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Article

The Impacts of the C/N Ratio on Hydrogen Sulfide Emission and Microbial Community Characteristics during Chicken Manure Composting with Wheat Straw

by
Shangying Cai
1,2,
Yi Ma
1,2,
Zhenkang Bao
1,2,
Ziying Yang
1,2,
Xiangyu Niu
1,2,
Qingzhen Meng
1,2,
Dongsheng Qin
3,
Yan Wang
1,2,
Junfeng Wan
1,2 and
Xiaoying Guo
1,2,*
1
College of Ecology and Environment, Zhengzhou University, Zhengzhou 450001, China
2
International Joint Laboratory of Environment and Resources of Henan Province, Zhengzhou 450001, China
3
Henan Sangao Agricultural and Animal Husbandry Co., Ltd., Xinyang 465200, China
*
Author to whom correspondence should be addressed.
Agriculture 2024, 14(6), 948; https://doi.org/10.3390/agriculture14060948
Submission received: 30 April 2024 / Revised: 10 June 2024 / Accepted: 10 June 2024 / Published: 18 June 2024
(This article belongs to the Special Issue Practical Application of Crop Straw Reuse in Agriculture)
Browse Figures
Versions Notes

Abstract

:
Wheat straw (WS) has long been subjected to rough treatment by traditional incineration, which not only results in the waste of biomass resources but also poses a risk of atmospheric pollution and is not conducive to the sustainable utilization of natural resources. With great humification potential, WS can be utilized as a valuable composting material. The study optimized the C/N ratio by mixing WS and chicken manure (CM) as composting raw materials, and found that this method could significantly improve the compost quality. In comparison to the conventional poplar woodchip (PW) conditioning, the incorporation of WS resulted in an elevated composting temperature, an extended high-temperature period, a more expeditious lignocellulose degradation, a notable enhancement in the organic matter content, a suppression of hydrogen sulfide production under low C/N ratio, and a promotion of elemental sulfur conversion, collectively contributing to an enhanced overall quality and environmental friendliness of the compost. Correlation analysis of microbial communities and environmental factors demonstrated that the mixed compost facilitated the growth of actinomycetes and sulfur-transforming bacteria. Additionally, structural equation model indicated that parameters such as temperature and pH value played a key role in the composting process.

1. Introduction

Straw is a common agricultural resource waste. Relevant studies have found that the total amount of straw in China reaches 900 million tons/year, accounting for about a quarter of the global total [1]. For a long time, the traditional means of disposal of agricultural straw waste in China has been based on open burning, which has a high environmental burden and low environmental performance [2], and the practice of straw burning poses a serious challenge to climate change and to maintaining the quality of natural resources and sustainable crop production systems. Straw burning produces harmful gases and particulate matter, causing air pollution, contributes to global warming as a carbon source, loses valuable nutrients retained in crop residues during burning, reduces surface organic matter, destroys soil structure, reduces soil fertility, and affects crop yields [3], and is detrimental to the subsequent development of tillage and sustainable agriculture. Therefore, efficient use of straw by returning straw to the field and biomass resource utilization has become a necessary disposal measure. As a soil conditioner, straw can increase soil water content and improve water use efficiency by improving the water holding capacity of the field [4]; in addition, straw returned to the field increases soil aggregates, which is conducive to improving soil organic matter [5,6]. However, if straw is directly returned to the field without rotting treatment, it may lead to poor soil permeability, accumulation of pathogenic bacteria, reduced seedling emergence, and aggravate heavy metal pollution [7,8]. As a carbon-rich biomass charcoal raw material, straw can be used as an activated carbon adsorbent [9] and as a natural agricultural fiber for building materials [10], but it is difficult to be widely utilized and its practical application is limited, and the application of straw feed is also faced with a number of problems resulting in a low utilization rate and failure of large-scale production.
Composting as a basic process of agricultural production can increase soil nutrients, improve soil and water conservation capacity, and achieve soil fertility recovery [11], and rotting compost is an important means of straw biomass resource recycling [12]. Wheat Straw (WS) has high humus propagation potential, high fluffiness, large surface aperture and pore size, and a large number of hydrophilic groups within the fibers, making it a good natural substrate for rapid composting, which is conducive to efficient bioconversion. Compared with biomass burning, straw composting treatment can significantly increase soil nutrient content, which is favorable to crop growth and has a positive effect on the improvement of soil carbon sink capacity [13]. Single straw composting is difficult to achieve better results, and the commonly used livestock manure because of its high moisture content and low C/N ratio, the original conditions of parameters are difficult to meet the demand for aerobic composting, and it is necessary to add carbon-enriched co-composting materials to regulate. Co-composting treatment is an effective method to improve the quality and efficiency of compost [14]. Chicken manure (CM) is rich in nitrogen, phosphorus, potassium and other nutrients required for plant growth, while straw can provide a carbon source and loose physical structure. Adding straw for composting treatment can not only regulate the carbon to nitrogen ratio and increase the porosity, but also use WS as a composting raw material to promote the formation of humus-rich fertilizer and improve the compost stability. Poplar wood chips (PW) have high lignin content, which is more slowly decomposed by microorganisms, and composting is time-consuming and has low bioconversion efficiency [15]. Therefore, in this study, WS was used as a composting auxiliary material for co-composting with CM, in order to realize the resourceful synergistic treatment of WS and CM, to improve the composting efficiency and to enhance the crop yield.
In this study, the effects of different C/N ratio on the characteristics of compost products were investigated by adjusting the mass ratio of WS auxiliaries to CM, analyzed the physicochemical properties of compost products, the transformation of sulfur-containing forms, the changes in the diversity of bacterial communities, and the correlation between microorganisms and environmental factors, with a view to providing scientific and technological solutions to improve the quality of the products, clarifying the mechanism of C/N ratio on the composting, and realizing a green, low-carbon and recycling utilization of agricultural straw resources and improving the treatment level of agricultural resources and wastes. The results of this study provide a scientific basis for improving product quality, clarifying the influence of C/N ratio on composting, realizing green, low-carbon and recycling utilization of agricultural straw resources, and improving the treatment level of agricultural resources and waste.

