Soil Greenhouse Gas Emissions and Nitrogen Change for Wheat Field Application of Composted Sewage Sludge

Abstract

This study aimed at assessing the character of greenhouse emissions under different rates of composted sewage sludge as a nitrogen fertilization substitute, founding the characteristic of soil nitrogen change and, ultimately, providing a theoretical basis for the rational utilization of composted sewage sludge in calcareous soil. Compost sludge as a nitrogen fertilizer substitute has emission reduction effects on N₂O, CH₄ and CO₂. Compared with that of the single fertilizer application, the GHGI under the 20% sludge replacement treatment was significantly reduced by 20.15%, and the global warming potential was significantly reduced by 11.53%, while the wheat yield was increased by 4.78%. Compost sludge as a nitrogen substitute significantly increased the soil organic carbon at the jointing stage and reached a 100% replacement ratio at the maturity stage. During the jointing and mature stages, the total nitrogen content of the soil increased significantly at the 100% replacement ratio, while the soil nitrate nitrogen content only increased significantly at the 50% and 100% replacement ratio. Our findings highlight the impact of sludge compost on greenhouse gas emissions and soil nitrogen and guide the use of sludge compost in wheat fields.

1. Introduction

Chemical fertilizers can promote crop growth and increase crop yield [1], and China is the world’s largest fertilizer consumer. According to data released by the National Bureau of Statistics, the annual application of chemical fertilizers in China reached 52.507 million t in 2020 [2], of which the annual application of nitrogen fertilizer (pure nitrogen) was 18.339 million t. Since China’s nitrogen fertilizer utilization rate is only approximately 30% [3], a large amount of nitrogen fertilizer input not only causes nitrogen loss from soil but also pollutes the atmosphere and waterbodies. Replacing or reducing nitrogen fertilizer application with an appropriate amount of organic fertilizer does not significantly affect the crop yield. In contrast, it can also improve the nitrogen utilization efficiency to a certain extent [4] and reduce the emission of greenhouse gases, such as N₂O.

According to statistics, 5% to 20% of the total global annual CO₂ emissions come from soil respiration [5]. Therefore, arable land is considered an important source of CO₂ loss to the atmosphere [6]. Numerous scientific papers have shown that have shown that fertilization significantly affects N₂O emissions through nitrification and denitrification processes [7,8].

Reducing nitrogen fertilizer usage, reducing greenhouse gas environmental pollution [9] and greenhouse effects [10] has been a focus of researchers. For example, the combined application of organic fertilizers in rice fields in southern China promoted CH₄ emissions in rice fields [11]. However, dryland farmland is the main source of CO₂ and N₂O emissions. Compared with a single application of chemical fertilizer, the combined application of organic fertilizer can reduce N₂O emissions [12] but increase CO₂ emissions in the soil [13].

These results suggest that studies on the effect of organic fertilizers on soil nitrogen status and GHG emissions are inconsistent [14]. Organic fertilizers may be a wise choice to reduce the intensive use of chemical fertilizers, protect the environment and further increase crop yields.

As a kind of organic biosolid with huge yield, composted sewage sludge is rich in nutrients and has high agricultural potential after harmless treatment [15]. Studies have shown that sludge application can increase crop yield while improving soil fertility [16]. In addition, the application of sludge instead of nitrogen fertilizer will also affect the warming potential and comprehensive greenhouse effect [17]. Calcareous soil is one of the main soil types in northern China, with high salt base saturation and can adsorb and deposit heavy metals. It has also been confirmed that harmless sludge has a low agricultural risk on this type of soil [18].

Previous studies have mainly focused on the effects of sewage sludge on soil fertility or heavy metal accumulation, little attention has been associated with soil GHGs and nitrogen and insufficient knowledge or controversy related to the ambiguous effect of organic materials, especial sludge compost alternative nitrogen fertilizer on GHG emission and soil nitrogen status [19,20]. Therefore, the objectives of this study were: (1) to evaluate the responses of Greenhouse gas emissions under different sludge compost application ratio and (2) to display the soil nitrogen change effected by sludge compost application in wheat field.

