Impact of Waste Tea Litter on NH₃ and CO₂ Emissions during Broiler Rearing

Abstract

Pollution generated by livestock and poultry rearing is an important environmental issue, and gas emissions during animal production are continuously increasing. A digital rearing chamber inspection system was designed in the present study in order to examine the waste tea litter’s impact on the growth performance and harmful gas emissions, such as ammonia (NH₃) and carbon dioxide (CO₂), during broiler rearing. Broilers were raised without litter and with waste tea litter. According to the results, broiler growth showed little difference between the two groups during the experimental period, and it was concluded that waste tea litter had no impact on broiler growth. Meanwhile, the gas emissions of the waste tea-litter group were lower than the non-bedding-materials group. In detail, the average concentrations of NH₃ and CO₂ of the non-bedding-materials group were 9.33 ± 3.65 ppm and 797 ± 107 ppm, respectively; while these concentrations in the waste-tea-litter group were 1.01 ± 0.35 ppm and 713 ± 69 ppm, respectively. According to the analysis of the litter properties, it was suggested that waste tea litter can reduce the moisture content in litter, and affect microbial and urease activity due to its low carbon nitrogen ratio (C/N), weak acid, and porous structure characteristics. In conclusion, this study showed the potential of waste tea litter in NH₃ and CO₂ emission reduction during broiler rearing.

1. Introduction

The livestock and poultry industries are quickly expanding, and harmful gases created during rearing, such as ammonia (NH₃) and greenhouse gases, are attracting more attention because they constitute a danger to the environment. High concentrations of ammonia and carbon dioxide (CO₂) affect the broilers’ growth, slaughter traits, and animal welfare [1]. At the same time, the public is concerned about the serious pollution of the surrounding environment [2,3,4]. A large amount of NH₃ and CO₂ emissions have serious negative impacts on public health and climate change [5], and it is urgent to reduce the production and emissions of NH₃ and CO₂ during livestock and poultry rearing.

In order to create an improved rearing environment, researchers have conducted [6,7,8,9] a series of modifications. During the broiler rearing, litter is often spread on the ground to prevent animals from directly contacting the ground, which provides a comfortable growth environment for broilers and reduces NH₃ emissions. It has been considered that the bedding materials can change the physicochemical properties of litter, with further effects on its microorganism activity, and finally influence the generation and emissions of NH₃ and CO₂. In chicken manure, 60–75% of nitrogen exists in the form of uric acid nitrogen, and the NH₃ produced is mainly due to the decomposition of uric acid by the uricase generated from microbial activities. Generally, the less active the poultry excrement microorganisms, the less carbon and nitrogenous substances in the excrement will be decomposed. Moreover, the absorption characteristics of litter can also affect gas emissions [10]. Common litters include straw, rice husk, wood shavings, sawdust, etc. [11,12,13].

Tea plantation areas around the globe have reached over 4 million hectares and the yield is more than 6 million tons. Tea is often used for the removal of unpleasant odors, sewage deodorization, and inhibition of odor emissions in solid waste disposal. It has been reported that tea’s gas adsorption and deodorization functions might be attributed to its rich tea polyphenol (TP) content, as it contains strong active carboxyl and oxhydryl groups that account for 18–34% of its dry weight [14,15,16]. Takahashi et al. [16] applied composite papers containing waste tea to remove the odor of several gases, such as hydrogen sulfide (H₂S) and NH₃. Patil et al. [17] found that tea grounds and other tea wastes could serve as low-cost adsorbents to remove part of the pharmaceutical contaminants in a water medium. At present, the utilization rate of tea wastes, such as residues from tea plantations and tea grounds from deep processing, only accounts for about 15% of total tea production. Therefore, we studied whether waste tea could be used as a litter additive to inhibit the emissions of NH₃ and CO₂ during poultry rearing.

The assessment of the regular emissions during the rearing process is very important in order to reduce them. Before field detection, rearing chamber detection is usually adopted for its easy accessibility [18,19,20]. In this work, a digital rearing chamber inspection system was developed for simultaneous monitoring of broiler growth conditions, as well as emissions of NH₃ and CO₂ with waste tea litter or without litter. The aim of this experiment was to determine whether waste tea litter could provide a new technique and method for emission reduction and control of NH₃ and CO₂ in livestock rearing.

