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Article

Effects of Pig Slurry Coupled with Straw Mulching on Soil Nitrogen Dynamics and Maize Growth

by
Yali Yang
1,2,†,
Dengchao Lei
1,2,†,
Yulan Zhang
1,2,
Zhe Zhao
3,
Hongtu Xie
1,2,
Fangbo Deng
1,2,
Xuelian Bao
1,2,
Xudong Zhang
1,2 and
Hongbo He
1,2,*
1
Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China
2
Key Lab of Conservation Tillage and Ecological Agriculture, Shenyang 110016, China
3
College of Water Conservancy, Shenyang Agricultural University, Shenyang 110866, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Agronomy 2025, 15(5), 1062; https://doi.org/10.3390/agronomy15051062
Submission received: 18 March 2025 / Revised: 15 April 2025 / Accepted: 25 April 2025 / Published: 27 April 2025
(This article belongs to the Special Issue Exploring Mechanisms and Technologies for Enhancing Nitrogen Efficiency in Maize Production)
Browse Figures
Versions Notes

Abstract

:
The balanced application of organic and chemical fertilizers is essential for maintaining soil fertility and crop productivity. To optimize nitrogen (N) balance and maize yield through integrated pig slurry and straw mulching management, a split-plot field experiment was conducted in Northeast China. The study included two straw treatments (straw mulching, S; no straw, NS) and three substitution levels of pig slurry for chemical fertilizer (0%, 20%, and 40%; denoted as M0, M20, and M40). Parameters evaluated included N balance, maize biomass, soil available N, and the mineral N to TN ratio (mineral-N/TN), measured across 0–100 cm at key maize growth stages. Results showed that pig slurry substitution significantly increased soil DON, mineral N, and mineral-N/TN in the topsoil (0–20 cm) at the maize seeding stage and decreased mineral-N/TN at the maize milk (10–40 cm) and maturity (80–100 cm) stages. Meanwhile, straw mulching reduced NH4+-N accumulation in the 0–10 cm of topsoil at the seeding stage, decreased NO3−-N in the 0–40 cm soil layer from the jointing to maturity stages, and lowered the mineral-N/TN ratio in the topsoil, thereby mitigating the risk of N leaching. Notably, the combination of pig slurry substitution and straw mulching slightly increased DON and NO3−-N in the topsoil while significantly reducing the mineral-N/TN in the deep soil layer at the seeding and milk stages. Pig slurry substitution significantly improved maize yield, N uptake, and N use efficiency (NUE). The highest maize yield (14,628 kg ha−1) was observed in the S-M20 treatment, representing a 19% increase compared to NS-M0. N balance analysis indicated that pig slurry substitution alone increased maize yield and N uptake but depleted soil N, whereas straw mulching maintained N surplus. The findings highlight that combining pig slurry with straw mulching optimizes soil N availability and improves sustainable N management and crop productivity in agroecosystems.

1. Introduction

Soil nitrogen (N) dynamics play a crucial role in determining soil fertility and crop productivity, particularly in intensively managed agricultural systems [1]. Proper application of chemical fertilizers can increase soil N content and promote crop yield [2]. However, the excessive and inappropriate use of chemical fertilizers in agricultural practices, particularly in China, has resulted in mismatches between fertilizer application and crop nutrient requirements, negatively impacting agricultural production [3]. Over-application of fertilizers often leads to reduced fertilizer efficiency, N loss, and environmental pollution [4,5,6]. Conversely, under-application fails to meet crop N demands, depletes soil N and organic matter, and accelerates soil degradation [7]. Therefore, sustainable management strategies are essential for maintaining soil health while minimizing environmental impacts such as N leaching risk.
Organic fertilizers, particularly animal manures, are nutrient-rich and contain diverse organic compounds that can serve as effective substitutes for chemical fertilizers [8]. When applied appropriately, manure can enhance soil nutrient content, improve grain quality, boost crop yield, and reduce reliance on chemical fertilizers [9,10]. Research has shown that the combined use of organic and chemical fertilizers improves N uptake and accumulation in crops by 16–19%, optimizing nutrient efficiency [11,12]. However, excessive application of organic manure leads to N losses through leaching and volatilization, raising environmental concerns [13,14]. For instance, Yang et al. [15] reported that increasing organic manure substitution from 30% to 50% resulted in a 39% increase in apparent N loss.
In addition to manure, crop straw is another essential organic input that plays a significant role in maintaining soil fertility. Straw returning enhances soil organic matter, regulates microbial N supply, and improves N retention capacity [16,17,18]. Due to its high carbon-to-nitrogen ratio (C/N), straw returning can mitigate N leaching by immobilizing excess N in microbial biomass [19,20]. However, this effect may also induce temporary N immobilization, creating competition between soil microbes and crops for available N, potentially reducing crop yields [21]. Several manure management practices have been shown to reduce N leaching by increasing the initial C/N through the addition of carbon (C)-rich materials (e.g., straw) [22]. The co-application of straw and organic manure improves soil N dynamics, enhances crop nutrient uptake, reduces N loss, increases soil fertility, and boosts crop yield [23,24,25,26].
Although numerous studies have investigated the combined application of organic fertilizers and crop straw, most focused on solid manure, while research on the integration of liquid manure (such as pig slurry) with straw remains limited. Pig slurry, a nutrient-rich liquid manure, contains a significant amount of NH4+-N and NO3−-N, which account for 46−93% of its N nutrient [27]. Its lower C/N ratio enhances nutrient availability for crop uptake but also increases the risk of N loss and soil infiltration. Therefore, integrating pig slurry with straw returning offers a promising approach to balance N availability, improve soil fertility, and enhance nitrogen use efficiency (NUE) in crop production systems [22,28]. However, the optimal ratio of pig slurry and straw application for balancing soil N supply and crop uptake remains unclear.
Here, we conducted a field experiment investigating the effects of substituting chemical fertilizer with pig slurry at various levels, both with and without straw mulching. The objective of this research is to optimize the ratio of organic and chemical fertilization to achieve N balance between soil and crops, ensuring both crop yield and soil fertility. We hypothesized that (1) partial substitution of chemical N fertilizer with pig slurry would improve N use efficiency by synchronizing soil N release with crop demand, and (2) combining slurry substitution with straw mulching would amplify these benefits by modulating the C/N ratio of exogenous inputs, therefore increasing crop yields and reducing N leaching losses.

