Effects of Combined Application of Organic and Inorganic Fertilizers on Physical and Chemical Properties in Saline–Alkali Soil

Abstract

To mitigate the issues of severe farmland soil salinization, the environmental degradation stemming from the overuse of chemical fertilizers, and suboptimal soil composition, a study was conducted to investigate the influence of different types and ratios of organic fertilizers on the physical and chemical attributes of saline–alkali soil. This study aimed to investigate the relationship between different types and proportions of organic fertilizers, soil moisture, organic fertilizer application rates, organic carbon molecular structure, and the soil environment in saline–alkali soils. Reducing the application of chemical fertilizers and substituting them with organic fertilizers can improve the soil quality of saline–alkali lands. The results indicated that replacing a part of the urea with organic fertilizer in saline–alkali farmland reduced the soil salinity by 11.1 to 22.8% in the 0–60 cm soil layer, decreased the soil pH by 0.11 to 1.52%, and increased the soil redox potential (Eh) values by 2.5 to 4.3% in the 0–20 cm layer of the mild and moderate saline–alkali soils. It also decreased the accumulation of the soil organic matter (OM) during the growing season. Compared to commercial organic fertilizers, natural organic fertilizers increased the accumulation of the soil soluble carbon (DOC) and nitrogen (DON), resulting in less soil salinity accumulation. When commercial organic fertilizer was applied in a 1:1 ratio with inorganic fertilizer, the salt accumulation was minimized. Compared to conventional fertilization, organic fertilizer reduced the accumulation of the NH₄⁺-N (ammonium nitrogen) and NO₃⁻-N (nitrate nitrogen) in the soil by 3.1 to 22.6%. In comparison to conventional chemical fertilizers, the application of organic fertilizer in the mild and moderate saline–alkali soils increased the accumulation of the DOC, DON, microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), and microbial quotient during the grain-filling stage. Specifically, it increased the DOC, DON, and DOC/DON by 12.7 to 26.7%, 12 to 59.3%, and 15.2 to 35.5%, respectively. The application of commercial organic fertilizer in the mild saline–alkali soils increased the MBC, MBN, MBC/SOC, and MBN/TN by 37.1, 65.6, 36.7, and 4.7%, respectively. Through analyzing the relative proportions of soil surface organic carbon functional groups during the grain filling period, we observed that, after the application of organic fertilizer, the OM in the mildly salinized soils primarily originated from terrestrial plant litter, whereas, in moderately salinized soils, the OM was mainly derived from microbial sources.

1. Introduction

OM is an important component of soil function and can affect the soil adsorption capacity, cation exchange capacity, and pH buffering capacity [1]. Globally, some soils in various countries have low OM, high alkalinity, and severe secondary salinization, which inhibit their crop growth and development due to drought and salt stress [2]. Therefore, it is necessary to implement reasonable irrigation and fertilization practices to reduce the salt accumulation in the surface layer and to improve fertilization efficiency [3]. Previous studies have shown that the long-term and heavy application of organic fertilizers increases soil salinization and the leaching of soluble salts [4]. Under the combined application of plastic film mulching and farmyard manure, soil salinity was reduced, soil moisture content was increased, the soil nutrients affected by salinization were improved, and the plant growth and yield were enhanced [5]. Applying organic fertilizer and proper irrigation can increase the content of OM and moisture while significantly enhancing nitrogen (N) absorption by prokaryotic microorganisms [6]. Increasing OM can boost the number of soil microorganisms [7], optimize and regulate the microbial community structure [8], and the microbial biomass increases with the increase in the application of organic fertilizers [9]. The dynamic characteristics of SOC (soil organic carbon) components after organic fertilizer application are a complex process influenced by various factors such as environmental conditions, the amount and type of organic fertilizer applied, and soil properties [10]. After the application of organic fertilizer, the soil pH and total nitrogen are altered, leading to changes in the physical components and chemical structure of the SOC [11].

Previous studies have investigated the effects of organic fertilizer application on the physical, chemical, and microbial properties of soil during the process of soil fertility improvement [12,13]. However, the impact of organic fertilizer application on soil physicochemical properties and organic carbon structure when cultivating salt-tolerant crops, such as sunflowers, in mild-to-moderate saline–alkali soils, while simultaneously reducing total nitrogen input, remains unknown. It is hypothesized that the application of specific types of organic fertilizers and their mixing ratios in mild-to-moderate saline–alkali soils can significantly reduce soil water deficit, decrease soil salinity content, and increase soil organic matter content.

2. Materials and Methods

2.1. Overview of the Study Area and Experimental Design

The experimental site was located in Wuyuan County, Bayannur City, Inner Mongolia Province, China. The annual average temperature ranged from 3.7 to 7.6 °C, with the highest temperatures occurring in July. Precipitation was scarce, with an annual average of 188 mm. The annual average evaporation ranged from 2032 mm to 3179 mm, and it was generally 10–30 times greater than precipitation. The soil physical properties are listed in

Table 1. Basic physicochemical properties of the soil.

	Soil Layer (cm)	BD (g/cm3)	θc (%)	θs (%)	Ψ (%)	EC (s/cm)	pH	OM (g/kg)	TN (g/kg)	NH4+-N (g/kg)	NO3−-N (g/kg)	TP (g/kg)	TK (g/kg)
F1	0–20	1.49	26.08	28.54	84.81	0.22	7.84	13.71	0.89	8.92	7.23	0.98	21.90
	20–40	1.45	24.57	29.35	86.17	0.21	7.84	9.75	0.69	13.34	25.68	0.67	17.88
	40–60	1.49	24.64	28.45	86.02	0.21	7.85	10.07	0.79	18.35	37.18	0.58	16.22
F2	0–20	1.43	29.51	30.91	84.21	0.32	8.03	8.43	0.66	10.51	3.23	0.66	15.69
	20–40	1.51	26.27	27.40	86.17	0.37	8.03	7.86	0.55	12.26	43.68	0.60	14.83
	40–60	1.47	27.94	32.36	87.18	0.45	8.02	7.02	0.47	19.23	58.25	0.57	16.19

Note for translation: BD refers to soil bulk density; θ_c represents field capacity; θ_s stands for saturated water content; ψ denotes porosity; EC is the soil (1:5) solution electrical conductivity; pH refers to the acidity or alkalinity of the soil (1:5) solution; OM indicates organic matter; TN stands for the total nitrogen; TP denotes the total phosphorus content in the soil; and TK stands for the total potassium content in the soil.

