Evaluating the Sustainability of Wheat–Maize System with a Long-Term Fertilization Experiment

Abstract

In light of the issue concerning excessive fertilization that prevails in the Huang-Huai-Hai Plain, through conducting a 13-year long-term positioning experiment, the sustainability of a wheat and maize double-cropping soil system under different fertilization strategies is evaluated using the triangular area method. The objective is to establish a theoretical basis for the development and implementation of appropriate fertilization practices in the Huang-Huai-Hai Plain. In the protracted long-term experiment, chemical fertilizer (F) was taken as the control (CK) and three distinct treatments combining organic and inorganic fertilizers were used: chemical fertilizer with straw mulching (FS), chemical fertilizer with cow dung (FM), and chemical fertilizer with cow dung and straw mulching (FMS). Between 2018 and 2019, a non-fertilization treatment was concurrently incorporated in parallel on the foundation of each existing fertilization treatment. The results indicated that following prolonged fertilization, the soil nutrient content, enzyme activity, and crop yield of each organic fertilizer treatment were significantly greater than those of the chemical fertilizer treatment alone, resulting in a more stable yield. After two years of discontinuation of fertilizer cultivation, the soil fertility indexes of each treatment exhibited a notable decline. However, the rate of decrease in soil fertility indexes for the three organic fertilizer treatments was lower compared to that of the single application of chemical fertilizer treatment, suggesting that long-term allocation of organic + inorganic fertilizers contributes to better preservation of soil fertility. Through an assessment of the soil system’s sustainability under various treatments, it becomes evident that following a two-year cessation of fertilization, the sustainability indexes of the soils subjected to three long-term organic + inorganic fertilizer treatments (1.26, 1.29, and 1.27) exceeded that of the soil treated solely with chemical fertilizer (1.00). These findings provide further evidence supporting the notion that the combined application of organic and inorganic fertilizers can enhance the soil system’s capacity for sustainable production in wheat–maize farmland within the Huang-Huai-Hai Plain.

1. Introduction

The Huang-Huai-Hai Plain, serving as the primary grain-producing region in China, predominantly cultivates wheat and maize, accounting for approximately 70% of the nation’s wheat and 30% of its maize output [1]. Fertilization, recognized as a key component of field management, significantly contributes to enhancing soil productivity and sustainability [2]. Sustaining high levels of productivity and sustainability in the intensely cultivated cropping systems of this region presents a formidable challenge for agricultural production. In recent years, there has been a prevalent trend of excessive utilization of inorganic fertilizers in order to achieve increased yields and economic advantages [3]. The research indicates that China exhibits the highest fertilizer application intensity globally, approximately five times greater than the average global application intensity [4]. The overuse of fertilizers not only results in environmental issues such as soil secondary salinization and heightened greenhouse gas emissions but also escalates the economic expenses associated with crop cultivation and significantly diminishes the sustainable production potential of agricultural land [5,6]. Hence, it holds significant theoretical and practical importance for the sustainable advancement of agriculture to assess the sustainability of soil systems in order to determine the appropriateness of fertilization practices.

Organic fertilizers contain a plethora of essential macro and microelements that offer ample nutrients for plant development [7]. Moreover, they contribute significantly to enhancing soil fertility and structure, boosting crop yields, and are extensively utilized within agricultural systems [8,9]. Nonetheless, the limited effective nutrient content in organic fertilizers coupled with their slow efficiency may not suffice to support the rapid growth of crops [10], potentially resulting in a substantial decrease in crop yield when relying solely on organic materials. For instance, the research has demonstrated that the utilization of organic materials alone resulted in yield reductions of 24% and 54% compared to the use of chemical fertilizers alone and the combined application of organic and inorganic fertilizers [11]. Additionally, a study over four consecutive years of wheat–maize location experiments revealed that the yield stability of a single application of organic fertilizer was found to be inadequate, whereas the yield stability of organic fertilizer partially substituting chemical fertilizer yielded the best results [12]. The integration of organic and inorganic fertilizers can enhance the synergy between chemical and organic fertilizers, leading to increased soil enzyme activity [13], enhanced soil nutrient levels, and improved soil structure [14]. This approach not only boosts crop yields [15] but also enhances the sustainability of agricultural ecosystems [16]. Thus, the blending of organic and inorganic fertilizers represents a significant fertilizer application strategy with the capacity to promote fertilizer efficiency, yield enhancement, and overall agricultural productivity.