2. Materials and Methods

2.1. Experimental Site and Materials

The field experiment was conducted from October 2020 to May 2021 on the farm of Henan University of Science and Technology, which is located in Luoyang City (34°41′ N, 112°27′ E) in the western part of Henan Province. The frost-free period is more than 210 days. The annual average temperature, rainfall, sunshine hours and humidity are 14.86 °C, 578.2 mm, 2204.9 h and 63%, respectively. The trial soil was a typical calcareous soil with non-hazardous sludge applied for 5 consecutive years. In addition, the tested sludge comes from a typical urban domestic sewage treatment plant in Luoyang and is decomposed by aerobic composting.

The water content of the composted sewage sludge is 42.85%. Electrical conductivity of the sludge is 0.92 mS × cm⁻¹. The content of cadmium, mercury, lead, chromium and arsenic in the selected sludge was 2.17, 0.058, 80.80, 232.87 and 27.95 mg × kg⁻¹, respectively, which met the “Contamination Control Standard for Agricultural Sludge Compost” (GB4284-2018). The test crop was wheat (Triticum aestivum L. Yumai 58), and the chemical fertilizers were urea (containing N 46%), superphosphate (containing P₂O₅ 16%) and potassium chloride (containing K₂O 60%). The physical–chemical properties of the soil and composted sewage sludge are shown in

Table 1. Basic physicochemical properties of the soil and sludge.

Parameter	pH	Soil Organic Carbon (g × kg−1)	Total Nitrogen (g × kg−1)	Available Phosphorus (g × kg−1)	Exchangeable Potassium (g × kg−1)
Soil	7.61 ± 0.03	0.42 ± 0.04	0.52 ± 0.02	0.014 ± 0.001	0.030 ± 0.002
Composted sewage sludge	7.28 ± 0.10	118.20 ± 5.70	8.32 ± 0.25	0.240 ± 0.008	3.27 ± 0.001

Note: The data are the mean ± standard deviation (n = 3).

.

2.2. Experimental Design

Six treatments were set up in the experiment: single application of fertilizer (S0), substitution of 20% nitrogen fertilizer with sludge (S1), substitution of 50% nitrogen fertilizer with sludge (S2), substitution of 100% nitrogen fertilizer with sludge (S3), substitution of 200% nitrogen fertilizer with sludge (S4) and substitution of 300% nitrogen fertilizer with sludge (S5). Among them, the single fertilizer application treatment adopts the local recommended fertilization rate—namely, 180 kg × ha⁻¹ N, 90 kg × ha⁻¹ P₂O₅ and 60 kg × ha⁻¹ K₂O in the wheat season.

Each treatment was repeated three times and arranged randomly, and each plot area was 2 m². On 27 October 2020, basal fertilizer was spread on the corresponding plot, and then the topsoil (0–20 cm) was manually turned and mixed with the basal fertilizer. The wheat was planted in artificial ditches, and the seeding rate was 15 kg × ha⁻¹. Top fertilizer was applied at the jointing stage of wheat on 13 March 2021. The specific fertilization scheme and fertilization amount are shown in

Table 2. Amount of fertilizer applied under different treatments (kg × ha⁻¹).

Treatment	Base Fertilizer (27 October 2020)				Topdressing (13 March 2021)
	Sludge	Urea	Superphosphate	Potassium Chloride	Urea
S0	0	120	90	60	60
S1	4150.08	84	90	60	60
S2	10,305.15	30	90	60	60
S3	20,610.30	0	90	60	0
S4	41,220.60	0	90	60	0
S5	61,830.90	0	90	60	0

. In addition, we used traditional methods to manage the crop field during the experimental run.

2.3. Sample Collection and Analysis

2.3.1. Gas Sample Collection and Analysis

The fluxes of N₂O, CH₄ and CO₂ were measured using a static chamber method. The box is composed of a top box (50 cm in height, 50 cm in length and 10 cm in width) and a base (cross-sectional area of 50 cm × 10 cm). The bottom of the base is embedded into the soil to approximately 5 cm in depth, and the upper part of the base has a water tank with a depth and width of 3 cm.

