Long-Term Straw Return Combined with Chemical Fertilizer Enhances Crop Yields in Wheat-Maize Rotation Systems by Improving Soil Nutrients Stoichiometry and Aggregate Stability in the Shajiang Black Soil (Vertisol) Region of North China Plain

Abstract

The sustainability of wheat-maize rotation systems in the North China Plain is challenged by the over-reliance on chemical fertilizers, which leads to the decline of soil organic matter and structural degradation, particularly in the unique Shajiang black soil (Vertisol). While straw return is widely recommended to mitigate these issues, the synergistic mechanisms of its long-term combination with chemical fertilizers on soil nutrient stoichiometry and aggregate stability remain inadequately quantified. A long-term field experiment was conducted with the five fertilization treatments including: (1) no fertilizer or straw (CK), (2) chemical fertilizer alone (NPK), (3) straw return chemical fertilizer (NPKS), (4) straw return with 10% straw-decomposing microbial inoculant combined with chemical fertilizer (10%NPKS), and (5) straw return with 20% straw-decomposing microbial inoculant combined with chemical fertilizer (20%NPKS) in the Shajiang black soil (Vertisol) region to investigate the effects of straw return combined with chemical fertilizers on soil organic carbon (SOC), total nitrogen (TN) and total phosphorus (TP) stoichiometry, aggregate stability, and crop yield in winter wheat-summer maize rotation systems of North China Plain. Our study demonstrated that the co-application of straw with a straw-decomposing microbial inoculant is a highly effective strategy for enhancing soil health and crop productivity, with its efficacy being critically dose-dependent. Our results identified the 10%NPKS treatment as the optimal practice. It most effectively improved soil physical structure by significantly increasing the content of large macroaggregates (>0.5 mm) and key stability indices (MWD, GMD, WA), while concurrently enhancing nutrient cycling, as evidenced by elevated SOC, TN, and shifted C/P and N/P stoichiometry. Multivariate analyses confirmed strong positive correlations among these soil properties, indicating a synergistic improvement in soil quality. Crucially, these enhancements translated into significant yield gains, with a notable crop-specific response: maize yield was maximized under the 10%NPKS treatment, whereas wheat yield benefited sufficiently from NPKS treatment. A key mechanistic insight was that 20%NPKS treatment, despite leading to the highest SOC and TN, induced a relative phosphorus limitation and likely caused transient nutrient immobilization, thereby attenuating its benefits for soil structure and yield. We conclude that co-applying straw with a 10% microbial inoculant combined with chemical fertilizer represents the superior strategy, offering a sustainable pathway to synergistically improve soil structure, nutrient availability, and crop productivity, particularly in maize-dominated systems.

1. Introduction

The sustainable intensification of agricultural production is critical for addressing global food security and environmental challenges. Cereal rotation systems, particularly wheat-maize rotations, serve as the foundation of grain production in key regions like the North China Plain (NCP) [1,2]. However, long-term intensive cultivation with insufficient organic inputs has triggered soil degradation in this region. This is especially evident in Shajiang black soils (Vertisols), where conventional practices dominated by excessive chemical fertilization disrupt nutrient cycling and aggregate stability, ultimately threatening agricultural sustainability [3,4,5]. These clay-rich soils are inherently prone to cracking, hardening, and nutrient imbalances, which collectively constrain crop productivity [6,7]. The decline in soil organic matter (SOM), deterioration of soil structure, and distorted nutrient stoichiometry under such management practices progressively undermine the soil’s productive capacity [8,9,10]. Thus, developing integrated strategies that simultaneously enhance crop yields and restore soil health has become a research priority.

Straw return is globally recognized as a key practice for enhancing soil fertility and crop productivity while improving straw resource utilization efficiency [11,12]. This approach mitigates the environmental pollution associated with open burning and delivers multiple agronomic benefits, supporting sustainable grain production [13,14]. As an important nutrient reservoir, straw replenishes nitrogen (N), phosphorus (P), and potassium (K) upon decomposition, helping close nutrient cycles and reducing dependence on mineral fertilizers [15,16]. More significantly, straw serves as a primary driver of the soil carbon cycle by substantially increasing organic carbon inputs, thereby enhancing both labile and recalcitrant soil organic carbon pools [17,18]. The stimulated microbial activity following straw addition can further accelerate the mineralization of native SOM and improve nutrient availability [19]. Extensive studies confirm that straw application promotes SOC sequestration, facilitates nutrient cycling, and improves soil structure [20,21]. It positively influences soil aggregation, water retention, and microbial activity [22], while also regulating the ecological stoichiometry of carbon (C), nitrogen (N), and phosphorus (P), which in turn affects nutrient availability and microbial metabolism [10,23]. Although the effects are context-dependent, meta-analyses consistently report significant yield increases of 6.78–27% under long-term straw incorporation [14,24,25,26]. The benefits of straw return are modulated by its interaction with mineral fertilizers. While chemical fertilizers supply readily available nutrients for immediate crop growth, they generally contribute little to improving the soil’s physical and biological properties [27,28]. In contrast, the decomposition of high-carbon straw can induce temporary nitrogen immobilization, potentially creating nutrient deficits if not properly managed [29,30]. Furthermore, straw incorporation alters C and nutrient distribution within soil aggregates, influencing microbial biogeochemical processes [31]. Therefore, combining straw return with chemical fertilizers is hypothesized to create a synergistic system where inorganic fertilizers meet immediate crop demands while straw builds long-term soil fertility and structure.