Soil serves as a crucial foundation for sustainable agricultural practices, with its quality playing a direct role in crop yield [17]. Assessing the effects of fertilizer application on the sustainability of soil–crop agroecosystems necessitates the utilization of scientifically valid methodologies. The sustainable yield index (SYI) of a crop serves as a key indicator in determining the ability of the soil ecosystem to support crop growth over time. In a study conducted by Zhang [18], it was observed that the sustainable yield index of maize reached a high level of 0.58 when a combination of organic and inorganic fertilizers was applied. The research has demonstrated a significant positive correlation between SYI and soil quality chemical indicators, thereby suggesting the potential utility of SYI in evaluating farmland productivity and soil sustainability [19]. However, the current use of SYI for assessing soil quality and sustainability is limited to soil or crop evaluations, which cannot objectively reflect the sustainability of agricultural systems.. Given the challenges faced by Chinese agriculture, including resource depletion and environmental degradation, there is a critical need to establish a comprehensive evaluation index for agricultural sustainability and offer recommendations for sustainable development. Hence, Kang [20] introduced the triangle area method as a novel approach to assess the sustainability of farmland soil systems by integrating soil and crop indicator attributes. This method offers a more comprehensive evaluation of farmland soil sustainability. Furthermore, up to now, there has been a paucity of research employing this method to assess the sustainability of the soil–crop system in the Huang-Huai-Hai Plain under diverse fertilization regimens.

Therefore, given the gradual impact of fertilization on soil properties and productivity, accurate conclusions regarding its effects can only be drawn through long-term field experiments. This study conducted an evaluation of the soil sustainability index through analysis of soil nutrient characteristics, soil microorganisms, crops, and other indicators under various fertilization measures, based on a long-term fertilization experiment spanning 2007 to now. The objective is to offer a theoretical foundation and technical assistance for the advancement of the green development of wheat–maize double-cropping mode, the optimal distribution and effective utilization of fertilizer resources, and the sustainable progression of double-cropping farmland soil systems in the Huang-Huai-Hai Plain region.

2. Materials and Methods

2.1. Profiles of Experimental Field

The experimental field was situated at the Experimental Station of Henan Normal University in Qianli Village, Zhaojing Town, Huojia County, Henan Province, with coordinates of 35°11′ N, 113°41′ E. The topography of the area is characterized by a flat terrain and a warm temperate continental monsoon climate. The average annual precipitation in the region is 532.3 mm, the average annual temperature is 16.0 °C, and the annual sunshine hours amount to 2311 h. The monthly precipitation data for the experimental period (2018–2020) is depicted in Figure 1, indicating that precipitation is predominantly concentrated in the summer months of June, July, and August. The crop planting scheme is the alternation of winter wheat and summer maize (on the same land), with the test field soil classified as clay loam. Immediately prior to the experiment (2007), nutrient content within the tillage layer (0–20 cm deep) of the soil was measured. The values observed were total N 1.28 g kg⁻¹, total P 0.9 g kg⁻¹, and organic matter 18.12 g kg⁻¹.

2.2. Experimental Design

This experiment began in 2007, the long-term fertilization test to carry out research. Utilizing four distinct fertilizer treatments: F(CK), nitrogen, phosphorus, and potassium fertilizer; FS, nitrogen, phosphorus, and potassium fertilizer with straw mulching; FM, nitrogen, phosphorus, and potassium fertilizer with cow dung, and FMS, nitrogen, phosphorus, and potassium fertilizer with cow dung and straw mulching (wheat and maize straw crushed and returned to the field in full). The same amount of nitrogen, phosphorus, and potassium fertilizer was applied to each fertilization treatment: wheat season N (270 kg ha⁻¹), P₂O₅ (120 kg ha⁻¹), and K₂O (180 kg ha⁻¹), with a 1:1 ratio of basal fertilizer to follow-up fertilizer. maize season N (280 kg ha⁻¹), P₂O₅ (60 kg ha⁻¹), and K₂O (60 kg ha⁻¹). In both FM and FMS treatments, cow dung was added at a rate of 4 m³ ha⁻¹ in both wheat and maize seasons, and the organic matter content of the cow dung was 40.1 g Kg⁻¹; the total N content was 25.1 g Kg⁻¹ with 12.57 g Kg⁻¹ of total phosphorus. Each treatment plot consisted of an area measuring 17.5 m² and was replicated three times. The testing period covered in this paper ranges from October 2018 to September 2020 and from four treatments into eight treatments: F, FS, FM, FMS, SF (Stop nitrogen, phosphorus, and potassium fertilizer), SFS (Stop nitrogen, phosphorus, and potassium fertilizer with straw mulching), SFM (Stop nitrogen, phosphorus, and potassium fertilizer with cow dung), SFMS (Stop nitrogen, phosphorus, and potassium fertilizer with cow dung and straw mulching). The wheat variety utilized in the study was AK 58, known for its cold and drought resistance, quality advantages, and suitability for cultivation in high middle-yielding fields. The maize variety used in the experiment was Zhengdan 958. The planting density for wheat was 2.25 million plants per hectare, while the planting density for maize was 64,000 plants per hectare.

