Evaluation of Maturity and Greenhouse Gas Emission in Co-Composting of Chicken Manure with Tobacco Powder and Vinasse/Mushroom Bran

Abstract

This study investigated the effects of different proportions (0%, 5%, 10%, 15%) of bulking agent (vinasse, mushroom bran, and tobacco powder) on maturity and gaseous emissions in chicken manure composting. The results showed that all of the treatments reached the standard of harmless disposal. With the exception of the control treatment, the CH₄, N₂O, and NH₃ emissions in the treatments that had been prepared using the addition of mixed bulking agents were effectively reduced by 2.9–30.6%, 8.30–80.9%, and 37.3–26.6%; their compost maturity also met the Chinese national standard. Specifically, 10% mushroom bran combined with 5% tobacco powder was the optimal combination for simultaneously improving the maturity and reducing greenhouse gas emission in chicken manure composting.

1. Introduction

Currently, the chicken manure (CM) production has risen sharply in China [1,2]. The improper processing of CM can easily lead to environmental pollution. For example, malodorous and greenhouse gases (GHGs) released from CM during its natural decomposition can severely reduce air quality and enlarge the greenhouse effect [3]. It has been reported that the CH₄ and N₂O emissions originating from livestock manure accounted for the 5–10% and 7% of the global emission amount of the CH₄ and N₂O, respectively [3].

Aerobic composting is an effective and resource recycling method that can harmlessly convert livestock manure into high-grade organic fertilizer [4,5]. However, a significant amount of the ammonia and GHGs are released during CM aerobic composting, which causes serious atmospheric pollution, and the loss of nitrogen and carbon leads to the decline of the quality of the final composting products [6]. In addition, CM has a high water content (>80%), a low carbon/nitrogen ratio (C/N) (<5), and a tight bulk density, which limit the feasibility being composted separately [1]. Therefore, it is necessary to add a certain amount of bulking agent to properly regulate the composting conditions and to boost composting processes.

Tobacco production in China reached 2.24 million tons in 2018, but 30~35% of tobacco were discarded as residue powder during the production process [7,8]. Tobacco residue powder with low C/N contains abundant proteins, polysaccharides, and nitrogen, which makes the tobacco powder a suitable nutrient source for microorganism growth and therefore can boost the aerobic degradation process [9]. Therefore, the reutilization of tobacco by means of composting is an effective waste resource utilization method. In order to make better use of the nitrogen-containing compounds (such as protein and amino acids) in tobacco and to make them degrade more rapidly, fillers with a high carbon content can be used for mixed composting. Further, the addition of a bulking agent into the composting system can make the composting system more porous, which is preferable for the uniform diffusion of oxygen [9].

Vinasse and mushroom bran are typical organic wastes in southwest China [10]. Vinasse results from the fermentation residues of sorghum, wheat, corn, and other grains during alcohol production and has high organic matter content, is rich nutrients, and has a certain amount of fermentation strains [11,12]. Mushroom bran has been widely used in compost due to its loose and porous structure, rich nutrients, and various enzymes (e.g., cellulase, esterase, etc.) [13]. Therefore, the mixed bulking agents (i.e., tobacco powder mixed with vinasse or mushroom bran) with a relatively low water content, high C/N, and good porosity can optimize the composting environment for microorganism growth by properly adjusting the bulk moisture and porosity [10]. Controlling the composting process under the optimized conditions can efficiently degrade the organic matter and can improve the compost maturity [1]. Relevant research has indicated that microorganism-enriched organic waste can effectively mitigate nitrogen loss and the emission of greenhouse gases (GHGs) through microbial assimilation during aerobic composting [14]. Therefore, this study used vinasse and mushroom bran mixed with tobacco powder as the combined bulking agents to improve the maturity and to reduce GHGs and NH₃ emissions during chicken manure composting.

The application of unstable and immature compost to the farmland may induce root hypoxia and ammonia poisoning. Thus, it is necessary to investigate the maturity of the compost by evaluating the compost color, smell, structure, pH, electric conductivity (EC), C/N, and seed germination rate index (GI). In addition, the CH₄ and N₂O are important GHGs, and a large amount of NH₃ emissions will reduce the nutrient content in the compost. Previous studies have shown that vinasse played a significant role in improving the maturity of compost and that the addition of mushroom bran can reduce GHGs emissions and can enhance composting maturity [15].