.

This study was conducted to investigate the effects of organic fertilizer types (commercial organic fertilizer, cow manure, and sheep manure), the ratios of organic-to-inorganic fertilizers on soil properties, and the sunflower growth and development in mildly and moderately salinized farmlands. The trial was conducted in May 2022 with spring irrigation performed according to the local irrigation schedule (sunflower), applying 160 mm of water before planting. Based on the analysis of the soil physicochemical properties, the soil type was Solonetz. The test crop was sunflower, specifically the Tongqing No. 6 variety, which was provided by Jiuquan Tongqing Seed Industry Co., Ltd. (Jiuquan, China). Two types of salinized farmland were selected: mildly saline F1 (salinity < 2‰) and moderately saline F2 (2‰ < salinity < 4‰) [14,15]. Three types of organic fertilizers were used in the experiment, including cow dung, sheep dung, and commercial organic fertilizer. The OM contents of sheep manure (SF), cow manure (CF), and commercial organic fertilizer (PF) were 32.85, 48.64, and 16.48%, respectively; the TN contents were 2.15, 1.95, and 11.58%, respectively; and the OM were 1.63, 1.57, and 11.76%, respectively. Four ratios of the organic N fertilizer to inorganic fertilizer were established: 25, 50, and 75%. The experimental design adopted was a completely randomized block design, where the experiment was divided into 2 blocks based on soil type (2 types). A random allocation of treatments was carried out within each block, where combinations of 4 different ratios of organic manure and 3 types of organic fertilizers are randomly assigned, along with 1 blank control treatment and 1 conventional fertilization treatment. This resulted in a total of 28 treatments. The organic fertilizer was substituted with urea as the base fertilizer at a rate of 60 kg/hm². The top-up urea (N 46%) was applied at twice the amount of the base urea. By substituting a portion of the urea with organic fertilizer, the total N input was reduced. The experimental plots were of a 10 m × 6.5 m size.

Table 2. The 2022 experimental design.

Treatment	Seedling Stage (Calculation of the Pure Nitrogen Content)		Flowering Stage
	Organic Fertilizer kg/hm2	Urea kg/hm2	Urea kg/hm2
CK (control)	0	0	0
NF (constant fertilizer)	0	60	120
CF100 (constant cow manure)	180	0	0
SF100 (constant sheep manure)	180	0	0
PF100 (constant commercial organic fertilizer)	180	0	0
CF25 (25% CF + 75% urea)	15	45	90
CF50 (50% CF + 50% urea)	30	30	60
CF75 (75% CF + 25% urea)	45	15	30
SF25 (25% SF + 75% urea)	15	45	90
SF50 (50% SF + 50% urea)	30	30	60
SF75 (75% SF + 25% urea)	45	15	30
PF25 (25% PF + 75% urea)	15	45	90
PF50 (50% PF + 50% urea)	30	30	60
PF75 (75% PF + 25% urea)	45	15	30

Note: The OM (%), TN (%), and TP (%) of the SF were 32.85, 2.15, and 1.63, respectively, while those of the CF were 48.64, 1.95, and 1.57, respectively. For PF, the corresponding values were 16.48, 11.58, and 11.76, respectively.

presents the detailed experimental design. For the constant chemical fertilizer treatment (NF) and constant organic fertilizer treatments (CF100, SF100, and PF100), the specific fertilizer application rate was 180 kg/hm² of N fertilizer per crop season (calculated as N), with P and K fertilizers applied once before sunflower sowing for the entire year.

2.2. Sample Collection and Measurement Methods

2.2.1. Measurement and Related Calculations of the Physical and Chemical Properties in the Soil

The soil sampling method used was random point layout, where the soil cores were used to collect samples from 0–20, 20–40, and 40–60 cm depths. The soil redox potential (Eh) was measured using the potentiometric method [16]. A portion of the soil samples was placed in aluminum boxes, dried at 105 °C, and then weighed to determine the soil moisture content using the oven-drying method [17]. The remaining soil samples were air-dried indoors for future testing. Soil samples were collected every 15 days during the growing season, with 3 repetitions for each treatment. The dried soil samples were then subjected to laboratory tests, where a 1:5 soil extract was prepared. The electrical conductivity (EC) and pH of the extract were measured [18]. The Ec of the soil solution was determined using the potential difference method with platinum and saturated calomel electrodes, while the soil pH was measured using a pH meter with the potentiometric method [19].

SSC = 2.2811 × 0.001 × EC − 0.0015,

(1)

where SSC (g/kg): 0–60 cm of soil salinity content.

$$SWC={\displaystyle \sum }BD\times h\times SWC,$$

(2)

where SSC represents the soil salt content within the 0–60 cm depth of the soil, BD represents soil bulk density (g·cm⁻³), SWC represents the soil mass moisture content (cm³·cm⁻³) in the 0–60 cm layer, and ‘h’ represents the soil layer thickness (cm).

$$SWDI=\left({\displaystyle \frac{\theta -{\theta }_{FC}}{{\theta }_{AWC}}}\right)10.$$

(3)

In the above formula, SWDI stands for the Soil Water Deficit Index, θ(%) represents the soil moisture content, θ_FC(%) represents the soil field capacity, and θ_AWC represents the available soil water capacity. The reference value was determined based on the soil texture with a typical value of 0.12 [20].