2.3. Sample Collection and Measurement

Soil samples were collected from the 0–30 cm soil layer at various stages of growth for wheat and maize from October 2018 to September 2020 (pre-sowing, seedling, jointing, flowering, and maturity stages of wheat and seedling, jointing, flowering, and maturity stages of maize). The samples were divided into two groups: one group was air-dried and sieved (2 mm and 0.15 mm) for analysis of soil nutrient indexes, while the other group was kept fresh for analysis of soil microbial indexes.

Soil nutrient indexes: Soil nutrient indexes: the total organic carbon analyzer [21] (TOC, Elementar, Langenselbold, Germany) was utilized to determine organic matter content; the continuous flowing analyzer [21] (SEAL AA3, Ludwigshafen, Germany) was employed to determine total nitrogen and phosphorus content; the alkaline hydrolysis diffusion method [22] was used to determine alkaline hydrolysis nitrogen content; and the molybdenum antimony colorimetric method [22] was used to determine available phosphorus content.

Soil microbial indicators: the phenol-sodium hypochlorite colorimetric method for soil urease activity [23] and the 3,5-dinitrosalicylic acid colorimetric method for soil sucrase activity [23].

Crop indexes: at the maturity of wheat (1 June 2019, and 27 May 2020) were measured by randomly selecting 1 m² of wheat with uniform growth for yield measurement with three replications. Additionally, 30 wheat plants were randomly collected to assess grain number per spike, thousand-grain weight, and overall yield. Similarly, at the maturity of maize (20 September 2019, and 18 September 2020), 5 m double rows were sampled for yield measurement with three replications, and 20 plants were randomly selected for cob analysis to determine grain number, thousand kernel weight, and yield.

2.4. Calculation of Soil Fertility Retention

In Equation (1), FCI is the fertility retention, F is the value of soil nutrients and soil microbial indicators for each fertilization treatment after long-term fertilization (2018), and NF is the value of soil nutrients and soil microbial indicators for each treatment corresponding to the cessation of fertilization.

$$FCI=1-(F-NF)/F$$

(1)

2.5. Calculation of Crop Yield Stability

The stability index for the wheat–maize annual crop from 2018 to 2020 was utilized to assess the sustainability of crop growth on the farmland, calculated by dividing the yield of each treatment after stopping fertilizer application by the average yield of each fertilizer treatment (2007–2018), and the closer the data is to 1.00 the higher the stability is.

2.6. Calculation of the Sustainability Index

The 10 evaluation factors, including fertilizer retention of total nitrogen, total phosphorus, organic carbon, available phosphorus, alkaline hydrolysis nitrogen, urease, and sucrase, as well as the stability of grain number, thousand-grain weight, and yield, will be categorized into soil nutrient indexes, soil microbial indexes, and crop indexes two years following the discontinuation of fertilizer cultivation in 2020. Subsequently, the soil nutrient index (SNI), soil microbial index (SMI), and crop index (CI) will be computed.. The formulas are as follows:

In Equation (2), I_ij is the index value of the jth parameter of the ith treatment; A_ij is the measured value of the jth parameter of the ith treatment; Th_ij is the critical value of the jth parameter. Determination of each critical value: the critical value of the crop yield index is 1.2 times the arithmetic mean, and the critical value of the rest of the index is the arithmetic mean. In Equation (3), SNI_ij is the value of the soil nutrient index, and j is the number of nutrient indicators. In Equation (4), SMI_ij is the value of the soil microbial index, and j is the number of microbial indicators. CI_ij in Equation (5) is the crop index value, and j is the number of crop indicators.

$$I_{ij}=A_{ij}/{Th}_{ij}$$

(2)

$${SNI}_{ij}=1/5{\sum }_{j=1}^5{I}_{ij}$$

(3)

$${SMI}_{ij}=1/4{\sum }_{j=1}^4{I}_{ij}$$

(4)

$${CI}_{ij}=1/3{\sum }_{j=1}^3{I}_{ij}$$

(5)

The soil sustainability index (SI) is calculated based on the soil nutrient index, soil microbial index, and crop index (Figure 2). The formula for its calculation is as follows:

$$\begin{gathered} SI=\mathrm{triangular}\mathrm{area}\left({\mathrm{S}}_{\Delta \mathrm{ABC}}\right)={\mathrm{S}}_{\Delta \mathrm{AOB}}+{\mathrm{S}}_{\Delta \mathrm{BOC}}+{\mathrm{S}}_{\Delta \mathrm{AOC}}=\mathrm{a}\mathrm{b}\mathrm{s}\mathrm{i}\mathrm{n}(120°)/2 \\ +\mathrm{b}\mathrm{c}\mathrm{s}\mathrm{i}\mathrm{n}(120°)/2+\mathrm{a}\mathrm{c}\mathrm{s}\mathrm{i}\mathrm{n}(120°)/2=\sqrt{3}(\mathrm{a}\mathrm{b}+\mathrm{b}\mathrm{c}+\mathrm{a}\mathrm{c})/4 \end{gathered}$$

(6)

2.7. Statistical Analysis

Microsoft Office 2019 and Origin 2017 were used for data organization and graphing, SPSS 26.0 for statistical analysis of data, and the LSD method for the significance test.

