Replacing Chemical Fertilizer with Separated Biogas Slurry to Reduce Soil Nitrogen Loss and Increase Crop Yield—A Two-Year Field Study

Abstract

The application of biogas slurry in agriculture production is regarded as a sustainable method for mitigating the environmental impacts of fertilization. To investigate the effects of biogas slurry application on soil nitrogen loss and crop yield, a field plot experiment was conducted within a wheat–maize rotation system. This study assessed the effects of three levels of biogas slurry nitrogen substitution, 50% (BSF), 100% (BS), and 150% (EBS), on the yield of silage maize and wheat, nitrogen use efficiency, and soil nitrogen loss. The findings revealed that in the first year (characterized by high rainfall), the application of the biogas slurry led to increased NH₃ emissions and nitrogen leaching, resulting in a notable rise in the annual nitrogen loss. Additionally, it was observed that as the amount of applied biogas slurry increased, the nitrogen loss also rose correspondingly. However, in the second year (a period of drought conditions), despite the elevated NH₃ emissions from the biogas slurry, there was a significant reduction in nitrogen leaching, which resulted in reductions of 14.2% and 20.0% in annual nitrogen loss for the BSF and BS treatments, respectively, with comparable nitrogen input to the fertilizer treatment. Throughout both years, the application of biogas slurry did not decrease the yield of silage maize and wheat, and notably, the BS treatment even enhanced the crop nitrogen utilization efficiency. Compared with other nitrogen fertilizer treatments, the EBS treatment did not increase crop yield even with an increased nitrogen application rate; it also reduced the nitrogen utilization efficiency and N loss. In conclusion, employing biogas slurry to replace chemical fertilizer (equivalent nitrogen substitution) during drought years can enhance nitrogen utilization efficiency, reduce nitrogen loss, and sustain crop yield. When applying biogas slurry in years with substantial rainfall, effective measures should be implemented to mitigate nitrogen loss.

1. Introduction

The sustainable recycling of fecal waste is recognized as the most efficient strategy to enhance the environmental conditions of livestock farms [1]. In a study of several large-scale dairy farms in Northern China, it was observed that following the solid–liquid separation of fecal waste in the majority of farms, the solid feces could be repurposed as bedding material or converted into organic fertilizer for convenient transportation, thus facilitating its effective treatment and utilization. Nevertheless, the cumbersome transportation and application of the liquid portion post separation pose significant inconveniences, exacerbating the challenges of reuse and leading to environmental pollution concerns, which present a substantial obstacle in large-scale pollution control efforts within livestock farming. It is estimated that a dairy farm housing 10,000 cows produces approximately 750 tons of fecal wastewater daily, highlighting the importance of proper handling to prevent secondary environmental pollution [2,3]. Research suggests that after anaerobic fermentation, fecal wastewater transforms into biogas slurry rich in essential nutrients like nitrogen, phosphorus, and potassium, suitable for use as fertilizer in agricultural fields. This approach not only reduces the reliance on chemical fertilizers but also mitigates the secondary pollution resulting from the complex treatment of fecal wastewater. Therefore, the application of biogas slurry in agriculture stands out as an effective method to cut costs, conserve resources, and minimize environmental pollution [4,5,6].

Northern China is a paramount agricultural production region in the country, characterized by an escalating level of intensification. The rapid increase in the use of chemical fertilizers and irrigation water has heightened crop yields but has also led to issues such as NH₃ emissions and nitrogen leaching [7,8,9]. Previous studies have demonstrated that substituting chemical fertilizers with organic fertilizer not only maintains and enhances crop yields but also reduce NH₃ emissions and nitrogen leaching. Research by Rahaman et al. [10] has indicated that the integrated use of biogas slurry and chemical fertilizer provides the greatest crop yield. Chastain [11] noted that the application of biogas slurry by farmers does not increase soil NH₃ emissions, while studies by Verdi et al. [12] highlighted a reduction in NH₃ emissions when substituting chemical fertilizers with biogas slurry using the injection method. The works of Karimi et al. [13] further support the efficacy of using biogas slurry to decrease nitrogen leaching. Additionally, research has shown that integrating organic and inorganic fertilizers in the winter wheat–summer maize rotation system in Northern China can prevent nitrate nitrogen migration to deeper soil layers, thereby reducing groundwater pollution [14]. Studies by Yaseen et al. [15] have proposed that employing biogas slurry could enhance crop yields and lessen NH₃ emissions, and do not exacerbate nitrogen leaching compared to chemical fertilizers. Therefore, substituting a portion of chemical fertilizers with biogas slurry in the winter wheat–summer maize rotation system (planting wheat in winter and harvesting the kernels, planting maize in summer and harvesting maize plants without kernels for silage) in Northern China can boost crop productivity, reduce NH₃ emissions, and reduce nitrogen leaching, emphasizing its effectiveness in farming practices and pollution reduction.