2.2.2. Determination of the OM, TN, DON, DOC, MBC, MBC, and MBN

The soil’s basic chemical–biological parameters indicators were measured following the methods outlined in “Soil Agrochemical Analysis Methods” [21]. The analysis of soil OM was obtained by measuring the SOC and then multiplying it by a certain conversion factor of 1.724. The soil sample was digested with potassium dichromate under heating, which oxidized the C in the OM into C dioxide while the dichromate ions were reduced to trivalent chromium ions. The remaining potassium dichromate was titrated with a standard solution of ammonium ferrous sulfate, and then the change in the amount of dichromate ions before and after the oxidation of SOC was used to calculate the OM [22]. TN, NH₄⁺-N, and NO₃⁻-N were determined using the Kjeldahl method and indophenol blue colorimetry [23,24,25], and the soil samples are mixed with concentrated sulfuric acid and catalysts (such as copper sulfate and potassium sulfate) and then digested at high temperature to convert the organic N into NH₄⁺. After cooling the digested samples, excess sodium hydroxide was added to convert the NH₄⁺-N into ammonia gas, which was then collected in a boric acid solution through a distillation apparatus. The collected ammonia reacted with boric acid to form ammonium borate, which was then titrated with a standard hydrochloric acid (0.1 mol/L). The TN was calculated based on the amount of hydrochloric acid consumed. Using a 2 mol/L potassium chloride solution to extract the soil, the NH₄⁺ adsorbed on the soil colloids and water-soluble NH₄⁺-N were extracted. In a strong alkaline medium, the NH₄⁺-N present in the soil extract reacted with hypochlorite and phenol to form the water-soluble dye indophenol blue. Within a N concentration range of 0.005 mol/L to 0.5 mol/L, the absorbance was directly proportional to the NH₄⁺-N content, allowing for its determination through a colorimetric method. At the wavelength of 625 nm, the absorbance was measured with a 1 cm absorber dish, and the corresponding NH₄⁺-N was found from the working curve. After extracting the soil with a saturated calcium sulfate solution, a portion of the extract was evaporated to dryness under slightly alkaline conditions. The residue was then treated with phenol disulfonic acid, and the NO₃⁻-N was colorimetrically determined at a wavelength of 420 nm. MBC and N MBN were measured using the chloroform fumigation-extraction method [26]. By fumigating the soil with chloroform to kill the microorganisms, the total amount of extractable OM was measured. The microbial biomass was calculated by subtracting the value measured from the non-fumigated soil from that of the fumigated soil. The DOC and DON were measured using ultraviolet spectrophotometry [27], and the soil sample was mixed with deionized water or an extractant and then agitated to obtain a clarified extract. A UV-Vis spectrophotometer was used, with the wavelength set at 254 nm (typically used for measuring the DOC) and 280 nm (typically used for measuring the DON).

The formulas for the accumulation of the OM, TN, NH₄⁺-N, and NO₃⁻-N in the 0–40 cm soil layer were obtained by utilizing the soil bulk density, multiplying it with the soil indicators from the 0–20 cm and 20–40 cm layers, and then accumulating the results by addition:

$$Microbial \mathrm{biomsass} \mathrm{C}:\mathrm{N} \mathrm{ratio}=MBC/MBN,$$

(4)

$$C/N=SOC/TN.$$

(5)

2.2.3. Determination of the SOC Functional Group Structure

Organic C functional groups are used to characterize the molecular structure of the SOC [28], and FTIR (Fourier transform infrared spectroscopy) technology is commonly used for analysis and the functional groups that mainly originate from aliphatic C, aromatic C, and polysaccharide C [29]. The organic functional groups used in different characteristic intervals were based on the existing literature [30]: aliphatic C: 2800–3010 cm⁻¹; aromatic C: 1580–1660 cm⁻¹; polysaccharide C: 1520–1546 cm⁻¹; and alcohols and phenols: 3620, 3448 cm⁻¹ [31,32]. Characteristic peaks: aliphatic C at 2930, 2925, and 2853 cm⁻¹; aromatic C at 1644, 1623, and 1620 cm⁻¹; polysaccharide C at 1530, 1163, 1073, and 1034 cm⁻¹; alcohols and phenols at 1879 and 1799 cm⁻¹; alcohol ethers at 1159; and silicates at 997 and 1030 cm⁻¹ [33,34,35]. The characteristic peaks in the infrared spectrum of the soil were identified to classify the organic functional groups as aliphatic C, aromatic C, polysaccharide C, alcohols, and phenols. The corrected peak areas were calculated by integration, dividing the corrected area of each peak by the sum of the areas of the five peaks, and then multiplied by 100 to obtain the relative peak areas. Careful preselection of the peaks was required to avoid strong mineral interference [36].

2.3. Statistical Analysis

Using SPSS Statistics 26, two-way ANOVA and repeated measures ANOVA were performed to analyze the effects of the organic fertilizer ratio and type on the monitoring indicators at different time points. First, the Shapiro–Wilk test was used for normality, and if p > 0.05, then the null hypothesis that the data come from a normal distribution could not be rejected. A normality test for variance was conducted. Marginal means were calculated and compared using the LSD method. Meeting the 95% confidence interval indicates that the test results have a statistically significant difference, which provides more accurate estimates than simple means [37]. Using Origin 2024, area integration was performed for each characteristic peak of the functional groups, and the average values, proportions, and graphs were calculated and plotted. Infrared peak area calculations were performed using a fitting method, which fits the actual spectral data to one or more Gaussian peaks and calculates the area under the fitted curve [36].

3. Results

3.1. Effects of the Organic Fertilizer Type and Ratio on the Water–Salt Characteristics of Salinized Soil

The average soil moisture content and EC of the soil layers during the crop growth period were calculated. The results are shown in Figure 1 and Figure 2. The application of organic fertilizers was observed to increase the soil moisture content in the 0–20 cm layer while reducing it in the 20–60 cm layers. Compared to conventional fertilization practices, the application of organic fertilizers primarily led to a decrease in the soil moisture content in the 0–40 cm soil layer. Additionally, the application of organic fertilizers was found to reduce the soil EC values. Compared to the conventional fertilization treatment, CF50, SF50, and PF50 reduced the soil salinity by 9.6, 13.5, and 5.1%, respectively, in the 0–60 cm layer of F1, as well as by 44, 23.3, and 26.2%, respectively, in F2.

Two-way ANOVA and repeated measures ANOVA were used, as shown in

Table 3. The significant differences in the marginal mean values of the SSC, SWC, SWDI, pH, Eh, OM, TN, C/N, NH₄⁺-N, NO₃⁻-N, DOC, DON, DOC/DON, MBC, MBN, MBC/MBN, MBC/SOC, and MBN/TN repeats in the mild saline–alkali land under different types of organic fertilizers and their ratios in 2022.