3. Results

3.1. Effect of Long-Term Fertilization on Soil Nutrient Properties

Soil nutrients serve as the essential foundation for the growth and development of plants. Various fertilization methods were found to have a notable impact on the levels of soil total nitrogen, total phosphorus, organic matter, available phosphorus, and alkaline hydrolysis nitrogen content (

Table 1. The soil nutrient content after long-term fertilization under different treatments prior to wheat sowing in 2018.

Treatment	Total Nitrogen (g Kg−1)	Total Phosphorus (g Kg−1)	Organic Matter (g Kg−1)	Available Phosphorus (mg Kg−1)	Alkaline Hydrolysis Nitrogen (mg Kg−1)
F	0.93 ± 0.03c	0.79 ± 0.01c	33.56 ± 0.05c	11.01 ± 0.04c	70.54 ± 0.18c
FS	1.03 ± 0.02b	0.84 ± 0.01b	39.49 ± 0.02b	13.69 ± 0.20b	72.49 ± 0.16b
FM	1.10 ± 0.01a	0.86 ± 0.01ab	40.13 ± 0.03a	13.99 ± 0.06ab	73.59 ± 0.24a
FMS	1.00 ± 0.02b	0.88 ± 0.01a	40.16 ± 0.09a	14.32 ± 0.13a	72.23 ± 0.13b

F, NPK fertilizer; FS, NPK fertilizer + straw mulching; FM, NPK fertilizer + cow dung; FMS, NPK fertilizer + cow dung + straw mulching; The value is mean ± standard error (n = 3). Different letters in the same column indicate significant differences between treatments, p < 0.05.

). Specifically, three long-term organic + inorganic fertilizer treatments (FS, FM, FMS) were observed to significantly enhance the levels of soil total nitrogen, total phosphorus, organic matter, available phosphorus, and alkaline hydrolysis nitrogen content. In comparison to the sole application of fertilizer (F), the soil exhibited significant increases in total nitrogen content by 10.75%, 18.28%, and 7.53%; total phosphorus content by 5.88%, 8.40%, and 10.76%; organic matter content by 17.68%, 19.59%, and 19.68%; available phosphorus content by 24.35%, 27.11%, and 30.03%; and alkaline hydrolysis nitrogen content by 2.77%, 4.33%, and 2.41%. These findings suggest that prolonged utilization of organic–inorganic fertilization methods can enhance soil fertility through the augmentation of soil nutrient levels.

Upon cessation of long-term fertilization treatments during the two wheat–maize cropping cycles of 2018–2020, a notable decline in soil organic matter, total nitrogen, total phosphorus, alkaline hydrolysis nitrogen, and available phosphorus contents was observed in the treatments (Figure 3). In comparison to pre-sowing levels, the concentrations of total nitrogen, total phosphorus, organic matter, available phosphorus, and alkali-hydrolyzable nitrogen in soil following the application of chemical fertilizer at the maturity stage of maize in 2020 exhibited a reduction of 33.03%, 19.16%, 23.20%, 30.06%, and 33.10%, respectively. Conversely, the total nitrogen content in soil subjected to a combined application of organic and inorganic fertilizers(SFS, SFM, and SFMS) decreased by 24.27%, 21.18%, and 21.00%; total phosphorus content by 5.40%, 0.48% and 3.41%; organic matter content by 10.81%, 10.69% and 12.57%, available phosphorus content by 24.12%, 24.66% and 21.96%; and alkali-hydrolyzable nitrogen content by 21.34%, 21.61%, and 21.66%. The rates of decline in soil organic matter, total nitrogen, and alkali-hydrolyzed nitrogen were found to be more pronounced in the SF treatment compared to the SFS, SFM, and SFMS treatments. Similarly, the reduction in soil total phosphorus content was nearly identical in the SF and SFMS treatments, both of which exceeded the decline observed in the SFS and SFM treatments. Furthermore, the decrease in soil available phosphorus content followed the order of SFMS > SF > SFS > SFM (

Table 2. The changing trend of soil nutrient content following the cessation of fertilization in 2018–2020.