Before sampling, the tank was filled with water to prevent air leakage at the junction of the base and top box. The sampling time was from 9:00 a.m. to 11:00 a.m. At 0, 20 and 40 min after covering the static chamber, 50 mL of mixed gas in the box was sampled with a syringe and injected into a vacuum air bag for storage. Within 2 days, the sample was measured in an Agilent 7890A GC system (Agilent Technologies Inc., Palo Alto, CA, USA) using flame ionization detector (FID) and electron capture detector (ECD).

In addition, to clarify the effect of sludge substitution of nitrogen fertilizer on the N₂O, CH₄ and CO₂ emissions of calcareous dryland wheat according to different wheat development stages, the wheat growth period was divided into the seedling stage (sowing to greening stage), middle stage (greening to jointing stage) and late stage (joining to maturity stage) to set the gas extraction frequency.

The frequency of air intake in the seedling stage (5 to130 days after sowing) was 7 days, the frequency of air intake in the middle stage (138 to 169 days after sowing) was 7 days, and the frequency of air intake in the later stage (181 to 195 days after sowing) was 14 days. At the same time, a plug-in temperature sensor was used to measure the soil temperature at 10 cm depth, the box temperature and the atmospheric temperature (see Figure 1). After sampling, the static box was removed to minimize the impact of the box on the crop and soil environment.

2.3.2. Collection and Analysis of Soil and Plant Samples

Soil samples were collected at the jointing stage and at the maturity stage, and some fresh samples were stored at 4 °C for the determination of soil ammonium nitrogen and nitrate nitrogen. Part of the soil samples was dried and passed through 0.85 mm and 0.15 mm soil sieves for the determination of other parameters. The soil pH was determined by using a pH meter at a soil–water ratio of 1:5. Organic matter was determined by the potassium dichromate external heating method. Total nitrogen was determined by Kjeldahl nitrogen determination.

Soil available phosphorus was determined by sodium bicarbonate extraction-colorimetry. Soil available potassium was determined by using ammonium acetate extraction. Ammonium nitrogen was extracted by potassium chloride-magnesium oxide distillation. Nitrate nitrogen was determined by potassium chloride leaching-magnesium oxide and zinc-ferrous sulfate reducing agent distillation [21]. When the wheat was harvested, the yield of wheat in each plot was calculated by harvesting, threshing, airing and weighing.

2.4. Calculations and Statistical Methods

The calculation formula of the greenhouse gas emission flux is [22]:

$$F=\mathsf{\rho }\times \mathrm{H}\times \frac{\mathsf{\Delta }\mathrm{c}}{\mathsf{\Delta }\mathrm{t}}\times \left(\frac{273}{273+\mathrm{T}}\right)$$

(1)

In the formula, F is the greenhouse gas emission flux (μg × m⁻² × h⁻¹); ρ is the density of the gas in the standard state (the corresponding values of CO₂, CH₄, N₂O are 1.96, 0.714 and 1.96 kg × m⁻³); H is the height of the airtight box (m); Δc/Δt is the change in gas concentration per unit time (μg × h⁻¹); and T is the average temperature in the airtight box (°C).

$$\mathrm{Ec}={{\displaystyle \sum }}_{i=1}^n(\frac{F_{i+1}+F_i}{2})\times ({\mathrm{t}}_{i+1}-{\mathrm{t}}_i)\times 24$$

(2)

In the formula, Ec is the cumulative emission flux (mg × m⁻²), F is the greenhouse gas emission flux; i is the number of monitoring events; t_i+1 − t_i is the time between two monitoring events (d); and n is the total number of monitoring events.

Global warming potential (GWP) is the warming potential of the total seasonal emissions (kg × ha⁻¹) of various greenhouse gases converted into carbon dioxide equivalents, and it is calculated according to the following formula [23]:

$$GWP=F_{{\mathrm{CH}}_4}\times 25+F_{{\mathrm{N}}_2\mathrm{O}}\times 298+F_{\mathrm{C}{\mathrm{O}}_2}$$

(3)

In the formula, the unit of GWP is t × ha⁻¹; F is the total emission of greenhouse gases; the global warming potential of CH₄ or N₂O on a centennial scale is 25 times and 298 times that of CO₂, respectively.