2. Materials and Methods

2.1. Digital Rearing Chamber Inspection System

The digital rearing chamber inspection system is shown in Figure 1. The rearing chamber dimensions were 1.2 m × 1.2 m × 1.2 m, with the feeding trough and waterer above the chamber. The air inlet and outlet were both 100 mm in diameter, and they were located on the left and right sides of the box. Three air inlets were arranged on the upper left side, and one air outlet was arranged on the lower right side. A 0.8 m L × 0.6 m W × 0.6 m H iron cage was placed inside. The iron cage was located on a stainless-steel base. An axial fan (SE-A100, Shenzhen, China) was installed at the air outlet for negative pressure ventilation. The broiler’s growth parameters were measured by weight sensors. An infrared acoustic-spectrum gas-detection analyzer (INNOVA1412, LumaSense, Ballerup Kommune, Denmark) and a multipoint gas sampling apparatus (INNOVA1409, LumaSense, Ballerup Kommune, Denmark) were used to detect the NH₃ and CO₂ concentrations. The INNOVA analyzer was challenged yearly and calibrated, as needed, with 0 ppm, 25 ppm NH₃ (balanced with air), and 3000 ppm CO₂ (N₂ balance) span calibration gases. The digital rearing chamber was connected to the analyzer sample inlet and outlet with two 3 mm tubes, and sample air was extracted from the digital rearing chamber, circulated through the analyzer sample chamber, and then returned to the digital rearing chamber. The ammonia detection limit of the photoacoustic analyzer was 0.2 ppm, and the sensitivity to water vapor was compensated. Before the start of the test, the gas-sampling instrument was zero-adjusted and calibrated with an S4000 gas dilution calibrator from Envoronics, Tolland, CT, USA, with a sampling interval of 10 min. A filter (diameter: 4.5 cm; 4.5 μm) was installed in the sampling pipe for gas sampling through a Teflon tube (1/4 inch).

2.2. Digital Rearing Chamber Inspection System

Gas determination can be influenced by air flow in the chamber. To avoid dead corners of ventilation, ANSYS was used for simulating the airflow field direction and mode inside the rearing chamber, and then determining the position, size, and number of inlets and outlets. Based on the ventilation rate (10–25 times/h), the rearing chamber volume (1.49 m³), and the air speed inside the rearing chamber (less than 1 m/s), a flow field simulation was run; the results are shown in Figure 2. The simulation results showed that when the inlets were on the left and the outlets were on the right, the air inside the chamber was distributed evenly, and the air circulation covered the entire rearing chamber space with no ventilation dead corners in the broiler activity area. This reduced the aggregation of NH₃ and CO₂ in the rearing chamber and ensured normal growth of the broilers. Moreover, check valves were installed at the inlets to prevent NH₃ and CO₂ from escaping.

2.3. Experiment Process

Ten AA broilers (Aviagen broiler) with good growth at 30 days of age were selected from the farm. In order to avoid the impact of standard litter on ammonia emissions and the physicochemical properties of chicken manure, the control group did not use standard litter, in which 5 broilers were placed in the rearing chamber without litter and the other 5 broilers in the chamber with waste tea litter. These broilers were bred in the farm for 10 days, including the first 3 days as the adaptation period. The experimental process was conducted in the lab with natural illumination and at temperatures of 22~25 °C controlled by an air conditioner. The chambers were ventilated in the same way to ensure the same amount of ventilation. All experiments with animals were carried out in strict accordance with the recommendations in the Guide for the Care and Use of Laboratory Animals of the Ministry of Science and Technology of the People’s Republic of China. The protocols were approved by the Committee on the Ethics of Animal Experiments of the Diagnostic Center for Animal Disease of Huazhong Agricultural University. The litter consisted of 800 g of low-grade, unfermented waste green tea purchased from Enshi Hubei China and spread on the dropping tray. Before the experiment, the rearing chambers were cleaned and disinfected. These broilers were fed with the same and excessive diet and water 3 times per day, and their weight, excreta, feed intake, and water intake, as well as the NH₃ and CO₂ concentrations in the rearing chambers, were measured.