Treatment	Organic Matter (g Kg−1)				Total Nitrogen (g Kg−1)				Total Phosphorus (g Kg−1)				Alkaline Hydrolysis Nitrogen (mg Kg−1)				Available Phosphorus (mg Kg−1)
	Mean Value	Slope	R2	Change Trend	Mean Value	Slope	R2	Change Trend	Mean Value	Slope	R2	Change Trend	Mean Value	Slope	R2	Change Trend	Mean Value	Slope	R2	Change Trend
SF	29.25	−0.3468	0.79 **	Decrease	0.73	−0.0113	0.89 **	Decrease	0.73	−0.0069	0.94 **	Decrease	54.39	−0.1752	0.81 **	Decrease	10.34	−0.5874	0.81 **	Decrease
SFS	37.55	−0.2425	0.83 **	Decrease	0.86	−0.0080	0.80 **	Decrease	0.83	−0.0032	0.76 **	Decrease	62.97	−0.1713	0.88 **	Decrease	12.13	−0.6733	0.93 **	Decrease
SFM	37.87	−0.2526	0.71 **	Decrease	0.88	−0.0087	0.73 **	Decrease	0.91	−0.0056	0.78 **	Decrease	64.31	−0.1656	0.90 **	Decrease	12.43	−0.6101	0.82 **	Decrease
SFMS	37.12	−0.2155	0.70 **	Decrease	0.89	−0.0090	0.72 **	Decrease	0.91	−0.0066	0.85 **	Decrease	63.01	−0.1867	0.90 **	Decrease	12.92	−0.8542	0.88 **	Decrease

SF, NPK fertilizer; SFS, NPK fertilizer + straw mulching; SFM, NPK fertilizer + cow dung; SFMS, NPK fertilizer + cow dung + straw mulching (The letter “S” indicates the cessation of fertilization). ** indicates a very significant decrease.

). These findings suggest that the combined use of organic and inorganic fertilizers demonstrates superior retention capabilities compared to singular applications.

3.2. Effect of Long-Term Fertilization on Microbiological Properties of Soil

The results of the experiment (Figure 4) indicate that the application of three different organic and inorganic fertilizer treatments (FS, FM, FMS) leads to a significant increase in soil urease and sucrase activities compared to the use of chemical fertilizer alone. Specifically, the soil urease and sucrase activities followed the order FMS > FM > FS > F for all treatments. The urease activities of the three organic and inorganic fertilizer treatments (FS, FM, FMS) were found to be 8.55%, 10.62%, and 12.16% higher, respectively, than those of the chemical fertilizer treatment (F), while the sucrase activities were 5.13%, 7.36%, and 7.98% higher than the chemical fertilizer treatment (F) on average. The above results indicated that long-term organic and inorganic fertilizer blending could improve soil fertility by increasing soil enzyme activity.

Upon cessation of long-term fertilization treatments during the two wheat–maize cropping cycles of 2018–2020, a notable decline in soil urease and sucrase activities was observed in the treatments (Figure 5). Soil urease and sucrase activities were reduced by 36.71% and 39.47%, respectively, and 26.94%, 22.69%, 26.61%, and 29.87%, 30.66%, and 30.02%, respectively, under the sole application of chemical fertilizer (SF) and the combined application of organic and inorganic fertilizers (SFS, SFM, and SFMS) at the maturity stage of maize in 2020, as compared with the pre-sowing in 2018. The findings suggest that the decline in soil urease activity was more pronounced in the F treatments compared to the FS, FM, and FMS treatments. Additionally, the rate of decrease in soil sucrase activity followed the order SF > SFM > SFMS > SFS (

Table 3. The trends in soil microbial characteristics after the discontinuation of fertilization in 2018–2020.

Treatment	Urease (g Kg−1)				Sucrase (g Kg−1)
	Mean Value	Slope	R2	Change	Mean Value	Slope	R2	Change
SF	3.91	−0.0671	0.89 **	Decrease	14.93	−0.2680	0.88 **	Decrease
SFS	4.68	−0.0513	0.84 **	Decrease	16.82	−0.1832	0.77 **	Decrease
SFM	4.87	−0.0433	0.61 **	Decrease	17.32	−0.2301	0.92 **	Decrease
SFMS	4.76	−0.0636	0.89 **	Decrease	17.94	−0.2274	0.91 **	Decrease

SF, NPK fertilizer; SFS, NPK fertilizer + straw mulching; SFM, NPK fertilizer + cow dung; SFMS, NPK fertilizer + cow dung + straw mulching (The letter “S” indicates the cessation of fertilization). ** indicates a very significant decrease.

). These results demonstrate that the integrated application of organic–inorganic fertilizers is more effective in preserving the stability of soil enzyme activity than the use of chemical fertilizers alone.

3.3. Effect of Long-Term Fertilization on Crop Yield

Table 4. The average yield under different fertilization treatments in 2007–2018 and the changes in crop indexes under the treatment of stopping fertilization in 2018–2020.

