Co-Application of Coated Phosphate Fertilizer and Humic Acid for Wheat Production and Soil Nutrient Transport

Abstract

The application of a diammonium phosphate coating effectively mitigates direct contact between the phosphate fertilizer and the soil, thus minimizing phosphorus fixation. Humic acid holds a pivotal role in augmenting soil quality and activating the soil’s phosphorus reserves. Notably, when combined with humic acid, diammonium phosphate significantly enhances the utilization efficiency of phosphate fertilizer. However, there is a paucity of literature exploring the dynamics of nutrient transport in soil when humic acid is paired with coated phosphate fertilizers. To assess the impact of the combined application of coated diammonium phosphate and humic acid on wheat yield enhancement, we conducted pot experiments along with leaching and ammonia volatilization simulation tests, aiming to elucidate the effects of this combination on nutrient transport. The study explored the effects of three distinct treatments: coated diammonium phosphate (CP), coated diammonium phosphate combined with humic acid (PHA), and coated diammonium phosphate combined with humic acid (CPHA). The investigation focused on analyzing their impacts on wheat yield, ammonia volatilization, soil-available phosphorus, nitrate nitrogen, ammonium nitrogen, soil-available potassium, as well as the mobilization and transport of calcium and magnesium in the soil. (1) Compared to the P treatment, the PHA and CP treatments significantly increased grain yield by 17.2% and 13.5%, respectively. The PHA treatment also increased effective panicle number by 12.9%. Overall, the CP, PHA, and CPHA treatments improved grain yield by 13.5%, 17.2%, and 19.1% compared to the P treatment. (2) The CP and PHA treatments reduced available phosphorus by 95.6% and 49.2%, calcium by 2.0% and 67.0%, and magnesium by 11.6% and 46.1% compared to the P treatment. Ammonium nitrogen decreased by 37.0% and 64.3%, while nitrate nitrogen increased by 14.0% in CP and slightly decreased by 0.8% in PHA. In the leaching solution, PHA and CP treatments reduced available phosphorus by 96.7% and 62.5%, increased calcium by 5.0% and 78.9%, decreased ammonium nitrogen by 2.2% and 43.4%, and decreased nitrate nitrogen by 10.6% and 13.0%. The PHA and CPHA treatments increased available phosphorus in the 0–20 cm soil layer by 1.4 times and 25.8%, respectively. (3) The CP treatment reduced ammonia volatilization by 87.0% compared to the P treatment, while the CPHA treatment further reduced it by 87.5% compared to the PHA treatment. The application of coated diammonium phosphate efficiently delays nutrient release and reduces nutrient leaching in the soil. Additionally, the integration of humic acid significantly improves the availability of phosphorus in the soil, minimizing phosphorus loss. Notably, the combined application of humic acid and coated diammonium phosphate leads to a significant increase in soil phosphorus content, subsequently enhancing soil nutrient availability, conserving fertilizer, and ultimately resulting in an improved wheat yield.

1. Introduction

Phosphorus is one of the essential nutrients for plant growth and development, serving as a structural component of macromolecules and participating in metabolic processes, playing a crucial role in achieving high crop yields [1]. However, phosphorus in soil is easily adsorbed by clay minerals or forms insoluble precipitates with cations such as calcium, magnesium, iron, and aluminum, resulting in a significant reduction in phosphorus availability. It is reported that over 43% of the world’s cultivated soils are limited in crop yield due to phosphorus deficiency [2]. To meet the phosphorus demand of crop growth, farmers have resorted to increasing the application of phosphate fertilizers to address the issue of low soil available phosphorus. Although effective in the short term, most of the phosphate fertilizers applied to the soil accumulate in a non-available form, resulting in a utilization rate of only 10% to 25% in China’s current season [3]. In pursuit of higher yields and economic benefits, the amount of phosphate fertilizers applied to the soil has gradually increased, far exceeding the growth needs of plants. This has led to a significant accumulation of phosphorus and a severe imbalance in soil nutrients. Excessive phosphorus enters surface and groundwater through runoff or infiltration, readily causing environmental problems such as water eutrophication. Non-point source pollution, primarily caused by phosphorus loss from farmland, is often the main source of phosphorus in water bodies [4]. Additionally, excessive phosphorus supply can lead to problems such as premature plant maturity, shortened vegetative growth, and limited stem and leaf growth. It can also induce deficiencies in trace nutrients such as iron, zinc, and magnesium [5], resulting in poor crop vegetative development and reduced yields. It is noteworthy to highlight that phosphate fertilizers such as monoammonium phosphate and diammonium phosphate not only efficiently supply crops with essential phosphate ions but also concurrently provide nitrogen nutrients. However, inappropriate application of these fertilizers can lead to the release of nitrogen in the form of ammonia volatilization from the field, thereby contributing to air pollution and exacerbating the pressing challenge of global greenhouse gas emissions. As such, the adoption of scientific and rational fertilization management practices is of paramount importance.