Although previous research has investigated the influence of separate vinasse or mushroom bran as a bulking agent on the GHGs and NH₃ emissions that are produced during the composting process, research concerning the addition of tobacco powder combined with microbially abundant materials (i.e., mushroom bran and vinasse) as a mixed bulking agent for CM composting to improve the nitrogen fixation, GHGs emissions, and composting maturity has not been reported. Herein, this study aims to investigate the effect of the tobacco powder combined with vinasse and mushroom bran as mixed bulking agents on the dynamic changes of GHG and NH₃ emissions and the compost maturity during CM composting.

2. Materials and Methods

2.1. Raw Materials and Experimental Setup

The composting experiment was conducted at the Shangzhuang Experimental Station of China Agricultural University. The main raw material for composting was CM, and the bulking agents included tobacco powder, vinasse, and mushroom bran. The CM was collected from the Beijing Yanqing County Kangzhuang chicken farm. Tobacco powder was collected from Guizhou Kaiyang Nanjiang Modern Agricultural Development Co. Ltd. Vinasse and mushroom bran were collected from the Zunyi Mushroom Garden in Guizhou Province. The vinasse and mushroom bran were dried in air and were crushed into small pieces of 1 to 5 cm diameter. The properties of the raw materials are shown in

Table 1. General characteristics of the initial composting materials.

Materials	TOC (g·kg−1)	N (g·kg−1)	Moisture Content (%)	pH	EC (ms·cm−1)	C/N
Chicken manure	344 ± 3.21	22.8 ± 0.32	77.3 ± 0.81	7.98 ± 0.12	7.02 ± 0.26	15.1
Tobacco powder	389 ± 1.11	30.3 ± 0.31	7.90 ± 0.21	6.03 ± 0.00	8.99 ± 0.10	13.1
Vinasse	474 ± 1.35	47.2 ± 0.89	8.26 ± 0.57	6.02 ± 0.01	2.74 ± 0.04	10.1
Mushroom residue	434 ± 2.41	18.9 ± 0.76	8.08 ± 0.15	7.49 ± 0.30	2.31 ± 0.11	22.9

Note: The data in the table were determined using dried samples.

. It is noteworthy that the C/N ratio of chicken manure is normally less than 5, but the experimental value of chicken manure obtained in this research was 15.1, which can probably be attributed to the fact that pre-treatment and storage methods for the raw materials of chicken manure can affect the of C/N ratio of chicken manure to a certain extent [16].

In this study, seven treatments were included. All of the treatments maintain the same total weight, of which 80% (w/w) was CM, and 20% (w/w) was bulking agent. For the control treatment, the bulking agent of tobacco powder accounted for 20%. The other six treatments were a combined bulking agent treatment, i.e., tobacco powder combined with vinasse or mushroom bran (

Table 2. Application rate of diverse agents in different treatments.

Treatments	Chicken Manure (%)	Tobacco Powder (%)	Vinasse (%)	Mushroom Residue (%)
CK	80	20	0	--
T1	80	15	5	--
T2	80	10	10	--
T3	80	5	15	--
T4	80	15	--	5
T5	80	10	--	10
T6	80	5	--	15

Note: Calculation based on a dry weight.

). The initial moisture content of the reactor body was controlled at 60%. After being mixed uniformly, the intermittent ventilation mode (i.e., ventilation every 30 min) was set with the ventilation rate of 0.4 L·kg⁻¹·DM·min⁻¹. The composting cycle for all of the treatments was 35 days, and it was turned manually on days 0, 3, 7, 14, 21, and 28. Solid samples were collected homogeneously after each manual turning.