One of the most significant benefits of straw return lies in its role in promoting soil aggregation [32,33]. As the fundamental units of soil structure, aggregates and their stability are key indicators of soil health [34]. Straw facilitates aggregation through both physical and biological pathways [22,35]: physically by acting as nucleation sites for micro-aggregate formation [36,37], and biologically by stimulating microbial production of binding agents such as fungal hyphae and extracellular polysaccharides [38,39]. In Vertisols, straw return with chemical fertilization has been shown to increase the proportion of 3–5 mm aggregates and enhance overall aggregate stability [40,41]. This improved aggregation enhances soil porosity, aeration, water infiltration, and erosion resistance, while also physically protecting organic carbon within aggregate cores, thereby contributing to long-term carbon storage [42,43,44]. While long-term straw return is known to improve soil aggregation, its combined effect with chemical fertilizers on soil nutrient dynamics and their interplay with aggregate stability remains poorly understood.

Soil ecological stoichiometry, reflecting the balance of C, N, and P, serves as a fundamental indicator of soil fertility and nutrient cycling [10,23,45]. Optimal C:N:P ratios are critical for regulating microbial metabolism, organic matter decomposition, and nutrient use efficiency [46]. In intensive cropping systems, however, imbalanced mineral fertilization often causes a disruption in this equilibrium that results in narrowed C:N and widened N:P ratios, thereby accelerating native SOC mineralization, promoting nutrient losses, and constraining long-term nutrient use efficiency [10,23]. Simultaneously, soil structural integrity—governed by aggregate stability—is essential for maintaining porosity, water infiltration, and root penetration [38,47]. SOC acts as a primary binding agent for macro-aggregate formation and stabilization; its depletion consequently weakens soil structure and degrades physical health [39]. Straw return addresses these issues by replenishing organic carbon and nutrients, thereby helping rebalance soil C:N:P stoichiometry. This realignment can mitigate nutrient limitations, support a more active microbial community [48], and improve the overall soil environment for crop growth.

Despite these recognized benefits, a critical knowledge gap persists regarding the long-term synergistic effects of straw return combined with chemical fertilizers on the interaction between soil nutrient stoichiometry and aggregate stability in the Shajiang black soil region. The Shajiang black soil, a Vertisol critical for regional grain production in the NCP, is highly vulnerable to structural degradation due to its clay-rich nature [3,4]. Poor management leads to hardening and reduced aggregate stability, severely constraining root growth and crop productivity [41]. While straw return is known to improve soil health, its long-term combined effects with fertilizers on the co-evolution of C:N:P stoichiometry and aggregate stability in this unique pedo-climatic context remain poorly quantified. Understanding how this integrated management recalibrates ecological stoichiometry and functionally links these biochemical improvements to macro-aggregate formation is essential, as this mechanistic interplay underpins key soil functions including water infiltration, nutrient retention, and root development.

Therefore, based on a long-term field experiment, this study aims to (1) quantify the impacts of long-term straw return combined with chemical fertilizers on soil C:N:P stoichiometry and aggregate distribution and stability; (2) unravel the interrelationships between stoichiometric balance and aggregate stability; and (3) elucidate how these soil improvements collectively contribute to enhanced crop yields in a wheat-maize rotation system. We hypothesize that the combined application, compared to either practice alone, will (i) significantly boost crop yield, (ii) create a more balanced soil C:N:P ratio by elevating SOC without inducing nitrogen limitation, and (iii) enhance the formation and stability of macro-aggregates. The findings will provide a mechanistic explanation for yield sustainability and a scientific basis for optimizing management strategies to enhance the productivity and ecological resilience of Vertisol-based cropping systems in the NCP.

2. Materials and Methods

2.1. Long-Term Experimental Site and Design

Long-term experimental station (115°06′ E, 32°55′ N) is located in the Agricultural Science Research Institute in Linquan County, Anhui Province, China. Since its establishment in 2010, the station has recorded climatic conditions featuring a mean annual temperature of 15.3 °C (with extreme highs reaching 41.4 °C) and an average yearly precipitation of 892 mm. Soil type is Shajiang black soil (Vertisol, USDA Soil Taxonomy) [41], with a winter wheat (from mid-November to early June)-summer maize (from mid-June to early November) cropping system. Pre-experiment soil analysis indicated acidic conditions (pH 5.72), with measured concentrations of 12.64 g kg⁻¹ organic matter, 80.16 mg kg⁻¹ alkali-hydrolyzable nitrogen, 16.92 mg kg⁻¹ available phosphorus, 116.7 mg kg⁻¹ exchangeable potassium, and 0.36 g kg⁻¹ total phosphorus.

The study began from November 2022 to June 2023. All fertilization treatments have been consistently applied since 2010. The experimental design employed a randomized complete block arrangement with six experimental treatments: (1) no fertilization (CK); (2) chemical fertilization (NPK); (3) straw return plus chemical fertilization (NPKS); (4) straw return with 10% straw-decomposing microbial inoculant plus chemical fertilization (10%NPKS); (5) straw return with 20% straw-decomposing microbial inoculant plus chemical fertilization (20%NPKS). Each treatment had three experimental plots with repetitions in randomly assigned plots (5 m × 10 m) within blocks, totaling 50 m² per plot. All farmland management measures aside from straw return were consistent between treatment groups at each site (Figure 1).