The study in this paper concentrates on exploring the impact of substituting chemical fertilizers with biogas slurry on crop productivity, nitrogen utilization efficiency, and soil nitrogen loss (particularly NH₃ emissions and nitrogen leaching) in the winter wheat–summer maize rotation fields of Northern China. These objectives lead to the following hypotheses:

• Substituting chemical fertilizers with biogas slurry would increase both crop yield and nitrogen utilization efficiency, contributing to sustainable agricultural practices.
• Substituting chemical fertilizers with biogas slurry would reduce nitrogen loss.

2. Materials and Methods

2.1. Site Description and Experimental Design

The experiment took place in October 2017 in Cao County, Shandong Province, China (34°78′ N, 115°59′ E). The experimental site was situated in the Yellow River alluvial plain with ample sunlight, where there is a typical warm-temperate continental monsoon climate featuring an average annual temperature of 14.3 °C, 2147.6 annual sunshine hours, and an average annual precipitation of 678.4 mm. The monthly average temperatures and precipitation levels at the study site from June 2018 to June 2020 are shown in Figure 1. Prior to the experiment, the sample plot underwent leveling procedures following a 1-year, 2-season crop cycle. The soil was calcareous and moist (Haplic Luvisol), with the following initial soil characteristics: pH of 8.7, 1:2.5 w/v ratio of soil–water, organic matter content of 18.1 g kg⁻¹, alkaline hydrolysis nitrogen content of 75.4 mg kg⁻¹, Olsen phosphorus content of 89.7 mg kg⁻¹, and available potassium content of 178.7 mg kg⁻¹.

The tested crop varieties included Jimai 22 for winter wheat and Yu Silage 23 for maize. Five distinct treatments were established in the experiment: no nitrogen fertilizer (CK), conventional fertilizer (100% fertilizer nitrogen, CF), biogas slurry supplement re–placing 50% of the fertilizer (50% biogas slurry nitrogen + 50% fertilizer nitrogen, BSF), exclusively biogas (100% biogas slurry nitrogen, BS), and enhanced biogas supply at 1.5 times (150% biogas slurry nitrogen, EBS). Each treatment was replicated three times, and a randomized block design was implemented with individual plots measuring 9 m long and 6 m wide, separated by 2 m between the blocks and 1 m between the plots. Fertilizer quantities for all treatments were calculated based on their nitrogen content, with appli–cation rates (N-P₂O₅-K₂O) set at 210-105-90 kg ha⁻¹ and 210-75-105 kg ha⁻¹ during the winter wheat and summer maize seasons, respectively. The nitrogen fertilizer was primarily urea, using a 1:1 ratio to track the nitrogen base. The biogas slurry (solid–liquid separation after anaerobic digestion of dairy sewage) used in the experiment was sourced from a local large-scale dairy farm, and its nutrient composition is detailed in

Table 1. Nutrient composition of the biogas slurry.

pH	Moisture Content (%)	N (%)	NH4+-N (%)	P2O5 (%)	K2O (%)
7.85~8.02	95.6~98.2	0.09~0.11	0.05~0.07	0.03~0.05	0.11~0.14

. Urea with a 46.4% N content served as the nitrogen source for all treatments, calcium biphosphate (46% P₂O₅) served as the phosphate fertilizer, potassium sulfate (50% K₂O) served as the potassium source, and any deficiencies in phosphorus and potassium in the biogas slurry were supplemented with chemical fertilizers. Fertilizer application and irrigation occurred twice each during both the winter wheat and summer maize seasons, with each crop growing season delivering 75 mm of a water/biogas slurry mixture, and the biogas slurry was applied concurrently with irrigation water. In the maize season, fertilization and irrigation were carried out during the sowing (mid June) and tasseling periods (early August), respectively. In the wheat season, fertilization and irrigation were carried out after sowing (mid October) and during the jointing and booting period (early April), respectively. Biogas slurry and water were thoroughly mixed and then broadcasted onto the soil surface. Urea was broadcasted onto the soil surface and then irrigated with either water or biogas slurry. Pest and weed control was carried out based on local farmer habits.