		SSC	SWC	SWDI	pH	Eh	OM	TN	C/N	NH4+-N	NO3−-N	DOC	DON	DOC/DON	MBC	MBN	MBC/MBN	MBC/SOC	MBN/TN
Proportion of Organic Fertilizer	0	2.62c	138.96a	−7.91a	8.49a	285.00a	7.46a	0.51d	8.93a	90.4a	59.6a	260b	26e	1.85a	118.40d	9.50d	12.92a	1.38c	1.17e
	25	2.53d	129.41e	−9.12d	8.50a	279.74c	7.37c	0.62a	6.86d	79.5b	53.9b	332e	39b	1.46e	160.50b	23.00a	7.00b	1.60b	2.32a
	50	2.47e	131.07d	−8.74b	8.47a	283.93a	7.20d	0.58b	7.16c	77.6c	53.7b	354c	39c	1.63c	234.20a	17.90b	13.14a	3.05a	1.64b
	75	2.67b	133.74b	−8.72b	8.45a	282.10b	6.96e	0.55c	7.35b	68.5e	55.5b	341d	32d	1.82b	111.20e	13.70c	8.16b	1.34c	1.44c
	100	2.82a	132.91c	−8.99c	8.45a	283.36b	7.38b	0.62a	6.80e	75.5d	61.0a	365a	42a	1.52d	134.20c	16.90b	7.96b	1.59b	1.32d
Type of Organic Fertilizer	CF	2.53c	126.57e	−9.62d	8.49a	277.90d	6.95e	0.56c	7.16d	68.6e	51.1b	377a	36b	1.84b
	NF	2.71a	144.81a	−6.98a	8.50ab	280.58c	7.89a	0.62a	7.32b	85.7b	65.1a	297b	27d	2.17a	160.03a	17.88a	9.07c	1.90a	1.68a
	CK	2.53c	133.10d	−8.84c	8.48ab	289.42a	7.04d	0.39d	10.54a	95.1a	54.0c	223d	25e	1.52d	116.7b	10.8b	10.92b	1.39b	1.13b
	PF	2.68a	132.27c	−8.87c	8.44b	286.23b	7.17c	0.62a	6.68e	83.0c	50.4d	295c	36c	1.40e	120.10b	8.20b	14.92a	1.36b	1.21b
	SF	2.65b	136.50b	−8.18b	8.47ab	282.72c	7.56b	0.59b	7.29c	74.2d	66.5a	372a	43a	1.59c
Proportion of Organic Fertilizer		***	***	***	NS	***	***	***	***	***	***	***	***	***	***	***	***	***	***
Eta		0.978	0.989	0.959	0.104	0.455	0.998	0.93	0.794	0.989	0.739	0.981	0.998	0.999	0.996	0.832	0.79	0.995	0.977
Type of Organic Fertilizer		***	***	***	NS	***	***	***	***	***	***	***	***	***	NS	NS	***	NS	NS
Eta		0.936	0.999	0.997	0.148	0.827	0.999	0.973	0.975	0.995	0.951	0.998	0.997	1	0.138	0.273	0.569	0.043	0.182
Proportion of Organic Fertilizer		***	***	***	NS	***	***	***	***	***	***	***	***	***
Eta		0.927	0.995	0.993	0.198	0.452	1	0.603	0.956	0.996	0.939	0.991	0.999	1

Note: SSC (g/kg): soil salinity content at 0–60 cm depth; CF: cow manure; NF: N fertilizer (e.g., urea); CK: control (blank); PF: commercial organic fertilizer; and SF: sheep manure. SWC (mm): the SWC at 0–60 cm depth; and SWDI: the SWDI at 0–60 cm depth. pH: the soil pH at 0–60 cm depth; Eh (mV): the Eh at 0–60 cm depth; OM (t/hm²): the OM at 0–40 cm depth; TN (t/hm²): the TN in soil at 0–40 cm; and C/N: the soil C-to-N ratio at 0–40 cm. 0, 25, 50, 75, and 100: proportions of the organic fertilizer in the soil. *** significant at the 95% confidence interval, where NS indicates no significant difference. The letters after the numbers represent the results of LSD comparison and analysis among treatments in ANOVA, and different letters indicate significant differences. NH₄⁺-N (Kg/hm²): the NH₄⁺-N accumulation at 0–40 cm; NO₃⁻-N (Kg/hm²): the NO₃⁻-N accumulation at 0–40 cm; DOC (Kg/hm²): the DOC accumulation in soil at 0–40 cm; DON (Kg/hm²): the DON accumulation in soil; DOC/DON: the DOC-to-DON ratio in soil at 0–40 cm; MBC (Kg/hm²): the MBC accumulation at 0–20 cm; MBN (Kg/hm²): the MBN accumulation at 0–20 cm; MBC/MBN: the MBC-to-MBN ratio; MBC/SOC (‰): the contribution rate of MBC in SOC; and MBN/TN (‰): the contribution rate of the MBN in TN.

and

Table 4. The significant differences in the marginal mean of the SSC, SWC, SWDI, pH, Eh, OM, TN, C/N, NH₄⁺-N, NO₃⁻-N, DOC, DON, DOC/DON, MBC, MBN, MBC/MBN, MBC/SOC, and MBN/TN repeats in the different types of organic fertilizers and their ratios in moderate saline–alkali land in 2022.

		SSC	SWC	SWDI	pH	Eh	OM	TN	C/N	NH4+-N	NO3−-N	DOC	DON	DOC/DON	MBC	MBN	MBC/MBN	MBC/SOC	MBN/TN
Proportion of Organic Fertilizer	0	4.29a	150.25d	−9.20d	8.57a	277.63d	6.63a	0.44c	8.77a	59.9e	58.8c	254e	21e	2.42a	108.55b	9.10b	12.06a	1.32b	1.21c
	25	3.17d	147.91e	−9.33e	8.46b	281.99c	5.58e	0.49b	6.65d	72.0b	56.0d	295d	27d	1.93c	84.10d	9.50b	8.99b	1.25bc	1.12c
	50	2.88e	151.27c	−9.09c	8.51ab	284.40b	6.34b	0.45c	8.36b	63.7d	61.9b	303c	30b	1.79d	116.20a	15.00a	7.77b	1.21c	2.22a
	75	3.23c	155.18b	−8.73b	8.44b	284.15b	6.10d	0.45c	7.94c	73.4a	54.4d	327b	29c	2.09b	83.20d	10.90ab	7.69b	1.22c	1.31b
	100	3.35b	156.55a	−8.60a	8.47b	287.53a	6.17c	0.52a	6.81d	65.9c	67.7a	339a	38a	1.62e	99.10c	9.60ab	10.45ab	1.76a	0.93d
Type of Organic Fertilizer	CF	2.48e	147.82c	−9.36c	8.43b	287.40a	6.26c	0.42c	8.64b	66.7d	56.4c	325a	30b	1.92d
	NF	4.19b	158.87a	−8.39a	8.56a	282.17b	6.39b	0.46b	8.34bc	69.7b	66.3a	268d	25d	2.17b	124.20a	10.00a	12.57a	1.35	1.24b
	CK	4.40a	141.64d	−10.01d	8.58a	273.08c	6.87a	0.43c	9.21a	50.2e	51.4d	241e	17e	2.68a	95.65b	11.25ab	8.73b	1.36	1.40a
	PF	3.98c	151.44b	−9.08b	8.51ab	280.42b	5.89e	0.51a	6.82d	67.5c	61.7b	302c	28c	1.94c	92.90b	8.20b	11.54a	1.29	1.18b
	SF	3.02d	158.93a	−8.38a	8.48b	285.74a	6.01d	0.51a	6.86c	72.1a	62.0b	322b	34a	1.71e
Proportion of Organic Fertilizer		***	***	***	NS	***	***	***	***	***	***	***	***	***	***	***	NS	***	***
Eta		0.984	0.997	0.996	0.208	0.798	0.999	0.92	0.962	0.991	0.899	0.99	0.998	1	0.953	0.689	0.437	0.955	0.986
Type of Organic Fertilizer		***	***	***	***	***	***	***	***	***	***	***	***	***	***	NS	NS	NS	NS
Eta		0.999	0.999	0.985	0.281	0.978	0.998	0.951	0.973	0.988	0.836	0.977	0.996	0.999	0.932	0.153	0.076	0.153	0.111
Type of Organic Fertilizer Proportion of Organic Fertilizer		***	***	***	NS	***	***	***	***	***	***	***	***	***
Eta		0.998	0.997	0.987	0.125	0.556	0.999	0.829	0.89	0.998	0.781	0.845	0.995	0.999