By isolating the fertilizer core from direct contact with the soil, the new type of controlled-release phosphate fertilizer can effectively reduce the adsorption reaction between the available phosphate and soil mineral components, preventing the formation of precipitates that lead to low soil phosphorus activity and thereby enhancing the utilization rate of phosphate fertilizers [6]. Compared to conventional phosphate fertilizers, controlled-release phosphate fertilizers can slow down the rate of nutrient release into the soil, making the nutrient release rate basically match the crop’s nutrient demand pattern, satisfying the crop’s nutrient needs in the middle and late growth stages. Chen et al. [7] found that the physical membrane layer on the surface of controlled-release phosphate fertilizers had effectively delayed the phosphorus release rate, prolonged the phosphorus release period, and mitigated environmental pollution issues caused by excessive phosphorus release in a short period. Diego F et al. [8] reached a similar conclusion, demonstrating that the application of coated superphosphate fertilizer extended the fertilizer release period and improved the fertilizer release performance. Abhijit S et al. [9] found that the application of controlled-release phosphate fertilizer significantly increased wheat yield and phosphate fertilizer utilization efficiency. Chen et al. [10] also reached a similar conclusion through pot experiments, showing that controlled-release phosphate fertilizer significantly increased maize yield compared to conventional diammonium phosphate. Although the slow-release properties of coated controlled-release phosphate fertilizers are less affected by uncontrollable factors such as soil texture, pH, and root exudates compared to soil-slow-release phosphorus reservoirs, there are still issues such as the inability of wheat to absorb phosphorus through diffusion due to its poor mobility after continuous release.

Functional substances added to fertilizers are also known as fertilizer synergists [11]. Humic acid (HA) is a substance composed of aliphatic, aromatic, and other components, with various functional groups attached to it. Some of these functional groups react with metal cations in the soil [12]. After the application of humic acid, the active functional groups within HA competitively adsorb metal ions present in the soil, including Fe³⁺, Al³⁺, Ca²⁺, and Mg²⁺. This competitive adsorption process mitigates the tendency of these metal ions to sequester phosphate ions in the soil, thereby reducing the loss of available phosphates [13]. This leads to an increase in the concentration of water-soluble phosphorus, acid-soluble phosphorus, and Olsen phosphorus in the soil. Furthermore, research has demonstrated that the increase in humic acid concentration notably reduces the formation of precipitates such as calcium phosphate under both acidic and alkaline conditions [14]. While the widely acknowledged effectiveness of humic acid in enhancing phosphorus availability in soil is undisputed, the specific mechanisms and pathways of its action remain a subject of ongoing debate and research. In addition, HA, as a plant growth regulator, can improve plants’ physiological characteristics, such as enhancing photosynthesis in leaves, reducing transpiration, and improving crop drought resistance, avoiding issues like dwarf plants, poorly developed reproductive organs, and low yield [15]. Furthermore, Ge X et al. [14] showed that the combination of phosphate and HA could promote the growth of crop roots and stems by facilitating the availability of soluble phosphorus sources. However, HA alone cannot meet the large nutrient demands of crops, and its sole application can lead to insufficient nutrient supply.

The current research focus on controlled-release fertilizers has shifted towards enhancing their slow and controlled-release functions to replace traditional coating-based technologies and address issues such as the limited functionality of traditional controlled-release fertilizers. However, previous studies mostly concentrated on the mechanism of quality improvement and efficiency enhancement when combining HA with controlled-release nitrogen fertilizers and conventional phosphate fertilizers. For instance, Li et al. [16] found that combining controlled-release urea with HA substances could improve soil nitrogen cycling microbial communities, reduce nutrient loss, and increase wheat yield. Sara D et al. [17] showed that the combined application of HA and superphosphate significantly increased root dry matter weight and crop yield compared to treatments without HA. By combining novel controlled-release phosphate fertilizers with fertilizer synergists like HA, it is theoretically possible to achieve efficient utilization of phosphate fertilizers and scientific application of fertilizer synergists. Related studies indicated a synergistic effect between diammonium phosphate coatings and HA on corn [7]. However, there is limited research on the effects of combining coated diammonium phosphate with HA on winter wheat and its nutrient movement in the soil. This study, through pot experiments, leaching, and ammonia volatilization simulation tests, investigated the impact of combining coated diammonium phosphate with HA on wheat yield and explored the movement of soil nutrients under this combined application. The findings provide a theoretical basis for the scientific application of coated diammonium phosphate with HA and a data foundation for further research on how this combination enhances soil phosphorus supply.

2. Materials and Methods

2.1. Experimental Materials

The experiment was conducted from October 2018 to June 2019 at the Scientific and Technological Innovation Park of Shandong Agricultural University in Tai’an, Shandong Province (117°13′ E, 36°20′ N). The tested soil was collected from the 0–20 cm plow layer of the experimental base of the National Engineering Research Center for High-efficiency Utilization of Soil and Fertilizer. The soil type is brown soil, classified as Typic Hapli-Udic Argosols in the Chinese soil system. The basic physicochemical properties of the tested soil are as follows: total nitrogen 0.66 g kg⁻¹, nitrate nitrogen 72.45 mg kg⁻¹, organic matter 12.1 mg kg⁻¹, ammonium nitrogen 9.45 mg kg⁻¹, total phosphorus 0.32 g kg⁻¹, available phosphorus 13.50 mg kg⁻¹, rapidly available potassium 92.32 mg kg⁻¹, and pH 7.83 (soil-to-water ratio of 1:2.5) [7].