2.2. Sampling and Analyses

2.2.1. NH₃ Emissions

NH₃ emissions were continuously monitored using the aeration method [16] for each sample plot from June 2018 to June 2020. Specifically, a rigid polyvinyl chloride (PVC) tube, 10 cm high and with an inner diameter of 15 cm, served as the aeration device. The device contained two sponge discs, each 2 cm thick and 16 cm in diameter, soaked in a solution of phosphoric acid (H₃PO₄) and glycerol (C₃H₈O₃), diluted in water to a total volume of 1000 mL, with a mixture ratio of 50 mL H₃PO₄ to 40 mL C₃H₈O₃. The bottom sponge layer was placed at a distance of 5 cm from the bottom, aligning the top layer with the top of the tube. Inserted into the soil to a depth of 1 cm, the device was protected with a rain shield positioned 20 cm above its top. Capture of the volatilized ammonia started on the day after fertilizer application. Three ammonia-capture devices were installed on the diagonal of each plot, and the samples were collected at 8:00 each morning. The lower layer of the sponge was quickly removed and placed into a sealed bag. Then, another pre-soaked sponge was placed on top, and the upper sponge was replaced every 2–4 days. The removed sponge was cut into pieces and placed into a 500 mL plastic bottle, in which 300 mL of a 1.0 mol L⁻¹ KCl solution was added to completely immerse the sponge pieces. After shaking the system for 1 h, the content of ammonium nitrogen in the leaching solution was measured with an automatic intermittent analyzer (Smartchem 200, AMS Alliance Co. Ltd., Rome, Italy). During the first week after fertilizer application, a sample was collected daily, and subsequently, a sample was collected every 2–5 days to monitor the amount of volatilized ammonia until no further ammonia was detected.

2.2.2. Nitrogen Leaching

This study conducted nitrogen leaching monitoring in each sample plot from June 2018 to June 2020 using the field leakage-pool method [17]. A leaching tank with dimensions of 140 cm in length, 70 cm in width, and 90 cm in height was dug in the central area of each group. A leaching bucket (diameter of 40 cm, height of 36 cm) was placed at a depth of 40 cm in the center of the leaching tank. The lid of the leaching bucket was equipped with several small holes to facilitate the inflow of the leaching solution. The leaching bucket was also equipped with a drainage pipe and a ventilation pipe connected to the ground. The leaching tank was enclosed with plastic film. A hole with a diameter of 20 cm was cut into the leaching bucket. Underneath the hole, a nylon bag (100 mesh) containing fine sand was placed over the bucket surface with a thickness of 5 cm to prevent soil particles from infiltrating the leaching bucket. We then covered the soil to a depth of about 25 cm, followed by laying a straw layer, as was carried out for all plots in this study, and finally covering it with soil again. After irrigation and heavy rainfall, at the end of each crop-growing season, the leaching solution was extracted from the leaching bucket using a vacuum pump, and its volume was recorded. After mixing the solution evenly, a sample was taken back to the laboratory in a small bottle, where an automatic intermittent analyzer (Smartchem 200, AMS Alliance Co. Ltd., Rome, Italy) was used to measure the content of NO₃⁻-N and NH₄⁺-N.

2.2.3. Crop Yield and Plant Nitrogen Content

During the silage maize harvest, three 9 m² sample plots were extracted from each plot for yield evaluation. The samples were directly weighed to calculate the yield based on their fresh weights and subsequently air-dried for a nitrogen content analysis. Similarly, at the winter wheat harvest, three 3 m² sample plots were selected from each plot for yield assessment. After air-drying, the samples were weighed to determine the seed and straw yields, followed by a nitrogen content analysis. The nitrogen content in the maize stover and that in the wheat kernels and stover, and the total nitrogen content across all treatments, were determined utilizing the H₂SO₄-H₂O₂ digestion Kjeldahl method [18].