Compost trials were conducted in a 60-L laboratory-scale stainless steel column reactor (0.6 m high and with a 0.36 m inner diameter) (Figure 1). The bottom of the fermentation tank was equipped with a stainless steel porous sieve plate (aperture of 1 cm). There were two ports in the lower part of the tank; one was connected to the ventilation control system for ventilation, through which the blower forces the air into the fermentation tank at a certain speed to supply the oxygen in reactor. The other is the leachate collection port. The temperature sensor is inserted in the tank, and the temperature information can be recorded and transmitted to the receiver through radio waves. An exhaust pipe is set on the top of the tank for gas collection, which is also used for discharging excess gas and water in the tank and for ensuring the connection with the ambient atmosphere.

2.2. Analytical Methods and Statistical Analysis

The temperature was automatically detected and recorded daily by means of the Testo temperature automatic monitoring system. Oxygen (O₂) concentrations were measured by a portable biogas meter, an oxygen saturation probe was used to determine the pile oxygen concentration (BIOGAS5000, Geotech, UK). The N₂O and CH₄ concentrations were determined by a gas chromatograph (3420A, Beifen, Beijing, China) equipped with an electron capture detector (ECD). NH₃ was collected through the boric acid absorption method, and its concentration was determined through titration with diluted sulfuric acid. The cumulative emissions were calculated using the trapezoid formula [17]:

$$A_{t\left(ab\right)}=\frac{\left(t_b-t_a\right)\times \left(F_{ta}+F_{tb}\right)}{2}$$

(1)

where A_t(ab) is the cumulative GHGs emission amount among the measurement days (between t_a and t_b), t_a and t_b are the dates of the two measurements, and F_ta and F_tb are the gas fluxes at the two measurement dates.

The cumulative emissions can be calculated as the sum of the day using the following formula:

$$\mathrm{Total} \mathrm{cumulative} \mathrm{emission}={\displaystyle \sum }{A}_{t\left(ab\right)}$$

(2)

In addition, the global warming potential (GWP) of CH₄ and N₂O are 25 (CH₄) and 298 (N₂O) times higher than that of CO₂ on a centennial scale [18]. The moisture content was determined by mixing about 5 g of fresh compost sample in an aluminum box, and this sample was dried to a constant weight at 105 °C; the percentage of the lost mass to the fresh weight of the sample was calculated. The weight of the pile was weighed before and after each pile turning. For other basic physical and chemical indicators, a compost sample (approximately 200 g) was collected and tested after each pile turning, and the sample was divided into two parts for subsequent analysis. The pH, EC (MP521 type, pH/EC meter), and E4/E6 (N4S, China UV spectrophotometer) of different samples were determined by referring to Jiang et al. [19]. For the determination of the NH₄⁺ and NO₃⁻, deionized water was used as the extraction solution. The content of the total organic carbon (TOC) and total nitrogen (TN) were determined using an elemental analyzer (Elementar Analysensysteme, Hanau, Germany). During the determination process, gas samples are collected from 10 am to 11 am daily. The solid samples were collected by the five-point sampling method after thorough pile turning. All of the measurements were measured in triplicate. The data values shown in all the figures and tables are the means ± standard deviations (n = 3) for each treatment. The analysis of variance (ANOVA) was utilized to test the significant differences among different treatments. Duncan’s procedure was adopted to conduct the multiple comparisons of means, and the level of significant difference was set as p < 0.05.

