization can enhance crop P uptake and yield; however, with the increasing rates of P fertilization, the growth rates of cotton P uptake and seed cotton yield became slower. That resulted in the reduced PNB, AE and PFP with the increasing P fertilization rates. FA addition significantly increased AE and PFP in this study, but not AE and PNB. These indicate that the main mechanism of FA addition on P fertilizer use efficiency was to promote cotton yield through increasing bolls per plant, compared with enhancing plant P uptake.

5. Conclusions

Our study showed that the soil P availability, cotton biomass, uptake and yield increased with the increasing P fertilization rates, but the PNB, AE and PFP decreased with increasing P fertilization rates. Considering the seed cotton yield and field P balance, the P fertilization rate was recommended to be around 100 kg P₂O₅ ha⁻¹ to get a higher seed cotton yield and keep the balance of P in this field. FA addition significantly increased seed cotton yield through increasing bolls per plant, thus promoting AE and PFP. Therefore, FA can be applied to enhance cotton yield by ensuring P balance in the cotton field in arid regions.