Effects of Substituting Organic Fertilizers for Chemical Nitrogen Fertilizers on Physical and Chemical Properties and Maize Yield of Anthropogenic-Alluvial Soil

Abstract

To resolve issues in the traditional agricultural production of the Ningxia irrigated area, where the sole pursuit of yield through extensive application of chemical nitrogen fertilizers has resulted in a deteriorated soil structure, reduced quality of anthropogenic-alluvial soil, and limited improvement in crop yield per unit area, a fixed-site experiment on substituting organic fertilizers for chemical nitrogen fertilizers was performed at the comprehensive experimental base of the NingXia Academy of Agriculture and Forestry Sciences during 2021–2024. Using conventional fertilization (N, P₂O₅, and K₂O application amounts of 450, 150, and 60 kg·ha⁻¹, respectively) as the control (CK), treatments of substituting organic fertilizers for 15% (T1), 30% (T2), 45% (T3), and 100% (T4) of chemical nitrogen fertilizers were used to analyze their effects on soil physical and chemical properties, as well as the maize yield in anthropogenic-alluvial soil. Substituting organic fertilizers for chemical nitrogen fertilizers increased the content of water-stable macroaggregates and the mean weight diameter (MWD) stability parameter in the soil. In 2024, the treatments of substituting organic fertilizers for chemical nitrogen fertilizers significantly increased MWD by 24.18–30.22% compared to the CK treatment. The soil’s available nitrogen content significantly decreased under the T4 treatment by 8.25–20.50% compared to CK treatment during 2021–2024. The organic matter (OM) content showed an increasing trend with the proportion of substitution of organic fertilizers for chemical nitrogen fertilizers; in 2024, the T3 and T4 treatments significantly increased OM by 5.98% and 6.60%, respectively, compared to CK. Furthermore, the available phosphorus and potassium contents also exhibited an increasing trend with the proportion of substitution of organic fertilizers for chemical nitrogen fertilizers. Based on the full dataset method, it was calculated that the T1 treatment consistently improved the soil quality index (SQI) during 2021–2024, with an increase of 9.31–18.29% compared to CK. The T1 treatment increased maize yield by 9.90% and 16.93% in 2023 and 2024, respectively, compared to CK. A random forest model identified the available nitrogen as the most critical physical and chemical indicator affecting SQI, followed by the available potassium. Linear fitting between the SQI and yield showed a highly significant positive correlation (R² = 0.6288, p < 0.01). Moreover, polynomial fitting of the proportion of substitution of organic fertilizers for chemical nitrogen fertilizers showed that SQI reached a maximum for a substitution proportion of 31.46%, while the maximum maize yield reached a proportion of 28.74%. Comprehensive analysis combining information and weight suggested an optimal proportion of substitution of organic fertilizers for chemical nitrogen fertilizers of 29.52%, achieving both an increase in SQI and maize yield in the anthropogenic-alluvial soil of the Ningxia irrigated area, while also achieving a rational utilization of organic fertilizer.

1. Introduction

Located in the inland northwest region of China, in the middle and upper reaches of the Yellow River, the Ningxia region currently has approximately 0.70 million ha of irrigated cultivated land. Long-term irrigation and cultivation have produced a special type of anthropogenic-alluvial soil, which has relatively high fertility and plays an extremely significant role in grain production [1,2]. However, production practices in recent years have revealed that traditional methods of solely applying chemical fertilizers frequently cause such issues as the breakdown of soil aggregate structure in anthropogenic-alluvial soil, severe nutrient loss, and low crop yields. These issues pose potential threats to the conservation of soil health in cultivated land and the stable improvement of crop yields, which seriously affect the sustainable and high-quality development of agriculture [3,4,5]. As a vital component of traditional agriculture, organic fertilizers not only provide the nutrients required for crop growth and increase the organic matter (OM) content in soil, but they also improve the soil structure and enhance soil fertility and crop yield [6,7,8]. Fueled by the vigorous development of the animal breeding industry in Ningxia, approximately 32 million tons of livestock and poultry manures are produced every year. Converting these manures into organic fertilizers, and the combined application of organic and chemical fertilizers to fields, will be of enormous significance for the safe utilization of livestock and poultry manures, the reduction and efficiency improvement of chemical fertilizers, and scientific soil fertilization [9].