2.4. Measurement Index and Method

After the experiment, 500 g of chicken litter was picked up from the dropping tray with the 5-point sampling method for later determination of the properties, including carbon–nitrogen ratio (C/N), pH, and moisture content. The chicken litter was weighted by an electronic analytical balance (AUY220, Shimadzu, Kyoto, Japan); total carbon (TC) was measured by a carbon–nitrogen analyzer (HT1300, Analytik Jena AG, Jena, Germany); total nitrogen (TN) was determined by an auto chemical analysis machine (SmartChem200, AMS Group Company, Treviolo, Italy) after digestion by the microwave digester (MARS-6, CEM Company, Charlotte, NC, USA); and the pH value was measured with a PHS-3E acidity meter (PHS-3E, Shanghai INESA Scientific Instrument Co., Ltd., Shanghai, China) after air drying, grinding, and addition of water; and the moisture content was measured using the oven-drying method.

3. Results and Discussions

3.1. Growth Performance Indexes

The broiler growth performance parameters are shown in

Table 1. Effect of non-bedding materials and waste tea litter on growth parameters of broilers.

Days	Growth Parameters
	Average Daily Weight/g		Average Daily Excretion/g		Average Daily Feed Intake/g		Average Daily Water Intake/g
	Non-Bedding Materials	Waste Tea Litter	Non-Bedding Materials	Waste Tea Litter	Non-Bedding Materials	Waste Tea Litter	Non-Bedding Materials	Waste Tea Litter
1	1861	1964	78	181	113	118	239	229
2	1958	1945	184	125	119	118	237	238
3	1963	1944	172	138	121	120	260	238
4	1964	1992	115	112	121	118	254	238
5	1955	1946	110	209	126	121	238	239
6	1980	1962	128	114	121	120	240	238
7	1991	1949	46	111	122	120	246	239
Notability analysis	NS (p = 0.815)		NS (p = 0.356)		NS (p = 0.446)		NS (p = 0.056)

. There was no significant change in the broiler weight period between the two groups. The daily average values of the broiler weight of the nonlitter group and the waste-tea-litter group were 1953 g/d ± 42 g and 1957 g/d ± 17 g (p > 0.05), respectively, showing no significant difference (p > 0.05). The control broilers were in good spirits, responded quickly, and had an appetite for feed. The chicken manure had a certain shape and was gray-brown. The broiler excreta increased as time passed, and the daily average defecation quantities of the nonlitter group and waste-tea-litter group were 119 g/d ± 49 g and 141 g/d ± 39 g (p > 0.05), respectively, and fluctuated during the experimental period, which might have been relevant to measuring time, broiler growth conditions, and other factors. The feed intake and water intake were maintained as stable. The daily average feed intakes of broilers in the nonlitter group and the waste-tea-litter group were 120 g/d ± 4 g and 119 g/d ± 1 g (p > 0.05), respectively, and the daily average water intakes were 245 g/d ± 9 g and 237 g/d ± 4 g (p > 0.05), respectively. The average daily feed intake and average daily water intake consumed on the local farms was basically the same. According to the analysis of the experiment results, waste tea litter had no significant impact on the broilers’ growth performance.

3.2. Measurement of NH₃ and CO₂ Levels

The changing trend in daily concentrations of NH₃ and CO₂ during the experiment is shown in Figure 3.

The emissions of each gas showed a significant difference (p < 0.05) between the waste-tea-litter group and the nonlitter group. As shown in Figure 3a, NH₃ concentrations in the nonlitter broiler group and the waste-tea-litter broiler group were increased sharply and increased slightly in the early stage of the experiment (2–3 days), respectively, and both were maintained as stable in the middle and later stages. Namely, in two conditions, the NH₃ concentrations tended to rise at first and became stable later, and the NH₃ concentrations in the nonlitter group were higher than in the waste-tea-litter group. As shown in Figure 3b, the daily average emission concentration of CO₂ in the waste-tea-litter group (713 ± 69 ppm) was significantly lower than that in the nonlitter group (797 ± 107 ppm). The highest NH₃ emission concentration in the non-litter group was 19 ppm, and the respective value was only 2.2 ppm in the waste-tea-litter group. Therefore, waste tea litter could lower the NH₃ and CO₂ emissions during the broiler rearing process. As reported [21], the NH₃ concentration in the air of broiler house should be lower than 25 ppm. When the concentration was higher than 75 ppm, the broiler production performance was reduced greatly; when it was higher than 50 ppm, both daily average feed intake and weight gain were decreased [22]. As shown in Table 1 and Figure 3, the maximum NH₃ concentration was 19 ppm in this experiment, not more than 25 ppm; and the emissions of NH₃ and CO₂ had almost no effect on the broilers’ growth performance.