A wheat-maize double cropping system was implemented with complete straw retention. The wheat cultivar ‘Hengguan 35’ was sown at 195 kg ha⁻¹ using 15 cm row spacing, while maize (‘Zhengdan 958’) was planted at 60,000 plants ha⁻¹ with 60 cm row spacing. Fertilization protocols utilized urea (46% N), single superphosphate (16% P₂O₅), and potassium sulfate (50% K₂O). For wheat, total nitrogen application reached 300 kg ha⁻¹ (120 kg ha⁻¹ basal application plus 180 kg ha⁻¹ split between jointing and booting stages), with single applications of 120 kg ha⁻¹ P₂O₅ and 100 kg ha⁻¹ K₂O as basal fertilizers. Maize received 250 kg ha⁻¹ nitrogen (100 kg ha⁻¹ basal plus 150 kg ha⁻¹ at bell-mouth stage) alongside 45 kg ha⁻¹ each of basal P₂O₅ and K₂O (

Table 1. The rates of chemical fertilizer and straw application for each treatments in wheat and maize season, respectively.

Treatment 1	N (kg ha−1)	P2O5 (kg ha−1)	K2O (kg ha−1)	Amounts of Straw-Decomposing Microbial Inoculant (kg ha−1)	Rates of Straw Return (t ha−1)
CK	0/0	0/0	0/0	0/0	0/0
NPK	300/250	120/45	100/45	0/0	0/0
NPKS	300/250	120/45	100/45	0/0	6/15
10%NPKS	300/250	120/45	100/45	150/150	6/15
20%NPKS	300/250	120/45	100/45	300/300	6/15

¹ CK, no fertilization; NPK, chemical fertilization; NPKS, straw return plus chemical fertilization; 10%NPKS, straw return with 10% straw-decomposing microbial inoculant plus chemical fertilization; 20%NPKS, straw return with 20% straw-decomposing microbial inoculant plus chemical fertilization. The harvested straw is all returned to the fields.

). Wheat straw was mechanically chopped and incorporated with basal fertilizers/straw decomposers prior to plowing, whereas maize received furrow-applied basal fertilizers with straw post-sowing. Detailed fertilization regimes are presented in Table 1.

2.2. Soil Sampling and Chemical Analysis

Soil samples were collected in mid-November 2023 after two complete rotation cycles (after maize harvest). Surface soil (0~20 cm depth) was sampled from each plot using the five-point sampling method. Following collection, gravel and plant residues were removed prior to air-drying and sieving for subsequent analysis. For subsequent analyses, the bulk soil sample was split via the quartering method. Soil aggregate analysis was performed on Subsample 1, which was gently broken (≤8 mm) and air-dried. Conversely, routine physicochemical analysis was conducted on Subsample 2, which was air-dried and then sieved through 2 mm and 0.15 mm meshes. The standard soil analysis procedures were conducted according to Zhang et al. [49], including: potentiometric pH determination at 2.5:1 soil-water ratio, Soil organic carbon (SOC) quantification via potassium dichromate wet oxidation, total nitrogen (TN) measurement using Kjeldahl digestion, and total phosphorus (TP) analysis through the perchloric acid-sulfuric acid-molybdenum-antimony-scandium colorimetric method and total potassium (TK) analysis through the perchloric acid-sulfuric acid-flame photometry method after HClO₄-H₂SO₄ digestion.

2.3. Soil Aggregate Stability Analysis

The water-stable aggregate composition was analyzed through wet-sieving technique following established protocols [50,51], with methodological refinements based on Tang et al.’s (2022) procedures [52]. Four distinct size fractions were isolated: 2–5 mm, 0.5–2 mm, 0.25–0.5 mm, and <0.25 mm. All fractions were oven-dried at 50 °C for mass determination, enabling subsequent calculation of aggregate mass percentages per size class [53]. Additionally, all aggregate sizes by wet-sieving were filtered by a 0.15 mm-sieve for SOC, TN and TP measurement according to Zhang et al. [49].

Soil aggregate stability was quantified using five established metrics: >0.25 mm water-stable aggregates (WA, %), mean weight diameter (MWD, mm), geometric mean diameter (GMD, mm), fractal dimension (FD), and mean weight-specific surface area (MWSSA, cm² g⁻¹). The corresponding calculation formulas are provided below [54,55]:

$$WA\left(\%\right)=\left(1-{\displaystyle \frac{w_{d<0.25}}{w_t}}\right)\ast 100\%$$

(1)

where $w_{d<0.25}$ is the mass of the <0.25 mm aggregates, and $w_t$ is the total mass of all aggregates.

$$MWD={\displaystyle \sum _{i=1}^n\left(x_i{w}_i\right)}$$

(2)

$$GMD=\exp {\displaystyle \sum _{i=1}^n\left(w_i\ln x_i\right)}$$

(3)

where $i$ is the total number of fractions, $w_i$ is the proportion of the total aggregates in the $i^{\mathit{th}}$ fraction, and $x_i$ is the mean diameter of the $i^{\mathit{th}}$ sieve size.

$$FD=3-{\displaystyle \frac{\lg \left(\frac{M_{i<x_i}}{M_t}\right)}{\lg \left(\frac{x_i}{x_{max}}\right)}}$$

(4)

where $i$ is the total number of fractions, $M_{i<x_i}$ is the cumulative mass of aggregates of $i$ less than $x_i$ size, $M_t$ is the total mass of all aggregates, $x_i$ is the aggregate size class, $x_{max}$ is the mean diameter of the largest aggregate class, and lg is decimal logarithm (log₁₀).

$$MWSSA={\displaystyle {\sum }_{i=1}^n\frac{6w_i}{x_i{p}_i}}$$

(5)

where n is the number of aggregate size fractions, $w_i$ is the proportion of the total aggregates in the $i^{\mathit{th}}$ fraction, and $x_i$ is the mean diameter of the $i^{\mathit{th}}$ sieve size, and $p_i$ = 2.65 g cm⁻³.