There have been extensive studies on the application of organic fertilizers to fields. For example, Zhao et al. [10] found that over 100 years of application of organic fertilizers to fields at the Rothamsted Experimental Station in the UK had significantly increased the organic carbon content in the soil and enriched the organic carbon pool. Liao et al. [11] performed a 35-year long-term fixed-site experiment to demonstrate that long-term application of high amounts of organic fertilizers significantly reduced the aggregate stability of red soil paddy fields, with the aggregate mean weight diameter (MWD) decreasing by 8.39%. In a 20-year long-term fixed-site experiment, Xie et al. [12] found that the treatment of a combined application of organic and chemical fertilizers increased organic carbon content in soil, in the plow layer of black soil by 56.6% and in gray desert soil by 143.1%, compared to conventional fertilization treatment. Li et al. [13] carried out an 18-year long-term fixed-site experiment on winter wheat–summer maize rotation on Loess-derived stratified old manured Loessial soil, revealing that a long-term combined application of organic and chemical fertilizers had no significant effect on the distribution of dry-sieved aggregate sizes, but it tended to increase the content of water-stable aggregates by >2 mm. Liang et al. [14] performed a three-year experiment on substituting organic fertilizers for chemical fertilizers in the semi-humid drought-prone region of the southern Loess Plateau, showing that substituting organic fertilizers for 19% inorganic fertilizers increased nitrate nitrogen content in soil in dryland wheat fields by 27.6% without reducing the wheat yield. Zhang et al. [15] conducted a three-year fixed-site study on medium-fertility land in the Hetao Irrigated Area, demonstrating that substituting organic fertilizers for 25% chemical nitrogen fertilizers increased the maize yield. Using a fixed-site experiment during 2018–2020, Zhou et al. [16] showed that when the ratio of organic to inorganic nitrogen application was 3:2, the crop yield was at a maximum (12,578 kg ha⁻¹) and nitrate nitrogen leaching (15.70 kg ha⁻¹) was also at an acceptable level. Fei [17] performed a two-year fixed-site experiment in the cinnamon soil farmland area of the southwestern Shanxi Basin, showing that substituting organic fertilizers for 30% chemical nitrogen fertilizers not only promoted maize growth and increased the maize yield but also improved the soil fertility supply capacity.

It is evident that the optimal proportion of substitution of organic fertilizers for chemical nitrogen fertilizers varies across different regions, fertility levels, and soil types. However, current research on substituting organic fertilizers for chemical nitrogen fertilizers in medium-to-high fertility anthropogenic-alluvial soil in Ningxia remains relatively limited, and the substitution proportions have not been clearly defined. Consequently, this study aims to utilize local abundant livestock and poultry waste for resource utilization and enhance soil fertility through combined organic and inorganic fertilization as the starting point. Using a rapid fermentation technology for manure, researched and developed by our team to produce organic fertilizers meeting safety standards, a four-year fixed-site experiment was performed on substituting organic fertilizers for different proportions of chemical nitrogen fertilizers. The effects on the soil’s physical and chemical properties, as well as the maize yield, were analyzed. Soil quality was comprehensively evaluated based on the full dataset method, and the optimal substitution proportion was determined by combining yield data with information weight analysis, with a view to providing a scientific basis for soil health cultivation, green agricultural development, and waste resource utilization.

2. Materials and Methods

2.1. Experimental Site Description

We conducted a four-year fixed-site experiment on the application of organic fertilizers to fields during 2021–2024 at the experimental base of the NingXia Academy of Agriculture and Forestry Sciences in Wanghong Town, Yongning County, Ningxia Hui Autonomous Region, China. The region has a mild, temperate arid climate, with an average annual temperature of 8.7 °C, average annual precipitation of 201.4 mm, and an average annual evaporation rate that is 8.6 times that of precipitation. The terrain is flat, characterized by an alluvial plain of the Yellow River. The soil parent material consists of alluvial and irrigated sediments, with the soil type predominantly being anthropogenic-alluvial soil. The soil texture is silty and sandy loam (International System) (Table 1). The OM content in soil is relatively low, while the available phosphorus (AP) and available potassium (AK) contents are relatively high (Table 2).

Table 1. Basic physical properties of irrigation-silted soil.

Soil Layer (cm)	Mechanical Composition/mm (%)			Bulk Density (g cm−3)	Water Stable Aggregate Content/mm (%)
	Sand 0.02~2	Silt 0.02~0.002	Clay <0.002		>2	0.25~2	<0.25
0~20	11.30 ± 1.80 a	59.40 ± 1.21 a	29.30 ± 0.70 a	1.35 ± 0.02 a	3.23 ± 1.32 a	34.32 ± 3.44 a	62.45 ± 4.75 b
20~40	13.57 ± 1.82 a	56.53 ± 1.77 a	29.9 ± 0.10 a	1.38 ± 0.08 a	1.12 ± 0.07 b	29.85 ± 2.23 ab	69.03 ± 2.3 ab
40~60	10.77 ± 1.16 a	58.63 ± 1.38 a	30.6 ± 1.80 a	1.32 ± 0.14 a	1.05 ± 0.16 b	24.7 ± 1.25 b	74.25 ± 1.31 a

Note: Different letters in the same column indicate that there are significant differences in different soil layer indexes (p < 0.05).

Table 2. Basic chemical properties of irrigated silt soil.

Soil Layer (cm)	Organic Matter (g kg−1)	Available Nitrogen (mg kg−1)	Available Phosphorus (mg kg−1)	Available Potassium (mg kg−1)	pH	Total Salt (g kg−1)
0~20	15.37 ± 0.48 a	36.50 ± 0.64 a	23.23 ± 0.42 a	135.67 ± 5.73 a	8.20 ± 0.06 a	0.50 ± 0.01 a
20~40	10.86 ± 0.62 b	31.19 ± 0.23 b	26.20 ± 0.46 a	153.33 ± 1.83 a	8.21 ± 0.13 a	0.47 ± 0.02 a
40~60	8.06 ± 0.10 c	19.55 ± 1.01 c	25.60 ± 0.58 a	149.33 ± 2.87 a	8.18 ± 0.21 a	0.47 ± 0.05 a

Note: Different letters in the same column indicate that there are significant differences in different soil layer indexes (p < 0.05).