Note: SSC (g/kg): soil salinity content at 0–60 cm depth; CF: cow manure; NF: N fertilizer (e.g., urea); CK: control (blank); PF: commercial organic fertilizer; and SF: sheep manure. SWC (mm): the SWC at 0–60 cm depth; and SWDI: the SWDI at 0–60 cm depth. pH: the soil pH at 0–60 cm depth; Eh (mV): the Eh at 0–60 cm depth; OM (t/hm²): the OM at 0–40 cm depth; TN (t/hm²): the TN in soil at 0–40 cm; and C/N: the soil C-to-N ratio at 0–40 cm. 0, 25, 50, 75, and 100: proportions of the organic fertilizer in the soil. *** significant at the 95% confidence interval, where NS indicates no significant difference. The letters after the numbers represent the results of LSD comparison and analysis among treatments in ANOVA, and different letters indicate significant differences. NH₄⁺-N (Kg/hm²): the NH₄⁺-N accumulation at 0–40 cm; NO₃⁻-N (Kg/hm²): the NO₃⁻-N accumulation at 0–40 cm; DOC (Kg/hm²): the DOC accumulation in soil at 0–40 cm; DON (Kg/hm²): the DON accumulation in soil; DOC/DON: the DOC-to-DON ratio in soil at 0–40 cm; MBC (Kg/hm²): the MBC accumulation at 0–20 cm; MBN (Kg/hm²): the MBN accumulation at 0–20 cm; MBC/MBN: the MBC-to-MBN ratio; MBC/SOC (‰): the contribution rate of MBC in SOC; and MBN/TN (‰): the contribution rate of the MBN in TN.

, in F1 and F2. The soil moisture content was observed to be 75 > 50 > 25% across different organic fertilizer ratios, and, in terms of organic fertilizer types, the order was SF > PF > CF. The application of organic fertilizer reduced the salt content in the 0–60 cm soil layer. In F1, the performance of the different proportions of organic fertilizer was in the order of 100 > 75 > 25 > 50%, and the performance of the different types of organic fertilizer was in the order of NF > PF > SF > CF > CK. Compared to the conventional fertilization treatment, the application of PF, SF, and CF reduced the salinity by 1.1, 2.21, and 6.64%, respectively. In F2, the performance of the different proportions of organic fertilizer was in the order of 100 > 75 > 25 > 50%, and the performance of the different types of organic fertilizer was in the order of CK > NF > PF > SF > CF. Compared to the conventional fertilization treatment, the application of PF, SF, and CF reduced the salinity by 4.53, 27.9, and 40.8%, respectively.

3.2. Effects of the Organic Fertilizer Type and Ratio on the Chemical Properties and Nutrients of Salinized Soil

An analysis of the pH and Eh in the 0–20, 20–40, and 40–60 cm soil layers across different salinized farmlands during the entire growth period of the sunflowers under urea N reduction with different organic fertilizer application ratios was carried out. As shown in Figure 3 and Figure 4, the coefficient of variation for the soil pH and Eh values between the different soil layers was under 0.01. Therefore, the average values of the three soil layers were calculated to represent the pH and Eh of the 0–60 cm soil, and a repeated measures ANOVA was performed on the marginal means at different monitoring time points to analyze the significant differences

Two-way ANOVA and repeated measures ANOVA were used, as shown in Table 3 and Table 4. Compared with conventional fertilization, the application of the organic fertilizer reduced the soil pH in F2, with the order of different organic fertilizer types being PF > SF > CF > NF. By analyzing the marginal mean values of the three soil layers monitored at different time points during the growth period, it was found that, in F1, the order was 25 > 50 > 75 = 100%, and the order of the different organic fertilizer types was NF > CF > SF > PF. In F2, the order was 50 > 25 > 100 > 75%, with the order of organic fertilizer types being NF > PF > SF > CF. The application of organic fertilizer to replace part of the inorganic fertilizer under N reduction reduced the soil pH and prevented excessive soil alkalization. In F1, the order of the different organic fertilizer types in terms of the soil Eh value was PF > SF > CF > CF. Compared to conventional fertilization, the application of PF and SF increased the soil Eh by 2 and 0.8%, respectively, while the CF decreased it by 1%. In F2, the order was CF > SF > CF > PF. The application of CF and SF increased the soil Eh by 1.9 and 1.3%, respectively, whereas the application of PF decreased it by 0.6%. The redox properties of the soil treated with different types of organic fertilizers were significantly affected by the initial salt content, leading to different soil Eh values.