2. Materials and Methods

2.1. Study Site and Collection of Samples

This study was conducted in April 2023 at the Changtu Modern Agricultural Experimental Station of the Institute of Applied Ecology, Chinese Academy of Sciences (42°48′ N, 123°57′ E), which is located in Liaoning Province, Northeast China. The mean annual precipitation and temperature were approximately 602.5 mm and 7 °C, respectively. The total precipitation and annual temperature in 2023 were 658 mm and 8.6 °C, respectively. The precipitation was 561.7 mm during the maize growing season, which mainly concentrated in June-August (Figure 1). The soil is classified as Alfisols with 25% clay, 28% silt, and 47% sand. The initial soil properties across four depth layers (0–20, 20–40, 40–60, and 60–100 cm) were as follows: soil water content—22.61%, 23.76%, 23.29%, and 20.32%; pH—5.25, 5.85, 6.36, and 6.69; soil organic carbon (SOC)—9.27, 7.64, 4.80, and 3.20 g kg−1; and total nitrogen (TN)—0.91, 0.72, 0.51, and 0.43 g kg−1.
The field trials were conducted using a split-plot design with three replications. The main plots contained straw mulching (S) and no straw (NS), with different manure treatments randomly assigned within each main plot, which were 0%, 20%, and 40% substitution of N chemical fertilizer by pig slurry (Table 1). There were six treatments: S-M0, S-M20, S-M40, NS-M0, NS-M20, and NS-M40. By substituting 40% of the N from chemical fertilizer, 170 t ha−1 of pig slurry was used, while the 20% substitution resulted in 85 t ha−1. All plots received the same total fertilizer amounts: N 220 kg ha−1, P2O5 76 kg ha−1, and K2O 110 kg ha−1. During the application process, water was added to ensure equal liquid content across all treatments. The maize straw was cut into 10–20 cm sections and mulched at the soil surface in the S plot, and all straw in the NS plot was removed before seeding.
The total N content of maize straw was 7.5 g kg−1, and the total C content was 450 g kg−1, with a C/N ratio of 60. The nutrient content of the pig slurry used in the experiment was as follows: TN, 1.22 g kg−1; TC, 6.10 g kg−1; C/N ratio, 5; dissolved organic carbon (DOC), 373.67 mg kg−1; dissolved organic nitrogen (DON), 520.83 mg kg−1; total phosphorus (TP), 0.085 mg kg−1; and total potassium (TK), 0.20 mg kg−1. Fermented pig slurry was applied one week before sowing, and chemical fertilizer was applied on the sowing day to supplement the remaining required nutrients. The water content of the pig slurry exceeded 95%. To ensure consistency in soil moisture across treatments, additional water applications of approximately 170 t ha−1 and 85 t ha−1 were applied to the 0% and 20% substitution treatments, respectively.

2.2. Sample Collection and Measurements

(1) Sample collection
Soil samples were collected following the principle of random sampling with depths of 0–10, 10–20, 20–40, 40–80, and 80–100 cm at the maize seedling (33 days after sowing, 13 June), jointing (56 days after sowing, 6 July), milk (90 days after sowing, 9 August), and maturity (138 days after sowing, 26 September) stages. The samples were passed through a 2 mm sieve by gently pressing the soil clods and removing fine roots, straw residue, and debris. The fresh soil samples were divided into two parts: one part was stored at 4 °C for analysis of NH4+-N, NO3−-N, and DON, and the rest was air-dried and used for determination of pH and TN. Plant samples were collected in three replications for each treatment at the seedling, jointing, milk, and maturity stages, with maize plants separated into roots, straws, cobs, and grains. Fresh plant samples were dried in an oven for 60 min at 105 °C to deactivate enzymes and then dried at 65 °C until a constant weight was reached for dry weight determination. Dry plant samples were ground through a 100 mesh sieve to measure TN.
(2) Soil physicochemical properties
Soil moisture content was determined by drying the samples at 105 °C until a constant weight was achieved. Soil pH was measured using a pH meter (Shjingmi, Shanghai, China) with a soil-to-water ratio of 1:2.5. DOC was extracted with ultrapure water at a ratio of 1:5, extracts were filtered through a 0.45 µm filter, and directly analyzed by multi N/C 3100 (Analytik Jena AG, Jena, Germany). Soil NH4+-N and NO3−-N were extracted with 2 mol L−1 KCl solution at a ratio of 1:5 and analyzed using a continuous flow auto-analyzer (AMS Alliance, Frépillon, France). Mineral N was calculated as the sum of NO3−-N and NH4+-N. The TN content of plant and soil samples was determined by dry combustion using an elemental analyzer (Elementar, Frankfurt, Germany).