2.4. Statistical Analyses

Laboratory measurement data was repeated three times. Statistical analyses were performed using SPSS 17.0 (IBM, Armonk, NY, USA) for variance computation, with mean separation achieved through LSD and Duncan’s tests (p = 0.05). Data visualization and multivariate analyses were implemented in OriginPro 2025b (OriginLab Corp., Northampton, MA, USA) including principal component analysis (PCA), redundancy analysis (RDA) and correlation heatmap analysis to identify dominant factors influencing aggregate stability across fertilization types.

3. Results

3.1. Soil Basic Chemical Properties Under Long-Term Different Straw Return Treatments

There were statistically significant effects of long-term straw return on soil basic chemical properties (Figure 2). Compared to CK treatment, the 10%NPKS and 20%NPKS treatments decreased significantly soil pH with the value of 13.20% and 12.05% (p < 0.05, Figure 2a), respectively, while the NPK and NPKS were no significantly difference (p > 0.05, Figure 2a). Moreover, SOC contents in the NPK treatment decreased significantly by 10.88%, while that of NPKS, 10%NPKS and 20%NPKS treatment increased significantly by 58.06%, 62.88% and 100.705 as compared with CK treatment, respectively (p < 0.05, Figure 2b). Similarly, compared to CK treatment, the NPK, NPKS, 10%NPKS and 20%NPKS increased significantly TN contents with 17–28 times (p < 0.05, Figure 2c). In addition, TP content was higher in the NPK treatment than that in CK and 20%NPKS treatments (p < 0.05, Figure 2d); however, TK content was higher in the 20%NPKS treatment than that in CK, NPK, NPKS and 20%NPKS treatments (p < 0.05, Figure 2e). Also, we found that the 10%NPKS and 20%NPKS treatments had lower pH and TP, and high SOC, TN and TK, showing straw-decomposing microbial inoculant can accelerate the decomposition and release of organic acids and nutrients in the straw.

3.2. Characteristic of Soil C:N:P Stoichiometry Under Long-Term Different Straw Return Treatments

Long-term straw return significantly affects soil C:N:P stoichiometric ratio (Figure 3). Compared to CK treatment, soil C/N and C/N/P of the NPK, NPKS, 10%NPKS and 20%NPKS treatments decreased significantly by 22–33 times (Figure 3a) and 22–39 times (Figure 3d), respectively. Moreover, compared to CK and NPK treatments, soil C/P in the NPKS, 10%NPKS and 20%NPKS treatments increased significantly 44.15–93.51%, 49.75–101.03% and 111.28–183.62%, respectively (p < 0.05, Figure 3b), and furthermore, the 20%NPKS treatment had higher C/P among all the straw return treatments. Similarly, the NPK, NPKS, 10%NPKS and 20%NPKS treatments increased significantly N/P with 14–28 times (p < 0.05, Figure 3c). Moreover, the 20%NPKS treatment had higher N/P among all the straw return treatments. In brief, although long-term straw return can decrease soil C/N, and it enhances soil C/P and N/P, which is closely related to the contents of SOC, TN and TP.

3.3. Distributions of Soil Aggregate Contents and Stability Under Long-Term Different Straw Return Treatments

Long-term straw return significantly affected the distributions of soil aggregate contents across different sizes (Figure 4). Compared with CK and NPK treatments, the NPKS, 10%NPKS and 20%NPKS treatments increased significantly 0.5–2 mm aggregate content with the contents of 68.89–77.41%, 48.06–55.53% and 55.03–62.85%, respectively, while those treatments decreased significantly 0.25–0.5 mm aggregate content with the contents of 41.78–45.67%, 55.78–58.74% and 40.30–44.29%, respectively, and the content of <0.25 mm aggregates with the contents of 32.02–32.07%, 44.92–44.97% and 40.12–40.16%, respectively (p < 0.05, Figure 4). Moreover, the 10%NPKS treatment increased significantly >2 mm aggregate content with the contents of 99.00%, 102.07%, 95.30% and 34.20% as compared to the CK, NPK, NPKS and 20%NPKS treatments, respectively (p < 0.05, Figure 4). We also found that soil aggregate contents of the CK, NPK and NPKS treatments decreased significantly with the increasing of aggregate sizes; however, soil aggregate contents of the NPKS, 10%NPKS and 20%NPKS treatments increased significantly with the increasing of aggregate sizes (Figure 4). Our results showed that long-term straw return increased the content of large macroaggregates (>0.5 mm), with the 10%NPKS treatment showing the greatest increase.