3. Results and Discussion

3.1. Changes of Temperature and Oxygen Concentration

Temperature is one of the key indicators reflecting composting progress and microbial activity [20]. The temperature changes of different treatments in this experiment were shown in Figure 2. The control treatment reached the maximum temperature of 70 °C at day 9, and the high temperature period (>55°C) lasted for 18 days. In separate vinasse treatments, the pile temperatures of the 5%, 10%, and 15% tobacco powder treatments (T1~T3) peaked on day 9 (73.6 °C), day 9 (74.3 °C), and day 1 (71.0 °C), respectively. The high temperature period of (T1~T3) lasted for 20 days, 24 days, and 26 days, respectively. In the mushroom bran-mixed tobacco powder treatments (T4~T6), the pile temperatures all peaked on day 9 and were 73.6 °C, 73.8 °C, and 73.7 °C, respectively. The high temperature period lasted for 19 days, 22 days, and 22 days, respectively. The organic matter in the reactor had been completely degraded at day 35, as the pile temperature had become stable after day 35. The pile temperature of all of the treatments met the requirements for national harmless sanitation standards (GB 7959-87) without significant difference among all of the treatments (p > 0.05). Compared to the mushroom bran mixed-tobacco powder treatments, the high temperature period of vinasse-mixed tobacco powder treatment lasted longer, and the compost process started earlier, which may be attributed to the more porous pile structure, microbial adsorption sites, and better aerobic conditions induced by the addition of the bacterial enriched mushroom [21,22]. Secondly, the higher C/N ratio of the mushroom bran is beneficial to prolonging the high temperature period of compost [23]. Compared to the control treatment, the average pile temperature of the T1~T6 treatments was higher, as was the high temperature period of the compost, which was mainly due to the relatively larger average particle size of the vinasse and mushroom bran, which favor the improvement of the ventilation conditions and of generating more heat through the aerobic reaction of microorganisms. Since the tobacco is powdery and the surface moisture content of the CM is relatively high, the tobacco powder is prone to sticking on the CM surface, and it is hard for the air to enter into the heap due to the formation of a protective film, inhibiting the metabolic reaction of aerobic microorganisms. It has also been reported that the addition of tobacco to the CM composting process can reduce the pile temperature and the duration of the high temperature period, which is not conducive to the rapid maturation of compost materials [13].

The kinetics of the O₂ concentration reflect the progress and stability of the compost. Sufficient O₂ ensures microbial respiration and favors the generation of stable and mature compost and vice versa. In the early stages of the composting process, the microbial aerobic activity was vibrant, the O₂ concentration dropped sharply from 17% to 12%, and the difference among treatments was not significant (p > 0.05). This is consistent with previous composting research and could be attributed to the rich content of easily degradable organic matter [17,24]. When the O₂ concentration increased from 10% to 18%, the microbial activity increased, and the degradation of the organic matter became faster. After 21 days, the microbial decomposition activity became more weakened, and the O₂ consumption gradually decreased with the decreasing degradable organic matter content [25]. In contrast, the pile O₂ concentration began to increase. By the end of the 35th day, the O₂ concentration was close to the environmental O₂ concentration, the degradation of the organic matter that could be easily decomposed by microorganisms had been completed, and the microorganisms continued to convert the complex organic matter into humus in the pile. In previous research, the temperature was significantly correlated with the O₂ concentration in the reactor environment. In addition, the kinetic trend of the O₂ concentration corresponded to that of the pile temperature during the composting process [26].

3.2. Analysis of Compost Maturity

Compost maturity is usually evaluated based on indicators according to Ministry of Agriculture Organic Fertilizer Standard (NY525-2012).

Table 3. Chemical properties and maturity index changes of the compost at the 35th day.

Treatment	pH		EC (ms·cm−1)		E4/E6		C/N		GI (%)
	Initial	Finish	Initial	Finish	Initial	Finish	Initial	Finish	Initial	Finish
CK	5.84 c	8.51 a	6.33 a	5.68 a	2.85 f	1.69 a	12.6 b	12.0 a	0.06 g	63.8 g
5% Vinasse	6.27 a	8.40 b	5.32 d	4.94 b	3.80 c	1.13 b	11.0 d	10.3 d	3.67 c	93.1 f
10% Vinasse	6.25 a	8.18 c	5.69 c	4.46 c	4.10 b	0.97 c	14.1 a	11.4 b	1.76 f	97.7 e
15% Vinasse	6.04 a	8.48 a	5.87 b	3.74 e	5.54 a	0.78 d	12.3 c	10.7 c	7.17 a	98.9 d
5% Mushroom residue	6.31 a	8.38 b	6.06 b	3.90 d	2.51 g	1.20 b	12.7 b	11.8 a	2.99 d	106 c
10% Mushroom residue	5.93 b	8.48 a	5.93 b	3.68 f	3.43 d	1.10 b	12.0 c	10.2 d	2.42 e	110 b
15% Mushroom residue	6.14 a	8.52 a	4.95 e	3.03 a	3.28 e	0.77 d	13.8 a	11.8 a	6.12 b	115 a