2. Materials and Methods

2.1. Physical and Chemical Properties of Compost

The wheat straw (WS) used in this study was purchased from Henan Zhumadian Yuhua Animal Husbandry Co., Ltd. (Zhumadian, China). The length of the straw was 3~10 cm after crushing. The poplar wood chips (PW) purchased from Heze City, Shandong Province, particle size of about 0.5~2 cm. The fresh chicken manure (CM) was taken from the intensive laying hen coop of Peiyuan Agricultural and Animal Husbandry Co., Ltd. (Yuzhou, China) in Zhuge Town, Yuzhou City, Henan Province. The physical and chemical properties of the raw material are reported in Table 1.

2.2. Experimental Design

CK: 250 kg CM + 45 kg PW (C/N ratio of about 11.3)
T1: 250 kg CM + 60 kg WS (C/N ratio of about 12.1)
T2: 250 kg CM + 75 kg WS (C/N ratio of about 13.5)
T3: 250 kg CM + 90 kg WS (C/N ratio of about 14.5)
The experiments were conducted with a view to controlling the C/N ratio by adjusting the quantity of WS introduced to the process. The objective was to study and analyze the effect of C/N ratio on the composting process. The experimental group was set up with different gradients of straw addition to facilitate a systematic exploration and quantification of the effect of WS on C/N. This was completed in order to precisely regulate the C/N ratio of waste and thus optimize the composting process and improve the efficiency of resource utilization and product quality.

2.3. Aerobic Composting Operation

After the composting materials were prepared according to the experimental design, they were evenly mixed and piled into cones with a diameter of about 1.5 m and a height of about 1 m. In order to collect the sulfur-containing gas (hydrogen sulfide) produced in the process of composting, a plastic cover with a size of 1.5 m × 1.5 m × 1.5 m was placed on each composting body for relative closure, and a vacuum pump was prepared for each pile. The alkali absorption bottle (NaOH: 1 M) and buffer bottle were set up in front of the vacuum pump to collect the acidic sulfur-containing gas produced in the process of composting.

2.4. Temperature Measurement and Sample Collection

The temperature of the pile was measured at 7 a.m. every day during the composting process. The temperature of the upper, middle and lower layers of the pile were measured four times from four different directions, and the average value was finally taken as the fermentation temperature of the pile. At the same time, the ambient temperature of the composting was recorded. After the temperature measurement, the alkali absorption liquid in the absorption bottles of different piles was collected and replaced with new alkali liquid to ensure that the absorption liquid has the ability to absorb acidic gas continuously. The collected alkali liquid was refrigerated and analyzed within two days. During the composting period, the compost was turned over every 3 days until the end of the composting, and the samples of the turned-over compost were collected on the 3rd, 6th, 9th, 12th, 15th, 18th, 21st, 24th, 27th, 30th, 33rd and 36th days of the composting. The samples were fully mixed, and one part was put into the refrigerator at 4 °C for the determination of sulphuration index; one part was air-dried and ground to 60 mesh screens for preservation; one part was baked at 105 °C to constant weight for the determination of the compost moisture content; one part was frozen at −80 °C for microbial analysis. After the sampling was completed, the moisture was supplemented to the compost according to the measured moisture content to maintain the appropriate moisture content for composting fermentation.

2.5. Determination Methods of Relevant Physical and Chemical Indicators

The determination methods of conductivity and pH in the compost samples were carried out according to the agricultural industry standard of the People’s Republic of China (NY/T 525-2021) [16]. The determination of organic matter in the fertilizer sample was carried out by the ignition loss method (HJ 761-2015) [17], the determination of sulfate was carried out by turbidimetric method of sulfate content, the determination of total sulfur was carried out by the Eschka-Ion chromatography method (HJ 769-2015) [18], and the determination of sodium sulfide in the absorption liquid was carried out by methylene blue spectrophotometric method (HJ 1226-2021) [19].The determination of cellulose, hemicellulose and lignin was carried out by Yuwan Wang method [20].