NH₃ is mainly produced by microorganisms’ decomposition of nitrogenous substances in litter, and the emission of NH₃ is related to the surface area of litter exposed to air [23,24] and the accumulation time [25]. In general, the smaller the exposed surface area of the litter or the slurry, the lower the NH₃ release [26]. Therefore, it can be shown that the NH₃ emission tendency in this experiment was as follows: due to a lower litter mass and a smaller litter surface area in the early stage, the NH₃ concentration was low; as the litter mass and the litter surface area increased, the NH₃ concentration increased rapidly; in the middle and late stages, fresh litter continued to cover up the original excreta, and the litter surface area became stable gradually; therefore, the NH₃ concentration showed little change, and the emission concentration was stable in the middle and late stages. The experimental results of the present study were consistent with that of Chepete et al. [24]. As shown in Table 1, the total defecation of each broiler in the waste-tea-litter group and the nonlitter group were 990 g and 832 g, respectively. According to the study of Chepete et al. [24], the more the excreta, the longer the accumulation time, and the more NH₃ was emitted. In this experiment, the NH₃ emission concentration in the waste-tea-litter based group was much lower than in the nonlitter group. Therefore, in addition to excreta, litter surface area, and accumulation time, the NH₃ emission also was related to the tea’s chemical properties (pH, moisture content, C/N, and chemical bond) The changes in these factors affected the generation and emission processes of NH₃ and CO₂ in the chicken litter.

3.3. Assessment of Litter Characteristics

In this study, after the broiler rearing experiment, the litter was sampled to measure its moisture content, pH, TC, TN, and C/N. The measurement results are shown in

Table 2. Physicochemical properties of litter treated with waste tea.

Type	Physicochemical Indices
	TC (g/kg)	TN (g/kg)	C/N	Moisture Content (%)	pH	Notability Analysis
Waste tea	529 ± 24	25.5 ± 4.1	20.1 ± 2.7	25.5 ± 4.1	5.8 ± 0.1
Waste tea litter	331 ± 20	25.2 ± 4.3	13.3 ± 2.0	25.2 ± 4.3	6.4 ± 0.2	NS (p = 0.866)
Non-bedding materials	254 ± 23	19.9 ± 2.1	12.8 ± 1.4	19.9 ± 2.1	7.8 ± 0.1

.

During broiler rearing, the NH₃ generation and emission mechanism is an extremely complex process. The moisture content and pH of litter, environmental temperature, wind speed, and the litter surface area affected the NH₃ generation and emission [27,28,29]. NH₃ mainly originates from the urease hydrolyzation of urea and uric acid, with a small quantity coming from the degradation of undigested protein. The reaction processes are shown in Formulas (1)–(3) [23,29,30]. Several enzymes and microorganisms participate in this process, and water and oxygen are essential. The balance relationship between NH₃(l) and NH₄+ in litter is affected by pH and temperature (T) (Formula (4)). A decrease in the litter pH can promote the transformation of NH₃ into NH₄+ ion, thus inhibiting NH₃ generation and emission [31,32,33]. The balance between the NH₃(l) and NH₃(g) on the litter surface is affected by wind speed (V) and the litter surface area (A). The reaction process is shown in Formula (5).

$${\mathrm{C}}_5{\mathrm{H}}_4{\mathrm{O}}_3{\mathrm{N}}_4+1.5{\mathrm{O}}_2+4{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{urease}}{\to }{\mathrm{NH}}_3+5{\mathrm{CO}}_2$$

(1)

$$\mathrm{CO}{({\mathrm{NH}}_2)}_2+{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{urease}}{\to }2{\mathrm{NH}}_3+{\mathrm{CO}}_2$$

(2)

$$\mathrm{Undigested}\mathrm{proteins}\to {\mathrm{NH}}_3$$

(3)

$${\mathrm{NH}}_4^{+}\stackrel{\mathrm{pH},\mathrm{T}}{\iff }{\mathrm{H}}^{+}+{\mathrm{NH}}_3\left(\mathrm{l}\right)$$

(4)

$${\mathrm{NH}}_3\left(\mathrm{l}\right)\stackrel{\mathrm{V},\mathrm{A}}{\iff }{\mathrm{NH}}_3\left(\mathrm{g}\right)$$