2.3. Method of Calculation

(1) Maize yield: at the maturity stage, 10 ears of maize were randomly selected from each sample area, and the total number of ears in the sample area was recorded. Ears were threshed, dried to a moisture content of 14%, and weighed to calculate the grain yield per unit area.
(2) The plant N uptake [29] was measured according to Equation (1):
N uptake (kg ha−1) = (Ʃ plant N contenti × plant dry matteri)/1000
In Equation (1), i denotes the straws (including leaves), roots, cobs, and grains.
(3) The N balance in the soil–plant system [30,31] was determined by calculating the difference between the total input N flows in crop production (chemical fertilizer N, pig slurry N, straw N, biological fixation of N, and atmospheric deposition N) and the total output N flows in crop production, according to Equation (2):
Nbalance = Ʃ (Ninputs) − Ʃ (Noutputs)
In Equation (2), atmospheric N deposition refers to atmospheric N deposition in China from 2010 to 2020 [32]; biological N fixation refers to the amount of nonsymbiotic N fixation per unit area in dryland farmland [33].
(4) The NUE, including N partial factor productivity, N agronomic efficiency, and N apparent recovery efficiency, was calculated as described in Supplementary Table S2.

2.4. Data Statistics and Analysis

Data processing was performed using Excel (2021), statistical analysis was conducted with SPSS 26.0 software (IBM Co., Armonk, NY, USA), and graph rendering was performed using Origin (2022). Linear mixed effects models were used to assess how straw management, manure substitution, and their interactions affected different forms of N, crop biomass, and N uptake by using the “lme4” and “lmerTest” packages in R (4.4.3). The normality of model residuals was tested using the Shapiro–Wilk test, and the homogeneity of variances was assessed by visual assessment with a residual diagram. Non-normal data were transformed using square root (sqrt) or logarithmic (log) transformations to improve model robustness, guided by the optimal lambda values derived from the Box–Cox analysis using the “MASS” package in R (4.4.3). One-way analysis of variance (ANOVA) was performed by Duncan’s multiple range test (p < 0.05) for post hoc test comparisons in SPSS 26.0 software.