Additionally, soil aggregate stability was significantly affected by long-term straw return (p < 0.05, Figure 5). Compared with CK and NPK treatments, the MWD values of the NPKS, 10%NPKS and 20%NPKS treatments increased significantly by 21.55–23.95%, 60.69–63.85% and 39.41–42.15%, respectively (p < 0.05, Figure 5a), and the GMD values of those treatments increased significantly by 44.71–47.33%, 92.39–95.87% and 65.40–68.40%, respectively (p < 0.05, Figure 5b), and the WSA values of those treatments increased significantly by 16.99–17.04%, 23.89–23.89% and 21.29–21.34%, respectively (p < 0.05, Figure 5c). In contrast, the Dm values of the NPKS, 10%NPKS and 20%NPKS treatments decreased significantly by 3.84–3.96%, 7.96–8.08% and 5.72–5.84%, respectively (p < 0.05, Figure 5d), while the MWSSA values of the NPKS, 10%NPKS and 20%NPKS treatments decreased significantly by 26.76–27.41%, 39.14–39.68% and 33.07–33.66% as compared to CK and NPK treatments, respectively (p < 0.05, Figure 5e). Furthermore, compared to NPKS and 20%NPKS treatments, the MWD and GMD values of the 10%NPKS treatment increased significantly by 32.19% and 15.26%, respectively (p < 0.05, Figure 5a,b), while the Dm value of the 10%NPKS treatment decreased significantly by 4.29% and 2.38%, respectively (p < 0.05, Figure 5d). Therefore, the MWD, GMD and WA values of the 10%NPKS treatment had higher increase; however, the FD and MWSSA values of the 10%NPKS treatment had higher decrease among all the straw return treatments, indicating that long-term straw return increased soil aggregate stability, among which the 10%NPKS treatment had the best effect.

3.4. Yields of Wheat and Maize Under Long-Term Different Straw Return Treatments

There were statistically significant effects of the long-term straw return treatments on wheat and maize yields (Figure 6). The yields of wheat and maize in this study were 11.17–49.17 kg 50⁻¹ m⁻² and 60.17–99.00 kg 50⁻¹ m⁻², respectively (Figure 6). Compared to CK treatment, the NPK, NPKS, 10%NPKS and 20%NPKS treatments significantly increased wheat yield with the values of 297.01–340.30% (Figure 6a), while those treatments significantly increased maize yield with the values of 39.34–64.54% (Figure 6b). Furthermore, compared to NPK treatment, the NPKS treatment significantly increased wheat yield by 10.90%, (p < 0.05, Figure 6a); however, 10%NPKS and 20%NPKS had no significant difference (p > 0.05, Figure 6a). Conversely, compared to NPK treatment, the NPKS treatment no significant difference (p > 0.05, Figure 6b); however, 10%NPKS and 20%NPKS significantly increased wheat yield by 17.89% and 18.09%, respectively (p < 0.05, Figure 6b). Thus, while long-term straw return can crop yields, it is also dependent on the crop and straw types as compared to no fertilizer and chemical fertilizer.

3.5. Relationship Between Soil Nutrient Stoichiometric Ratio, Aggregate Stability and Crop Yield

Correlation heatmap analysis revealed significant relationships between soil nutrients, their stoichiometric ratios, and both aggregate stability parameters and crop yield (Figure 7). Soil pH was negatively correlated with SOC, TN, C/P, N/P, P1, MWD, GMD, WSA, WY, and MY, but positively correlated with P4, FD, and MWSSA (p < 0.05). In contrast, both SOC and TN exhibited inverse correlation patterns relative to pH. The ratios C/P and N/P showed positive correlations with P2, MWD, GMD, WSA, and MY, whereas they were negatively correlated with P3, P4, FD, and MWSSA (p < 0.05). Furthermore, P1 and P2 were positively associated with MWD, GMD, and WSA, but negatively correlated with FD and MWSSA (p < 0.05). In comparison, P3 and P4 demonstrated opposite trends relative to P1 and P2 (p < 0.05). Among the aggregate stability indicators, MWD, GMD, and WSA were mutually positively correlated, while all three were negatively correlated with FD and MWSSA (p < 0.05). In terms of yield components, both WY and MY were positively related to P2, GMD, and WSA, and negatively related to P4, FD, and MWSSA (p < 0.05).

3.6. Contribution of Soil Nutrient Stoichiometric Ratio, Aggregate Stability to Crop Yield Under Long-Term Diferent Straw Return Treatments

Principal component analysis (PCA) was conducted to decipher the multivariate associations among selected soil aggregate parameters, nutrient metrics, and five distinct fertilization treatments, with the outcomes visualized in the biplot of PC1 (accounting for 64.8% of total variance) and PC2 (11.2% of total variance) (Figure 8). The fertilization treatments were CK (no fertilization), NPK (chemical fertilization), NPKS (straw return coupled with chemical fertilization), 10%NPKS (straw return supplemented with 10% straw-decomposing microbial inoculant plus chemical fertilization), and 20%NPKS (straw return with 20% straw-decomposing microbial inoculant plus chemical fertilization). The vectors in the biplot denote various soil attributes: soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), total potassium (TK), their stoichiometric ratios (C/N, C/P, N/P), as well as soil aggregate-related indices including mean weight diameter (MWD), geometric mean diameter (GMD), >0.25 mm water-stable aggregate (WSA), fractal dimension (FD), and mean weight-specific surface area (MWSSA).

Notably, the biplot exhibits distinct clustering patterns for the treatments. NPKS, 10%NPKS, and 20%NPKS are aggregated in the positive quadrant of PC1, showing strong positive correlations with SOC, TN, MWD, GMD, WSA, and ratios such as N/P and C/P. This indicates that these treatments, which integrate straw return and microbial inoculants, substantially promote soil organic matter accumulation, nutrient availability, and aggregate stability. In contrast, CK is located in the negative quadrants of both PC1 and PC2, correlating with pH and C/N, which reflects the low soil fertility under no fertilization. The NPK treatment is situated in the positive quadrant of PC2, associated with TP and MWSSA, implying that sole chemical fertilization mainly influences soil phosphorus content and the specific surface area of soil aggregates. In summary, PCA effectively differentiates the effects of various fertilization strategies on soil aggregate parameters and chemical properties, underscoring the benefits of straw return combined with microbial inoculants in enhancing soil quality.