2.2.4. Soil NO₃⁻-N and NH₄⁺-N

At the end of the experiment, soil samples were collected at a depth of 100 cm. A soil drill was used to collect soil samples in five levels of 0–20, 20–40, 40–60, 60–80, and 80–100 cm in an “S” shape in each experimental plot. The samples were mixed at the same level and brought back to the laboratory to measure the soil NO₃⁻-N, NH₄⁺-N, and moisture content. The moisture content was determined by oven-drying the soil at 105 °C to a constant weight. NO₃⁻-N and NH₄⁺-N were extracted with 2 mol L⁻¹ KCl and measured using an automatic intermittent analyzer (Smartchem 200, AMS Alliance Co. Ltd., Rome, Italy).

2.3. Calculations

2.3.1. NH₃ Emissions

The soil NH₃ emission rate was calculated using the following equation [16]:

V = [M/(A × D)] × 10⁻²

(1)

where V is the ammonia volatilization rate (kg ha⁻¹ d⁻¹), M is the average (NH₃-N, mg) measured with a single device using the venting method, A is the cross-sectional area of the capture device (m²), and D is the time (d) of each continuous capture.

The cumulative emissions were calculated using the linear interpolation method.

The emission factor of the NH₃ emissions (NH₃ EF) was calculated using the following formula:

NH₃ EF (%) = (E_F − E₀)/R_F × 100%

(2)

where E_F and E₀ are the annual or seasonal NH₃ emissions (kg NH₄⁺-N ha⁻¹) from the N-fertilized and CK plots, respectively; and R_F represents the annual or seasonal application rate of N fertilizer (kg N ha⁻¹).

2.3.2. Nitrogen Leaching

Nitrogen leaching was calculated as follows [17]:

W = P × C/A × 10⁻⁶

(3)

where W is the amount of NO₃⁻-N or NH₄⁺-N leaching, kg NO₃⁻-N m⁻² or kg NH₄⁺-N m⁻²; P is the volume of leaching solution, L; C is the concentration of NO₃⁻-N or NH₄⁺-N in the leaching water samples, mg L⁻¹; and A is the area of the leakage pool (0.98 m²).

The total nitrogen leaching amount was the sum of all nitrogen leaching (NO₃⁻-N and NH₄⁺-N) amounts during each crop growth season or year.

The rate of nitrogen leaching was calculated using the formula below:

N leaching rate (%) = (N_F − N₀)/R_F × 100%

(4)

where N_F and N₀ are the annual or seasonal nitrogen leaching (kg NO₃⁻-N ha⁻¹) from the N-fertilized and CK plots, respectively; and R_F represents the annual or seasonal application rate of N fertilizer (kg N ha⁻¹).

2.3.3. Nitrogen Use Efficiency

The nitrogen use efficiency (NUE) was calculated using the formula below [18]:

NUE (%) = (U_F − U₀)/R_F × 100%

(5)

where U_F and U₀ are the annual or seasonal aboveground N uptakes measured at harvest (kg N ha⁻¹) from the N-fertilized and CK plots, respectively; and R_F represents the annual or seasonal application rate of N fertilizer (kg N ha⁻¹).

2.3.4. Nitrogen Balance

Soil nitrogen accumulation was calculated as follows:

Q = H × ρ × C × 10⁻⁶

(6)

where Q is the amount of soil NO₃⁻-N or NH₄⁺-N accumulation, kg NO₃⁻-N ha⁻¹ or kg NH₄⁺-N ha⁻¹; H is the soil depth, cm; ρ is the soil density, kg m⁻³; and C is the content of soil NO₃⁻-N or NH₄⁺-N, mg kg⁻¹.

The N balance were calculated using the following formula [19]:

Apparent N loss = Fertilization N-Input (kg N ha⁻¹) − N uptake in the harvested crop (kg N ha⁻¹) − soil residual N (0–100 cm, kg N ha⁻¹)

(7)

where “N uptake in the harvested crop” and “soil residual N” both refer to the portion derived from the application of nitrogen fertilizer; these values were obtained by subtracting the non-nitrogen treatment from the nitrogen fertilizer treatment.

2.4. Statistical Analyses

The data obtained from this experiment were processed and graphed using Excel 2016 and analyzed using the one-way AVNOVA tool in SPSS 22.0 software for analysis of variance (ANOVA), and Duncan’s method was chosen for multiple comparisons, with a significance level of 0.05.

3. Results

3.1. NH₃ Emissions

The cumulative NH₃ emissions from all treatments increased rapidly within 7 days after fertilization, reaching their maximum value after 15 days of fertilization (S1).