The coated phosphate fertilizer, comprising bio-based polyurethane membrane and diammonium phosphate granules with a nutrient content of N 17.2% and P₂O₅ 44.0%, was meticulously crafted by the National Engineering Research Center for High-efficiency Utilization of Soil and Fertilizer. This process involved initially heating and water-spraying the diammonium phosphate granules for surface modification, followed by application of 0.5% polyolefin wax upon drying. Subsequently, a robust bio-based polyurethane membrane, derived from castor oil and isocyanate, was uniformly coated over the wax layer, utilizing a small-scale rotary drum coating method [7]. The coated controlled-release urea (N 43.2%) was produced by Kingenta Ecological Engineering Group Co., Ltd., Linyi, China; humic acid from a mineral source extracted by alkali-soluble acid extraction (N 2.0%, K₂O 3.0%) [18]; other fertilizers included ordinary urea (N 46.0%), diammonium phosphate (N 18.0%, P₂O₅ 46.0%), and potassium chloride (K₂O 60.0%), which were purchased from local fertilizer dealers. The winter wheat cultivar used in the experiment was “Jimai 22”, with a growth period of 239 days and a 1000-grain weight of 40 g.

2.2. Experimental Design

2.2.1. Pot Experiment

A terracotta pot with a diameter of 30 cm, a height of 36 cm, and a drainage hole at the bottom was selected for the pot experiment. Initially, 1 kg of sand was added to the bottom of each pot, and then the soil and fertilizer were thoroughly mixed and filled into the pot, with 19 kg of air-dried soil in each pot. According to the experimental requirements, tap water was used to maintain the soil moisture content above 60% of the field water holding capacity. Each pot was sown with 45 seeds, and thinning was performed after emergence to 36 plants per pot. Weeding, pest and disease control, and other management measures were identical to traditional local farmer practices. The pot experiment consisted of five treatments: (1) phosphorus blank control treatment (CK); (2) conventional phosphorus application treatment (P); (3) coated diammonium phosphate treatment (CP); (4) conventional diammonium phosphate combined with humic acid treatment (PHA); and (5) coated diammonium phosphate combined with humic acid treatment (CPHA). Each treatment had four replicates. The application rate of fertilizers was set at N–P₂O₅–K₂O 225–150–75 kg·hm⁻², while the amount of humic acid used was 45 kg·hm⁻². Except for the type of phosphate fertilizer, the amounts of nitrogen, phosphorus (excluding CK), and potassium fertilizer remained consistent. The ratio of fast-acting nitrogen to controlled-release nitrogen was 4:6 (

Table 1. Description and quantities of different fertilizer treatments.

Treatment	Description	Quantity
		Diammonium Phosphate (g/pot)	Coated Diammonium Phosphate (g/pot)	Coated Urea (g/pot)	Urea (g/pot)	Potassium Chloride (g/pot)	Humic Acid (g/pot)
CK	Phosphorus blank control treatment	-	-	7.53	4.70	3.00	-
P	Conventional diammonium phosphate	7.83	-	7.44	1.60	3.00	-
CP	Coated diammonium phosphate	-	8.18	4.26	4.70	3.00	-
PHA	Conventional diammonium phosphate with humic acid	7.83	-	7.53	1.40	2.73	5.40
CPHA	Coated diammonium phosphate with humic acid	-	8.18	4.26	4.46	2.73	5.40

). According to local farmers’ fertilization habits, sowing was performed using the method of simultaneous sowing and fertilization, and all fertilizers were mixed and applied as a basal fertilizer in a single application.

2.2.2. Leaching Experiment

In this study, leaching experiments were set up to investigate the movement of phosphorus in the vertical direction. The leaching experiment was conducted using a self-made leaching device, which consisted of three main components: a leaching stand, soil columns, and a collection system. The experiment comprised six treatments, each replicated three times: (1) Phosphorus blank control (CK), (2) Humic acid application alone (HA), (3) Regular diammonium phosphate application (P), (4) Coated diammonium phosphate treatment (CP), (5) Diammonium phosphate combined with humic acid (PHA), and (6) Coated diammonium phosphate combined with humic acid (CPHA). The main component of the leaching device, the soil column, was made of PVC pipe (55 cm height × 7.0 cm diameter). Each soil column was filled with approximately 1994 g of soil (soil layer height of 40 cm, with a soil bulk density of 1.296 g/cm³). Fertilizers were applied in the upper 0–20 cm layer of soil, followed by covering the top with filter paper. The lower outlet for leachate was equipped with a funnel and a conical flask to collect the filtrate. The bottom of the soil column was lined with filter paper and gauze to prevent soil and impurities from entering the funnel and blocking the outlet. The fertilization rates for the soil columns were set at 0.20 g P₂O₅/100 g soil and 0.28 g humic acid/100 g soil. Specifically, 2.79 g of humic acid, 4.33 g of diammonium phosphate, and 4.69 g of coated diammonium phosphate were used. To minimize interference from other fertilizers, no additional nitrogen or potassium fertilizers were added to the soil columns. To accurately simulate the movement of fertilizer nutrients through soil in conjunction with water, 772 mL of water was administered on the 1st, 5th, 10th, and 15th days of the experimental period, mimicking typical irrigation volumes. Subsequently, leaching water samples were collected on each of these specified days (1st, 5th, 10th, and 15th) and subjected to analysis to determine the nutrient content present within [16]. After the experiment, when the soil columns were completely dried, the entire PVC pipe was sectioned at the 0–20 cm soil layer. Considering the limited mobility of phosphorus in soil and the root distribution characteristics of wheat [19] (about 80% of the roots are distributed in the 0–20 cm soil layer) [20], the 20–25 cm soil layer was further subdivided into 20–22.5 cm and 22.5–25 cm sections for soil available phosphorus analysis.