(5)

During this experiment, the litter moisture content in the waste-tea-litter group (59.3% ± 1.3) was decreased by 13.7% when compared with the nonlitter group (73% ± 4.2). An ideal litter material should feature high hygroscopicity, a reasonable drying time, and high fragility. The litter can quickly separate urine and manure in the excrement [34], thus blocking the NH₃-generation process, because the surface area of fresh urine or slurry exposed to air greatly influences the NH₃ emissions. The urea and uric acid in manure is decomposed by urease to generate NH₃, while tea polyphenols can reduce the urease activity, thus decreasing the decomposition of urea and uric acid, and inhibiting NH₃ generation. Meanwhile, tea absorbs the urine in broiler litter, which reduces the contact between urea and urease and the contact between urine and air, and increases the NH₃ diffusion distance and transmission resistance, finally lowering NH₃ emissions [35]. The waste tea litter also decreases the moisture content in litter; decreased moisture content will lower the microorganism activity and weaken the decomposition of nitrogenous substances in litter, thus reducing NH₃ generation. Miles et al. [36] found that when the moisture content in chicken litter was lower than 40%, NH₃ release was directly proportional to moisture content, and low moisture content inhibited microorganism growth; when the moisture content in chicken litter was higher than 60%, the microorganism activity in the anaerobic environment was inhibited, and the NH₃ release was inversely proportional to moisture content; when the moisture content in chicken litter was 40–60%, it was ideal for microorganism activity, and NH₃ release increased and then decreased as the moisture content rose; when the litter moisture content was lower than 10%, no NH₃ was produced. Therefore, during the poultry-rearing process, the litter’s moisture content is often controlled at 25–35% to keep the poultry house dry. In this study, litter moisture content in the waste-tea-litter group was 59.3% ± 1.3, though it reduced the moisture content of litter, but it still was slightly higher than expected.

As shown in Formula (4), the litter pH value and temperature are also some of the main affecting factors in NH₃(l) generation. The environmental temperatures in the two rearing conditions were consistent, with a lab temperature of 25 °C. As shown in Table 2, the pH values of litter samples in the nonlitter group and waste-tea-litter group were 7.8 ± 0.1 and 6.4 ± 0.2, respectively. The pH value of waste tea was 5.8 ± 0.1, appearing mildly acidic. As the tea was mildly acidic, H+ increased in the broiler litter in the waste-tea-litter group, making the balance relationship in Formula (4) shift to the NH₄⁺ form. In addition, the microorganisms had lower activity in a low-pH environment, and the decomposition rates of urea and protein were slowed down [23]. Generally, litter additives can control NH₃ generation and emission, and mainly include adsorbent, microorganism, inhibitor, and acidifier types. Among them, the acidifier has been recognized as one of the most effective methods to prevent nitrogen from flowing away to the atmosphere, and has been popular in poultry-rearing processes. Theoretically, as the bacteria for transforming uric acid or urea into NH₃ grows well in a high-pH environment, reducing the pH value of litter in litter can reduce the amount of bacteria and the NH₃ generation. The pH value of litter has been proved as significant in the growth and survival of pathogens. Moreover, studies proved that litter acidification also could inhibit the urease activity [37]. Urease is a muramidase mainly existing in manure bacteria, and its activity is affected by pH value. Studies showed that urease inhibitor in livestock litter can inhibit the original microorganism bacteria activity, thus inhibiting the NH₃ generation [38]. Fernando and Roberts [39] reported that tea extract had an equivalent inhibiting effect on urease as the urease inhibitor did. The microbial urease is a nickel-bearing enzyme, and needs six Ni₂⁺ atoms to come into play [40]. Studies have showed that adding nickel can affect the generation of urease [41]. It has been reported that tea is capable of absorbing heavy metals [42], since some chemical substances in tea can react with the Ni₂⁺ atom [43,44]. This nickel-absorption process might inhibit the activity of bacteria that produce uricase and urease, and reduce the generation of urease. Therefore, this are probably why the mildly acidic tea with tea polyphenols can inhibit the growth of urease bacteria and urease actinomycetes, as well as urease activity [45], and weaken the reaction processes given in Formula (1) and Formula (2), which further reduce the decomposition of urea and uric acid.