Based on the two-dimensional redundancy analysis (RDA) between crop yield and soil nutrients under different fertilization treatments (Figure 9a), the following results were observed. The first two RDA axes explained a substantial proportion of the variance in the relationship between soil properties and crop yields. Treatments such as NPK and NPKS, along with their variants (10%NPKS and 20%NPKS), were distinctly separated from the control (CK) along the first RDA axis, indicating a clear fertilization effect. Soil parameters including SOC, TN, TP, and TK showed positive correlations with crop yields (WY and MY), particularly under straw-return treatments (NPKS, 10%NPKS, 20%NPKS). Specifically, SOC and TN vectors were closely associated with the NPKS and 20%NPKS treatments, suggesting that these treatments significantly enhanced soil organic carbon and total nitrogen, which in turn supported higher wheat and maize yields. The ratios C/N, C/P, and N/P also contributed to the explained variance, with C/N and C/P appearing more influential in differentiating the straw-amended treatments from the mineral-only NPK treatment. Notably, the 20%NPKS treatment was positioned closer to the yield vectors (WY and MY) than the 10%NPKS and NPKS treatments, indicating a stronger positive effect on crop production, likely due to the enhanced straw decomposition and nutrient release facilitated by the higher dosage of microbial inoculant. In contrast, the CK treatment was located in the opposite direction of most nutrient vectors, confirming the limitations in soil nutrient availability and crop yield under unfertilized conditions. Overall, the RDA illustrates that integrated fertilization practices, especially straw return combined with decomposing microbial inoculants (20%NPKS), significantly improve soil nutrient status and are strongly associated with increased crop yields.

Similarly, to explore the multivariate relationships between crop yield and soil aggregate stability under different fertilization treatments, redundancy analysis (RDA) was performed, and the results are presented in a two-dimensional biplot (Figure 9b). The RDA axes, RDA1 and RDA2, accounted for 95.06% and 4.94% of the total variance, respectively, indicating that RDA1 was the dominant explanatory axis. In the RDA biplot, fertilization treatments exhibited distinct spatial distributions. The CK treatment was clustered in the negative region of RDA1, reflecting its weak associations with both crop yield and soil aggregate stability indicators. The NPK treatment (chemical fertilization) was located in the positive region of RDA1 but relatively distant from the key aggregate stability and yield variables. In contrast, NPKS (straw return plus chemical fertilization), 10%NPKS (straw return with 10% straw-decomposing microbial inoculant plus chemical fertilization), and 20%NPKS (straw return with 20% straw-decomposing microbial inoculant plus chemical fertilization) were positioned in the positive region of RDA1, showing close correlations with crop yield and aggregate stability parameters. Regarding the variables, wheat yield (WY) and maize yield (MY) were positively correlated with large aggregate fractions (P1: >2 mm, P2: 0.5–2 mm) and aggregate stability indices, including mean weight diameter (MWD), geometric mean diameter (GMD), and >0.25 mm water-stable aggregate (WA), all of which were oriented along the positive direction of RDA1. Conversely, small aggregate fractions (P3: 0.25–0.5 mm, P4: <0.25 mm), fractal dimension (FD), and mean weight-specific surface area (MWSSA) were aligned in the negative direction of RDA1, indicating their negative associations with crop yield. These patterns suggest that fertilization regimes profoundly influenced soil aggregate stability and, consequently, crop productivity. The CK treatment, due to the absence of nutrient input, resulted in poor soil aggregate structure, which limited crop yields. While the NPK treatment improved soil conditions to some extent, it was less effective than treatments involving straw return. NPKS, 10%NPKS, and 20%NPKS enhanced crop yields by promoting the formation of large, stable aggregates (evidenced by higher MWD, GMD, and WA). Notably, the incorporation of straw-decomposing microbial inoculants (10%NPKS and 20%NPKS) further optimized soil aggregate stability, implying that combining straw return with microbial inoculants can effectively synergize soil aggregate stability and crop productivity.

4. Discussion

4.1. Effects of Long-Term Straw Return on Soil Basic Chemical Properties and C:N:P Stoichiometry

The practice of straw incorporation exerts influence on soil properties by increasing organic matter content and altering pH via mineralization [42]. Furthermore, it facilitates the release of soil nutrients, notably nitrogen, phosphorus, and potassium, from the decomposed straw, thereby functioning as an effective soil amendment that improves the soil’s nutrient pool [56]. In this study, long-term straw return combined with a straw-decomposing microbial inoculant (10%NPKS, 20%NPKS) significantly reduced soil pH (Figure 2), likely due to the accumulation of acidic metabolites during straw decomposition and enhanced leaching of base cations [57]. Concurrently, these treatments significantly increased soil organic carbon (SOC) and total nitrogen (TN) contents, while total phosphorus (TP) remained statistically unchanged across all treatments (Figure 2). As a result, elevated C/P and N/P ratios were observed under the straw return with inoculant treatments (Figure 3). Our results demonstrate that the integrated application of straw with decomposing inoculant and chemical fertilizer (10%NPKS, 20%NPKS) most effectively enhanced soil fertility in the Shajiang black soil, as indicated by significant increases in SOC and TN compared to other treatments. The superior performance of these integrated treatments can be attributed to the synergistic effect between straw decomposition and nutrient supply [58]. The microbial inoculant likely accelerated straw decomposition, releasing organic acids and metabolites that contributed to SOC accumulation, while the chemical fertilizer provided readily available nitrogen that prevented microbial immobilization from limiting crop availability [59].