Greenhouse gas emission intensity (GHGI) is the ratio of global warming potential to crop yield, and the formula is as follows [24]:

$$GHGI=GWP/\mathrm{Y}$$

(4)

In the formula, GWP is the global warming potential (t × ha⁻¹, calculated as CO₂); and Y is the crop yield (t × ha⁻¹).

2.5. Data Analysis

Between-treatment comparisons were performed with analysis of variance and multiple comparisons using the mean of three replicates. Excel 2010 was used for data sorting and statistical analysis, SPSS 17.0 was used for significant analysis and multiple comparisons (p < 0.05), and Origin 2021 was used for graph drawing.

3. Results

3.1. N₂O, CH₄, CO₂ Fluxes

The N₂O flux from the farmland during the wheat growth period is shown in Figure 2. During the whole growth period, the N₂O emission flux in each fertilizer treatment ranged from −65.51 to 53.00 μg × m⁻² × h⁻¹, and the average daily emission was as follows: S0 > S1 > S2 > S3 > S5 > S4. The average N₂O emission flux of the S0 treatment was 5.22 μg × m⁻² × h⁻¹ higher than that in the other treatments. The average N₂O emission fluxes in the sludge replacement treatments were 1.05 to 3.60 μg × m⁻² × h⁻¹ lower than that in S0. The S0 and S2 treatments showed peak emissions on the 116th day after sowing (21 February 2021), and the S1, S3, S4 and S5 treatments showed peak emissions on the 45th day after sowing (12 December 2020).

The CH₄ emission flux in this study is shown in Figure 2. During the whole growth period, the CH₄ emission flux in each fertilizer treatment ranged from −85.85 to 133.49 μg × m⁻² × h⁻¹, and the average daily emission was as follows: S0 > S3 > S1 > S2 > S4 > S5. The average CH₄ emission flux in the S0 treatment was 10.66 μg × m⁻² × h⁻¹, which was higher than that in the other treatments. The average CH₄ emission fluxes in the sludge replacement treatments were 7.15 to 13.85 μg × m⁻² × h⁻¹ lower than that in S0. The composted sewage sludge replacement treatments showed a peak CH₄ emission on the 145th day after sowing (22 March 2021), while the S0 treatment showed a peak emission on the 45th day after sowing (12 December 2020).

As shown in Figure 2, the CO₂ emissions during the whole growth period of wheat were essentially the same across all treatments. The CO₂ emission flux of each fertilization treatment in the wheat season ranged from −48.63 to 390.57 mg × m⁻² × h⁻¹, and the average daily emission was as follows: S0 > S2 > S5 > S4 > S3 > S1. The average CO₂ emission flux in the S0 treatment was 137.25 mg × m⁻² × h⁻¹, and the average CO₂ emission flux in the sludge replacement treatments was 2.96% to 19.56% lower than that in S0. The S0, S1, S3, S4 and S5 treatments showed a peak CO₂ emission on the 138th day after sowing (15 March 2021), which may be related to the fertilizer applied on 13 March 2021. The S2 treatment showed a peak CO₂ emission on the 159th day after sowing (5 April 2021).

3.2. Cumulative Emissions at Different Growth Stages

As shown in Figure 3, in the wheat seedling stage, the cumulative N₂O emission range of each fertilization treatment was −0.03 to 0.12 kg × ha⁻¹, and the order of magnitude was S1 > S0 > S4 > S5 > S2 > S3. In the middle stage of wheat, the cumulative N₂O emission range of each fertilization treatment was 0.02 to 0.15 kg × ha⁻¹, and the order of magnitude was S1 > S2 > S0 > S5 > S3 > S4. In the later stage of wheat, the cumulative N₂O emission range of each fertilization treatment was 0.01 to 0.03 kg × ha⁻¹, and the order of magnitude was S1 > S5 > S2 > S0 > S3 > S4.