C/N is another important factor, as the NH₃ in litter is mainly generated by bacteria’s decomposition of organic nitrogen. This catabolic pathway can provide the energy required for bacterial growth, which can occur in both aerobic and anaerobic conditions, but is higher in the aerobic conditions. Adding straw and other litters can increase the number of degradable carbon substances, which is favorable for aerobic conditions, and induces anabolism [8]. Studies [46] showed that the C/N ratio in pig manure slurry in an anaerobic environment was between 4 and 10 with almost no anabolism; the C/N ratio in livestock litter varied from 1:1 to 27:1. As the litter C/N increased, released CO₂–C and NH₃–N quantities tended to decrease [47]. Therefore, the C element can be increased and the N element can be decreased in litter by controlling the C/N in the litter to fix NH₄⁺ in an organic form, thus reducing NH₃ generation and emission. Studies showed that by adding litter with high carbon organics in litter, the ammonium nitrogen can be transformed into organic nitrogen during the additive decomposition. If the added litter has a high C/N ratio, microorganisms can use inorganic nitrogen (ammonium and nitrate) in the litter, which can be fixed to a new microbial biomass under microbial action [48]. As shown in Table 2, carbon content in the waste tea was 529 ± 24, which enhanced the nitrogen-fixing capacity of the microorganisms, and reduced the volatilization of NH₃. The C/N in the litter in the waste-tea-litter group (13.3 ± 2.0) was higher than that in the nonlitter group (12.8 ± 1.4), but the difference was not significant. Therefore, for the broiler litter in the waste-tea-litter group, the litter moisture content and pH had a more obvious effect on the NH₃ emissions compared with C/N.

To sum up, the physical and chemical properties of waste tea could affect the NH₃ generation and emission process in the litter. (1) The best C/N range of microorganisms was 25–35, while the C/N of the waste tea was 20.1 ± 2.7. Therefore, C/N was not the main affecting factor in NH₃ emissions under the experimental conditions. (2) The moisture content of the waste tea was as low as 8.4% ± 1.4, while the moisture contents of the chicken litter and waste tea litter were 73.1% ± 4.2 and 59.3 ± 1.3, respectively. Due to good hygroscopicity of the tea, the moisture required for microorganism activity was decreased, and the microorganism activity was inhibited. (3) Thanks to special porosity, tea could provide oxygen for aerobic bacteria, and the bacteria had a better capacity to transform NH₃ into organics under aerobic conditions through nitrogen assimilation or nitrogen immobilization. (4) When the weakly acidic tea was mixed with the litter, the H⁺ in the litter increased, which enabled the balance relationship between NH₄⁺ and NH₃ toward the NH₄⁺, and reduced the production of NH₃. (5) The NH₃ volatilization was related to wind speed and the litter surface area. The wind speeds in the two rearing chambers in the rearing experiment were the same. Therefore, the litter surface area was the key. The total excreta of each broiler in the waste-tea-litter group and the nonlitter group was 990 g and 823 g, respectively. Although NH₃ volatilization was in positive correlation with the litter surface area and excreta mass, the NH₃ emissions in the waste-tea-litter group was much lower than in the nonlitter group. The reason might be that the porous surface structure of the waste tea caused the physical absorption of NH₃. The high-balance moisture regain of waste tea could help absorb the NH₃ in the air that was dissolved in manure and urine.

4. Conclusions

In this paper, a digital rearing chamber inspection system was designed under lab conditions for real-time monitoring of concentrations of the harmful gases NH₃ and CO₂, as well as growth performance indexes, during broiler growth process. (1) The nonlitter group and the waste-tea-litter group showed no significant difference in terms of growth conditions. (2) C/N was not the main affecting factor in NH₃ emissions under the experimental conditions. (3) The tea’s low moisture content and weakly acidic nature led to reduced ammonia emissions. (4) With special physical properties and chemical compositions, waste tea might affect the generation and emissions of NH₃. The production of NH₃ in the litter was the result of microorganisms (total bacteria, ammonia-producing bacteria, and fungus) activity. The treatment of litter might affect some bacterial populations, and was favorable for the survival or growth of other bacterial populations. However, the corresponding relationship between the special physical tissue and chemical composition of the tea and NH₃ emission reduction remains unclear. Furthermore, this experiment still had some defects, such as a short rearing period and a small sample size. Therefore, further studies are needed to study the ammonia-producing microbiological mechanism in livestock litter and the mechanism of the tea emission-reduction process.