3. Results

3.1. Soil Available Nitrogen

During the maize growth period, the DON content in the 0–20 cm soil layer exhibited a continuous decline, with an overall reduction of 81.78% at the maturity stage compared to the seedling stage (Figure 2). At the seedling stage, straw mulching significantly decreased DON content in the 20–80 cm soil layer. Manure management significantly affected DON in the 0–20 cm and 40–100 cm soil layers (p < 0.05) (Figure 2a). Specifically, the substitution of pig slurry significantly increased the DON content in the 0–20 cm soil layer, whereas it decreased in the 40–100 cm soil layer (p < 0.05) (Table S3). Furthermore, these effects were also significantly (p < 0.05) influenced by the interaction of manure and straw management in the 0–40 cm soil layer, which was mainly reflected in the treatments with straw mulching combined with pig slurry substitution rather than no straw treatments (Table S3). Compared to S-M0, S-M40 and S-M20 significantly increased DON content by 122.81% and 28.72%, respectively (Table S3). At the jointing stage, straw mulching decreased DON content in the 0–20 cm and 40–80 cm soil layers, with a gradual decrease from the soil surface downward (Figure 2b). At the milk stage, straw mulching significantly reduced the DON content in the 0–20 cm soil layer (p < 0.05) by 9.52 mg kg−1 (Figure 2c). At the maturity stage, straw mulching significantly decreased DON content in the 20–80 cm soil layer (Figure 2d).
Throughout the maize growth period, the NO3−-N content in the 0–20 cm soil layer gradually decreased, with an overall reduction of 76.74% at the maturity stage compared to the seedling stage (Figure 3). At the seedling stage, pig slurry substitution significantly increased NO3−-N content in the 0–20 cm soil layer (p < 0.05), with 40% and 20% pig slurry substitution treatments increasing NO3−-N content by 47.21% and 2.51%, respectively, compared to M0 (Figure 3a). There was an interactive effect between straw and manure management. S-M40 exhibited the highest increase in NO3−-N content, by 98.22%, compared to S-M0 (Table S3). Conversely, in the 80–100 cm soil layer, NO3−-N content was also influenced by the interaction of straw and manure management, with the highest NO3−-N level observed under NS-M40 (Table S3). At the jointing, milk, and maturity stages, straw mulching significantly reduced NO3−-N content, mainly in the 0–40 cm soil layer (p < 0.05) (Figure 3b–d). Pig slurry substitution significantly decreased NO3−-N content in the 80–100 cm at jointing and milk stages and in the 10–20 cm and 40–80 cm at maturity stage (Figure 3). Additionally, the interaction between straw and manure management had a significant effect on soil NO3−-N content in the 80–100 cm at the milk stage (S-M40, S-M20 < S-M0) and 40–80 cm at the maturity stage (NS-M40 < NS-M20) (Figure 3).
From the seedling stage to the jointing stage, soil NH4+-N content gradually decreased with increasing soil depth, whereas no significant difference was observed across soil depths during the milk and maturity stages (Figure 4). At the seedling stage, straw management significantly affected NH4+-N content in the 0–10 cm and 40–100 cm soil layers, while manure management also had significant effects in the 0–10 cm soil layer (p < 0.05) (Figure 4a). Specifically, straw mulching reduced NH4+-N content, while pig slurry substitution increased it. A significant effect of the interaction between straw and manure management was observed in the 0–10 cm and 40–100 cm soil layers (p < 0.05) (Figure 4a). Pig slurry substitution treatments significantly increased NH4+-N content, excepting S-M40 in the 0–10 cm soil layer (Table S4). Compared to chemical fertilizer alone, the combined application of pig slurry substitution with straw mulching reduced NH4+-N content in the 40–100 cm soil layer, whereas pig slurry substitution alone increased NH4+-N accumulation (Table S4). At the jointing stage, straw mulching significantly decreased NH4+-N content in the 0–10 cm surface soil layer (p < 0.05) (Figure 4b). At the milk stage, straw mulching significantly increased NH4+-N content in the 10–20 cm soil layer (p < 0.05) (Figure 4c). At the maturity stage, manure management had a significantly negative effect on NH4+-N content in the 20–80 cm (p < 0.05) (Figure 4d). Furthermore, significant straw and manure interactions were found in the 10–20 cm and 40–80 cm soil layers (Figure 4d).
The mineral-N/TN decreased across the maize growth period, with higher values observed in the 0–20 cm soil layer during the seedling and jointing stages and lower values during the filling and maturity stages (Figure 5). At the seedling stage, straw management negatively affected mineral-N/TN in the 0–10 cm and 80–100 cm soil layers, while manure management showed significantly positive effects on mineral-N/TN in the 0–20 cm and 80–100 cm soil layers (p < 0.05) (Figure 5a). A significant interaction of straw and manure was observed in the 10–40 cm and 80–100 cm soil layers. Specifically, the mineral-N/TN of S-M40 was significantly higher than S-M20 in the 10–40 cm soil layer (Table S4). Compared to chemical fertilizer alone, the combined application of pig slurry substitution with straw mulching reduced mineral-N/TN in the 80–100 cm soil layer (p < 0.05), whereas pig slurry substitution alone had no significant effect (Table S4). At the jointing and milk stages, straw mulching significantly decreased mineral-N/TN in the 0–20 cm soil layer (p < 0.05) (Figure 5b, c). At the maturity stage, straw mulching had a significantly negative effect on mineral-N/TN in the 0–10 cm soil layer, while pig slurry substitution significantly decreased mineral-N/TN in the 10–40 cm soil layers (Figure 5d). In addition, a significant effect of straw and manure interaction was found in the 10–20 cm soil layers. Only the combined application of pig slurry substitution with straw mulching reduced the mineral-N/TN, while pig slurry substitution alone showed no significant effect (Table S4).