A statistical analysis of the cumulative amounts of the OM and TN in the 0–40 cm soil layer on 20 June, 20 July, and 25 August during the growth period, as shown in Figure 5 and Table 3, demonstrated that the proportion and type of the organic fertilizer had significant differences in the OM, TN, and C/N ratio, with the type of organic fertilizer having the greatest impact. Under different ratios of organic-to-inorganic fertilizer, the OM and TN in the 0–40 cm soil layer were in the order of 100 > 25 > 50 > 75%. There was no significant difference in the soil’s TN content between the 25% organic fertilizer ratio and full organic fertilizer application during the growth period. The C/N ratios were in the order of 25 > 50 > 75 > 100%. The analysis demonstrated that the soil C/N ratio was less than 0–15, indicating that, as the amount of organic fertilizer increased, the release of N accelerated, which was beneficial for plant N absorption. The TN and OM content decreased with an increase in the organic fertilizer proportion (whereas the TN content decreased). Compared with conventional fertilization, the application of different CF, SF, and PF ratios under N reduction conditions reduced the OM by 11.9, 4.2, and 9.1%, respectively; reduced the TN by 9.7, 4.8, and 0%, respectively; and reduced the soil C/N by 2.2, 0.4, and 8.7%, respectively.

As shown in Table 4, under different ratios of the organic-to-inorganic fertilizer application in F2, the OM and TN values demonstrated trends in the order of 50 > 100 > 75 > 25% and 100 > 25 > 50 > 75%, respectively. There was no significant difference in the soil’s TN content between the 50% organic fertilizer ratio and 75% organic fertilizer application during the growth period. The C/N ratios were in the order of 50 > 75 > 100 > 25%. Compared with NF, the application of the different CF, SF, and PF ratios under N reduction conditions reduced OM by 15.3, 18.7, and 20.3%, respectively, as well as increased the TN by −8.7, 10.9, and 10.9%, respectively. The soil C/N ratios were reduced by −3.6, 17.7, and 18.2%, respectively.

3.3. Impact of the Organic Fertilizer Types and Ratios on the Inorganic Nitrogen and Active Organic Carbon and Nitrogen Components in the Salinized Soil

Figure 6 and Figure 7, as well as Table 3 and

Table 5. The soil 0–20 cm MBC and MBN content under different organic fertilizer ratios in 2022.

	MBC	MBN	MBC/MBN	MBC/SOC	MBN/TN		MBC	MBN	MBC/MBN	MBC/SOC	MBN/TN
F1PF25	160.5	23.0	7.0	1.60	2.32	F2PF25	84.1	9.5	8.9	1.25	1.12
F1PF50	234.2	17.9	13.1	3.05	1.64	F2PF50	116.2	15.0	7.7	1.21	2.22
F1PF75	111.2	13.7	8.1	1.34	1.44	F2PF75	83.2	10.9	7.6	1.22	1.31
F1PF100	134.2	16.9	7.9	1.59	1.32	F2PF100	99.1	9.6	10.3	1.76	0.93
F1CK	120.1	8.2	14.6	1.36	1.21	F2CK	124.2	10.0	12.4	1.35	1.24
F1NF	116.7	10.8	10.8	1.39	1.13	F2NF	92.9	8.2	11.3	1.29	1.18

Note: MBC (kg/hm²): the MBC at 0–20 cm; MBN (kg/hm²): the MBN at 0–20 cm; and the MBC/MBN, MBC/SOC (‰), and MBN/TN (‰): same as Table 3.

, show the results of an analysis of the average values of the NH₄⁺-N, NO₃⁻-N, DOC, DON, MBC, and MBN in the 0–20 and 20–40 cm soil layers during the growth period of the salinized farmland. As shown in Table 3, under different fertilizer application ratios in F1, the NH₄⁺-N and NO₃⁻-N levels demonstrated no significant differences. The different types of organic fertilizers demonstrated the following orders: NF > PF > SF > CF and SF > NF > CF > PF. Compared to conventional fertilization, the application of different CF, SF, and PF ratios under N reduction conditions reduced the soil NH₄⁺-N by 20, 13.4, and 3.2%, respectively, as well as reduced the soil NO₃⁻-N by 21.5, −2.6, and 22.6%, respectively. The DOC and DON levels demonstrated the following orders: 50 > 75 > 25% and 75 > 50 > 25%. Compared to NF, the application of different CF, SF, and PF ratios under N reduction conditions increased the soil DOC by 26.9, 25.3, and −0.7%, respectively, as well as increased the soil DON by 33.3, 59.3, and 33.3%, respectively. The MBC and MBN levels demonstrated orders of 50 > 25 > 75% and 25 > 50 > 75%, respectively, with the MBN increasing as the proportion of organic fertilizer increased (and TN decreased). When the organic fertilizer ratio was 50%, the soil MBC and MBN contents were highest. Compared to the NF, which reduced the soil MBC and increased MBN content, the application of commercial organic fertilizer increased the MBC and MBN by 37.1 and 65.6%, respectively.

As shown in Table 4, under different fertilizer application ratios in F2, the NH₄⁺-N and NO₃⁻-N levels demonstrated orders of 75 > 25 > 50% and 50 > 25 > 75%, respectively. Compared to conventional fertilization, the application of different CF, SF, and PF ratios under N reduction conditions reduced the soil NH₄⁺-N by 20, 13.4, and 3.2%, respectively, as well as reduced the soil NO₃⁻-N by 21.5, −2.6, and 22.6%, respectively. For F2, the DOC and DON levels demonstrated trends of 100 > 75 > 50 > 25% and 100 > 50 > 75 > 25%, respectively. Under N reduction conditions with different CF, SF, and PF ratios, the soil DOC increased by 21.3, 20.1, and 12.7%, respectively; the soil DON increased by 20, 36, and 12%, respectively; and the soil DOC/DON decreased by 13, 26.9, and 11.9%, respectively. Compared to NF, the PF resulted in a lower DOC and DON accumulation in the soil. The MBC, MBN, and MBN/TN demonstrated a 50 > 75 > 25% trend. There were no significant differences in the soil MBC, MBN, or MBC/SOC between the organic fertilizer ratios of 25 and 75%; however, there were significant differences in the MBN/TN. Compared to conventional inorganic fertilizer application, the use of commercial organic fertilizer increased the MBC, MBN, MBC/SOC, and MBN/TN by 3, 37.2, 5.4, and 18.6%, respectively, whereas the MBC/MBN decreased by 24.4%. Compared to the NF, the use of commercial organic and inorganic fertilizers under normal crop growth conditions resulted in decreased soil MBC, MBC/SOC, and MBN/TN.