Treatment	Average Production for 2007–2018		Treatment	2019 (Wheat)		2019 (Maize)		2020 (Wheat)		2020 (Maize)
	Wheat	Maize		Crop Yield (kg ha−1)	Sustainability Index	Crop Yield (kg ha−1)	Sustainability Index	Crop Yield (kg ha−1)	Sustainability Index	Crop Yield (kg ha−1)	Sustainability Index
F	6986.67	8862.50	SF	6105.00c	0.87	8461.00b	0.95	5872.00c	0.84	7461.00c	0.84
FS	7931.67	9283.0	SFS	7743.35b	0.98	9034.0a	0.97	7389.25b	0.93	8683.00b	0.94
FM	8134.5	9082.50	SFM	7821.46ab	0.96	8925.00a	0.98	7523.75a	0.92	8749.00a	0.96
FMS	8103	9123.00	SFMS	7916.00a	0.98	9070.00a	0.99	7550.00a	0.93	8735.00a	0.96

The value is mean (n = 3). Different letters in the same column indicated significant differences between treatments, p < 0.05.

is the average yield under different fertilization treatments from 2007 to 2018 and the change in crop index under the treatment of stopping fertilization from 2018 to 2020. The impact of three distinct organic and inorganic fertilizer blending treatments on crop yields demonstrated statistical significance. Specifically, in 2019, the yield of wheat and maize under the combined treatment of organic and inorganic fertilizers was significantly higher than that of fertilizer alone. The average crop yield in 2007–2018 and the crop yield in 2020 have the same rule.. Furthermore, the stability of crop yield was observed to be greater under the combined organic and inorganic fertilizer treatments compared to the use of chemical fertilizer alone. In 2019, with regard to the soil treatment groups, the wheat yields in those where fertilization had been ceased (SF, SFS, SFM, SFMS) were 24.66%, 2.37%, 3.85%, and 2.31% lower, respectively, compared to the yields in the fertilized soil groups (F, FS, FM, FMS). Similarly, the maize yields in the former were 4.53%, 2.68%, 1.73%, and 0.58% lower. Moving on to 2020, for the soil where fertilization was discontinued (SF, SFS, SFM, SFMS), the wheat yields were 9.75%, 3.41%, 4.30%, and 1.36% lower when contrasted with those in the fertilized soil (F, FS, FM, FMS). Likewise, the maize yields in the unfertilized soil groups were 6.92%, 2.03%, 0.71%, and 1.66% lower. These findings suggest that the integration of organic and inorganic fertilizers not only enhances crop yield significantly but also improves the stability of crop yield.

3.4. Effect of Long-Term Fertilization on the Sustainability Index

Following a two-year cessation of fertilization during the 2020 maize season, it was observed that the soil nutrient index of 0.90 and soil microbial index of 0.89 when using chemical fertilizer alone (SF) fell below the critical threshold of 1.00. Conversely, the soil nutrient indexes (1.03, 1.03, 1.04) and soil microbial indexes (1.03, 1.05, 1.03) achieved with the combination of organic and inorganic fertilizers (SFS, SFM, SFMS) exceeded the critical threshold of 1.00. The crop indexes recorded under each treatment (SF, SFS, SFM, SFMS) were 0.84, 0.90, 0.91, and 0.91, respectively, all of which were below the critical threshold of 1.00. The sustainability indexes of the combined application of organic and inorganic fertilizers (SFS, SFM, SFMS) were calculated to be 1.26, 1.29, and 1.27, respectively, surpassing the sustainability index of treatment SF (1.00). On average, the soil nutrient index, soil microbial index, crop index, and sustainability index were found to be 14.81%, 16.48%, 7.93%, and 27.33% higher, respectively, under the combined application of organic and inorganic fertilizers compared to chemical fertilizer alone (

Table 5. Soil nutrient index, soil microbial index, crop index, and sustainability index after two years of stopping fertilization (2020 maize season).

Treatment	Soil Nutrient Index	Soil Microbial Index	Crop Index	Sustainability Index
SF	0.90	0.89	0.84	1.00
SFS	1.03	1.03	0.90	1.26
SFM	1.03	1.05	0.91	1.29
SFMS	1.04	1.03	0.91	1.27
critical value	1.00	1.00	1.00	1.30
CV (%)	6.67%	7.45%	3.84%	11.62%

CV represents the coefficient of variation.

, Figure 6). The findings indicate that utilizing a combination of organic and inorganic fertilizers may contribute to the preservation of soil ecosystem sustainability, as opposed to relying solely on chemical fertilizers.

4. Discussion

4.1. Effect of Long-Term Fertilization on Soil Nutrient Properties

Soil organic matter is a crucial component of the soil’s solid phase, serving a significant function in nutrient provision and the prevention of nutrient leaching. In the context of this particular study, the soil in question is clay loam. Straw deposits in clay loam tend to become compacted by clay particles, impeding their normal decomposition process. This, in turn, can have adverse effects on the growth of crop roots and the generation of root exudates [24]. Conversely, the combined application of organic and inorganic fertilizers presents several benefits. It can stimulate the formation of soil aggregates, thereby enhancing the overall soil structure. This improved structure creates a more favorable environment for the growth and respiration of crop roots [25]. As crops thrive, they contribute to a greater influx of organic residues, particularly from roots in both the aboveground and underground portions, into the soil. This influx serves to augment the soil organic matter content [26]. Moreover, the utilization of organic fertilizer can act as a catalyst for the metabolism of soil microorganisms. This heightened microbial activity leads to an increase in metabolites, consequently resulting in a rise in soil organic carbon content [27]. Collectively, all of these factors elucidate that in comparison to the sole application of chemical fertilizer (F), the combined application of organic and inorganic fertilizers (FS, FM, FMS) can remarkably enhance the soil organic matter content.