The cumulative N₂O emissions in the S0, S1 and S4 treatments were concentrated in the seedling stage, and the cumulative N₂O emissions in the S2, S3 and S5 treatments were concentrated in the middle stage. At the seedling stage of wheat, the cumulative CH₄ emission range of in each fertilization treatment was −0.15 to 0.32 kg × ha⁻¹, and the order of magnitude was S0 > S3 > S1 > S4 > S5 > S2. In the middle stage of wheat, the cumulative CH₄ emission range in each fertilization treatment was 0.08 to 0.22 kg × ha⁻¹, and the order of magnitude was S2 > S3 > S0 > S1 > S5 > S4. In the later stage of wheat, the cumulative CH₄ emission range in each fertilization treatment was −0.10 to 0.02 kg × ha⁻¹, and the order of magnitude was S0 > S1 > S2 > S3 > S4 > S5.

The cumulative CH₄ emissions under the S0 treatment were concentrated in the seedling stage, and the cumulative CH₄ emissions of the other sludge replacement treatments were concentrated in the middle stage of wheat. At the seedling stage of wheat, the cumulative CO₂ emission range of each fertilization treatment was 2226.88 to 3052.32 kg × ha⁻¹, and the order of magnitude was S4 > S0 > S5 > S3 > S2 > S1.

In the middle stage of wheat, the cumulative CO₂ emission range of each fertilization treatment was 1852.30 to 2493.97 kg × ha⁻¹, and the order of magnitude was S0 > S2 > S1 > S5 > S3 > S4. In the later stage of wheat, the cumulative CO₂ emission range of each fertilization treatment was 579.86 to 1015.12 kg × ha⁻¹, and the order of magnitude was S2 > S5 > S0 > S1 > S3 > S4. The cumulative CO₂ emission of each treatment was the largest at the wheat seedling stage, and all showed a downward trend with the progression of the wheat growth period.

3.3. Cumulative Emissions, GHGI and GWP

During the wheat growth period, CO₂ was the largest greenhouse gas emitted from farmland soil, and CH₄ gas had the lowest cumulative emissions. The cumulative emission of N₂O in each treatment ranged from 0.08 to 0.36 kg × ha⁻¹, and after replacing nitrogen fertilizer with sludge, the cumulative N₂O emission was reduced. The cumulative emission of CH₄ in each treatment ranged from −0.11 to 0.53 kg × ha⁻¹. Among the treatments, the cumulative CH4 emissions of S1 and S2 were 82.86% and 24.14% higher than that of S0, respectively.

The cumulative CO₂ emission range of each treatment was 5036.99 to 6261.96 kg × ha⁻¹, and compared with the S0 treatment, the S1 treatment had a more marked effect on CO₂ emission reduction, and the cumulative emission was reduced by 19.56%.

Both GWP and GHGI were significantly affected by nitrogen fertilization (

Table 3. The wheat yield, cumulative emissions, GWP and GHGI under different treatments.

Treatment	Yield (kg × ha−1)	Ec-N2O (kg × ha−1)	Ec-CH4 (kg × ha−1)	Ec-CO2 (kg × ha−1)	GWP (t × ha−1)	GHGI
S0	7173.34 ± 569.37 ab	0.36 ± 0.27 a	0.29 ± 0.18 b	6261.96 ± 186.23 a	6.35 ± 0.27 a	0.83 ± 0.00 d
S1	7516.21 ± 643.39 a	0.30 ± 0.23 a	0.53 ± 0.01 a	5036.99 ± 810.15 b	5.62 ± 0.25 cd	0.66 ± 0.02 e
S2	6540.08 ± 470.86 bc	0.26 ± 0.23 a	0.36 ± 0.01 b	6076.49 ± 116.09 a	6.11 ± 0.18 ab	0.88 ± 0.02 c
S3	6373.81 ± 462.81 bc	0.16 ± 0.15 a	0.16 ± 0.07 c	5389.83 ± 821.71 ab	5.10 ± 0.20 bc	0.93 ± 0.04 b
S4	6298.99 ± 570.31 c	0.15 ± 0.01 a	0.05 ± 0.00 d	5484.48 ± 322.51 ab	5.50 ± 0.36 d	0.90 ± 0.08 bc
S5	5294.31 ± 454.27 d	0.08 ± 0.00 a	−0.11 ± 0.01 e	5956.46 ± 0.01 ab	5.98 ± 0.00 b	1.20 ± 0.00 a

Note: Different lowercase letters in the same column indicate significant differences among different treatments (p < 0.05).