2.2.3. Ammonia Volatilization Experiment

The ammonia volatilization experiment was conducted using a self-assembled apparatus consisting of an air pump, a reaction chamber, an absorption bottle, and connecting hoses. A 1% boric acid solution was used to absorb the ammonia gas, as illustrated in Figure 1. The experiment comprised six treatments, each replicated three times: (1) Full blank control (Control), (2) Humic acid application alone (HA), (3) Diammonium phosphate application alone (P), (4) Coated diammonium phosphate application alone (CP), (5) Diammonium phosphate combined with humic acid (PHA), and (6) Coated diammonium phosphate combined with humic acid (CPHA). The fertilization rates were set at 0.20 g P₂O₅/100 g soil and 0.28 g humic acid/100 g soil. Each reaction chamber contained 100 g of soil. Therefore, the amount of diammonium phosphate used was 0.43 g, the coated diammonium phosphate was 0.47 g, and the amount of humic acid for treatments with humic acid was 0.28 g. To minimize interference from other fertilizers, no additional nitrogen or potassium fertilizers were added to the reaction chambers. The diammonium phosphate, coated diammonium phosphate, or humic acid were uniformly mixed with the soil in the reaction chambers according to the treatment. Each treatment was sprayed with an equal amount of water and then the containers were immediately sealed. Air was pumped into the chambers at a flow rate of 8.3 L min⁻¹ [21]. According to the experimental requirements, the boric acid solution in the absorption bottle was replaced daily, and the amount of ammonia absorbed was determined through titration. This allowed for the calculation of the ammonia volatilization rate.

2.3. Sample Collection

After wheat harvest, the number of wheat ears in each pot was accurately counted before harvesting. Then, the entire wheat plant was collected using scissors. In the laboratory, the plant straw and wheat grains were placed in an oven at 105 °C for 15 min to kill the enzymes. Afterwards, they were transferred to a 65 °C oven for drying until a constant weight was achieved. The dried samples were then weighed to calculate the yield. The soil columns were placed on a leaching frame with a funnel attached at the bottom. Leachates were collected by slowly adding water using a graduated cylinder on the 1st, 5th, 10th, and 15th day. When nearing the end of the collection, the receiving status was maintained for at least 3 h until no more liquid droplets were observed, at which point the collection was stopped [16]. The leachates were used for water-soluble nutrient determination. The soil samples collected after stratification during the cultivation period were further air-dried and then ground through a 2 mm sieve for preservation and later measurement of soil available phosphorus. The ammonia volatilization test was conducted daily by ventilating the samples for 1 h at a flow rate of 8.3 L min⁻¹. The amount of ammonia absorbed in the boric acid solution was measured to calculate the ammonia volatilization rate and observe the volatilization pattern.

2.4. Sample Measurement and Analysis Methods

The available phosphorus in the soil was extracted using 0.5 mol L⁻¹ sodium bicarbonate solution, and the extract was determined using a fully automatic chemical analyzer (Smartchem 200, Alliance Instruments Co., Ltd., Frepillon, France). The soil nitrate and ammonium nitrogen were extracted using 0.01 mol/L CaCl₂ (soil-to-water ratio of 1:10) and then measured using a continuous flow analyzer (SEAL, AA3, SEAL Analytical Co., Ltd., Berlin, Germany). The collected leachates were directly analyzed for water-soluble nutrient content. Phosphorus content was determined using a fully automatic chemical analyzer (Smartchem 200, Alliance Instruments Co., Ltd., Frepillon, France); potassium content was measured using a flame photometer (F-100, Shanghai Precision Scientific Instruments Co., Ltd., Shanghai, China); calcium and magnesium contents were determined using an atomic absorption spectrophotometer (AA370MC, Shanghai Precision Scientific Instruments Co., Ltd., Shanghai, China).

2.5. Statistical Analyses

The experimental data were organized and summarized using Microsoft Excel 2016, while SigmaPlot 12.5 was employed for graph plotting. Analysis of Variance (ANOVA) was utilized to analyze the data, and Duncan’s Multiple Range Test (p ≤ 0.05) was applied for mean separation. All analyses were conducted using SAS version 9.2 (SAS, 2012). To demonstrate visually the intricate contribution relationships between yield indicators in pot experiments, nutrient content from leaching experiments, and different fertilizer treatments, this study employed the Chord Diagram as a visualization tool. Chord analysis drawings were performed using Origin 2021. The lowercase letters in the figure indicate the statistical significance of data differences between different treatments.

3. Results

3.1. Winter Wheat Yield under Different Treatments

The application of coated phosphate fertilizer combined with humic acid had varying degrees of influence on wheat yield, aboveground biomass, and the number of effective ears (

Table 2. Yield and yield composition of wheat relative to different treatments.

Treatment	Yield (g/pot)	Increase Yield Compared to P Treatment (%)	Aboveground Biomass (g/pot)	Number of Spikes (p−1)
CK	44.2 c	−13.84	79.3 d	44.8 c
P	51.3 b	-	93.0 c	57.3 b
CP	58.2 a	13.45	106.2 b	59.8 ab
PHA	60.1 a	17.15	119.7 a	64.8 a
CPHA	61.1 a	19.10	122.3 a	62.3 ab

Note: Means followed by a same lowercase letter in the same column were not significantly different (p < 0.05).