The combination of organic and inorganic fertilizers is advantageous for the enhancement of total and available nutrient accumulation in soil [28,29]. The current research findings indicate that the soil’s total nitrogen and alkaline hydrolysis nitrogen levels were notably elevated when organic and inorganic fertilizers were used in combination, as opposed to the use of chemical fertilizers alone. Organic fertilizer not only enhances soil nitrogen levels but also supplies fresh carbon, which serves as a source of energy for soil nitrogen immobilization upon incorporation into the soil [30]. In the Huang-Huai-Hai Plain, the maize growing season is characterized by high temperatures and abundant rainfall, leading to rapid conversion of nitrogen into NH4⁺ and NO3⁻ forms in the soil. However, these nitrogen forms are susceptible to loss through ammonia volatilization and nitrate leaching. In contrast, organic nitrogen mineralization occurs at a slower rate, resulting in lower loss rates and higher retention rates in the soil, ultimately enhancing fertilizer efficiency [31]. Additionally, the study demonstrates that the combined application of organic and inorganic fertilizers leads to a significant increase in soil phosphorus content. Chemical fertilizers containing inorganic phosphorus can become immobile in the soil, making it unsuitable for plant absorption and use. However, incorporating organic fertilizer into the soil can boost organic matter levels and phosphorus content [32,33]. This process can release phosphorus that was previously fixed in the soil, potentially because of the carbon dioxide released during organic matter mineralization. The carbon dioxide dissolves in water to form carbonic acid, which can help dissolve certain primary minerals and enhance phosphorus effectiveness [34].

Following the cessation of fertilizer application, there were significant declines in soil total nitrogen, total phosphorus, organic matter, available phosphorus, and alkaline hydrolysis nitrogen contents across all treatments. However, the rate of decrease in these soil parameters was found to be lower in the treatment combining organic and inorganic fertilizers compared to the treatment with chemical fertilizers alone. After two years of stopping fertilizer cultivation (2020 corn season), it was observed that the reductions in soil nutrient indexes resulting from the combined application of three types of organic and inorganic fertilizers were less pronounced compared to the treatment involving chemical fertilizers alone. In addition, the levels of soil organic matter, total nitrogen, total phosphorus, available phosphorus, and alkaline hydrolysis nitrogen were found to be significantly elevated in the three types of organic and inorganic fertilizer combined application in comparison to the chemical fertilizer treatment alone (SF). The depletion of nutrients in the soil following a single application of chemical fertilizer treatment was not adequately replenished, leading to a deficiency in soil nutrients [35]. In contrast, the gradual release of nutrients from organic fertilizers due to the slow decomposition of organic materials helped maintain nutrient balance and effectiveness over an extended period, ultimately enhancing soil fertility and crop yield [31,36,37]. The gradual decrease in soil nutrient content following the application of organic–inorganic fertilizer treatment can be attributed to this factor as well.

4.2. Effect of Long-Term Fertilization on Microbiological Properties of Soil

Soil enzymes are primarily generated by soil microorganisms and animal and plant residues, playing a crucial role in soil microbiological processes. Urease, a key enzyme, is closely linked to soil nitrogen supply, as it facilitates the conversion of inert organic nitrogen into a form that plants can utilize [38]. Conversely, soil sucrase levels are indicative of soil organic carbon storage and serve as a valuable indicator of soil fertility [39]. Prolonged use of synthetic fertilizers may lead to a decline in soil enzyme activity [36], whereas the application of organic fertilizers has been shown to enhance soil enzymatic function [40,41]. The current research findings indicate that the concurrent application of organic–inorganic fertilizers (FS, FM, FMS) resulted in a notable enhancement of soil urease and sucrase activities. This phenomenon can be attributed to the decomposition of organic fertilizer, which supplied ample energy and carbon sources to support soil microbial activities, thereby stimulating their metabolic processes and proliferation, ultimately leading to heightened soil enzyme activities [42]. In addition, the augmentation of organic matter content contributed to the amelioration of soil physicochemical properties, creating a more favorable habitat for the proliferation of microorganisms and soil fauna. The prior research has demonstrated a strong positive relationship between soil enzyme activities and nutrient levels [43,44], with soil sucrase and urease activities being influenced by the presence of nitrogen, phosphorus, and organic matter in the soil [45]. Consequently, the simultaneous use of both organic and inorganic fertilizers can lead to a substantial enhancement in soil nutrient levels, as well as an indirect increase in soil enzyme activity. The relationship between soil enzyme activity and soil nutrient levels suggests a mutually reinforcing effect, highlighting the potential benefits of utilizing both organic and inorganic fertilizers to enhance soil enzyme activity, nutrient availability, and ultimately promote crop growth. Additionally, the secretion of inter-root compounds during crop growth can stimulate the metabolism and reproduction of microorganisms, further enhancing enzyme activity. Consequently, strategic fertilization practices can establish a positive feedback loop among soil, crops, and microorganisms.