2.6. High-Throughput Sequencing of Microorganisms

The second generation of high-throughput sequencing technology was used to sequence the 16S rRNA and 18S rRNA gene sequences of composting microorganisms. Total DNA was extracted from the microbial community according to the instructions of E.Z.N.A.® soil DNA kit (Omega Bio-tek, Norcross, GA, USA). The extraction quality of DNA was detected by 1% agarose gel electrophoresis, and the DNA concentration and purity were determined by NanoDrop2000. Bacterial 16S rRNA was amplified by PCR from V3-V4 variable regions using primers 338F (5′-ACTCCTACGGGAGGCAGCAG-3′) and 806R (5′-GGACTACHVGGGTWTCTAAT-3′). After mixing the PCR products of the same sample, the PCR products were recovered by 2% agarose gel, purified by AxyPrep DNA Gel Extraction Kit (Axygen Biosciences, Union City, CA, USA), detected by 2% agarose gel electrophoresis, and quantitatively detected by QuantusTM Fluorometer (Promega, Madison, WI, USA). The purified amplified fragments were constructed into PE 2 × 300 library and PE 2 × 250 library for 16S and 18S, respectively, according to the standard operation of Illumina MiSeq platform (Illumina, San Diego, CA, USA). The sequencing was performed by using Illumina’s MiseqPE300 platform (Shanghai Meiji Biomedical Technology Co., Ltd., Shanghai, China).

2.7. Data Processing and Analysis

Origin2022b software was used to analyze the data of temperature, pH, moisture, organic matter, sulfate, total sulfur and hydrogen sulfide. The sequencing results were subjected to analysis using the Meiji biological cloud platform, developed by Shanghai Meiji Biomedical Technology Co., Ltd. (Shanghai, China). A structural equation model of different influencing factors was developed using SPSS Statistics 26 to assess the complex relationships of the composting process. The values entered in the experiment are based on 2 replications.

3. Results and Discussion

3.1. Physical and Chemical Characterization of Compost

Temperature variations in the composting process are closely related to the microbial activity, organic matter conversion and degradation of the compost [21], which is a key parameter to measure the efficiency and stability of the composting process [22]. At the early stage of composting, the compost body contained more easily degradable organic matter, which led to the rapid propagation of the microorganisms. Compost fermentation enters into a high temperature period within a few hours to a day at the beginning. The rapid increase in temperature, due to microbial respiration to release heat and rapid decomposition of degradable organic matter [23,24]. Typically, the higher the temperature of the pile, the higher the microbial activity and the better the high temperature inactivation of pathogenic microorganisms [25].
The changes in temperature, pH, conductivity and water content during composting with different C/N ratios are shown in Figure 1. From Figure 1a, after 24 h, CK, T1, T2 and T3 all entered the high temperature period, and the temperature of the heap body on days 1–8 was between 60 and 70 °C, with the temperature order of T1 > T2 > T3 > CK, respectively. On days 10–17, the temperature of group T1 (low C/N ratio) fluctuated and decreased continuously, and the temperature of group T3 (high C/N ratio) increased to the highest. Combined with the change in water content, it can be seen that the microbial activity in group T3 increased, and thus the water content decreased with the increase in temperature. From the 17th day to the end of composting, the temperature of T1 group was always the highest, showing the trend of T1 > T2 ≈ T3 > CK.
It shows that adjusting the C/N ratio can increase the composting temperature, and the effect of low C/N ratio treatment group is more obvious, and the C/N ratio is too high, which will affect the microbial metabolism inside the composting body and accelerate the heat dissipation of the composting body. The duration of high temperature in the composting process is about 30 days, with the consumption of organic substrate in the heap, the high-temperature bacteria gradually stop their activities, and at the same time the temperature of the heap decreases, and the compost enters into the stage of decomposition, where lignin and other difficult-to-biodegradable organic matter is further degraded and converted into more stable humus [26]. Zhang et al. also found that increasing the proportion of straw composting can prolong the thermophilic period [27]; the longer the high temperature period of composting, the higher the degree of compost decomposition, which has a better potential for the effective elimination of antibiotic contamination [28], and therefore composting safety is improved. The longer the high temperature period of composting, the higher the degree of compost decomposition. From Figure 1a, it can be seen that the duration of the high temperature period is about 30 days, which fully meets the health and safety requirements of killing weed seeds and pathogenic microorganisms (7 days), which was also reported by Zhou et al. [29].
As can be seen in Figure 1b, pH increased in T1, T2, and T3 groups on days 3–6, and in CK group on days 3–9, which was primarily attributable to the copious quantities of ammonia generated by protein decomposition, the mineralization of organic nitrogen, and the degradation of organic acids during the initial stages of the high-temperature period [30,31,32,33,34]. Compared with the CK group, the pH fluctuation of the experimental group with adjusted C/N ratio was smaller, and the high C/N ratio group had the smallest change, indicating that adjusting the C/N ratio had a certain buffering capacity for the pH change of the compost pile. On the 12th–24th days of composting, the pH of the CK group increased continuously to about 9.3. At the end of composting, the pH of all treatment groups was within the standard requirement (8.5), and similar changes were reported in the study of Wong et al. [35].
At the initial stage, the electrical conductivity (EC) showed T1 > T3 ≈ T2 > CK, and the EC of the CK, T1, T2, and T3 groups all showed an increasing trend in the first 15 days of composting. That is, straw increased the EC values in the initial composting environment, which may be due to the fact that straw is more susceptible to mineralization and releases more ions compared to poplar wood chips during co-composting [36]. The increase in EC values may be due to the concentration effect during composting and the conversion of complex compounds into simple compounds (such as nitrates, nitrites and volatile fatty acids) during the high temperature period [37]. The EC values of all treatment groups showed a tendency to increase and then decrease. The decrease in EC may be due to the volatilization of ammonia and the precipitation of mineral salts during the cooling and decomposition periods, in addition to the fact that humus has a high cation exchange capacity to immobilize ions (Ca2+ and Mg2+). Therefore, a decrease in EC may also be caused by humus action [38]. The EC value reflects the salinity in the compost, and high salinity may have phytotoxic effects on plant growth. It is widely acknowledged that the EC of compost should be below 4.0 mS/cm, which represents the critical value for safe use [39]. This finding is consistent with the results of the present study.
The compost pile was replenished with 5 to 8 L of water per day during the composting process, while the pile was rehydrated every 3 days according to the moisture content of each treatment group after sampling was completed, to try to ensure that the moisture content of each treatment group was maintained at approximately 55%. From Figure 1d, it can be seen that the moisture content was basically maintained in the range of 40–55%. Due to the intensive microbial activity and high metabolic intensity in the compost in the first 20 days, the heat generated makes the water evaporate. Furthermore, the pumping effect of the vacuum pump takes away some of the water.