Note: (data are the means of three replicates, and bars with a different letter (a–g) in different treatments indicate a significant difference at p < 0.05.)

shows the physical and chemical properties and maturity of the initial and maturation stages of composting. The pH of different treatments increased from 5.84 to 6.27, 8.18, and 8.52, and the difference among the treatments was not significant (p > 0.05), indicating that the improvement of the compost quality is related to the pH increase. An alkaline compost product can induce the precipitation of heavy metals and can prevent heavy metals from entering the plant root system to some extent [27,28]. The initial EC of the different treatments ranged from 4.95 to 6.33 ms·cm⁻¹ and decreased to 3.03–5.68 ms·cm⁻¹ at the end of the composting process. Among them, the EC of the control treatment, T1, and T2 were all higher than the compost safety limit of 4 ms·cm⁻¹, which could be attributed to the originally higher EC of tobacco. Toxic substances (e.g., nicotine) in tobacco powder, and the high-content salt can hinder its further utilization. Therefore, it is necessary to increase the proportion of the bulking agent to optimize the composting process.

The E4/E6 value can indicate the stabilization process of humus. Among all of the treatments, the E4/E6 decreased from the initial 2.51~5.54 to 0.77~1.69. In addition, the C/N ratios in the treatments of the T1~T3 were relatively higher because vinasse can introduce more small molecular substances that are not completely anaerobically decomposed and thus benefited the form of large molecular humus substances and improved the humification degree of the products. In terms of the change of C/N, it can be seen that the C/N ratios of the composting treatments were reduced from 11.03~14.11 to 10.19~12.04, which meets the basic standard (10~15) of compost maturity. However, the C/N ratio between treatments was not significantly different (p > 0.05). The reduction of the C/N ratio in T1~T6 is significantly higher than that of the control treatment. After the tobacco powder was added in the CM compost, the porosity of the pile porosity increased, which favored the improvement of the microbial oxygen environment and the degradation efficiency of the organic matter.

GI is also an important index for evaluating compost maturity and biological toxicity. It is generally believed that compost with GI > 50% is basically non-toxic to crops, compost with GI > 80% is completely non-toxic to crops, and compost with GI > 100% is beneficial to crops [29]. The initial GI of the control treatment was only 0.06%, and the initial GI of the auxiliary substituted treatments was between 1.76% and 7.17% (Table 3), indicating that tobacco powder has a serious inhibitory effect on seed germination and is extremely toxic. After the composting process was finished, the GI of the control treatment rose to 63.8%, reaching the standard of causing no harm to the crops. The GI of the bulking agent-substituted treatments rose to 93.1%~115.3% and reached the standard of being harmless and beneficial to crops. The GI of the mushroom bran replacement group was higher than that of the vinasse replacement group, as mushroom bran may contain a higher amount of nutrients and enzymes that are beneficial to reducing the toxicity of CM, acid substances, and aromatic compounds in the tobacco powder.