2.2. Test Materials

The indicator crop was a dual-purpose grain and forage maize variety (Xianyu No. 1225, Figure 1). The organic fertilizers used in the experiment were derived from large quantities of beef cattle manure produced by the surrounding livestock and poultry breeding, rapidly decomposed and fermented using the “molecular membrane” (Nano-black film, which is waterproof, moisture-permeable, deodorizing, and nitrogen-retaining) aerobic composting technology. The total nutrient content of the fertilizer was 4.35% (total nitrogen 2.00%, total phosphorus 1.28%, total potassium 1.07%), with an OM content of 45%, mechanical impurities < 0.5%, moisture content 20.7%, pH 7.99, and total salt content of 11.78 g/kg, meeting the reference standards of NY/T 525-2021 [18]. The germination index of maize seeds for the fermented organic fertilizer was 70%, and the mortality rate of ascarid eggs was 98%, meeting the reference standards of NY/T 3442-2019 [19]. The chemical fertilizers used were all purchased from the market: urea (N 46%), calcium superphosphate (P₂O₅ 12%), and potassium sulfate (K₂O 50%).

Figure 1. Aerial view of field experiment.

2.3. Experimental Details

Referring to the long-term fixed-site experiment by Xu et al. [20], and based on local conventional fertilization (application amounts of N, P₂O₅, and K₂O of 450, 150, and 60 kg ha⁻¹, respectively), treatments were applied in a randomized block design, using the principle of equivalent nitrogen input (phosphorus and potassium mineralization rates are relatively slow and thus disregarded). The treatments included substituting organic fertilizer nitrogen for 15%, 30%, 45%, and 100% of chemical nitrogen fertilizers (Table 3). Each treatment was replicated three times, resulting in a total of 15 plots, each with an area of 60 m² (10 m × 6 m). Sowing was performed under a suitable soil temperature and moisture conditions. All phosphorus, potassium, and organic fertilizers were applied as base fertilizers. A total of 40% of the total chemical nitrogen fertilizer is applied as a basal application, while the remaining 60% is topdressed at the maize 3–4 leaf stage and 11–12 leaf stage, respectively. Fertilization measures remained consistent each year. During the growth period, irrigation was carried out using water sourced from the Yellow River (pH 7.56, mineralization degree 0.56 g L⁻¹). The irrigation volume was approximately 5250 m³ ha⁻¹, applied over four irrigation times. The sowing density was 90,000 plants/ha. Diseases, pests, and weeds were controlled before sowing and during the key growth period. The previous crop was wheat.

Table 3. Experimental design (kg ha⁻¹).

Treatments	Fertilization Measure	Organic Fertilizer Amount	Organic Fertilizer N	Chemical Fertilizer Application
				N	P2O5	K2O
CK	100% chemical nitrogen	0	0	450	150	75
T1	15% organic fertilizer nitrogen + 85% chemical fertilizer nitrogen	3375	67.5	382.5	150	75
T2	30% organic fertilizer nitrogen + 70% chemical fertilizer nitrogen	6750	135	315	150	75
T3	45% organic fertilizer nitrogen + 55% chemical fertilizer nitrogen	10,125	202.5	247.5	150	75
T4	100% organic fertilizer nitrogen	22,500	450	0	150	75

2.4. Determined Indicators and Methods

2.4.1. Determination of Soil’s Physical and Chemical Properties

The experiment was initiated in early April 2021. Prior to the trial, three soil profiles with a depth of 80 cm were excavated from the selected field for soil diagnosis and investigation. Simultaneously, soil bulk density (BD) was measured using the cutting ring method. Soil samples were collected from three depth layers: 0–20, 20–40, and 40–60 cm. Approximately 500 g of each sample was placed into zip-lock bags and brought back to the laboratory to air-dry naturally. Fine and hair roots that were slightly visible were removed using electrostatic attraction by a straightedge. The samples were then prepared using the quartering method, fully ground with a wooden stick, and successively passed through 1 and 0.25 mm soil sieves for the determination of soil physicochemical properties. Another portion, approximately 2000 g, was placed into plastic boxes (20 cm × 14 cm × 13 cm) of undisturbed soil and transported to the laboratory for analysis of the soil’s mechanical composition and aggregate properties. From 10 October 2021 to 2024, sampling was conducted during the crop harvest period. Soil BD was measured first, followed by the excavation of the plow layer’s (0–20 cm) soil profile to collect samples for analyzing soil physicochemical properties and aggregate indicators.