The cumulative magnitude of NH₃ emissions within each treatment followed the sequence of EBS > BS > BSF > CF > CK, evident in both the maize and wheat seasons (except for the 2018–2019 wheat season), as illustrated in Figure 2. However, there were no significant differences among the BS, BSF, and CF treatments. Throughout the two-year study period, NH₃ emission levels remained relatively consistent across the two maize seasons for all treatments. However, in the second wheat season, NH₃ emissions were notably higher compared to those in the first wheat season, being particularly pronounced in the BS, BSF, and EBS treatments. On average, over the two years, the NH₃ emissions of the BSF, BS, and EBS treatments increased by 9.41%, 16.9%, and 34.2% during the maize season, respectively, in comparison to those of the CF treatment. In contrast, in the wheat season, only the EBS treatment showcased a 19.9% increase in NH₃ emissions versus those of the CF treatment. Furthermore, NH₃ emissions decreased by 20.9% for the BSF treatment and 2.48% for the BS treatment during the same season.

The NH₃ EF value in the two maize seasons remained relatively consistent across all nitrogen fertilizer application treatments during the two-year study period. Conversely, higher EFs were observed in the second wheat season compared to the first wheat season, being particularly notable in the biogas slurry substitution treatments (BSF, BS, and EBS), as depicted in Figure 3. On average, over the two-year period, the conventional fertilizer (CF) exhibited the lowest NH₃ EF at 4.85%, lower than that of BSF at 5.47% and that of BS at 5.96% in equivalent nitrogen treatments in the maize season. In contrast, CF displayed the highest NH₃ EF at 5.04% during the wheat season, surpassing BSF at 3.72% and BS at 4.88% in equivalent nitrogen treatments. Notably, due to the higher nitrogen application rate in EBS, which was 1.5 times that of CF, the resulting NH₃ EF of EBS was lower than that of CF.

3.2. Nitrogen Leaching

Our 2-year research results showed that all treatments exhibited a consistent trend, with the leachate volume and inorganic nitrogen concentration being significantly higher in the first year of both the maize and wheat seasons (Table S1), resulting in a higher nitrogen leaching rate in the first year (Figure 4). Notably, the biogas slurry substitution treatments exhibited higher nitrogen leaching than the CF treatments during the initial year, with a reversal of results in the subsequent year. In the two-year averages, BS and EBS treatments exhibited increased nitrogen leaching by 11.6% and 24.4%, respectively, during the maize season relative to CF. Meanwhile, the BSF treatment showed reduced nitrogen leaching by 6.6% compared to CF. In contrast, during the same season, BSF, BS, and EBS showed increases of 22.2%, 25.3%, and 72.0% in nitrogen leaching, respectively. Nitrate nitrogen accounted for over 90% of the total nitrogen leaching in all nitrogen fertilization treatments, except for the EBS treatment in the second year of the maize season.

In both the maize and wheat seasons, all nitrogen fertilizer treatments displayed significantly higher nitrogen leaching rates in year 1 compared to year 2 (Figure 5). Specifically, during year 1, the nitrogen leaching rate was greater in the biogas slurry substitution treatments than in the CF treatment. The two-year average leaching rates varied across treatments, with the treatment hierarchy being observed as BS (8.32%) > EBS (6.80%) ≈ CF (6.61%) > BSF (5.64%) in the maize season, and EBS (10.9%) ≈ BS (10.8%) ≈ BSF (10.4%) > CF (7.73%) in the wheat season.

3.3. Crop Yield and NUE

Throughout the 2-year study duration, the crop yields exhibited significant increases in the nitrogen fertilization treatments compared to the CK treatments, with this being evident in both the maize and wheat seasons (Figure 6). The results indicated that the application of biogas slurry did not result in decreased maize and wheat yields. Notably, the variations in crop yields between the nitrogen fertilization treatments were not statistically significant. However, concerning interannual variability, both the maize and wheat yields showed an improvement in year 2 compared to year 1 within the nitrogen fertilization treatments.

Consequently, the NUE for all nitrogen fertilizer application treatments exhibited an improvement in the second year compared to the first year (Figure 7). Notably, regarding interannual variability, the crop NUE significantly rose in all nitrogen fertilizer application treatments during the second year compared to the first year, with this being particularly notable in the wheat season, wherein the NUE increased by 5.23 to 16.8 percentage points. Interestingly, both the maize and wheat NUE rates were notably higher in the CF, BSF, and BS treatments with equivalent nitrogen applications compared to the EBS treatment with 1.5 times the nitrogen quantity.