3.3. Ammonium Nitrogen and Nitrate Nitrogen Content

Good nitrogen retention during the composting process is critical for generating a high-quality compost product as organic fertilizers. Reducing nitrogen loss can mainly be achieved by inhibiting ammoniation, reducing the nitrogen emission in the form of NH₃, increasing nitrification, and promoting the existence of nitrogen in the form of nitrate, which is easily utilized by plants [30]. The changes in the NH₄⁺-N and NO₃⁻-N contents of different treatments in this study are shown in Figure 3. The NH₄⁺-N content decreased slowly from day 0 to the 28th day and rapidly decreased from the 28th to the 35th day. The former was attributed to the assuasive ammoniation at high temperatures, and a large amount of NH₃ volatilization caused the consumption of NH₄⁺-N. The latter was ascribed to the violent action of nitrifying the bacteria converting the NH₄⁺-N to NO₃-N [31]. In the six groups, the content of NH₄⁺-N decreased, which were T1 (60.5%), T2 (59.9%), T3 (58.3%), T4 (49.9%), T5 (72.1%), and T6 (74.6%). Compared to CK, T1~T6 showed a higher NH₄⁺-N content than that of the control treatment (52.7%), indicating that the mixed bulking agents reduced the bulk NH₄⁺-N content. Among them, the bacterial enriched mushroom bran has a certain amount of adsorption and binding sites. In the early stage of composting, the NO₃⁻-N content was high, but the high temperature inhibited the activity of the nitrifying bacteria [32]. The original material contained local anaerobic areas, and there was a temporary denitrification that thus made the NO₃⁻-N content drop sharply in the early stage. However, it is worth noting that the content of NO₃⁻-N increased from day 0 to day 7 in the 5% tobacco treatment (T3). This could be because the tobacco powder contains a large amount of protein and polysaccharides that are able to be decomposed by thermostable microorganisms, promoting part of the transformation of NH₄⁺-N to NO₃⁻-N [33]. After that, the nitrifying bacteria gradually resumed their activity, and the NO₃⁻-N content increased. However, during the composting period (after 21 days), the NO₃⁻-N content of the vinasse replacement group (T1~T3) continued to increase, while the NO₃⁻-N content of the mushroom bran substitution group (T4~T6) decreased. The reason for this may be that the high concentration of the NH₄⁺-N content in the pile can inhibit nitrifying microbial activities [27]. Compared to the control treatment, the average NO₃⁻-N content of the bulking agent substitution group (T1~T6) were all higher than that of the control but showed a negligible difference among them (T1~T6).

3.4. Characteristics of Pollutant Gas Emissions during Composting

3.4.1. Methane (CH₄)

The CH₄ emission during the composting process was shown in Figure 4. In the early stage of composting, a large amount of oxygen is consumed, causing a local anaerobic environment due to the violent aerobic activities of the microorganisms. When the aerobic metabolism is complete, small molecular organic acids (formic acid, acetic acid, propionic acid, etc.) can present in the pile [34]. After the high temperature period (7 days), the CH₄ emission rate decreased. Pile turning can induce the fluctuation of the emission rate because pile turning improved the environmental oxygen conditions, and the severe aerobic activity formed a local anaerobic area and produced CH₄ [35]. As the degradable organic matter decreased, O₂ gradually returned to the environmental value, and the CH₄ emission rate also gradually decreased [25]. The cumulative CH₄ emissions from the T1~T6 were between 32.2~33.0 mg·kg⁻¹ DM and 23.6~32.8 mg·kg⁻¹ DM, respectively. Compared to the control treatment (34.0 mg·kg⁻¹ DM), the CH₄ emission reduction rate of the T1~T6 was 2.9~5.3% and 3.3~30.6%, respectively. Therefore, the CM compost with tobacco powder substituted by auxiliary treatments can reduce CH₄ emissions because mushroom bran has a strong physical adsorption capacity for CH₄ fixation [36]. However, if the proportion of mushroom bran is too large, the water retention and thermal insulation performance of the reactor will be reduced. Therefore, the optimal substitution ratio of the microorganism-enriched mushroom bran is between 10% and 15%.

3.4.2. Nitrous Oxide (N₂O)

As shown in Figure 5, the N₂O emissions mainly occurred in the early composting stage (0–7 days). Quite a few studies have also shown that higher N₂O discharge was attributed to a high NO_x-N content in the early stage of composting to which the active denitrifying bacteria contribute a lot [37]. All of the treatments showed the highest discharge of the N₂O on the 1st day. The N₂O emission rate in the vinasse substitution group (13.47~37.01 mg·kg⁻¹ DM·d⁻¹) was higher than that of the mushroom bran substitution group (9.5~15.7 mg·kg⁻¹ DM·d⁻¹). The N₂O emission decreased with the increasing proportion of bulking agents. The N₂O emission of control treatment was the highest when it reached 37.54 mg·kg⁻¹ DM·d⁻¹, indicating that the addition of bulking agents can effectively reduce N₂O emissions [38].