3.2. Characterization of Changes in Sulfur-Containing Substances in Compost

The transformation of sulfur-containing substances between different forms in the composting process is a complex process. As the temperature rises in the composting process, sulfur-containing organic compounds decompose under the action of heterotrophic microorganisms to generate sulfur-containing gases, which is the main cause of odor and sulfur loss in the compost [40]. Organic sulfides are sequentially degraded to form methyl mercaptan, sulfur-containing amino acids (cysteine and methionine), and hydrogen sulfide. Subsequently, sulfur-oxidizing bacteria catalyze the oxidation of reduced-state sulfides to various inorganic sulfur compounds [41]. CM is rich in -SH-based proteins, and the ammonification of proteins is also accompanied by desulfurization processes [42]. Relying on natural ventilation is generally difficult to meet the oxygen demand of aerobic microorganisms, and anaerobic fermentation is likely to occur and produce hydrogen sulfide and other malodorous gases. Increasing the vacuum pump to pump out the gas produced by composting makes a pressure difference between the inside and outside of the composting hood, thus improving the ventilation and oxygen supply of the pile.
The hydrogen sulfide (H2S) emitted during the composting process, the cumulative collection of hydrogen sulfide (H2S accumulation), and the variation of sulfate (SO42−) and total sulfur (Total S) in the compost are shown in Figure 2. From Figure 2a, it can be seen that the trend of H2S collection was similar in each treatment group. The H2S emission data on average from the CK, T1, T2, and T3 groups were analyzed for significance. The results indicated that the differences between the groups were not statistically significant (p > 0.05). Additionally, there may be some differences between the T1 and T3 groups (p = 0.06), but these differences were not significant at the statistical level (p > 0.05). Combined with the H2S accumulation in Figure 2b, it can be seen that there was no significant difference (p > 0.05) in the collection of H2S among the treatment groups in the first 15 days of composting. On days 15–33, there was no significant difference (p > 0.05) in the cumulative amount of hydrogen sulfide collected between the T2 and T3 groups (medium C/N ratio and high C/N ratio), and there was no significant difference (p > 0.05) between the cumulative amount of hydrogen sulfide collected between the CK and T1 groups (low C/N ratio). The cumulative amount of hydrogen sulfide collected by the T1 group was significantly lower than that of the T2 group (p < 0.05). At the end of composting, the cumulative amount of hydrogen sulfide collected in CK, T1, T2 and T3 groups were 139.43 mg, 134.43 mg, 144.83 mg and 150.69 mg, respectively. Low C/N ratio conditions inhibit the growth of anaerobic organisms in CM [43], and the cumulative amount of hydrogen sulfide collected in the T1 group (Low C/N) was reduced by 3.59% compared to the CK group. This was mainly due to the low original C/N and high CM ratio in the CK group, which resulted in poor oxygen diffusion due to the denser piles [44], and good air permeability due to the expansion and high porosity of the WS compared to PW [45], but at higher WS ratio also resulted in the difficulty of decomposition in a short period of time, which may form an anaerobic environment that was conducive to the growth of the sulfate-reducing bacteria, leading to an increase in the amount of H2S collected. The above results indicated that low C/N ratio had an inhibitory effect on hydrogen sulfide production.
From Figure 2c, it can be seen that there was no significant difference (p > 0.05) in the sulfate content of each treatment group on the 3rd day of composting. The trends of T2 and T3 groups on days 3–27 of composting were consistent, and the trends of T1 and CK groups were similar. From the 27th day to the end of composting, there was no significant difference (p > 0.05) between the sulfate content of T1 and T3 groups. In addition to that, the sulfate content of T2 group showed a continuous increasing trend, while the sulfate content of CK group continued to decrease. This phenomenon, due to the difficulty of humification of PW, may be related to the loss of sulfur from CM in the CK group during composting, which occurs in the form of ammonia, hydrogen sulfur, and other substances produced by the degradation of proteins and other substances. As shown in Figure 2d, the total sulfur content of CK, T1, T2 and T3 were CK (2.60 g/kg) > T3 (1.30 g/kg) > T1 (1.160 g/kg) > T2 (0.20 g/kg), respectively, as the composting was carried out up to the 3rd day. The total sulfur content in the heap showed a fluctuating increasing trend due to the concentration effect and organic matter consumption during the composting process. At the end of composting, the total sulfur content of each treatment group was T1 (6.25 g/kg) > T3 (5.22 g/kg) > T2 (4.92 g/kg) > CK (3.41 g/kg), respectively. The total sulfur content of CK, T1, T2 and T3 treatment groups at the end of composting increased by 0.30, 4.39, 23.60 and 3.02 times, respectively, compared with that at the beginning of composting. The organic fertilizer samples were weighed at the end of composting and total sulfur was accounted for before and after composting, and the calculation results showed that the total sulfur loss was 57.03%, 44.75%, 48.45% and 42.02% in CK, T1, T2 and T3 groups, respectively (see Table 2). Adding straw to adjust the C/N ratio can better retain the sulfur in CM and straw, effectively reduce the total sulfur loss rate, and the high C/N ratio test group has a better retention effect on total sulfur (p < 0.05).
There was no significant difference between the CK group and the T1 and T2 groups (p > 0.05), which may be attributed to the substantial error in the initial total sulfur determination. However, a significant difference was observed between the CK group and the T3 at the statistical level (p < 0.05), indicating that the high C/N ratio condition exhibited a notable ability to reduce sulfur loss. In conclusion, the results indicated that the use of straw for composting products has a low rate of sulfur loss, a higher ecological benefit, and a promising application prospect.