However, the sole application of chemical fertilizer with straw but without specialized microbial inoculant (NPKS) resulted in a moderate increase in SOC but had a minimal effect on TN sequestration compared to the NPK treatment. This can be explained by the slower decomposition of straw in the absence of specialized decomposing microbes, leading to limited nitrogen release from the straw and reduced nitrogen retention in stable organic forms [39,46]. Moreover, the sole application of chemical fertilizer (NPK) resulted in a moderate increase in TN but had a minimal effect on SOC sequestration. This aligns with the established understanding that chemical fertilizers primarily supply readily available nutrients for plant uptake but contribute little to the stable soil organic carbon pool [28,58]. In contrast, straw return alone (S) elevated SOC but led to a relative nitrogen deficit, as reflected in a higher C:N ratio. This phenomenon represents a typical “nitrogen mining” effect, where microbial decomposition of high C:N straw immobilizes available nitrogen, potentially competing with crops for this essential nutrient [11,29].

The synergy observed in the 20%NPKS treatment is therefore critical. The chemical fertilizer in this combined practice likely provided the immediate N supply necessary to meet both crop demand and support the microbial decomposition of the straw without inducing a net N immobilization [6,9]. This facilitated the more efficient conversion of straw-derived carbon into stable SOC and microbial biomass, thereby optimizing the C:N ratio to a level conducive for both microbial activity and plant growth [23]. Furthermore, the 20%NPKS treatment effectively balanced the C:N:P stoichiometry towards a more optimal state. The significant increase in SOC without a proportional surge in total P (which remained relatively stable across treatments) led to a widened C:P ratio. This shift suggests an enhanced capacity for the soil to retain organic carbon and may reduce the fixation of available phosphorus, improving its long-term bioavailability [38]. Thus, this balanced stoichiometry under 20%NPKS creates a more favorable biochemical environment for soil biota and plant roots.

4.2. Enhancement of Soil Aggregate Content and Stability

Our results clearly demonstrate that the long-term incorporation of straw, particularly when combined with a straw-decomposing microbial inoculant (10%NPKS and 20%NPKS treatments), fundamentally reshapes soil aggregate architecture and enhances its stability (Figure 4 and Figure 5). The most striking observation is the shift in aggregate size distribution, where all straw-return treatments promoted the formation of larger macroaggregates (>0.5 mm) at the expense of smaller microaggregates (Figure 4). This phenomenon can be attributed to the increased input of organic matter, which serves as a primary binding agent for microaggregates into larger units [60]. The added straw provides a carbon substrate for soil microbes, which in turn produce organic glues such as polysaccharides and hyphae that cement primary particles [31,33]. Notably, the application of microbial inoculant (10%NPKS and 20%NPKS) significantly amplified this effect compared to straw return alone (NPKS). The 10%NPKS treatment was particularly effective, even reversing the natural decline in aggregate content with size observed in the control and NPK treatments (Figure 4). This indicates that the microbial inoculant does not merely accelerate decomposition but fosters a more efficient and robust aggregation process, likely by enriching specific microbial consortia that are highly proficient in producing binding agents [31,38].

The superior performance of the inoculant-amended treatments, especially 10%NPKS, is unequivocally confirmed by the comprehensive suite of aggregate stability indices. The significant increases in mean weight diameter (MWD), geometric mean diameter (GMD), and >0.25 mm water-stable aggregates (WA), coupled with the marked decreases in the fractal dimension (FD) and mean weight-specific surface area (MWSSA) indices, collectively point to the formation of stronger, more resilient aggregates (Figure 5). The fact that the 10%NPKS treatment outperformed both the NPKS and 20%NPKS treatments reveals a non-linear, optimal dosage effect. This suggests that a moderate application rate (10%) may establish a more balanced and functionally synergistic microbial community, maximizing the production of stabilizing agents. In contrast, a higher dose (20%) might lead to competitive exclusion or an imbalance in microbial succession, thereby reducing the efficiency of aggregate formation [38,61]. This optimal microbial community at the 10% rate likely enhances the humification of straw-derived carbon and its subsequent integration into stable aggregate fractions [39]. Therefore, our findings underscore that the co-application of straw with a tailored microbial inoculant is a critically important strategy for improving soil structure, surpassing the benefits of straw or chemical fertilizer alone. The 10%NPKS treatment emerged as the most promising practice, effectively enhancing soil aggregation and stability. This improvement is crucial for enhancing soil porosity, erosion resistance, and ultimately, carbon sequestration potential in agricultural systems.

4.3. Synergistic Effects on Crop Yield

The significant yield enhancements of both wheat and maize under long-term straw return treatments underscore the pivotal role of organic amendments in sustaining agricultural productivity (Figure 6). The markedly higher yields in all NPK and straw-return treatments compared to the unfertilized control (CK) highlight the fundamental necessity of nutrient supplementation, whether chemical or organic, for crop production [62,63]. Notably, the response to specific straw management practices exhibited a distinct crop-specific pattern. For wheat, NPKS treatment provided a significant yield advantage over NPK, likely due to improved soil physical properties and a more balanced nutrient supply [48,64]. However, the addition of a straw-decomposing microbial inoculant (10%NPKS and 20%NPKS) did not confer a further significant yield increase over NPKS (Figure 6a). This suggests that for wheat, the benefits of straw incorporation might be primarily physical and nutritional, with the natural soil microbial community being sufficient for decomposition within the wheat growing cycle [65]. The potential for the inoculant to cause a rapid, intense release of nutrients early in the season might not synchronize perfectly with the nutrient uptake pattern of wheat, or its longer growth period may allow for adequate natural decomposition [66].