). During the growth period of wheat, all composted sewage sludge replacement treatments reduced the GWP; among them, the GWP of the S1, S3, S4 and S5 treatments was 11.50%, 19.69%, 13.39% and 5.83% lower than that of S0, respectively. Moreover, the range of GHGI of each treatment was 0.66 to 1.20. Compared with that of the S0 treatment, the GHGI of the S1 treatment was significantly reduced by 20.48%, and the wheat yields of the S1 treatment were 4.78% higher than those of the S0 treatment.

3.4. Soil Mineral Nitrogen Content

To clarify the effect of composted sewage sludge replacement of nitrogen fertilizer on soil available nitrogen, the contents of soil ammonium nitrogen and nitrate nitrogen were analyzed at the joining stage and maturity stage (Figure 4). At the jointing stage of wheat, the soil nitrate nitrogen content increased with the increase in the ratio of sludge replacing nitrogen fertilizer, and compared with that of the S0 treatment, the nitrate nitrogen contents of the S2, S3, S4 and S5 treatments were significantly increased by 24.23% to 202.30%.

The ammonium nitrogen content showed a decreasing trend, and the ammonium nitrogen content of the S1 treatment was significantly lower than that of S0 by 53.39%. During the maturity stage of wheat, the contents of ammonium nitrogen and nitrate nitrogen in the soil of each treatment increased with the increase in the composted sewage sludge replacement ratio, and the nitrate nitrogen content of the S2 and S3 treatments was 33.33% and 139.47% significantly higher than that of S0, respectively. The ammonium nitrogen content of the S1 treatment was 105.88% significantly higher than that of S0.

3.5. Soil Organic Carbon, Total Nitrogen and C/N

As shown in

Table 4. Dynamic changes in soil organic carbon, total nitrogen and soil C/N under each sludge replacement treatment.

Treatment	Joining Stage			Maturity Stage
	SOC (g × kg−1)	TN (g × kg−1)	C/N	SOC (g × kg−1)	TN (g × kg−1)	C/N
S0	5.45 ± 0.65 e	0.76 ± 0.07 c	11.14 ± 1.12 bc	5.11 ± 0.41 c	0.74 ± 0.04 c	7.02 ± 1.01 b
S1	6.76 ± 0.59 d	0.76 ± 0.05 c	10.23 ± 0.68 cd	5.29 ± 0.14 c	0.75 ± 0.05 c	7.92 ± 0.48 b
S2	6.99 ± 0.33 d	0.76 ± 0.05 c	12.64 ± 0.26 a	5.32 ± 0.48 c	0.75 ± 0.02 c	8.99 ± 0.93 a
S3	10.86 ± 0.66 c	1.00 ± 0.07 b	11.77 ± 0.86 ab	7.08 ± 0.60 b	0.97 ± 0.07 b	7.78 ± 0.69 b
S4	12.04 ± 0.92 b	1.42 ± 0.04 a	8.82 ± 0.93 e	8.72 ± 0.60 a	1.22 ± 0.09 a	7.34 ± 0.12 b
S5	14.12 ± 0.28 a	1.50 ± 0.09 a	9.72 ± 0.24 de	8.43 ± 0.17 a	1.19 ± 0.01 a	7.99 ± 1.07 ab

Note: Different lowercase letters in the same column indicate significant differences among different treatments (p < 0.05).