3.3. Analysis of the Effect of C/N Ratio on Changes in Cellulose, Hemicellulose and Lignin in Composting

The high temperature stage is accompanied by the most abundant and intensive metabolic activity of the microbial community, which is critical for lignocellulose degradation [46]. During the cooling and maturation stages, as cellulose and hemicellulose decrease, lignin begins to be degraded by mesophilic microorganisms to produce humus, which is critical for compost quality. Fungal communities are widely recognized as the most important organisms responsible for the mineralization and decomposition of lignocellulosic materials during the composting process [47,48,49]. Among them, the Ascomycetes is the dominant phylum in the lignocellulose-degrading flora, and members of this phylum may compete with endogenous microorganisms for nutrients. Members of the genus Ascomycetes play an important role in lignocellulose degradation during composting, and their high abundance facilitates the degradation of organic matter [50].
From Figure 3a,b, it can be seen that cellulose and hemicellulose showed a decreasing trend in all treatment groups. The highest cellulose degradation efficiency was observed in the first 15 days, and the degradation rate of cellulose at the end of composting was CK (19.69%) < T1 (22.71%) < T2 (23.91%) < T3 (24.03%), respectively. The highest degradation efficiency of hemicellulose was observed from day 15 to the end of composting with degradation rates of T3 (32.66%) < CK (40.08%) < T1 (41.41%) < T2 (53.16%), respectively. Statistical analysis revealed that the cellulose degradation rate was not significantly different in the CK, T1, and T2 groups (p > 0.05) and that there were significant differences between T2 and T3 (p < 0.05). In contrast, the hemicellulose degradation rate was not significantly different between any of the groups (p > 0.05). The degradation of large molecules of organic carbon, such as lignocellulose, is a key factor in determining the compost maturation process [49]. If lignocellulose is not adequately degraded, application of compost products usually leads to severe crop diseases and decaying root systems. At the initial stage of composting, the degradation efficiency of lignocellulose was the lowest. This was primarily due to the difficulty of combining cellulose wrapped in straw with enzymes, resulting in low enzyme activity. As the temperature increased, the enzyme activity was enhanced, resulting in accelerated degradation efficiency of cellulose and hemicellulose [51,52]. The results showed that adjusting the carbon and nitrogen ratio of WS as auxiliary materials could improve the degradation efficiency of cellulose and hemicellulose in composting, and the higher the C/N ratio was, the more favorable it was to the degradation of cellulose and hemicellulose within a certain range. The lignin content changes in T1, T2 and T3 during the composting process showed fluctuating changes, and the degradation rates were 4.62%, 1.05% and 8.40%, respectively. In contrast, the lignin content of the CK group showed a slow growth trend, which might be related to the concentration effect and organic matter consumption during the composting process. As the composting process proceeded, the CM content in the fertilizer samples decreased rapidly, while the auxiliary material (poplar wood chips) was difficult to degrade due to the relatively large particles, and the auxiliary material (23.83% lignin content) in the samples was on the high side, which led to a continuous increase in the lignin content of the samples. To accelerate the degradation of lignocellulose into humus, subsequent inoculation of microbial agents should be considered during the co-composting of WS and CM to optimize the composting procedure and improve the composting efficiency [53].