). Compared to the non-phosphorus treatment (CK), normal phosphorus application significantly increased wheat grain yield. The grain yield of the other treatments increased by 16–38% compared to CK, while the aboveground biomass and the number of effective ears increased by 17–54% and 28–45%, respectively. When compared to the P treatment, the application of ordinary diammonium phosphate combined with humic acid (PHA) had increased the grain yield by 17.2%, aboveground biomass by 28.7%, and the number of effective ears by 12.9%. The application of coated diammonium phosphate (CP) or coated diammonium phosphate combined with humic acid (CPHA) had increased the grain yield by 13.5% and 19.1%, aboveground biomass by 14.2% and 31.5%, and the number of effective ears by 4.4% and 8.7%, respectively. When comparing PHA and CPHA with P and CP treatments without humic acid, the grain yield had increased by 17.2% and 5.0%, the aboveground biomass had increased by 28.7% and 15.2%, and the number of effective ears had increased by 13.1% and 4.2%, respectively. Under the experimental conditions, the application of coated diammonium phosphate or its combination with humic acid had increased wheat grain yield, with the best yield-increasing effect observed when coated diammonium phosphate was combined with humic acid.

3.2. Different Fertilization Treatments on Soil Ammonia Volatilization and Soil Mineral Nitrogen Loss

Under the experimental conditions, there was no ammonia volatilization in the non-fertilized treatment (Figure 2A). In the PHA treatment, the amount of ammonia volatilization gradually decreased within the first 10 days of cultivation, reaching the lowest level on the 11th day. From the 12th day to the end of the cultivation period, the amount of ammonia volatilization in the PHA treatment gradually increased, resulting in a 27.0% increase in cumulative ammonia volatilization compared to the P treatment. Compared to the P and PHA treatments, the CP treatment significantly reduced ammonia volatilization (Figure 2B). Both the CP and CPHA treatments exhibited ammonia volatilization starting from the 7th day, and their cumulative ammonia volatilization was significantly reduced by 87.0% compared to the P treatment. The addition of HA did not significantly affect the amount of ammonia volatilization in the coated diammonium phosphate treatment.

Application of diammonium phosphate significantly increased soil ammonium nitrogen content, with varying impacts on ammonium nitrogen content in different soil layers (Figure 3A₁). Compared to P treatment, PHA treatment reduced ammonium nitrogen content in the 0–20 cm and 20–22.5 cm soil layers by 11.9% and 27.8%, respectively, while increasing it in the 22.5–25 cm layer by 20.2%. In contrast, CP treatment increased ammonium nitrogen content in the 0–20 cm layer by 151.2% compared to P treatment, while reducing it in the 20–22.5 cm and 22.5–25 cm layers by 65.7% and 28.8%, respectively. CPHA treatment, on the other hand, enhanced ammonium nitrogen content in the 0–20 cm layer by 135.4% compared to P treatment, while reducing it in the 20–22.5 cm and 22.5–25 cm layers by 46.2% and 9.0%, respectively. Under the experimental conditions, both the application of coated diammonium phosphate alone and its combination with HA significantly increased ammonium nitrogen content in the 0–20 cm soil layer and reduced it in deeper layers. Specifically, CPHA treatment increased ammonium nitrogen content in the 0–20 cm layer by 226.3% compared to PHA treatment, while reducing it in the 20–22.5 cm and 22.5–25 cm layers by 38.9% and 24.3%, respectively. Moreover, CPHA treatment increased ammonium nitrogen content in the 20–22.5 cm and 22.5–25 cm layers by 56.6% and 27.8%, respectively, compared to CP treatment.

During the first day, ammonium nitrogen content in the leachate of all treatments did not vary significantly. However, on the 5th and 10th days, ammonium nitrogen content in the leachate significantly increased, with CK and CP treatments showing the fastest growth (Figure 3A₂). On the 15th day of the leaching experiment, ammonium nitrogen content in the P treatment leachate sharply increased, while it remained relatively stable in other treatments. Over the entire cultivation period, PHA treatment reduced ammonium nitrogen content in the leachate by 23.6% to 63.5% compared to P treatment, while CPHA treatment reduced it by 40.5% to 85.1% compared to CP treatment. Under leaching simulation conditions, the total amount of ammonium nitrogen in the leachate varied among different treatments (Figure 3A₃). Specifically, PHA, CP, and CPHA treatments reduced ammonium nitrogen content in the leachate by 63.5%, 37.0%, and 64.3%, respectively, compared to P treatment. Among them, CPHA treatment reduced ammonium nitrogen in the leachate by 2.2% compared to PHA treatment and by 43.4% compared to CP treatment. Under the experimental conditions, it is evident that coated diammonium phosphate can significantly prolong the release period of ammonium nitrogen, while the addition of HA can significantly reduce the content of ammonium nitrogen in the leachate.