The mechanical composition was determined using a Mastersizer No. 2000 laser particle size analyzer [21]. The BD was determined by the cutting ring method [21]. The pH was determined with a pH meter (Phs-3C, Lei Ci, Shanghai, China) at a water:soil ratio of 2.5:1. Total salt content and water sample mineralization degree were determined using a conductivity meter (DDS-11, Lei Ci, Shanghai, China), with the total salt (TS) content calculated from the linear equation between conductivity and salt concentration (y = 3.0023x + 0.1456, where y represents TS in soil and x represents soil conductivity at 25 °C) [22]. The organic carbon content was determined by the potassium dichromate–concentrated sulfuric acid heating method [21]. The available nitrogen (AN) content was determined by the alkali hydrolysis diffusion method [21]. The AP content was determined by the 0.5 mol L⁻¹ sodium bicarbonate extraction–molybdenum–antimony anti-colorimetric method [21]. The AK content was determined by the 1 mol L⁻¹ ammonium acetate solution extraction–flame photometer method [21]. The soil’s water-stable aggregate content was calculated by the Savinov method of classification, using wet sieving and weighing [23]. This study is categorized into >2, 0.25–2, and <0.25 mm groups, and represented by R_>2, R_0.25–2, R_<0.25, respectively.

2.4.2. Determination of Maize Yield

The maize harvest began on 10 October every year. Its yield was measured by the actual harvest in quadrats, using the method of the Ministry of Agriculture, and converted to yield per hectare [24]. The plot area of the test treatment is 60. The area of the experimental plot is 60 m² (10 m × 6 m), and the total area is about 1500 m².

2.4.3. Calculation of Related Indicators

Water-stable aggregate parameters of soil: the MWD and geometric mean weight diameter (GMD) of soil were calculated as follows:

$$\mathrm{MWD} = {\displaystyle \sum _{i=1}^n\overline{x_i}{\mathrm{W}}_{\mathrm{i}}}$$

$$\mathrm{GMD}=\exp ({\displaystyle \sum _{i=1}^n{w}_i}\ln \overline{x_i})$$

In the formulas, $\overline{x_i}$ is the mean diameter of aggregates in the i-th size class (mm), W_i is the mass fraction of aggregates in the i-th size class (%), and lnX_i is the natural logarithm of the mean diameter of aggregates in the i-th size class (mm) [23].

Calculation of soil quality index (SQI): The SQI was calculated using the full dataset method. Linear indicator scores were calculated using the “more is better”-type indicator scoring function and the “less is better”-type indicator scoring function [25]:

$$S_i={\displaystyle \frac{x_i-x_{\min }}{x_{\max }-x_{\min }}}$$

$$S_i={\displaystyle \frac{x_{\max }-x_i}{x_{\max }-x_{\min }}}$$

In the formulas, S_i denotes the linear indicator score (0–1), x_i is the measured value of the indicator, x_max is the maximum value of the indicator, and x_min is the minimum value of the indicator.

Principal component analysis was performed on the 12 soil indicators tested in this study to extract the common factor variance of each indicator. The weight value of each indicator was obtained by calculating the ratio of the common factor variance of the indicator to the sum of the common factor variances. The SQI was calculated using the following formula, with a higher SQI value meaning higher soil quality [26]:

$$SQI={\displaystyle \sum _{i=1}^n{{\displaystyle w}}_i}{{\displaystyle S}}_i$$

In the formula, W_i denotes the weight value of the i-th indicator, S_i is the score of the i-th indicator, and n represents the number of indicators in each dataset (n = 12 in this study).

2.5. Statistical Analysis

Experimental data were organized using Excel 2003 software. Differences under different years and treatments were compared using analysis of variance, the least significant difference method, and Duncan’s method (p < 0.05, n = 3). Graphs were plotted using Origin 21. Random forest algorithm modeling was performed using SPSS-AU 3.0. Simultaneously, the information weight (coefficient of variation, n = 75) comprehensive evaluation method was employed to objectively assign weights to yield and SQI solutions. Graphs were plotted using Origin 21 software.

3. Results

3.1. Effects of Substituting Organic Fertilizers for Chemical Nitrogen Fertilizers on Soil’s Physical and Chemical Properties

Substituting organic fertilizers for chemical nitrogen fertilizers had little effect on the soil BD. Although there were significant differences among treatments in 2024 (p < 0.05), there were none among treatments in 2021–2023 (p > 0.05). Specifically, the T4 treatment significantly reduced the soil BD by 5.83% compared to the CK treatment (Figure 2A). The R_>2 content showed no significant differences among treatments during 2021–2023 (p > 0.05). In 2024, the T1 and T2 treatments increased the R_>2 content by 62.35% and 68.67%, respectively, compared to CK (p < 0.05). Moreover, the R_>2 content tended to increase with the higher proportion of substitution of organic fertilizers for chemical nitrogen fertilizers (Figure 2B). The R_0.25–2 content significantly differed among treatments and across years (p < 0.05). In 2024, the T3 and T4 treatments increased the R_0.25–2 content by 31.33% and 27.33%, respectively, compared to the CK (Figure 2C). The R_<0.25 content showed significant differences after 2022 (p < 0.05). Unlike the CK, the R_<0.25 content tended to decrease over the years in the treatments substituting organic fertilizers for chemical nitrogen fertilizers, with the T3 and T4 treatments showing the largest decreases (Figure 2D). The stability parameters of MWD and GMD were calculated based on the content of water-stable aggregates of different particle sizes. The MWD showed no significant differences among treatments during 2021–2022 (p > 0.05). In 2024, compared to the CK, the T1, T2, T3, and T4 treatments significantly increased the MWD by 24.18%, 30.22%, 25.33%, and 26.31%, respectively (Figure 2E). The GMD showed significant differences among treatments during 2022–2024 (p < 0.05). In 2023, the T4 treatment significantly increased the GMD by 19.52%, 7.45%, 5.68%, and 5.84% compared to the CK, T1, T2, and T3 treatments, respectively. In 2024, the T3 treatment showed the highest value, significantly increasing the GMD by 34.00% and 14.98% compared to the CK and T1 treatments, respectively (Figure 2F). It is evident that substituting organic fertilizers for chemical nitrogen fertilizers increased the soil’s > 0.25 mm water-stable macroaggregate content and improved aggregate stability, with the increase being more pronounced at higher proportions of substitution.