After composting was finished, the cumulative N₂O emissions in T1~T6 were 90.32, 59.08, 27.13, 76.16, 36.39, 18.83 mg·kg⁻¹ DM. Compared to CK treatment (98.45 mg·kg⁻¹ DM), its cumulative emission amount in T1~T6 was reduced by 8.30%, 40.0%, 72.46%, 22.7%, 63.1%, and 80.9%, respectively. Additionally, statistical analysis showed that the difference in cumulative N₂O emission among T1~T6 is significant (p > 0.05), and the 15% mushroom bran substitution treatment exhibited the best N₂O emission reduction effect. Therefore, the CM composted with tobacco powder substituted with vinasse or mushroom bran showed reduced N₂O emissions, which could be ascribed to the idea that vinasse or mushroom bran with abundant active aerobic microorganisms can inhibit the nitrification [39,40] and denitrification processes [41], thereby reducing N₂O dissipation [37].

3.4.3. Ammonia (NH₃)

NH₃ emission mainly occurred during the thermophilic stage (Figure 6). The sharp NH₃ emission change was ascribed to the pile turning, which can improve the oxygen content [38]. The cumulative NH₃ emission of the vinasse and mushroom bran substitution groups was 43.1~67.9 g·kg⁻¹ DM and 35.0~53.3 g·kg⁻¹ DM, respectively. Compared to the control treatment, the cumulative NH₃ emissions were reduced by −37.3~12.9% and −7.74~26.6%, respectively. It is noteworthy that the higher NH₃ emission reduction effect was found in those groups with a higher substitution rate (10% and 15%) of the mushroom bran. However, the addition of less vinasse (5% and 10%) can reduce ammonia emission. It is noteworthy that mushroom bran has a low nitrogen content (19.0 g·kg⁻¹ DM) and a loose and porous structure [14]. Herein, the porous structure of the bulking agent can adsorb NH₃ while providing a more adequate oxygen environment and can affect the NH₃ emissions by changing the NH₄⁺-NO₃⁻ balance by influencing the activity of the nitrifying bacteria.

However, compared to the control treatment, the more cumulative NH₃ emissions, which were mainly generated after 21st day, were found in those treatments of 15% vinasse and 5% mushroom bran treatments. On the one hand, the 15% vinasse treatment did not fully function in the early composting stage. After that, the fermentation microorganisms adapted to the pile environment and further degraded organic matter during the maturation stage. On the other hand, the organic matter could not be completely degraded at the early stage due to the insufficient replacement (5% mushroom bran) of the auxiliary materials. During the following cooling and maturation stages, the pile was mainly composed of difficult-to-degrade N-containing organic matter. During this stage, various enzymes contained in mushroom can promote the degradation process and were conducive to the ammoniation of some microorganisms to produce NH₃, but the yield was limited.

3.5. Carbon and Nitrogen Loss and Greenhouse Effect during Composting

No leachate was produced during the composting process, carbon and nitrogen were mainly lost through the volatilization of gases such as CO₂, CH₄, NH₃, and N₂O. The carbon and nitrogen loss are shown in

Table 4. Analysis of carbon and nitrogen balance during composting.

Treatments	Carbon Loss (%)			Nitrogen Loss (%)			Greenhouse Gas Emission
	CH4-C	CO2-C	TOC	N2O-N	NH3-N	TN	CH4		N2O		GHGs
							kg t−1 DM	kg CO2-eq t−1 DM	kg t−1 DM	kg CO2-eq t−1 DM	kg CO2-eq t−1 DM
CK	0.05	26.1	57.1	0.38	18.3	55.2	0.0432	1.08	0.32	94.8	95.9
5% Vinasse	0.06	28.6	31.2	0.36	18.3	26.4	0.0460	1.15	0.292	87.1	88.3
10% Vinasse	0.04	21.2	54.3	0.22	18.1	43.5	0.0480	1.20	0.195	58.1	59.3
15% Vinasse	0.06	28.7	36.4	0.10	29.5	26.8	0.0484	1.21	0.080	23.9	25.1
5% Mushroom residue	0.05	19.6	41.8	0.30	19	48.7	0.0480	1.20	0.287	85.6	86.8
10%Mushroom residue	0.05	25.7	30.4	0.17	17.3	18.2	0.0404	1.01	0.131	39.1	40.1
15% Mushroom residue	0.06	23.1	36.8	0.08	18.9	26.4	0.0504	1.26	0.060	17.9	19.2