The different treatments had varying impacts on the nitrate nitrogen content in different soil layers during the leaching experiment (Figure 3B₁). Compared to P treatment, PHA treatment reduced soil nitrate nitrogen content by 2.9% in the 20–22.5 cm soil layer and increased it by 10.2% in the 22.5–25 cm soil layer. In comparison to P treatment, CP treatment reduced soil nitrate nitrogen content by 2.7%, 29.3%, and 59.7% in the 0–20 cm, 20–22.5 cm, and 22.5–25 cm soil layers, respectively. CPHA treatment, on the other hand, reduced soil nitrate nitrogen content by 3.9%, 35.4%, and 47.6% in the 0–20 cm, 20–22.5 cm, and 22.5–25 cm soil layers compared to P treatment. When compared to PHA treatment, CPHA treatment reduced soil nitrate nitrogen content by 3.9%, 33.5%, and 52.5% in the 0–20 cm, 20–22.5 cm, and 22.5–25 cm soil layers, respectively. Moreover, CPHA treatment reduced soil nitrate nitrogen content by 8.6% in the 20–22.5 cm soil layer and increased it by 29.9% in the 22.5–25 cm soil layer compared to CP treatment. Under the experimental conditions, the application of coated diammonium phosphate or its combination with HA reduced the amount of nitrate nitrogen in the soil.

During the leaching experiment process, the amount of nitrate nitrogen in the leachate showed different changes among the various treatments (Figure 3B₂). During the first and fifth days of the leaching experiment, the amount of nitrate nitrogen in all treatments increased slowly, with CK treatment showing the highest nitrate nitrogen content in the leachate. On the 10th day, the nitrate nitrogen content in the leachate of all treatments except CK increased significantly, with PHA treatment reducing the nitrate nitrogen content in the leachate by 7.2% compared to P treatment, and CP treatment reducing it by 5.4% compared to CPHA treatment. On the 15th day of culture, the amount of nitrate nitrogen in the leachate of CP and PHA treatments continued to increase rapidly, while other treatments remained relatively stable. The impact of different treatments on the total amount of nitrate nitrogen in the leachate varied (Figure 3B₃). Compared to P treatment, PHA and CP treatments increased the amount of nitrate nitrogen in the leachate by 10.9% and 14.0%, respectively. Although the difference in nitrate nitrogen content in the leachate between CPHA and P treatments was not significant, CPHA treatment reduced the amount of nitrate nitrogen in the leachate by 10.6% and 13.0% compared to PHA and CP treatments, respectively.

3.3. Different Fertilization Treatments on Phosphorus Content in Soil and Leachate in Leaching Experiments

Phosphorus application significantly increased the content of soil available phosphorus, and different treatments had varying impacts on the soil available phosphorus content (Figure 4A). Compared with treatment P, PHA treatment increased the soil available phosphorus content by 140.8% in the 0–20 cm soil layer, while it decreased by 34.9% in the 20–22.5 cm soil layer, and increased by 23.4% in the 22.5–25 cm soil layer. In comparison with treatment P, CP treatment raised the soil available phosphorus content by 99.7% in the 0–20 cm soil layer, while it decreased by 35.4% and 62.2% in the 20–22.5 cm and 22.5–25 cm soil layers, respectively. Under the experimental conditions, compared with the application of ordinary phosphate fertilizer alone, the application of coated phosphate fertilizer or the combined application of HA could significantly increase the soil available phosphorus content in the 0–20 cm soil layer while reducing phosphorus leaching. Specifically, CPHA treatment increased the soil available phosphorus content by 151.7% in the 0–20 cm soil layer compared to treatment P, and it decreased by 34.9% and 57.9% in the 20–22.5 cm and 22.5–25 cm soil layers, respectively. Although the difference in soil available phosphorus content between CPHA and CP treatments in the 20–22.5 cm soil layer was not significant, the soil available phosphorus content in the 0–20 cm and 22.5–25 cm soil layers increased by 25.8% and 11.2%, respectively. Compared with PHA treatment, CPHA treatment increased the soil available phosphorus content by 4.2% in the 0–20 cm soil layer, while it decreased by 22.5% and 65.9% in the 20–22.5 cm and 22.5–25 cm soil layers, respectively.

Under simulated leaching conditions, different phosphorus application treatments had varying impacts on phosphorus leaching in the soil. No significant phosphorus leaching was observed in any treatment until the 5th day of cultivation. After the addition of leaching water on the 10th day, the phosphorus content in the leachate of treatments P and PHA increased significantly, but no significant leaching was observed in treatments CP and CPHA (Figure 4B). Compared with treatment P, the phosphorus content in the leachate of treatment PHA decreased by 56.5% and 49.2% on the 10th and 15th days, respectively, and the total phosphorus content gradually increased during the entire cultivation process. In terms of the total phosphorus content in the leachate (Figure 4C), the total phosphorus content in the leachate of treatments P and PHA increased significantly compared to treatment CK. The phosphorus content in the leachate of treatments CP and CPHA decreased by 95.6% and 98.3%, respectively, compared to treatment P. Compared with treatment PHA, the total phosphorus content in the leachate of treatments CP and CPHA decreased by 91.3% and 96.7%, respectively. The total phosphorus content in the leachate of treatments PHA and CPHA decreased by 49.2% and 62.5%, respectively, compared to treatments P and CP.

3.4. Different Fertilization Treatments on the Content of Potassium, Calcium, and Magnesium in Leachate during Leaching Experiments

The amount of potassium in leachate from different treatments showed a steady increase (Figure 5A₁). During the entire cultivation process, the amount of potassium in the leachate from PHA and CPHA treatments was consistently higher than that from P and CP treatments. The addition of HA increased the amount of potassium in the leachate by 2.7% to 29.5%. Compared to P, the amount of potassium in the leachate from CP decreased by 9.6% to 25.1%. Different treatments had varying effects on the amount of potassium in the leachate (Figure 5A₂). There was no significant difference in the amount of potassium in the leachate between PHA and P treatments. However, the amount of potassium in the leachate from CP and CPHA treatments decreased by 9.6% and 7.1% compared to P treatment. Although the difference between CPHA and CP treatments was not significant, the amount of potassium in the leachate from CPHA treatment was reduced by 7.7% compared to PHA treatment. Under the experimental conditions, the application of coated phosphate fertilizer could effectively reduce the amount of potassium in the leachate, and the addition of HA further reduced the amount of potassium in the leachate, effectively reducing the leaching loss of available potassium in the soil.