, composted sewage sludge as a replacement for nitrogen fertilizer can increase the soil organic carbon (SOC) and total nitrogen (TN) contents to above that of S0 at the joining and maturity stages (Table 4). The SOC content of the composted sewage sludge replacement treatments significantly increased by 24.04% to 159.08% at the jointing stage; however, at the maturity stage, the SOC content only began to increase significantly when the sludge replacement ratio exceeded 50%.

The soil TN content of the S3 treatment increased significantly at the joining stage and maturity stage compared with that of the S0 treatment. The soil C/N of the S4 and S5 treatments decreased significantly by 20.83% and 12.75% at the jointing stage, respectively, compared with that of the S0 treatment. The soil C/N of the composted sewage sludge replacement treatments increased at the maturity stage, and the soil C/N of the S2 treatment was significantly increased by 28.06%.

4. Discussion

4.1. Replacing Nitrogen Fertilizer with Organic Fertilizer Affects Soil Greenhouse Gas Emissions

Applications of organic fertilizers affect the soil C/N ratio and microbial activities; moreover, the nitrogen supply capacity of organic fertilizers is different, which in turn affects farmland greenhouse gas emissions [25,26]. Soil N₂O is the product of nitrification and denitrification by microorganisms. Studies have shown that, compared with the single application of chemical fertilizer, the application of organic fertilizer will promote N₂O emissions. The main reason is that the application of organic fertilizer in farmland provides exogenous carbon and nitrogen to the soil, which affects the activity of soil microorganisms and then affects nitrification and denitrification [27].

According to Zerssa et al. [28], N₂O emissions were reduced when the mineral fertilizers were combined with compost. This can be explained by the replacement of the mineral N by organic N and, consequently, by an initial microbial immobilization of N [29] or slow release of N from the organic fertilizer. In our study, composted sewage sludge substituting nitrogen fertilizer reduced soil N₂O emissions in wheat fields.

These data can be explained in the context of the research by Sosulski et al. [30], who detected a higher content of extractable organic carbon in the arable soils treated organically than in those under conventional mineral fertilization, and a low C-content inhibits denitrification, which ultimately leads to a decrease in N₂O emissions. In addition, the nitrogen in the composted sewage sludge is mainly organic nitrogen, whose content is low, and there are fewer reaction substrates directly supplied to nitrification and denitrification, and thus the production and emission of N₂O are also lower [31].

The absorption and emission of CH₄ in soil are also affected by fertilization methods [32]. For example, in the early stage of organic fertilizer application, soil CH₄ emissions can be promoted. The reason is related to the high content of soil organic matter in the early stage of fertilization, which provides a sufficient carbon source for CH₄ production after decomposition [33]. Some studies have reported that the soil CH₄ emissions in dryland are low because dryland soil has good permeability and does not easily produce an anaerobic environment; the decomposition rate of soil organic matter is high, and soil organic carbon does not easily accumulate, which affects CH4 emissions.

Therefore, drylands are regarded as important weak CH₄ sinks [34,35]. The results of this study showed that the average daily emission of soil CH₄ in each sludge replacement treatment was lower than that in the single application of chemical fertilizer, which may be caused by the application of topdressing nitrogen fertilizer during the jointing period [36]. Therefore, the effects of chemical fertilizers and organic fertilizers on soil CH₄ uptake in dryland farmland are not consistent, and further research is needed.

Organic matter and organic matter produced by the decomposition of organic fertilizers are the main sources of CO₂ in soil [37]; however, the use of organic fertilizers has different effects on CO₂ emissions. In this study, the average daily CO₂ emission fluxes from soils treated with sludge were lower than those of soils treated with a single application of chemical fertilizers, shown that organic fertilizers can reduce soil CO₂ emissions. The reason may be related to the increase of sludge addition, the content of heavy metals in the soil increases, which may reduce the microorganism diversity and richness [38], thus, reducing the release of CO₂.

However, it has no negative effect on crop growth. Similar results are also confirmed in reference [39]. The study confirmed that the soil CO₂ emission under different species of organic fertilization is different [5]. However, some studies have confirmed that organic fertilizer or organic–inorganic combined application has no effect on CO₂ emission reduction in farmland and even promotes CO₂ emission [40] because organic fertilizer is an effective method to increase the soil carbon and nitrogen substrate content and enhance microbial respiration.