3.2. Maize Nutrient Absorption

Straw mulching reduced maize biomass at the seedling stage (p < 0.05) but had no significant effect at the later stage (Figure 6a). Pig slurry substitution had a significantly positive effect on maize biomass at the seeding and jointing stages (Figure 6a). Manure management had a significant effect on maize yield (p < 0.01), with 20% substitution being greater than 40% substitution, and both higher than chemical fertilizer alone (Figure 6b). The S-M20 treatment achieved the highest yield of 14,628.76 kg ha−1, which was significantly higher than that of the NS-M0 and S-M0 treatments (p < 0.05), representing a 19.39% increase compared to NS-M0 (Table S5). The yields under NS-M40, S-M40, and NS-M20 treatments were also higher than those under NS-M0 and S-M0, though the differences were not statistically significant. However, straw N uptake of the S-M40 and NS-M20 treatments was significantly higher than NS-M0 and S-M0 (p < 0.05) (Figure 6c). During the growth period of maize, the straw mulching treatment resulted in lower root and straw biomass, as well as lower N uptake at the seedling stage. In contrast, at the milk and maturity stages, the effects of pig slurry substitution combined with straw mulching became more pronounced, with significant improvements observed in root and straw biomass, as well as N absorption (Figure S2). At the maturity stage, total N uptake in maize was significantly higher in the NS-M20 and S-M20 treatments compared to NS-M0 (p < 0.05), while no significant differences were observed among the other treatments (Table S5). Additionally, in general, fertilization management significantly affected the N partial factor productivity, N agronomic efficiency, and N apparent recovery efficiency (Table S2). The NUE of treatments with pig slurry was higher than treatments without substitution, with NS-M20 showing the highest NUE (Table S2). Compared with NS-M0, the partial factor productivity, N agronomic efficiency, and N apparent recovery efficiency of NS-M20 were increased by 12.42%, 19.04%, and 59.36%, and the S-M20 improved by 19.39%, 29.73%, and 40.62%, respectively (Table S2).
Throughout the maize-growing period, the relative change in farmland soil N showed an N deficit under the NS-M40 and NS-M20 treatments, indicating that total N input was lower than the N uptake required for maize growth, resulting in a net N deficit (Table 2). Among these, the NS-M20 treatment exhibited the highest N deficit, with a loss of −50.81 kg ha−1, accounting for 23.09% of the total N applied. In contrast, all straw mulching treatments exhibited an apparent soil N surplus (Table 2). The N surpluses under the S-M40, S-M20, and S-M0 treatments were 44.28, 33.35, and 74.65 kg ha−1, respectively, accounting for 20.13%, 15.16%, and 33.93% of the total N applied. Among these, the N surplus in the S-M0 treatment was the highest (Table 2).

4. Discussion

4.1. Effects of Straw Mulching and Pig Slurry on Migration of Soil Nitrogen Along Soil Profile

Soil N content is a crucial indicator of soil fertility, with available N serving as a key parameter reflecting the N supply potential for crop growth. This can be significantly influenced by different fertilization regimes, which have a notable impact on these factors. For example, Gong et al. [34] found that combining organic and inorganic fertilizers at a 1:1 ratio enhanced the total organic N content in the soil and increased the net N mineralization rate, thereby improving N availability. Similarly, Yang et al. [35] reported that the combined application of organic and inorganic fertilizers resulted in significantly higher concentrations of NO3−-N and NH4+-N compared to the application of chemical fertilizers alone. In our study, we observed that the concentrations of DON, NO3−-N, and NH4+-N in the topsoil during the maize seedling stage were significantly increased under pig slurry substitution, as well as the mineral-N/TN ratio, which indicates a significant increase in N availability to crops (Figure 2a, Figure 3a, Figure 4a and Figure 5a). However, due to the low demand for N during maize growth at the seedling stage, a high available N content can increase the risk of leaching [36]. Compared to chemical fertilizer alone, as time progressed, the mineral-N/TN value of several deep soil layers significantly decreased under treatments with pig slurry substitution at the milk and maturity stages, indicating a reduced risk of N leaching (Figure 5c,d). The variations in N leaching due to pig slurry substitution across different growing periods may be influenced by climate, soil microorganisms, and crop N uptake. Initially, due to the high mineral content of pig slurry, soil microorganisms accelerate the mineralization process and decompose organic matter following pig slurry application, thus increasing the N leaching risk in the early stage [37,38]. Nevertheless, N leaching requires high water input to be generated; in semi-arid fields with rain-fed patterns, spring precipitation is typically low [39]. Subsequently, as crops at the intermediary and later stages require more N and produce exogenous litter, which is high in C/N ratio, soil microorganisms may shift their focus from mineralization to assimilation, therefore decreasing the leaching risk [40,41]. Although high precipitation occurs from the jointing to milk stages in our study, the low risk of leaching indicates a minimal environmental impact from pig slurry. Ultimately, from the foregoing information, it can be found that the leaching risk associated with pig slurry substitution primarily exists in the early stages after its application. In this period, high water input, including precipitation and irrigation, should be avoided.
In addition to avoiding excessive water input after slurry application, combining slurry with crop straw can effectively reduce the risk of N leaching. Unlike pig slurry, straw mulching decreased NH4+-N and mineral-N/TN. Interestingly, compared to chemical fertilizer alone, the combination of pig slurry substitution and straw mulching slightly increased DON and NO3−-N in the topsoil (Figure 2 and Figure 3). This phenomenon may be attributed to straw mulching reducing rainwater infiltration, as evidenced by the observed increase in surface soil moisture content (Table S1). In contrast, combining slurry with crop straw significantly reduced the mineral-N/TN in the deep soil layer, a pattern that was also observed during the milk and maturity stages (Figure 5a,c). This might be achieved by adjusting the soil C/N ratio through the combination, which regulates microbial N immobilization and nutrient retention, promoting the incorporation of NH4+-N and NO3−-N into microbial biomass, thereby effectively reducing N leaching risk while enhancing long-term soil N storage [42,43,44]. Considering the dynamics of mineral N in the soil and changes in N cycling, the decrease in Ureolysis and Nitrate_reduction genes likely explains the significant reduction in NH4+-N content under straw mulching treatments (Figure S3). Furthermore, straw mulching significantly reduced the contents of DON, NO3−-N, and mineral-N/TN in the soil at the jointing and milk stages (Figure 2b,c, Figure 3b,c and Figure 5b,c), contributing to a further decrease in N leaching risk, particularly during growing periods with high precipitation. This effect can be attributed to the improvement of soil structure and increased soil porosity due to straw mulching, which facilitates maize root growth and exudation [45]. Maize root exudates enhance the activity of urease in the rhizosphere soil, promoting the conversion of soil organic N into mineral N, thereby reducing DON content [46]. Moreover, from temporal dimensions, seasonal changes of precipitation and temperature would significantly influence soil N dynamics [47,48]. The humid and warm conditions resulting from high precipitation and temperature accelerate the decomposition of straw, consuming more NO3−-N and thereby inducing a significant decrease in NO3−-N under straw mulching treatments after the jointing stage [49]. However, due to the limitations of short-term experiments in reflecting long-term N dynamics, future studies should focus on long-term effects and systematically evaluate N dynamics, environmental risks, and underlying microbial mechanisms.