3.4. Analysis of the Effect of C/N Ratio on the Change of Organic Matter in Composting

During aerobic putrefaction and fermentation, organic matter serves as a carbon source required for microbial activity and metabolism. One part of the organic matter is decomposed to produce carbon dioxide and water, while the other part exists in the form of stable organic matter. As shown in Figure 4, the organic matter content of all treatment groups showed a decreasing trend and the highest organic matter degradation efficiency was observed in the first 27 days. At the end of composting, the organic matter contents of CK, T1, T2 and T3 groups were 32.74%, 52.51%, 53.08% and 52.25%, respectively, which were higher than the national organic fertilizer standard (NY 525/T-2021). The reduction in organic matter in each treatment group before and after composting was 49.77%, 28.75%, 29.87% and 25.48%, respectively. The organic matter of the CK group exhibited a greater reduction during the composting process in comparison to the T1, T2, and T3 groups. This phenomenon was largely attributed to the sampling procedure employed. The quantity of PW incorporated was comparatively lower, and it exhibited minimal involvement in the humification process throughout the composting process. Consequently, to ensure the accuracy of the experiment, the samples measured in the CK sampling were primarily CM itself. Consequently, the organic matter change process of composting in the CK group can be attributed primarily to the organic matter change of CM. This explains why the decrease in organic matter degradation rate was more pronounced in the CK group. The results show that straw composting can effectively reduce the loss of organic matter, which is conducive to the production of higher quality compost products. It is worth noting that at the end of composting, the organic matter content of T1, T2 and T3 treatments showed an increasing trend, which may be due to the decrease in water content of the three treatments and the phenomenon of organic matter concentration with the composting process [54]. A compost product with a high concentration of organic matter can mitigate the adverse effects of heavy metals on plants [55]. Additionally, the product is rich in a variety of nutrients essential for plant growth, which can effectively promote the healthy growth of plants and improve the quality and yield of crops [56].