Figure 2. Effects of substituting organic fertilizers for chemical nitrogen fertilizers on soil’s physical properties. Different lowercase letters denote significant differences between substituting organic fertilizers for chemical nitrogen fertilizers in the same year at p < 0.05. (A) BD, bulk density; (B) R_>2, water-stable aggregates of >2 mm; (C) R_0.25–2, water-stable aggregates of 0.25–2 mm; (D) R_<0.25, water-stable aggregates of <0.25 mm; (E) MWD, mean weight diameter; and (F) GMD, geometric mean diameter.

Substituting organic fertilizers for chemical nitrogen fertilizers had little effect on the soil’s pH and TS content. Overall, there were no significant differences among treatments or across years (p > 0.05), but pH and TS content showed an increasing trend (Figure 3A,B). Soil OM content showed no significant differences among treatments during 2021–2023 (p > 0.05); however, in 2024, the T3 and T4 treatments significantly increased the OM content by 5.98% and 6.60%, respectively, compared to CK (Figure 3C). The possible reason is that organic fertilizer contains 45% organic matter. The larger the amount of chemical fertilizer that is replaced by organic fertilizer, the more it helps to increase the soil’s organic matter content. Soil’s AN content showed significant differences among treatments and across years (p < 0.05). The T4 treatment reduced the AN content compared to CK, with a reduction range of 8.25–20.50% (Figure 3D). The AP and AK contents showed significant differences among treatments in 2023 and 2024 (p < 0.05). Compared to the CK, substituting organic fertilizers for chemical nitrogen fertilizers significantly increased the AP content, with increases of 24.27–38.51% in 2023 and 33.36–41.15% in 2024. The AK content showed an increasing trend for the treatments substituting organic fertilizers for chemical nitrogen fertilizers, with the T4 treatment increasing it by 21.21–42.25% compared to CK (Figure 3E,F). It is evident that increasing the proportion of substitution of organic fertilizers for chemical nitrogen fertilizers may increase the potential risk of raising soil’s pH and TS accumulation, while significantly promoting the OM, AP, and AK contents. However, increasing the proportion of substitution reduces the soil’s AN content.

Figure 3. Effects of substituting organic fertilizers for chemical nitrogen fertilizers on soil chemical properties. Different lowercase letters denote significant differences between substituting organic fertilizers for chemical nitrogen fertilizers in the same year at p < 0.05. (A) pH, pH value; (B) TS, total salt; (C) OM, organic matter; (D) AN, available nitrogen; (E) AP, available phosphorus; and (F) AK, available potassium.

3.2. Effects of Substituting Organic Fertilizers for Chemical Nitrogen Fertilizers on SQI

To comprehensively evaluate changes in soil quality under different treatments substituting organic fertilizers for chemical nitrogen fertilizers, the full dataset method was employed. A total of 12 indicators underwent dimensionless standardization to a range of 0–1. These indicators included the physical parameters BD, R_>2, R_0.25–2, R_<0.25, MWD, and GMD, and the chemical parameters pH, TS, OM, AN, AP, and AK. The common factor variance of each indicator was extracted based on the principles of eigenvalue ≥1 and cumulative interpretation rate ≥ 90%, and the weight of each indicator was calculated (Table 4).

Table 4. Common factor variance and weights of soil quality evaluation indicators.

Indexes	2021		2022		2023		2024
	Communality	Weight	Communality	Weight	Communality	Weight	Communality	Weight
pH	0.997	0.087	0.858	0.076	0.643	0.059	0.861	0.076
TS	0.958	0.084	1.000	0.088	0.998	0.092	0.969	0.085
OM	0.975	0.085	0.845	0.075	0.923	0.085	0.971	0.086
AN	1.000	0.087	0.94	0.083	0.985	0.091	0.971	0.086
AP	0.797	0.069	0.995	0.088	0.813	0.075	0.881	0.078
AK	0.989	0.086	0.836	0.074	0.987	0.091	0.978	0.086
BD	0.779	0.068	0.887	0.078	0.57	0.053	0.982	0.087
R>2	0.998	0.087	1.000	0.088	0.944	0.087	0.964	0.085
R0.25–2	0.995	0.087	0.983	0.087	0.995	0.092	0.903	0.08
R<0.25	0.996	0.087	0.984	0.087	0.991	0.092	0.933	0.082
MWD	1.000	0.087	0.992	0.088	0.963	0.089	0.996	0.088
GMD	0.985	0.086	0.986	0.087	1.000	0.092	0.931	0.082