Two years after the cessation of fertilization, a notable decline in soil urease and sucrase activities was observed across all treatments. This decline can be attributed to the reduced presence of microorganisms and crop plants within the soil ecosystem post-fertilization cessation, leading to a decrease in soil enzyme activities. Furthermore, the decrease in soil nutrient content subsequent to the cessation of fertilization also contributed to the significant decrease in soil enzyme activities. Nevertheless, the reductions in soil microbial indices were less pronounced when organic and inorganic fertilizers were applied compared to chemical fertilizers alone (Table 3). This enhancement in soil enzyme stability is attributed to the long-term combined application of organic and inorganic fertilizers, which facilitates the adsorption of soil enzymes by clay particles and their combination with soil humus, forming stable organic–inorganic complexes [46].

4.3. Impact of Long-Term Fertilization on Crop Yield and Sustainable Production

The presence of carbon, nitrogen, and phosphorus in soil plays a crucial role in determining crop yield [47,48]. Organic fertilizers, which contain high levels of these essential nutrients, are increasingly being recognized as an effective management strategy for enhancing crop productivity [49]. In this particular study, it was observed that the soil within the testing site exhibited alkaline characteristics. Interestingly, the decomposition process of organic matter present in organic fertilizers gives rise to substances like organic acids. These acidic by-products play a crucial role in counteracting the alkalinity of the soil. As a result, the pH value of the soil is effectively reduced, steering it closer to the neutral range. Such a soil condition adjustment proves to be highly advantageous for the growth and development of the majority of crop species [50]. Moreover, the combination of organic and inorganic fertilizers can further optimize soil conditions by enhancing nutrient levels and enzyme activities, thereby providing the necessary resources for improved crop growth. This integrated approach has been shown to significantly boost yields of wheat and maize [51,52]. The utilization of organic fertilizers has been found to decelerate the senescence process in crop roots and leaves, lengthen the duration of photosynthesis in crops, enhance seed quality through prolonged seed filling, and ultimately boost crop yields [53,54]. Additionally, the research has demonstrated that the simultaneous application of organic and inorganic fertilizers (FS, FM, FMS) can result in a substantial increase in wheat and maize yields. Furthermore, the stability and sustainability metrics of organic and inorganic fertilizer treatments surpassed those of chemical fertilizer treatments following the cessation of fertilization practices. The findings suggest that both organic and inorganic fertilizer treatments have the potential to enhance crop yields, maintain yield stability, and reduce inter-annual fluctuations. Furthermore, the utilization of a sustainability index in this study allows for a more comprehensive evaluation of the soil system sustainability of double cropping farmland in the Huang-Huai-Hai Plain, avoiding the limitations of single-indicator assessments.

5. Conclusions

After 11 years of continuous fertilization of wheat–maize rotational cropland in the Huang-Huai-Hai Plain, it was discovered that using a combination of organic and inorganic fertilizers has the potential to enhance soil fertility and sustain agricultural soils. The application of both types of fertilizers led to a notable increase in soil nutrient levels and enzyme activities, resulting in a significant improvement in crop yields compared to solely using inorganic fertilizers. After a two-year period following the cessation of fertilization, it was observed that there was a notable decrease in soil nutrient content and enzyme activity. The rate of decline in organic and inorganic fertilizers was found to be lower than that of chemical fertilizers alone. Additionally, the stability of wheat and maize yields was higher when utilizing organic and inorganic fertilizers compared to chemical fertilizers alone. These findings suggest that the application of organic and inorganic fertilizers leads to improved soil fertility and more consistent crop yields. Following the cessation of fertilization, sustainability indexes were calculated for each treatment (SF, SFS, SFM, SFMS) as 1.08, 1.26, 1.29, and 1.27, respectively. These values fell below the critical threshold of 1.30, indicating an unsustainable soil ecosystem. However, the sustainability indexes for the combination of organic and inorganic fertilizers exceeded those for chemical fertilizers alone. In conclusion, the combined application of organic and inorganic fertilizers represents a fertilization paradigm that is capable of sustaining the long-term viability and ecological balance of the farmland soil system in the Huang-Huai-Hai Plain.