. The TOC loss ratios were within 30.4~61.8%, and the TN loss ratios were in range of 18.2~58.4%. In addition, the TN loss ratios of the T1~T6 were less than that of the control treatment, identifying the enhanced carbon and nitrogen retention effect induced by adding tobacco and tobacco derived bulking agents. The carbon loss in different treatments was mainly in the form of CO₂-C (accounting for 23.1~28.7% of the TOC loss), while the carbon loss in the form of CH₄-C was very small (only 0.04~0.06% of the TOC loss). In terms of nitrogen loss, NH₃-N was the main form (accounting for 17.3~29.5% of TN loss), and N₂O-N accounted for 0.08~0.38% of TN loss. The proportion of CH₄-C loss was lower than that achieved by Jiang et al. [35], which can mainly ascribed to the reason that during the composting process, not only a variety of different auxiliaries and CM were introduced, but also the ventilation and oxygen supply conditions were suitable; thus, it was not easy to generate enough anaerobic conditions to facilitate CH₄ production. The loss proportion of NH₃ and N₂O was also lower than that in a previous report, which was probably attributed to the higher proportion of fine tobacco powder with the greater specific surface area favoring gas adsorption.

GHGs are one of the main causes of global warming. It was expected that the global surface temperature will have increased by 0.9~3.5 °C in 2100. Of the pollutant gases that are generated during the composting process, CH₄ and N₂O are the main GHGs. The 100-year global warming potentials (GWPs) of CH₄ and N₂O are 25 times and 298 times that of equimolar CO₂, respectively. However, CO₂ is discharged within a closed interval during the composting process. As such, their inclusion as GHGs when calculating the greenhouse effect is still controversial. Therefore, CO₂ was not included in the list of GHGs in this article. The GHGs emission equivalent of the control treatment was 95.87 kg CO₂-eq t⁻¹ DM, the vinasse substitution group decreased by 7.90~73.8%, and the mushroom bran substitution group reduced by 9.46~80.0% (Table 4). The GHGs reduction effect of mushroom bran was better than that of vinasse. The main contributor to the GHGs emission equivalent of each treatment in this study was the N₂O (accounting for 93% to 98% of the total emission equivalent). In addition, the N₂O emission equivalents were significantly different in each treatment (p < 0.01). For instance, the bulking agent substitution group decreased by 8.07~81.1% compared to the control treatment, while the CH₄ emission equivalents were not significantly different in each treatment (p > 0.05). This shows that the addition of vinasse and mushroom bran in CM composting can effectively reduce N₂O emissions but that it had little effect on the release of CH₄ emissions. It is noteworthy that mushroom bran, tobacco powder, and vinasse bran were all free of charge, as they are the organic waste in the local area. Herein, the addition of these organic wastes as bulking agents savings can be achieved with the price of the compost feedstock, e.g., the addition of 20% bulking agent can save 20% cost of the compost feedstock (chicken manure).

4. Conclusions

Both vinasse and mushroom bran are typical organic wastes in southwest China that are cheap and that have high organic matter content, are rich nutrients, and have a certain amount of fermentation strains, meaning that they can serve as an ideal bulking agent for the co-compost of CM and tobacco powder. CM co-composted with tobacco powder partially substituted by vinasse and mushroom bran can reach the harm-free standard of composting products. Carbon loss was mainly in the form of CO₂-C, and nitrogen loss was mainly in the form of NH₃-N. The loss rate of TOC and TN in the auxiliary substitution group was lower than that of control treatment. The mixed bulking agent combining the vinasse and mushroom bran exhibited strong performance in enhancing the CM compost maturity, retaining the carbon and nitrogen, and reducing the CH₄, N₂O, CO₂, and NH₃ emissions. The 10% mushroom bran combined with 5% tobacco powder was the optimal combination for simultaneously improving the maturity and reducing greenhouse gas emissions in chicken manure composting. The achieved compost product can be used as an amendment for acidic soils and base fertilizer due to their high pH and abundant nutrients, and further effort should be placed on composting these materials at the commercial scale.