In the early stage of cultivation, the amount of calcium in the leachate increased slowly, and remained stable after the 5th day (Figure 5B₁). During the entire cultivation process, the amount of calcium in the leachate from PHA and CPHA treatments, which had HA added, was significantly higher than that from P and CP treatments, which did not have HA added. Compared to P, the amount of calcium in the leachate from CP decreased by 2.0% to 15.0%. Different treatments also had varying effects on the total amount of calcium in the leachate (Figure 5B₂). Although the difference in the amount of calcium in the leachate between CP and P treatments was not significant, the calcium content in the leachate from PHA and CPHA treatments increased by 67.0% and 75.4% compared to P treatment, respectively. The amount of calcium in the leachate from CPHA treatment increased by 78.9% and 5.0% compared to CP and PHA treatments, respectively. Under the experimental conditions, the amount of calcium in the leachate from treatments with HA added was generally higher than that from treatments with only phosphate fertilizer applied, indicating that adding HA could increase the risk of calcium leaching from the soil. The application of coated diammonium phosphate could mitigate the calcium leaching caused by fertilization.

During the cultivation process, the amount of magnesium in the leachate from PHA and CPHA treatments rapidly increased on the 5th day, while the amount of magnesium in the leachate from other treatments continued to increase slowly (Figure 5C₁). Throughout the entire process, the amount of magnesium in the leachate from HA, PHA, and CPHA treatments was significantly higher than that from treatments without HA addition. Compared to P, the amount of magnesium in the leachate from CP decreased by 11.6% to 29.2%. Different treatments had varying effects on the total amount of magnesium in the leachate (Figure 5C₂). Specifically, the amount of magnesium in the leachate from CP treatment decreased by 11.6% compared to P treatment, while the amount from PHA and CPHA treatments increased by 46.1% and 46.7%, respectively, compared to P treatment. The amount of magnesium in the leachate from CPHA treatment increased by 65.9% compared to CP treatment, but there was no significant difference compared to PHA treatment.

4. Discussion

4.1. Coated Diammonium Phosphate on Wheat Yield and Soil Nutrient Transport

Phosphorus participates in the synthesis and metabolism of organic compounds in plants [22]. The supply intensity of soil available phosphorus is crucial for the growth and yield formation of winter wheat throughout its growth stages [16], especially as the second largest nutrient supply element after nitrogen, it has a significant regulatory effect on the number of effective ears per unit area, one of the three main factors that constitute yield [23]. However, after the application of phosphate fertilizer into the soil, more than 80% of phosphorus cannot be absorbed by plants due to chemical reactions such as precipitation and adsorption [24]. The results of this experiment showed that the application of coated diammonium phosphate increased the content of available phosphorus and ammonium nitrogen in the 0–20 cm soil layer by 99.7% and 151.2%, respectively, compared with ordinary diammonium phosphate. The total amount of soil available phosphorus and cumulative ammonia volatilization decreased by 18.8% and 87.0%, respectively. The effective tiller number increased by 4.36%, and grain yield and above-ground growth increased by 13.45% and 14.2%, respectively. The coated diammonium phosphate exhibits excellent nutrient controlled-release effects, ensuring sustained and slow nutrient release, significantly extending the supply time of available phosphorus. This significantly improves the intensity of the soil phosphorus supply, aligning the nutrient patterns of fertilizers with the growth needs of winter wheat to meet the phosphorus requirements of winter wheat at different growth stages and promoting the formation of grain yield. This finding is consistent with the conclusions of Chen et al. [25]. Meanwhile, leaching experiments showed that the application of coated diammonium phosphate reduced the content of various nutrients in the leachate to varying degrees. Compared with P treatment, the total phosphorus, ammonium nitrogen, potassium, and magnesium in the leachate from CP treatment decreased by 95.6%, 63.5%, 9.6%, and 11.6%, respectively. The physical membrane layer on the surface of coated diammonium phosphate effectively reduces the contact area between diammonium phosphate and soil, both avoiding the invalidation of phosphorus due to fixation in the soil and reducing the risk of water pollution caused by soil leaching. Compared with urea, the amount of ammonia volatilization from diammonium phosphate varies greatly with pH changes [26]. The coating of diammonium phosphate greatly avoids ammonia volatilization (Figure 2B), reducing nutrient losses.