4.2. Replacing Nitrogen Fertilizer with Organic Fertilizer Affects Wheat Yield and Soil Nutrients

Replacing nitrogen fertilizer with organic fertilizer is an important way to reduce nitrogen fertilizer application in farmland; however, the effect of different organic fertilizer replacement ratios on crop yield is not the same. This experiment showed that with the increase in the sludge replacement ratio, wheat yield showed first an increasing and then a decreasing trend. It may be that the replacement ratio of organic nitrogen is too high, and thus the amount of nitrogen held by the microorganisms in the early stage will be insufficient, and the amount of nitrogen released in the middle and late stage will be too small, which is not sufficient to meet the nutrient requirements of the crops in the late growth stage [41].

Regarding the N mineralization potential of composted sewage sludge, some studies have indicated that the total nitrogen mineralization rate of sludge and sludge compost for 90 days is 14.25% and 1.15%, respectively [42], and it can be seen that the nitrogen mineralization rate of sludge is relatively small, so the negative relationship between the amount of sludge added and crop yield, which seems to be little affected by soil availability of nutrients. Therefore, only an appropriate amount of organic fertilizer can replace nitrogen fertilizer to improve crop yield.

Replacing nitrogen fertilizer with organic fertilizer is also an important measure to fertilize soil [43]. On the one hand, it can maintain and increase soil nitrogen nutrients [44], and on the other hand, the input of organic fertilizer increases the soil carbon pool [45]. The results of this experiment showed that compared with the single application of chemical fertilizer, the application of sludge increased the content of soil SOC and TN, and the content of ammonium nitrogen in the soil increased significantly at the end of the experiment. It may be that the combined application of organic nitrogen and inorganic nitrogen enhances the soil nitrogen mineralization and immobilization capacity and enhances the potential supply of soil nitrogen [46].

At the same time, we study showed that the content of heavy metals in soil and wheat grains increased with the increase of sludge replacement ratio. However, under the condition of maximum application of sludge compost, the contents of arsenic, cadmium, lead and chromium in the soil were 14.52, 0.15, 38.42 and 91.09 mg × kg⁻¹, respectively, which did not exceed the “Soil environmental quality risk control standard for soil contamination of agricultural land” (GB 15618-2018).

In addition, the contents of arsenic, cadmium, lead and chromium in wheat grains under this treatment were 0.07, 0.06, 0.09, 0.30 mg × kg⁻¹, respectively, which did not exceed the “National food safety standard limits of contaminants in foods” (GB 2762-2017) (These results are unpublished.). These results indicate that sludge compost is relatively safe for soil and crops during the test application period, Liang et al. [47] also verified this. Due to the electrical conductivity of the sludge is 0.92 mS × cm⁻¹ (less than 4 mS × cm⁻¹), which will not inhibit plant growth [48].

5. Conclusions

(i)

Replacing nitrogen fertilizer with composted sewage sludge can reduce the emissions of N₂O and CO₂ in calcareous wheat fields. At the same time, when the proportion of nitrogen fertilizer replaced by composted sewage sludge does not exceed 50%, the cumulative emissions of CH₄ in dryland wheat fields are increased. After replacing nitrogen fertilizer with composted sewage sludge, the GWP of the dryland wheat field decreased, and the GHGI was the smallest when the composted sewage sludge replacement ratio was 20%.

(ii)

The application of 20% composted sewage sludge to replace nitrogen fertilizer in a dryland wheat field can increase the wheat yield while not significantly increasing greenhouse gas emissions and can achieve a stable and increased wheat yield. There was a positive correlation between the soil total nitrogen content and sludge compost addition amount; at the same time, the soil mineral nitrogen content also increased in the mature stage.

(iii)

When composted sewage sludge replaced 20% of nitrogen fertilizer, the N₂O and CO₂ emissions in calcareous wheat fields were reduced; however, the CH₄ emissions were not favorable.