3.4. The Correlation between Crop Yield, NUE, and Nitrogen Loss

In the maize season, crop yield was positively correlated with NUE and negatively correlated with nitrogen leaching and loss. Similar results were observed in the wheat season (Figure 8).

3.5. Nitrogen Balance

The two-year nitrogen balance results for the winter wheat–summer maize rotation system treated with nitrogen fertilizer are shown in

Table 2. Nitrogen balance in the nitrogen fertilizer application treatment. The different lowercase letters are significantly different at p < 0.05.

Treatment	N Fertilizer kg N ha−1	Crop N Uptake kg N ha−1	Soil Residual N kg N ha−1	Apparent N Loss kg N ha−1
CF	840	329.6 ± 16.2 a	320.9 ± 104.1 ab	189.5 ± 104.2 a
BSF	840	323.9 ± 51.9 a	280.1 ± 60.8 ab	236.0 ± 110.3 a
BS	840	351.3 ± 54.4 a	266.6 ± 107.5 b	222.1 ± 76.5 a
EBS	1260	303.3 ± 26.7 a	530.7 ± 204.6 a	426.0 ± 182.4 a

. The results show that there was no significant difference in nitrogen uptake among all treatments. Compared with the CON treatment, the soil residual N of the BSF and BS treatments decreased by 12.7% and 16.9%, respectively, while that of the EBS treatment increased by 65.4%. Although the apparent N loss of the BSF, BS, and EBS treatments increased by 24.5%, 17.2%, and 124.8% compared to the CON treatment, respectively, the differences between treatments are not significant. This indicates that the excessive application of biogas slurry did not increase the crop N uptake but rather led to an increase in the nitrogen loss.

4. Discussion

4.1. Effect of Biogas Slurry Substitution on N Loss

Soil NH₃ emissions and nitrogen leaching are the primary pathways of nitrogen loss from agricultural fields in North China [14,20]. In this study, during the first year, the application of biogas slurry did not increase the annual NH₃ emissions (Table S2). In fact, both the BSF and BS treatments led to a reduction in annual NH₃ emissions. However, it significantly increased annual nitrogen leaching, which in turn resulted in a significant increase in the annual nitrogen loss (NH₃ emission + nitrogen leaching). In contrast, the second year saw a different outcome. The use of biogas slurry then increased the annual NH₃ emissions, but it also significantly decreased the annual nitrogen leaching, thereby reducing the overall nitrogen loss (with the exception of the EBS treatment) (Table S2). This discrepancy can be attributed to nitrogen leaching constituting 76.4% to 84.3% of nitrogen losses in the first year, whereas this ranged from 26.1% to 51.1% in the second year. This study revealed that water inputs, such as irrigation and rainfall, were the critical factors influencing NH₃ emissions and nitrogen leaching under identical conditions. Water inputs accelerate the downward leaching of nitrogen in the biogas slurry, thereby reducing NH₃ emissions and increasing nitrogen leaching [21,22,23,24]. Rainfall volumes were observed to be 775 mm and 402 mm in the first and second years (Figure 1), respectively, indicating that rainfall was likely the predominant factor driving the aforementioned trends.

In the second year, with lower rainfall, NH₃ emissions accounted for 48.9% to 73.9% of the nitrogen losses, with the application of biogas slurry treatments accounting for 60.8% to 73.9% of the total losses. The NH₃ emissions increased proportionally as the biogas slurry application rates became higher. This observation aligns with the findings of previous studies [25,26]. The alkaline nature of the soil in this study, combined with the surface application of digestate, likely contributed to the higher NH₃ emissions. The existing literature emphasizes that soil pH and nitrogen availability are key determinants of NH₃ emissions post manure application [26,27]. The study area’s soil pH measured at 8.74, and the combination of watering and applying alkaline manure with a high NH₄⁺ content led to substantial NH₃ emissions. Acidification and the deep application of biomass have been identified as effective strategies for controlling NH₃ emissions from soil [23,25]. Additionally, this study demonstrated that biogas application during low-rainfall periods, be it in the maize or wheat seasons, significantly reduced nitrogen leaching, which is consistent with previous research. This reduction is attributed to the increased soil organic carbon and improved carbon-to-nitrogen ratio (Table S3), enhancing the soil’s nitrogen retention and subsequently reducing nitrogen leaching [28,29].