4.2. Effects of Straw Mulching and Pig Slurry on Maize Yield, Nitrogen Uptake, and Nitrogen Balance

Compared to the chemical fertilizer application alone, the combined use of organic manure improves soil nutrient availability and enhances crop nutrient uptake, ultimately leading to higher yields [50,51]. Our results showed that substituting 20% and 40% of chemical N with pig slurry significantly increased the final biomass, N uptake, NUE, and yield of maize (Figure 6), which is consistent with the findings of Geng et al. [52]. However, compared to the 20% substitution, the 40% substitution exhibited a decreasing trend in maize yield and N uptake (Figure 6b,c), coinciding with the findings of Xia et al. and Zhai et al. [53,54], who reported that crop yield responses to organic fertilizer substitution follow a nonlinear pattern. Excessive substitution may disrupt the synchronization between soil N release and crop nutrient demand, leading to suboptimal yields. Instead, higher yields are achieved when soil N release patterns align with crop nutrient demands.
On the basis of a better substitution ratio, additional straw mulching can further benefit maize production. Previous research consistently indicated that the combined application of straw and manure with chemical fertilizer enhances maize yield stability and promotes agricultural sustainability [55]. In our study, the highest yield was observed in S-M20, which showed an increasing trend compared to NS-M20 (Figure 6b). Since manure enhances early-stage nutrient utilization but leaves residual nutrients in the soil [56], integrating straw incorporation with manure application can stabilize and retain these residual nutrients, prolonging their availability [57]. This practice ensures a sustained nutrient release that aligns more closely with crop nutrient demands during later growth stages [58], thereby improving maize nutrient absorption efficiency. Moreover, straw mulching significantly increased soil moisture from the seedling to milk stages (Table S1), mitigating the N uptake limitations caused by drought [59]. Although the interaction between pig slurry substitution and straw mulching did not show a significant impact on maize growth in our study, the addition of pig slurry significantly enhanced associated degradation functions (i.e., Chitinolysis, Aromatic_compound_degradation, Cellulolysis, and Ligninolysis) in carbon cycling (Figure S3) due to its nutrient-rich characteristics and abundant microbial pool [60]. By intensifying the degrading ability of microorganisms to break down recalcitrant organic matter, N release from maize straw and litter was facilitated, thereby benefiting maize growth in the later stages [61]. In conclusion, substituting chemical fertilizers with organic manure and straw mulching not only reduces fertilizer usage but also enhances the efficiency and sustainability of agroecosystems.
In terms of N balance, pig slurry substitution for chemical fertilizers without straw mulching increased maize yield and total N uptake (Figure 6b,c), but this led to N deficiency and depletion of existing soil N (Table 2), which is not conducive to long-term sustainability. In contrast, the straw mulching treatments exhibited N surplus (Table 2), as straw contains a significant amount of organic N, which contributes to soil N input and, upon mineralization, can partially replace chemical fertilizer requirements [62]. Similarly, Christensen et al. [63] reported that long-term straw incorporation enhances total soil N content, compensating for N deficits in the soil. However, since N in straw primarily exists in organic form, its nutrient release and accumulation effects require an extended period [64,65,66]. S-M0 had the highest N surplus, but the N uptake and yield of maize were lower than those of the treatments with pig slurry (Table 2 and Figure 6b,c). From an economic perspective, compared to NS-M0, the S-M0 treatment resulted in a slight benefit reduction, but substituting chemical fertilizer with pig slurry significantly improved economic benefits, with the highest benefit observed under the 20% substitution rate combined with straw mulching (Table S6). Thus, practical applications must consider integrating straw N with other N sources to optimize nutrient availability and economic benefits.