The radar charts of the soil indicator scores varied during 2021–2024. In 2021, under the T4 treatment, the scores of the TS, pH, AN, and R_<0.25 indicators were reduced, but the OM, AP, and R_0.25–2 scores increased; under the T1 treatment, the pH, R_<0.25, and MWD scores increased. In 2022, under the T1 treatment, the OM, AN, R_<0.25, R_0.25–2, MWD, and GMD scores increased, while under the T2 treatment, the AP, AK, and BD scores increased. In 2023, under the T1 treatment, the AN, AP, and GMD scores increased, and under the T4 treatment, the OM score increased. In 2024, under the T1 treatment, the AN, R_>2, R_0.25–2, and GMD scores increased. Overall, the T1 treatment increased the AN scores the most, while the T4 treatment increased the OM scores the most (Figure 4).

Figure 4. Radar I.c. of soil indicator scores under the substituting organic fertilizers for chemical nitrogen fertilizers.

In 2021, the SQI was highest under the T1 treatment, significantly increasing by 9.31% compared to CK. There were no significant differences between the T2 and T3 treatments and CK, while the T4 treatment significantly reduced the SQI by 12.97% compared to CK—the reductions increased as the proportion of substitution of organic fertilizers for chemical nitrogen fertilizers increased. In 2022, the SQI was also highest under the T1 treatment, increasing by 12.36% compared to CK. There were no significant differences in SQI among the T2, T3, and CK treatments, while the T4 treatment significantly reduced the SQI by 9.62% compared to CK. In 2023 and 2024, the SQI across treatments followed the order T1 > T2 > T3 > CK > T4. In 2023, the T1 treatment increased SQI by 11.43% compared to CK, while in 2024, it increased by 18.29% compared to CK. Furthermore, SQI changed across the years during 2021–2024: increasing by 1.48% for CK and by 9.81% for the T1 treatment (Figure 5).

Figure 5. Effects of substituting organic fertilizers for chemical nitrogen fertilizers on SQI. Different lowercase letters denote significant differences between substituting organic fertilizers for chemical nitrogen fertilizers in the same year at p < 0.05.

3.3. Effects of Substituting Organic Fertilizers for Chemical Nitrogen Fertilizers on Yield

The maize yield significantly differed among treatments and across years (p < 0.05). In 2021, the yield from the T1 treatment was the highest, with no significant differences compared to CK (p > 0.05); in contrast, the yield from the T2, T3, and T4 treatments was reduced by 2.92–42.72% compared to CK. In 2022, there were no significant differences among the T1, T2, and CK treatments (p > 0.05), while the yield from the T3 and T4 treatments was 16.81% and 43.68% lower (p < 0.05), respectively, than for CK. In 2023, the yield from the T1 and T2 treatments was 9.90% and 6.98% higher, respectively, than for CK, while the yield from the T3 and T4 treatments was 9.94% and 42.10% lower, respectively, compared to CK. In 2024, the yield from the T1 treatment was significantly higher by 16.93% compared to CK (p < 0.05), but with no significant differences between the T2 and CK treatments (p > 0.05). The yield from the T3 and T4 treatments was 16.43% and 38.46% lower, respectively, than for CK. Notably, the interannual yield changes suggested that the CK yield remained relatively stable. In 2024, the yield from the T1 treatment increased by 15.06% compared to 2021, while the other treatments showed yield increases of 1.79–13.05% (Figure 6). This reveals that the annual application of organic fertilizers improved the maize yield.

Figure 6. Effects of substituting organic fertilizers for chemical nitrogen fertilizers on yield. Different lowercase letters denote significant differences between substituting organic fertilizers for chemical nitrogen fertilizers in the same year at p < 0.05.

3.4. Analysis of the Effects of Soil’s Physical and Chemical Properties on Yield

Random forest modeling was performed using the soil’s physical and chemical properties as independent variables and the SQI as the dependent variable. The results indicated that the AN and AK in soil affected the SQI significantly (p < 0.01 and significantly p < 0.05, respectively). Thus, the AN is the most critical factor affecting the SQI (Figure 7). Linear fitting of the relationship between the SQI and yield for 2021–2024 revealed an extremely significant positive relationship (p < 0.01, Figure 8). Thus, an increase in the SQI significantly improves the maize yield, demonstrating that soil provides fundamental conditions such as nutrients, water, and physical and chemical support for crops. However, continued monitoring is essential in the follow-up, as yield may stabilize when SQI reaches a certain value.

Figure 7. Key physical and chemical properties of soil affecting SQI. * denotes significant level (p < 0.05), ** denotes extremely significant level(p < 0.01).

Figure 8. Linear regression analysis of SQI and maize yield.