4.2. Improving Effects of Biogas Slurry Substitution on Crop Yield, Nitrogen Use, and N Loss Reduction

The application of biogas slurry has shown benefits in enhancing soil structure, optimizing crop nutrition, and facilitating nutrient absorption by crops. Research conducted by Yin et al. [30] and Maurer et al. [31] demonstrated that biogas slurry treatments, in comparison to chemical fertilizer application, not only increased crop yield but also significantly increased the nitrogen use efficiency. Conversely, studies by Liu et al. [32] and Sherman et al. [33] reported that substituting chemical fertilizer with biogas slurry did not lead to a decrease in yield. Meanwhile, in this study, we found that replacing chemical fertilizer with biogas slurry had no detrimental effects on silage maize and wheat grain yields. Interestingly, the substitution with an equal amount of biogas slurry containing the same nitrogen content actually improved wheat yield and nitrogen utilization. This improvement is possibly attributed to wheat’s preference for ammonium–nitrogen [34], which promotes nitrogen absorption, leading to enhanced utilization. However, we observed that despite the increase in nitrogen input, the EBS treatment did not lead to a higher yield. This resulted in a significant reduction in the NUE. It is possible that this was due to an excessive amount of nitrogen residue in the soil (Table 2).

Combining the results over the two years, it was observed that the nitrogen losses in CF, BSF, BS, and EBS treatments were 1.81, 2.35, 3.02, and 2.59 times higher, respectively, in the first year compared to the second year. Notably, the nitrogen losses in the BSF, BS, and EBS treatments increased by 11.7%, 33.4%, and 56.5%, respectively, relative that in to the CF treatment (Table S2). Conversely, in the second year, only the EBS treatment led to increased nitrogen losses compared to the CF treatment, with reductions of 14.2% and 20.0% being observed for the BSF and BS treatments, respectively (Table S2). This highlights that the use of biogas slurry as a substitute for chemical fertilizers elevates nitrogen losses during rainy periods, whereas drought conditions in wheat–maize rotation systems in North China lead to decreased nitrogen losses. Thus, it is advisable to avoid applying biogas slurry during wet seasons and on humid soils or implement effective strategies to mitigate these nitrogen losses.

In summary, the application of biogas slurry with equal nitrogen inputs in the first wet year and the following dry year enhanced the annual NUE of the crops, compared to chemical fertilizers, all while maintaining their yields. Additionally, during dry periods, there was a reduction in soil nitrogen losses, with the most favorable outcomes being achieved by replacing chemical fertilizers with biogas slurry while maintaining the same nitrogen input. Our calculations indicate that, in dry years, biogas slurry application could conserve 382–467 m³ ha⁻¹ of irrigation water. Therefore, the application of biogas slurry on farmlands not only addresses the challenges of managing farm manure and sewage but also contributes to water and fertilizer conservation for crop production, serving as a valuable strategy for promoting sustainable and environmentally friendly agricultural practices in North China. However, it is also important to be aware of the leaching risks associated with the excessive application of biogas slurry. Consideration should be given to its long-term effects and the potential for other environmental issues, such as heavy-metal and antibiotic contamination.

5. Conclusions

Different amounts of rainfall are the main reason for the interannual variation in soil nitrogen losses in farmlands to which biogas slurry is applied. In rainy years, utilizing biogas slurry led to an increase in soil nitrogen losses (NH₃ emissions + nitrogen leaching), while in dry years, it resulted in reduced nitrogen losses but elevated NH₃ emissions, both in maize and wheat seasons. Under equivalent nitrogen application conditions, the application of biogas slurry not only sustained the crop yield but also increased the annual NUE by enhancing the wheat’s NUE in the wheat–maize rotation system, simultaneously reducing the soil nitrogen residue without increasing the apparent nitrogen loss. Excessive application of the biogas slurry increased the nitrogen loss and reduced the nitrogen utilization efficiency, and did not improve the crop yield. During drought periods, using biogas slurry to replace chemical fertilizer may be an effective practice in sustaining crop yields in North China, alongside enhancing nitrogen utilization and decreasing nitrogen losses.