3.5. Analysis of the Effect of C/N Ratio on the Diversity of Bacterial Communities in Composting

Changes in microbial communities at the phylum level and genus level during composting with different C/N ratio treatments in the composting process are as shown in Figure 5. The dominant bacteria at the phylum level were Firmicutes, Actinobacteriota, Proteobacteria, Halanaerobiaeota, Bacteroidota, Chloroflexi and Deinococcota (Figure 5a). Among them, Firmicutes and Actinobacteriota were the major taxa (total abundance more than 50%) in all stages of compost fermentation, mainly due to their high resistance to harsh environments. The Firmicutes can produce heat-resistant endospores and plays a key role in the efficient degradation of cellulose, lignin and hydrolyzed sugars [57,58]. Microbial activities in the high temperature period of the compost were intensive and the diversity of the community was high, and the cellulose and hemicellulose were degraded rapidly [59], and the pathogenic microorganisms were inactivated by the high temperature.
As the composting process progressed, the relative abundance of the Firmicutes, which is the first dominant phylum, showed an overall decreasing trend, while the relative abundance of the Actinobacteriota increased, which signaled compost maturity [60]. Similar results were found by Steger et al. and Wei et al. who also reported that Actinobacteriota played an important role in the later stages of composting, which could potentially be a critical indicator of compost maturity [61,62]. At the end of composting CK and T3 groups, Actinobacteriota replaced Firmicutes as the first dominant phylum. Some previous studies reported an increase in the abundance of Actinobacteriota during the bio-oxidation and maturation stages of compost [63]. It mainly appeared in CK (days 18–36), T1 (days 18–36) and T3 (days 24–36), and it was mainly involved in lignocellulose decomposition during composting. A study by Bao et al. found that Actinobacteriota have a suite of carbohydrate-active enzymes for the breakdown of lignocellulose [64], which are essential for degrading organic matter and stimulating hydrolytic enzymes to degrade lignocellulose [29]. Actinobacteriota mainly appeared in CK and T1 groups on day 18 and T2 and T3 groups on day 30. It has been reported that Actinobacteriota, Proteobacteria and Bacteroidota play important roles in organic matter degradation and sulfur cycling [65], in which sulfur oxidizing bacteria are aerobic bacteria in Proteobacteria, which oxidize reduced inorganic sulfur compounds to sulfate, thus facilitating sulfur cycling [66]. Halanaerobiaeota, mainly present in the T1 and T2 rotting phases, are extreme microorganisms capable of reducing sulfate to sulfide under high salt conditions [67,68], and are associated with high moisture content in T1 and T2 groups during the rotting phase that impedes oxygen transport within the heap. The Chloroflexi and the Deinococcota were mainly present in the CK group (days 18–36) and the T3 group (day 36). In conclusion, it has been demonstrated that microbial communities undergo continuous succession at various stages of composting, and therefore the abundance of corresponding microbial species can be altered by inoculation with natural or synthetic bacterial agents [69,70,71], which in turn can regulate the process of succession and produce high-quality compost products. The application of compost can alter the composition of the soil microbial community. This occurs through the decomposition of organic matter in the compost, which in turn provides nutrients and facilitates recycling [72]. The production of growth hormone-like substances can also be stimulated by the compost, which in turn promotes plant growth and development [73].
At the genus level, the relative abundance of Bacillus increased steadily in the T1 group from day 3–12 of composting, while the CK, T2 and T3 groups all showed a decreasing and then increasing trend. Staphylococcus is a facultative anaerobic bacterium belonging to Firmicutes, which produces volatile sulfur compounds, and was mainly present in the T1 group (day 6), the T2 and T3 treatment groups, with a maximum abundance of less than 2% in the CK group. Thermobifida was detected only in T1 (1.65%) on day 6 of composting, and it was detected simultaneously in CK (48.63%), T2 (29.53%), and T3 (2.74%) groups on day 12 of composting and was enriched during composting, further indicating that the compost was well rotted. In addition to that, Thermobifida and Saccharomonospora play an important role in the degradation of cellulose and hemicellulose [74]. Oceanobacillus was predominantly found in the CK (day 6) and T1 groups (day 12 and 24 days). Lactobacillus, Romboutsia, Corynbacterium and Atopostipes were mainly present in the CM and in the first 6 days of the composting process, with Lactobacillus being mainly associated with the production of hydrogen sulfide, and with less than 1% relative abundance at the end of the composting process. Pseudogracilibacillus and Jeotgalicoccus were predominantly present in the first 3 days of composting. Georgenia and Truepera were present in the putrefaction stage, whereas Truepera was associated with lignocellulosic degradation and humification, further suggesting that the compost was entering into the putrefaction stage. The composting process is accompanied by complex biochemical reactions, which can be enhanced by adding mixed fungi or isolating natural microorganisms from the composting process for mixed inoculation to improve humification efficiency [75]. The thermophilic fungal group isolated from composting has strong cellulolytic ability [76]. Zhu et al. also found that fungal pretreatment can promote the formation of humus in the co-composting process [77], and subsequent studies at the level of fungal communities can be carried out to clarify the relevant mechanisms.
In conclusion, the analysis of microbial communities in the composting process at the phylum and genus levels revealed that the composting system gradually tends to a mature state with the passage of time. This process is confirmed by the corresponding microbial community structure succession, which may be affected by environmental factors. Further analysis is required to determine the extent of this influence. Furthermore, anthropogenic control factors, such as compost pretreatment and additives, may also influence the succession of microbial communities.

3.6. Association Analysis between Microorganisms and Environmental Factors in Compost with Different C/N Ratios

In order to understand the relationship between microbial communities and physicochemical index characteristics in the composting process, the top 20 species in abundance were selected at the genus level for Spearman association analysis with environmental factors, and the clustering mode of environmental factors and species hierarchies were averaged. As shown in Figure 6, Brachybacterium, Thermobifida, Saccharomonospora, Halocella, norank_f_Limnochordaceae, Lactobacillus, Corynebacterium, Gallicola, Jeotgalicoccus, Oceanobacillus, Georgenia, unclassified_f_Planococcaceae, and Truepera are closely related to sulfur transformation. Among them, Oceanobacillus, Georgenia, unclassified_f_Planococcaceae were positively correlated with total sulfur, and Brachybacterium, Thermobifida, Saccharomonospora, Halocella, norank_f_Limnochordaceae, and Truepera were positively correlated with the total sulfur in fertilizer samples, which may promote the sulfur oxidation. Lactobacillus, Corynebacterium, Gallicola, and Jeotgalicoccus showed highly significant negative correlation with sulfur. In addition, Staphylococcus, Brachybacterium and Saccharomonospora showed a significant positive correlation with pH, suggesting that alkaline environments are more suitable for the growth and reproduction of these bacteria. Thermobifida, Saccharomonospora, Halocella, Brachybacterium, norank_f_Limnochordaceae, and Truepera had a highly significant negative correlation with temperature (T). Thermobifida, Oceanobacillus and norank_f_Limnochordaceae were positively correlated with EC. Lactobacillus were negatively correlated with EC. Gallicola, Georgenia and Truepera were highly significantly positively correlated with EC. Microbial communities exhibit significant differences in abundance under varying environmental conditions. The incorporation of inorganic materials, such as clay and natural minerals, into the co-composting system can facilitate the conversion and degradation of organic matter by modifying the composting microenvironment [78,79]. This, in turn, accelerates the composting process and improves the quality of the final product by leveraging the relationship between environmental factors and microorganisms.