3.5. Recommended Optimal Proportion for Substituting Organic Fertilizer Nitrogen

When establishing univariate quadratic equations for the relationship between the proportion of substitution of organic fertilizers for chemical nitrogen fertilizers and the SQI and maize yield for 2021–2024, the R² of the equation between the proportion of substitution and the SQI ranged within 0.6123–0.7185, while R² of the equation between the proportion of substitution and the yield exceeded 0.7. Both equations exhibited moderate-to-high fitness levels, indicating certain explanatory capabilities. Using dy/dx = 0, the calculated proportions of substituting organic fertilizers for chemical nitrogen fertilizers for achieving the maximum SQI were 33.33%, 30.00%, 32.50%, and 30.00%, respectively, with an average of 31.46% (Figure 9A). The calculated proportions of substituting organic fertilizers for chemical nitrogen fertilizers to achieve the maximum maize yield were 26.77%, 27.78%, 30.17%, and 30.25%, respectively, with an average of 28.74% (Figure 9B). Based on the dual objectives of SQI and yield, the information weight comprehensive evaluation method was employed to objectively assign weights to the yield and SQI solutions (Table 5). The unique identified optimal proportion of substitution of organic fertilizers for chemical nitrogen fertilizers was 29.52%.

Figure 9. Linear regression analysis of the proportion of substitution of organic fertilizer nitrogen for chemical fertilizers with yield and SQI. (A) Substitution ratio and SQI; (B) Substitution ratio and yield.

Table 5. Calculation results of information weight method.

Target Value	Mean Value	Standard Deviation	Coefficient of Variation (%)	Weight (%)
SQI	0.61	0.05	8.68	28.64
Yield	10,114.927	2188.84	21.64	71.36

4. Discussion

4.1. Effects of Substituting Organic Fertilizers for Chemical Nitrogen Fertilizers on Soil’s Physical Properties

Soil BD and aggregates are important soil physical properties. They not only reflect the compactness and porosity of soil, but also characterize the soil’s structure and stability, playing a significant role in soil fertility and crop growth [27]. This study indicated that substituting organic fertilizers for chemical nitrogen fertilizers had little effect on soil BD during 2021–2023, with no significant differences among treatments. However, in 2024, substituting organic fertilizers for 100% chemical nitrogen fertilizers (T4) significantly reduced BD compared to the CK treatment. Furthermore, substituting organic fertilizers for chemical nitrogen fertilizers significantly increased the R_>2 content in 2024. Specifically, the treatments substituting organic fertilizers for 15% chemical nitrogen fertilizers (T1) and for 30% (T2) increased the R_>2 content by 62.35% and 68.67%, respectively, compared to CK. Simultaneously, substituting organic fertilizers for 45% chemical nitrogen fertilizer (T3) significantly increased the R_0.25–2 content in 2024. Comparison of the water-stable aggregate stability parameters revealed that substituting organic fertilizers for chemical nitrogen fertilizers significantly increased the MWD in 2024, with an increase of 24.18–30.22% compared to CK. The changes in the above indicators are likely because, following the application of organic fertilizers to fields, microbial decomposition produces humus or metabolic substances, e.g., polysaccharides, cellulose, water-soluble organic carbon, fulvic acid carbon, and humic acid carbon. These substances not only fill the pores between the soil particles but also act as adhesives, promoting the aggregation of soil particles into larger aggregates, increasing the MWD of aggregates, and making the soil structure looser, which contributes to reduced soil BD [28,29,30].

4.2. Effects of Substituting Organic Fertilizers for Chemical Nitrogen Fertilizers on Soil Chemical Properties

The TS content in soil and pH value are key indicators for assessing the degree of soil salinization. Zhu et al. [31] demonstrated that substituting organic fertilizers for partial chemical fertilizers could reduce the TS content of coastal saline soil, with TS content gradually decreasing with a greater organic fertilizer application amount, but the effect on soil pH was not significant. Other studies have proven that applying organic fertilizers to soil also resulted in varying degrees of salt accumulation, but the accumulation gradually decreased with increasing application amounts, while the pH also exhibited an increasing trend [32]. The results of this study, however, suggest that substituting organic fertilizers for chemical nitrogen fertilizers increased the soil’s pH and showed a potentially increasing trend in TS content, with higher risks associated with higher proportions of substitution. This finding differs from the results of Zhang et al. [33]. The discrepancy may be attributed to the relatively slow mineralization of organic fertilizers in the soil in this study, resulting in lower release of organic acids, such as oxalic and humic acids, which were insufficient to offset the direct alkalinization effect of organic fertilizer, thereby leading to an increase in the soil’s pH [33]. The increase in TS content may be attributed to the inherently high salt content of organic fertilizers (11.78 g kg⁻¹) or the formation of ammonium salts through the combination of salts produced during the decomposition of organic fertilizers with anions in the soil, thereby increasing the soil’s TS content [34].