5. Conclusions

This study systematically evaluates the effects of integrating straw mulching with pig slurry substitution on soil N dynamics, maize productivity, and agroecosystem sustainability in Northeast China. Key findings are synthesized as follows:
Soil N dynamics and retention: pig slurry substitution (20–40%) increased topsoil NH4+-N and NO3−-N availability during the seedling stage. Straw mulching reduced mainly in the 0–40 cm soil layer compared to non-straw treatments. The combined use of pig slurry and straw mulching reduced NO3−-N and leaching risks in the 40–100 cm soil layer.
Maize yield and NUE: the 20% pig slurry substitution combined with straw mulching (S-M20) achieved the highest maize yield (14,628.76 kg ha−1) and NUE, representing a 19.39% increase over conventional chemical fertilization (NS-M0). In contrast, 40% substitution (S-M40) reduced yields by 9.16% compared to S-M20, highlighting the critical role of synchronizing nutrient supply with crop growth stages.
N balance and economic viability: straw mulching treatments consistently exhibited soil N surpluses (33.35–74.65 kg ha−1), ensuring long-term fertility, whereas non-straw treatments faced N deficits (−50.81 to −11.24 kg ha−1), risking soil degradation. Economically, S-M20 delivered the highest cost-effectiveness, balancing yield gains with reduced fertilizer inputs.
This finding corroborates the notion that organic fertilization, especially when combined with straw, provides a more sustainable approach to managing soil fertility, reducing dependency on chemical fertilizers. Ultimately, the integration of manure-like pig slurry and straw offers a promising strategy for sustaining soil health, improving N cycling, and ensuring long-term agricultural productivity.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agronomy15051062/s1, Supporting information: (include Table S1: Effects of straw mulching and fertilization management on pH and moisture content of 0–100 cm soil depth; Table S2: Effects of straw mulching and fertilization management on PFPN, NAE, and NRE; Table S3: One-way ANOVA analysis of TN, DON and NO3−-N in different soil depth under different treatments; Table S4: One-way ANOVA analysis of NH4+-N and Mineral N/TN in different soil depth under different treatments; Table S5: One way ANOVA analysis of maize biomass and nitrogen uptake at different growth stages under different treatments.; Figure S1: TN content in 0–100 cm soil depth of maize under different treatments and vegetative stages; Figure S2: Biomass and nitrogen uptake of maize root and straw at different growth stages under different treatments; Figure S3. Prediction of soil microbial function). References [67,68,69] are cited in the Supplementary Materials.

Author Contributions

Conceptualization, Y.Y. and H.H.; methodology, D.L.; formal analysis, D.L.; investigation, D.L. and Z.Z.; writing—original draft, Y.Y.; writing—review and editing, all authors; project administration, Y.Y., F.D., and Y.Z.; funding acquisition, Y.Y. and Y.Z.; supervision, Y.Y., X.B. and H.H.; validation, Y.Z., H.X. and X.Z.; All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (32401446, 42207370), the China Postdoctoral Science Foundation (2023M733673), and the Doctoral Start-up Foundation of Liaoning Province (2023-BSBA-310).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data sets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.