3.4. Impact of Organic Fertilizer Types and Ratios on the Organic Carbon Structure in Salinized Soil

Based on the characteristic absorption peaks in the infrared spectrum [34], the peak at 3620 cm⁻¹ was primarily attributed to the stretching vibration of the hydroxyl groups (C–OH) originating from plant residues [33]; the peak near 3424 cm⁻¹ corresponded to –NH [38], which was sourced from amino acids; the absorption peak at 2875 cm⁻¹ was caused by the asymmetric stretching vibration of the aliphatic CH₃ and CH₂ groups, with the aliphatic compounds being mainly derived from plant and animal biological tissues; the overtone peak at 2516 cm⁻¹ and the peak at 1797 cm⁻¹ indicated the presence of alcohols and phenols to O–H/N–H; the peak at 1644 cm⁻¹ corresponded to the stretching vibration of the C=O bonded to aromatic rings in lignin; the peak at 1434 cm⁻¹ was caused by the stretching vibration of the C=C or C=N double bonds in aromatic amides arising from microbial degradation products; the highest peak at 1032 cm⁻¹ originated from the stretching vibration of C–O bonds in polysaccharides [35] with carbohydrate compounds that were typically sourced from plant carbohydrates; and the peak at 800 cm⁻¹ may represent the out-of-plane bending vibration of CO₃²⁻, while the absorption peak at 779 cm⁻¹ was attributed to the stretching vibration of minerals.

An analysis of the changes in the five functional groups, including hydroxyl, phenolic, aromatic, carboxyl in polysaccharides, aromatic amide, and aliphatic, of the organic carbon in the surface soil during the crop-filling stage under different organic and inorganic fertilizer application ratios was carried out. As shown in Figure 8 and

Table 6. The characteristics of the changes in the relative proportions of the organic functional groups under different organic–inorganic fertilizer ratios and inorganic fertilizer ratios in 2022.

Treatment	Alcohol, Phenol	Aliphatic	Aromatic	Aromatic Amide	Carboxyl Group	Aliphatic/Aromatic	Aromatic/Aromatic Amide	Alcohol, Phenol/Aromatic Amide	Carboxyl Group/Aliphatic
	O–H/N–H	C–H	C=O/C=C	C=O/C=N	C–O
	3620/1797	2875	1644	1434	1032
F1PF25	2.39	12.13	2.53	17.01	65.94	4.79	0.15	0.14	5.43
F1PF50	7.39	28.21	3.73	13.58	47.09	7.56	0.27	0.54	1.67
F1PF75	21.69	15.39	2.58	9.84	50.49	5.96	0.26	2.20	3.28
F1PF100	11.56	15.98	5.06	16.80	50.61	3.16	0.30	0.69	3.17
F1CK	10.33	46.61	2.46	9.82	30.78	18.92	0.25	1.05	0.66
F1NF	3.67	40.19	11.70	8.73	35.72	3.44	1.34	0.42	0.89
F2PF25	10.04	26.14	2.26	11.86	49.70	11.57	0.19	0.85	1.90
F2PF50	7.46	25.84	2.67	13.31	50.72	9.68	0.20	0.56	1.96
F2PF75	9.31	13.88	2.36	13.33	61.12	5.87	0.18	0.70	4.40
F2PF100	9.00	45.99	2.40	11.54	31.07	19.18	0.21	0.78	0.68
F2CK	35.81	16.98	2.87	9.21	35.13	5.91	0.31	3.89	2.07
F2NF	6.52	19.89	2.86	14.14	56.59	6.95	0.20	0.46	2.85

Note: Unit is %. Alcohols, phenols, aliphatics, aromatics, aromatic amides, and carboxyl groups are represented as O–H/N–H, C–H, C=O/C=C, C=O/C=N, and C–O, respectively.

, compared to conventional fertilization treatment, applying PF in F1 increased the soil organic functional groups of the aromatic amides and carboxyl groups by 12.7 to 94.8 and 31.8 to 84.6%, respectively. Conversely, the aliphatic and aromatic groups decreased by 29.7 to 61.7 and 68.1 to 78.4%, respectively. Notably, the proportions of the aromatic amides and carboxyl groups decreased as the proportion of organic fertilizer (TN) increased. In F2, the combined application of the different proportions of PF and inorganic fertilizer reduced the proportions of the aromatic compounds and aromatic amides by 6.6 to 17.5 and 5.7 to 16.1%, respectively, while increasing the content of the alcohols and phenols by 14.4% to 54%. When the proportion of organic fertilizer was ≤50%, it increased the aliphatic and aliphatic/aromatic by 29.9–31.4 and 39.3–66.5%, respectively, while reducing the carboxyl and carboxyl/aliphatic by 10.4–12.2 and 31.2–33.3%, respectively.

4. Discussion

The application ratio and type of organic fertilizer significantly affected the soil moisture, salinity, and water deficit in the 0–60 cm layer during the growing period. Among these factors, the initial soil salinity had the greatest impact on the soil salinity accumulation. The effect of the organic fertilizer ratio on the soil salinity changes was greater than that of the organic fertilizer type. Our results indicate that the substitution of partial urea with organic fertilizer in saline farmlands can reduce salt accumulation. This may be due to the enhanced soil permeability and water retention capacity resulting from the application of organic fertilizer [39], or it could be due to the impact of organic fertilizer on the soil structure by the lowering of soil pH levels, which reduces salt accumulation [40]. Under the different types of organic fertilizers, the soil salinity showed a lower level when the SF was applied compared to the PF. This may be due to the fact that crops fertilized with PF required a greater amount of water, and, during the process of crop water absorption, more salt was transported to the root zone.

In F1 and F2, the substitution of organic fertilizers for partial urea in the conventional fertilization treatments had varying effects on the soil salinity depending on the type and proportion of organic fertilizer used. In F1, the application of the CF, SF, and PF all resulted in decreased water content in the 0–60 cm soil layer. In F2, only SF application alleviated the soil water deficit, while the application of CF and PF led to a decrease in the average soil water content during the growing season compared to conventional fertilization. Contrary to general research findings where organic fertilizer application minimizes the water stress on crops [41], the main reason for the observed increase in the soil water deficit during the growing season in this study was due to the climatic and soil conditions in the study area. The growing of sunflowers in this region was characterized by a prolonged state of water deficit [42]. The application of organic fertilizer enhanced the crop growth, leading to increased water demand. Therefore, during the application of organic fertilizer, the soil water deficit increased during the growing season. Additionally, due to the terrain and climate of the experimental area, excessive irrigation resulted in soil salt return during the crop growth period and lodging during adverse weather conditions. Therefore, it is crucial to pay attention to soil water and N management during sunflower cultivation in this region.