Soil OM is a vital component of soil. This study showed that substituting organic fertilizers for chemical nitrogen fertilizers had no significant effect on the OM content in soil during 2021–2023. However, in 2024, substituting organic fertilizers for 45% and 100% chemical nitrogen fertilizers significantly increased the OM content in soil by 5.98% and 6.60%, respectively, compared to CK. This is largely consistent with numerous previous findings [35,36]. The reason may be that treatments substituting organic fertilizers for high proportions of chemical nitrogen fertilizers significantly increased the input of organic materials, thereby enhancing microbial activity and significantly increasing the organic carbon content in soil [37]. There were significant differences in the soil’s AN, AP, and AK contents when substituting organic fertilizers for chemical nitrogen fertilizers; specifically, AN content showed a decreasing trend when the substitution proportion exceeded 15%, and a significant decreasing trend when the proportion reached 100%. Furthermore, there were significant differences in the AN content across years. The AP and AK contents showed an increasing trend when substituting organic fertilizers for chemical nitrogen fertilizers. Compared to CK, the AP content increased by 33.36–41.15% in 2024, while AK significantly increased for the 100% substitution of organic fertilizers. During 2021–2024, AK under the T4 treatment increased by 21.21–42.25% compared to CK. This is consistent with the results of a long-term fixed-site experiment of Chen et al. [38] and Liang et al. [14] in the drylands of the Loess Plateau. The reason may be that the application of organic fertilizers introduces a portion of AP and AK into the soil; certainly, this may be a limitation for the rigor of our experimental design. Follow-up studies will include compensatory treatments for phosphorus and potassium, as well as experiments on elemental interactions, to further clarify the independent effects of each factor. Otherwise, the nitrogen and carbon in organic fertilizer are more easily decomposed, and their mineralization losses are much lower than those of inorganic fertilizers [39].

4.3. Effects of Substituting Organic Fertilizers for Chemical Nitrogen Fertilizers on SQI and Yield

The effects of substituting organic fertilizers for chemical nitrogen fertilizers on the SQI and crop yield exhibit a clear proportion dependency [40]. In this study, the SQI was highest under the treatment substituting organic fertilizers for 15% chemical nitrogen fertilizers in 2021, increasing by 9.31% compared to CK. Similarly, during 2022–2024, the SQI was highest under the treatment substituting organic fertilizers for 15% chemical nitrogen fertilizers, with a notable increase of 18.29% in 2024 compared to CK. This is consistent with the findings of Wang et al. [41] for vegetables and Ma et al. [42] for wheat. In addition, our random forest model revealed that AN was the most critical factor affecting the SQI, followed by AK. This differs from the conclusion of Li et al. [43], who statistically analyzed 415 references and found that the soil’s OM was the core indicator in soil quality evaluation. This discrepancy may be attributed to the unique type of anthropogenic-alluvial soil and irrigation conditions in our study. The overall fluctuation of OM in anthropogenic-alluvial soil is relatively small, as its formation and changes are long-term processes. Under border irrigation conditions, the leaching of AN from the soil is significant, making its changes more sensitive and noticeable in the short term. Thus, the significant impact of AN may be more easily detected in the SQI evaluation system.

In the arid farmland of central Gansu, Xie et al. [44] found that substituting organic fertilizers for 37.5% chemical fertilizers promoted soil nutrient accumulation and a stable maize yield compared to sole application of chemical fertilizers. In an oasis irrigated area, Bai et al. [45] found that substituting equivalent nitrogen organic fertilizers for 25% and 50% chemical fertilizers increased the fresh ear yield of waxy maize; Li et al. [46], using a 30-year long-term fixed-site experiment on black soil, showed that the treatment substituting organic manures for 70% chemical nitrogen fertilizers effectively improved the soil’s physical and chemical properties, enhanced soil quality, and increased the maize yield. Our study, in contrast, revealed that substituting organic fertilizers for 15% chemical nitrogen fertilizers over four consecutive years significantly increased the maize yield by 16.93%, which is largely consistent with previous findings. It may be that the SQI was improved under this treatment, and there was an extremely significant positive correlation between the SQI and the yield (Figure 7), thereby increasing the maize yield. However, the proportion of substitution was relatively low compared to previous studies. The reason may be attributed to the relatively low soil fertility in this region, where excessively high proportions of substitution could lead to a mismatch between the nutrient supply and crop demand, thereby inhibiting the crop yield.

5. Conclusions

Compared to conventional fertilization, the treatments substituting organic fertilizers for chemical nitrogen fertilizers significantly increased the content of water-stable macroaggregates sized 0.25–2 mm and the MWD of soil. Additionally, this enhanced the soil’s OM, AP, and AK contents, with greater increases at higher proportions of substitution. The SQI reached its highest under the treatment substituting organic fertilizers for 15% chemical nitrogen fertilizers, as calculated using the full dataset method. During 2021–2024, the SQI showed sustained annual improvement, increasing by 9.31–18.29%. In addition, the maize yield was also significantly increased by 16.93% in 2024. Based on the relationship between the yield and the SQI, and incorporating information weight fitting, it was concluded that substituting organic fertilizers for 29.52% chemical nitrogen fertilizers could improve both the soil quality and maize production, while effectively utilizing livestock and poultry waste. This practice demonstrates strong potential for widespread adoption.