Conversely, NPKS treatment showed no significant benefit over NPK, the 10%NPKS and 20%NPKS treatments significantly boosted yields (Figure 6b). Maize, with its higher growth rate and greater nutrient demand during a warmer period, may benefit greatly from the accelerated decomposition and nutrient mineralization facilitated by the inoculant. This rapid nutrient release likely better synchronizes with the high nutrient demand of maize, preventing temporary nutrient immobilization that can sometimes occur with untreated straw [67]. The superior performance of the inoculant-amended treatments for maize underscores the importance of tailoring straw management strategies to the specific physiological demands and growing environments of different crops. Therefore, while straw return is a beneficial practice, the efficacy of a straw-decomposing microbial inoculant is crop-dependent, offering a significant advantage for maize but not for wheat under the conditions of this long-term experiment.

4.4. Contributions of Soil C:N:P Stoichiometric Ratio and Aggregate Stability to Crop Yield

Multivariate analyses (correlation, PCA and RDA) elucidate the mechanistic pathways through which long-term straw return enhances crop yield, primarily by concurrently improving soil structure and nutrient cycling (Figure 7, Figure 8 and Figure 9). We observed a coherent nexus of soil quality improvement, evidenced by strong positive correlations among SOC, TN, C/P, N/P ratios, macro-aggregate content, and stability indices (MWD, GMD, WSA) (Figure 7). This suggests that straw amendments, particularly when combined with a microbial inoculant, transform the soil environment not merely by increasing nutrient quantities but by driving the formation of stable macroaggregates, likely through the accelerated production of microbial-derived binding agents like glomalin and polysaccharides [38,68]. A critical finding was the central role of shifted soil stoichiometry. The significantly increased C/P and N/P ratios, while indicating a relative P limitation, also reflect a carbon-rich, microbially active environment [38]. The positive correlation of these ratios with both aggregate stability and maize yield implies that the physical benefits of improved soil structure—such as enhanced porosity and root exploration—may initially outweigh the potential phosphorus limitation for a high-demand crop like maize [69]. Furthermore, long-term straw returning to the field can increase soil organic carbon storage by enhancing the conversion of straw-derived carbon into mineral-associated fungal residues, ultimately leading to increased crop yields [70].

Additionally, the 20%NPKS treatment, despite achieving peak SOC, TN, and the most altered stoichiometry (Figure 2 and Figure 3), underperformed in aggregate stability and yield (Figure 5 and Figure 6). This paradox suggests that the higher inoculant dose induced an imbalance, where excessive microbial activity led to transient nutrient immobilization and a pronounced P limitation, hindering the translation of nutrient abundance into plant-available resources and stable aggregates [69]. In contrast, the 10%NPKS treatment consistently emerged as optimal, achieving the best synergy (Figure 6). The moderate inoculant dose fostered a more balanced microbial community—potentially enhancing fungal contributions to stable aggregation—and a proportional nutrient release that avoided severe immobilization [71,72]. This created an environment where improved soil structure and synchronized nutrient supply synergistically boosted yields, a effect most pronounced in maize, which is highly responsive to such rapid improvements. In summary, the enhanced crop yields, especially for maize, are a direct result of the synergistic improvements in soil organic matter, aggregate stability, and balanced nutrient stoichiometry, with the 10%NPKS treatment being identified as the superior strategy for integrating these benefits.

5. Conclusions

This long-term study establishes that the integrated application of straw with a straw-decomposing microbial inoculant is a highly effective strategy for enhancing soil health and crop productivity, the efficacy of which is critically dependent on achieving an optimal dosage. Our findings robustly identify the 10%NPKS treatment as the superior management practice, as it uniquely fostered a synergistic improvement in soil physical structure, nutrient cycling, and crop yield. This synergy was manifested through a significant increase in the formation and stability of macroaggregates—driven by the accelerated provision of organic carbon and microbial-derived binding agents—coupled with a balanced enhancement of soil organic carbon and total nitrogen. A key insight from this research is the elucidation of the non-linear, dose-dependent response. While the higher inoculant dose (20%NPKS) maximized SOC sequestration and elevated total nutrient pools, it also induced a pronounced shift in soil ecological stoichiometry, elevating C/P and N/P ratios to levels suggesting the onset of phosphorus limitation. This, combined with the potential for transient nutrient immobilization by an excessively stimulated microbial biomass, ultimately attenuated the agronomic benefits of the 20%NPKS treatment, underscoring that “more is not always better”. Furthermore, the crop-specific yield responses—with maize showing a greater dependence on the inoculant-driven acceleration of nutrient cycling and structural improvements, while wheat benefited substantially from straw return alone—highlight the importance of tailoring management practices to the dominant cropping system. Therefore, we strongly recommend the co-application of straw with a 10% straw-decomposing microbial inoculant as a sustainable practice to achieve synergistic gains in soil quality and grain production, particularly in systems dominated by high-demand crops like maize. Future research should prioritize elucidating the specific shifts in microbial community structure and function, particularly the fungal-to-bacterial ratio and the dynamics of key functional genes, that underpin these observed differential effects to further refine and optimize microbial inoculation strategies.