Acknowledgments

We thank Yuwei Fu and Ying Zhou for their help in fieldwork and experimental procedures. We would like to thank Jinsong Zhao from Huazhong Agricultural University for his guidance and help in the data analysis.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Seasonal variation in precipitation and temperature during the 2023 growing period. The blue bars represent daily precipitation (mm) on the right y-axis. The red and cyan lines indicate the highest and lowest daily temperatures (°C), respectively, on the left y-axis.
Figure 1. Seasonal variation in precipitation and temperature during the 2023 growing period. The blue bars represent daily precipitation (mm) on the right y-axis. The red and cyan lines indicate the highest and lowest daily temperatures (°C), respectively, on the left y-axis.
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Figure 2. DON content in the 0–100 cm soil profile of maize cultivation under different treatments at the (a) seedling stage, (b) jointing stage, (c), milk stage, and (d) maturity stage. F values from linear mixed effects models on the effects of straw management (S) and manure management (M) and their interactions (S × M) on the 0–10, 10–20, 20–40, 40–80, and 80–100 cm soil layers were presented. * p < 0.05; ** p < 0.01; *** p < 0.001.
Figure 2. DON content in the 0–100 cm soil profile of maize cultivation under different treatments at the (a) seedling stage, (b) jointing stage, (c), milk stage, and (d) maturity stage. F values from linear mixed effects models on the effects of straw management (S) and manure management (M) and their interactions (S × M) on the 0–10, 10–20, 20–40, 40–80, and 80–100 cm soil layers were presented. * p < 0.05; ** p < 0.01; *** p < 0.001.
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Figure 3. NO3−-N content in the 0–100 cm soil profile of maize cultivation under different treatments at the (a) seedling stage, (b) jointing stage, (c) milk stage, and (d) maturity stage. The rest of the notes are the same as in Figure 2 and were omitted for clarity. * p < 0.05; ** p < 0.01; *** p < 0.001.
Figure 3. NO3−-N content in the 0–100 cm soil profile of maize cultivation under different treatments at the (a) seedling stage, (b) jointing stage, (c) milk stage, and (d) maturity stage. The rest of the notes are the same as in Figure 2 and were omitted for clarity. * p < 0.05; ** p < 0.01; *** p < 0.001.
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Figure 4. NH4+-N content in the 0–100 cm soil profile of maize cultivation under different treatments at the (a) seedling stage, (b) jointing stage, (c) milk stage, and (d) maturity stage. The rest of the notes are the same as in Figure 2 and were omitted for clarity. * p < 0.05; ** p < 0.01; *** p < 0.001.
Figure 4. NH4+-N content in the 0–100 cm soil profile of maize cultivation under different treatments at the (a) seedling stage, (b) jointing stage, (c) milk stage, and (d) maturity stage. The rest of the notes are the same as in Figure 2 and were omitted for clarity. * p < 0.05; ** p < 0.01; *** p < 0.001.
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Figure 5. Ratio of mineral-N to TN content in the 0–100 cm soil profile of maize cultivation under different treatments at the (a) seedling stage, (b) jointing stage, (c) milk stage, and (d) maturity stage. The rest of the notes are the same as in Figure 2 and were omitted for clarity. * p < 0.05; ** p < 0.01; *** p < 0.001.
Figure 5. Ratio of mineral-N to TN content in the 0–100 cm soil profile of maize cultivation under different treatments at the (a) seedling stage, (b) jointing stage, (c) milk stage, and (d) maturity stage. The rest of the notes are the same as in Figure 2 and were omitted for clarity. * p < 0.05; ** p < 0.01; *** p < 0.001.
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Figure 6. The maize biomass (a), maize yield (b), N uptake by maize organs (roots, straws, cobs, and grains) (c), and total N uptake of maize (d) in different treatments across the growing seasons. Different letters indicate significant differences among the six treatments (Duncan, p < 0.05). The rest of the notes are the same as in Figure 2 and were omitted for clarity. * p < 0.05; ** p < 0.01; *** p < 0.001.
Figure 6. The maize biomass (a), maize yield (b), N uptake by maize organs (roots, straws, cobs, and grains) (c), and total N uptake of maize (d) in different treatments across the growing seasons. Different letters indicate significant differences among the six treatments (Duncan, p < 0.05). The rest of the notes are the same as in Figure 2 and were omitted for clarity. * p < 0.05; ** p < 0.01; *** p < 0.001.
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Table 1. Nutrient inputs in different fertilization regimes.
Table 1. Nutrient inputs in different fertilization regimes.
TreatmentInput of Chemical Fertilizer (kg ha−1)Input of Pig Slurry (kg ha−1)
NP2O5K2ONP2O5K2O
NS-M022076110000
NS-M2017660.8884415.222
NS-M4013245.6668830.444
S-M022076110000
S-M2017660.8884415.222
S-M4013245.6668830.444
Note: NS-M0: no straw with chemical fertilizer alone. NS-M20: no straw with pig slurry substitution for 20% of the chemical fertilizer. NS-M40: no straw with pig slurry substitution for 40% of the chemical fertilizer. S-M0: straw mulching with chemical fertilizer alone. S-M20: straw mulching with pig slurry substitution for 20% of the chemical fertilizer. S-M40: straw mulching with pig slurry substitution for 40% of the chemical fertilizer.
Table 2. Nitrogen balance in the soil−plant system.
Table 2. Nitrogen balance in the soil−plant system.
TreatmentN InputsN OutputsDifference
(Inputs−Outputs)
NfertilizerNpig slurryNstrawNdepositionNbio fixationNmaize uptake
(kg ha−1)(kg ha−1)(kg ha−1)(kg ha−1)(kg ha−1)(kg ha−1)
NS-M0220001615249.071.93
NS-M201764401615301.81−50.81
NS-M401328801615262.24−11.24
S-M0220067.51615243.8574.65
S-M201764467.51615285.1533.35
S-M401328867.51615274.2244.28
Note: N inputs came from chemical fertilizers, pig slurry, straw mulching, biological fixation, and atmospheric deposition. N outputs included only uptake by the above-ground portion of maize.
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MDPI and ACS Style

Yang, Y.; Lei, D.; Zhang, Y.; Zhao, Z.; Xie, H.; Deng, F.; Bao, X.; Zhang, X.; He, H. Effects of Pig Slurry Coupled with Straw Mulching on Soil Nitrogen Dynamics and Maize Growth. Agronomy 2025, 15, 1062. https://doi.org/10.3390/agronomy15051062

AMA Style

Yang Y, Lei D, Zhang Y, Zhao Z, Xie H, Deng F, Bao X, Zhang X, He H. Effects of Pig Slurry Coupled with Straw Mulching on Soil Nitrogen Dynamics and Maize Growth. Agronomy. 2025; 15(5):1062. https://doi.org/10.3390/agronomy15051062

Chicago/Turabian Style

Yang, Yali, Dengchao Lei, Yulan Zhang, Zhe Zhao, Hongtu Xie, Fangbo Deng, Xuelian Bao, Xudong Zhang, and Hongbo He. 2025. "Effects of Pig Slurry Coupled with Straw Mulching on Soil Nitrogen Dynamics and Maize Growth" Agronomy 15, no. 5: 1062. https://doi.org/10.3390/agronomy15051062

APA Style

Yang, Y., Lei, D., Zhang, Y., Zhao, Z., Xie, H., Deng, F., Bao, X., Zhang, X., & He, H. (2025). Effects of Pig Slurry Coupled with Straw Mulching on Soil Nitrogen Dynamics and Maize Growth. Agronomy, 15(5), 1062. https://doi.org/10.3390/agronomy15051062

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