The application of organic–inorganic fertilizer combined with the N reduction in F1 had no significant impact on the soil acidity and alkalinity. Partially replacing inorganic fertilizer with organic fertilizer for N reduction decreased the soil pH and prevented excessive alkalization [43]. Using a full-dose organic fertilizer, compared to a combination of organic and inorganic fertilizers, increases the soil pH and oxidative properties [44]. Extremely high or low soil pH is detrimental to plant growth. High acidity (below 5.5) can trigger the toxic aluminum or manganese release, inhibiting crop root growth [45], whereas intense aluminum ions can weaken the root absorption of scarce micronutrient cations, such as copper, iron, manganese, and zinc [46]. The level of Eh depends on the relative concentrations of the oxidized and reduced substances in the soil solution. The main factors that influence Eh include soil aeration, soil moisture conditions, the metabolic processes of plant roots, the content of readily decomposable OM in the soil, and microbial activity. The initial soil salinity, the proportion of organic fertilizer, and the type of organic fertilizer have significant differential effects on the Eh. In this study, the application of organic fertilizer increased the Eh, which is beneficial to the decomposition of the OM in the soil [47]. Therefore, this may result in no significant increase in the accumulation of the OM during the growth period of crops in the study of saline–alkali land in arid areas when compared with conventional fertilization.

Water management in saline–alkali soils may be constrained by climatic and topographical factors that affect the stability and decomposition rate of the OM. In contrast to conventional inorganic fertilizer treatments, the application of organic fertilizer to saline–alkali soils for sunflower cultivation resulted in reduced OM, which is contrary to some studies that have suggested that organic fertilizer increases OM [48]. The environmental conditions of saline–alkali soils (e.g., salinity, poor aeration, and low oxygen) may affect the OM stability and accumulation. Limited water management in saline–alkali soils can affect soil OM stability and decomposition rates if increased crop water demand after organic fertilizer application alters the soil moisture content. Soil salinity inhibited the DOC and DON components, such as DON and MBN, which is similar to previous studies on the influence of soil pH on soil microbial community diversity [49,50]. Compared to conventional fertilization treatments, the application of organic fertilizer under N reduction conditions reduced the NO₃⁻-N and increased the NH₄⁺-N, which is consistent with studies on continuous biochar and organic fertilizer application in the saline–alkali farmlands in northern China [51]. NH₄⁺-N is often more readily absorbed and utilized by plants (dryland crops) than NO₃⁻-N [52]. Thus, in organic fertilizer-mediated N reduction, increased soil NH₄⁺-N may lead to an increased plant utilization of NH₄⁺-N and reduced NO₃⁻-N [53]. Compared with commercial organic fertilizers, natural organic fertilizers increase the DOC and DON accumulation in saline–alkali soils. Existing research has shown that long-term fertilization significantly affects soil-active organic carbon components, which are the primary drivers of changes in soil enzyme activity, thus further influencing organic carbon decomposition and nutrient cycling [54]. This provides nutrients and growth-essential OM for soil microorganisms, thereby promoting microbial proliferation and growth, as well as increasing the MBC and MBN contents and affecting the soil nutrient cycling rates [55].

The application of organic fertilizer increased the content of the OM and altered the composition of SOC, while the N as an inorganic fertilizer treatment did not significantly differ in the chemical properties. The quality, function, and stability of SOC depend on its chemical composition and molecular structure, which can be reflected by the characteristics of SOC functional groups [56]. Aliphatic C groups, aromatic C groups, and polysaccharides in organic fertilizers increase the content of proteins and carbohydrates in soil OM after the composting process [57]. This affects the soil cation exchange and metal complexation reactions, biological activity, structural stability, as well as enhances the soil’s water retention capacity [57]. Organic fertilizers can improve unstable organic carbon functional groups and compounds but may increase insoluble OM [58]. In F1, organic fertilizer application results in more stable OM and more active microbial decomposition [59], reducing the impact of saline–alkali conditions on SOC mineralization [55], thus indicating faster C cycling rates [34,60].

However, the statement that organic fertilizers increase insoluble OM contradicts earlier information. In F1, the OM became more stable, and the microbial decomposition was more active after organic fertilizer application [57], reducing the effect of the saline–alkali conditions on SOC mineralization [61], thus indicating faster C cycling rates. As the proportion of organic fertilizer increased in F1, the microbial decomposition activity reflected a greater C cycling rate. Applying commercial organic fertilizers increased the soil aromatic contributions, enhanced the presence of refractory OM, stabilized OM, and reduced nitrogenous functional group contributions for the OM originating from terrestrial plant litter. In F2, as the organic fertilizer proportion increased (with reduced TN), the microbial decomposition activity decreased along with the C cycling rates owing to the salt and water stress impacting soil microbial activity [18]. Compared to the natural organic fertilizers, the commercial organic fertilizers reduced the OM aromatic and aromatic amide contributions, decreased the refractory OM, and increased the OM stability.

5. Conclusions

The application of organic fertilizer in the place of partial urea in salinized farmland can reduce soil salinity in the 0–60 cm layer, lower soil pH, prevent excessive soil alkalization, and enhance soil oxidation. Compared to natural farm manure, commercial organic fertilizers accumulate less soil salinity and are more suitable for large-scale use in saline–alkali soils. Full-dose organic fertilizer use, compared to the combined application of organic and inorganic fertilizers, may increase soil pH and oxidation, potentially exacerbating soil alkalization.

Replacing some inorganic fertilizers with organic fertilizers while reducing the TN content enhanced the accumulation of the OM content during the growing season. Compared to commercial organic fertilizers, the natural organic fertilizers increased the accumulation of the DOC and DON in the saline–alkali soils, providing energy for microorganisms. An analysis of the soil surface organic functional groups during the crop-filling stage revealed that the soil salinity affected the structure of the organic carbon molecules. In the mildly saline–alkali soil during the filling stage, as the organic fertilizer proportion increased, the microbial decomposition became more active, reflecting a faster C cycling rate. The application of commercial organic fertilizers enhanced the stability of the OM. In moderately saline–alkaline soils, the microbial decomposition activity decreased with lower proportions of organic fertilizer, similar to the C cycling rate. Commercial organic fertilizers also increased the OM stability, with more OM originating from the microorganisms and an increased C cycling rate. Therefore, soil quality assessments must consider the varying proportions and types of organic fertilizers, and they should integrate the soil water–salt balance, nutrient levels, and microbial properties to determine the optimal proportions of mild-to-moderate organic fertilizer use.