4.2. Humic Acid on Wheat Yield and Soil Nutrient Transport

Humic acid (HA) contains a large number of functional groups and active substances, which can improve the soil aggregate structure and promote plant growth and metabolism to a certain extent after being applied to the soil [18]. As shown by the experimental results, compared with P and CP treatments, the soil available phosphorus content increased by 32.2% and 15.2%, respectively, and the wheat grain yield increased by 17.17% and 5.0% in the PHA and CPHA treatments. The addition of HA can significantly increase the soil available phosphorus content and thus promote yield formation. The reason may be that HA can promote the growth of crop roots, stimulate the release of bio-organic acids from the roots into the soil, activate precipitated phosphates, effectively reduce phosphorus fixation in the soil, thereby increasing the absorption of nutrients such as phosphorus by the roots and promoting the increase of wheat yield. This is consistent with the conclusion of Qin et al. [27]. that HA can promote the transformation of soil phosphorus and increase the availability of soil phosphorus. The content of available phosphorus and ammonium nitrogen in the leachate was reduced by 49–63% and 43–64%, respectively, with the addition of HA. It can be seen that adding HA can improve soil structure, store soil nutrients, and have a certain fertilizer-preserving effect. This is consistent with the conclusion of Michael Susic [28] that HA can increase water and fertilizer retention. Under the experimental conditions, the addition of HA increased the amount of calcium and magnesium in the leachate. Liu et al. [29] found that HA and phosphate can be adsorbed in the soil simultaneously and compete for common surface positions. The addition of HA can compete with phosphate for Ca²⁺, Mg²⁺, Fe³⁺, and Al³⁺, reducing the precipitation of calcium-phosphorus compounds, allowing a large amount of unprecipitated calcium and magnesium in the soil to enter the leachate [30,31]. Meanwhile, the H+ produced by its decomposition can also change the soil micro-domain pH and inhibit the conversion of water-soluble phosphorus to hydroxyapatite [29,32,33].

4.3. Coated Diammonium Phosphate Combined with Humic Acid on Wheat Yield and Soil Nutrient Transport

The application of coated diammonium phosphate combined with HA can increase soil available phosphorus content and promote crop yield formation. Under the same phosphorus level, the treatment of coated diammonium phosphate combined with HA significantly increased the winter wheat grain yield by 19.10% compared with conventional fertilization, and showed a yield advantage over the treatment with only coated phosphate fertilizer and the treatment of ordinary diammonium phosphate combined with HA. The significant yield-increasing effect of coated diammonium phosphate combined with HA on winter wheat is consistent with previous research results [34]. The reason may be that both coated diammonium phosphate and HA can promote wheat yield separately. When they are applied in a certain proportion, on the one hand, the coated phosphate can control the slow release of nutrients and reduce the loss of nitrogen and phosphorus nutrients, providing sufficient effective nutrients for the entire growth period of wheat and increasing the effective tiller number of winter wheat. On the other hand, HA, as a biostimulant, can promote wheat growth [35], improve photosynthesis, enhance the absorption of nutrients, and improve the nutritional status of crops [11], thereby increasing the utilization rate of soil phosphorus by winter wheat. The release process of nutrients from coated fertilizers is directly or indirectly affected by factors such as soil temperature and water content. HA can enhance soil water retention by consolidating soil particles, participating in the formation of soil clay-organic complexes, containing hydrophilic cellulose and polyol groups. The combined application of the two is conducive to regulating the release properties of coated diammonium phosphate. In addition, studies have shown that the slow-release characteristics of coated phosphate fertilizer form a continuous low-phosphorus source supply in the soil, leading to the formation of “heterogeneous” nutrient supply in the soil due to the poor phosphorus mobility [36,37,38]. The dual stimulatory effects of low-phosphorus effects and HA can effectively promote the growth of winter wheat roots and enhance the secretion of low-molecular-weight organic acids, which not only expands the absorption area of the roots but also improves the availability of phosphorus in the soil. The results of this experiment show that the application of coated diammonium phosphate combined with HA maintained the topsoil at a higher nutrient content level, reducing the content of available phosphorus, ammonium nitrogen, and potassium in the leachate by 98.3%, 64.3%, and 7.1%, respectively. However, compared with the treatment of coated diammonium phosphate alone, it increased the soil available phosphorus content in the 0–20 cm and 20–25 cm soil layers, indicating that the combined application of the two can enhance soil fertility retention and increase the mobility of phosphorus in the topsoil to a certain extent. This allows nutrients to be directly absorbed by plant roots, reducing nutrient leaching and waste during vertical nutrient transport, and has a significant effect on improving fertilizer utilization efficiency and fertility retention for income enhancement (Figure 6). However, in actual farmland environments, given the complex, variable, and highly heterogeneous nature of soil conditions, ensuring that the combination of coated diammonium phosphate and humic acid consistently demonstrates high phosphorus fertilizer use efficiency across various soil types remains fraught with uncertainties.

5. Conclusions

The combination of coated diammonium phosphate and humic acid can not only maintain soil fertility but also increase wheat yield. Compared with other treatments, coated diammonium phosphate combined with humic acid has the best yield increase effect, thereby providing a solid theoretical foundation for reducing fertilizer application and enhancing efficiency. Through the leaching device and ammonia volatilization simulation test, it was found that the coated diammonium phosphate combined with humic acid significantly reduced the amount of ammonia volatilization compared with the ordinary diammonium phosphate treatment, and significantly reduced the contents of available phosphorus, ammonium nitrogen, and potassium in the leachate. However, the coated diammonium phosphate treatment significantly increased the content of available phosphorus in the 0–20 cm and 20–25 cm soil layers. The coating of diammonium phosphate can delay the release of nutrients and reduce the leaching loss of nutrients. Humic acid can improve the availability and mobility of soil phosphorus. The combination of the two can reduce the leaching of soil nutrients with water, increase the content of soil available phosphorus, and effectively improve the nutrient supply intensity of the plowing layer.