3.7. Correlation Analysis between Composting Characteristics and Environmental Variables

Structural equation modeling was used to quantify the correlation interactions among the factors affecting the composting process in order to identify the key drivers of the composting process and to analyze the relationships among the variables, as shown in Figure 7. SO42−, H2S, H2S accumulation, TS, and OM were selected as composting characteristic parameter variables, while temperature, pH, EC, and moisture were represented as environmental variables. The results showed that temperature and organic matter were significantly positively correlated (p < 0.01). With the increase in temperature, the degradable materials in compost additives could be further decomposed, and the content of organic matter continuously increased. The pH was significantly negatively correlated with temperature (p < 0.01), and there was a significant negative correlation between organic matter and the accumulation of H2S and total sulfur (p < 0.01), which means that temperature could affect the transformation process of the material through the effect of organic matter degradation, and the transformation of sulfur-containing substances could be affected by temperature. That is, temperature can affect the transformation process by influencing the degradation of organic matter, which plays a role in the transformation and accumulation of sulfur-containing substances. In addition, pH can have an indirect effect on sulfate transformation by affecting conductivity. In conclusion, it was shown that temperature, pH, and organic matter are the core covariates in this composting process and play a crucial role in composting by influencing other material transformation processes through direct and indirect effects. Related studies have also found that temperature is important to the compost humification process and that pH may be a key factor influencing bacterial communities [80]. Wang et al. found that pH adjustment and inoculation with mycorrhizal fungi improved the humification and succession of fungal communities during the cooling phase of composting [81]. Gao et al. also found that compost characteristics are a key factor in compost maturity, which may be regulated by influencing fungal community processes [82]. The correlation between EC and moisture with temperature and pH was found to be weak, and considering the key role of EC and moisture in composting as reported in the literature, it was hypothesized that there was a potential effect, which might be due to the differences in the experimental design with different emphasis on different factors. Further studies on environmental variables, microbial communities, functional genes, and enzyme reactions should be carried out on this basis in the future to clarify the mechanism of action between the factors and provide more scientific theoretical guidance for composting.

4. Conclusions

(1)
The use of WS to adjust the C/N ratio can promote the increase in compost temperature during composting and prolong the thermophilic period of composting, and the low C/N ratio group has a more obvious effect on the increase in aerobic compost fermentation temperature.
(2)
At the end of composting, the cumulative collection amount of hydrogen sulfide in groups CK, T1, T2 and T3 was 139.43 mg, 134.43 mg, 144.83 mg and 150.69 mg, respectively. Compared with group CK, the cumulative collection amount of hydrogen sulfide in low C/N ratio treatment group (group T1) was reduced by 3.59%. Adjustment of C/N ratio by straw addition significantly increased sulphate and total sulfur content in organic fertilizers and reduced sulfur loss. At the end of composting, the total sulfur loss of CK, T1, T2 and T3 treatment groups was 57.03%, 44.75%, 48.45% and 42.02%, respectively.
(3)
The contents of cellulose, hemicellulose and organic matter all showed a decreasing trend in the composting process. Adding straw to adjust the C/N ratio could improve the degradation efficiency of organic matter and cellulose and improve the composting quality. At the end of composting, the organic matter contents of CK, T1, T2 and T3 groups were 32.74%, 52.51%, 53.08% and 52.25%, respectively, which met the technical standard of organic fertilizer (NY 525/T-2021). The results indicated that adding straw to adjust C/N ratio could effectively increase the content of organic matter in organic fertilizer.
(4)
Regulation of the C/N ratio for co-composting treatment has been shown to promote maturation and the production of high-quality compost products. This occurs through alteration of the microbial community succession process, which in turn promotes the corresponding microbial sulfur transformation and cellulose degradation processes.

Author Contributions

Conceptualization, X.G. and Y.W.; methodology, S.C., X.G. and Y.W.; software, Y.M., Z.B. and S.C.; validation, S.C., Z.Y. and Q.M.; investigation, S.C., Z.B., X.N. and D.Q.; resources, X.G. and J.W.; data curation, S.C. and X.G.; writing—original draft preparation, S.C. and X.G.; writing—review and editing, S.C., X.G. and Y.W.; supervision, X.G. and Y.W.; project administration, Y.W., J.W. and X.G.; funding acquisition, Y.W. and X.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Key Research and Development Program of China (NO. 2021YFD1700900); Special Program for Central-Guided Local Scientific and Technological Development of Henan Province (YDZX20214100001834); National Natural Science Foundation of China (NO. 41601524).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The datasets generated for this study are available upon request from the corresponding author.

Acknowledgments

All subject programs are thanked for funding this study.

Conflicts of Interest

Author Dongsheng Qin was employed by the company Henan Sangao Agricultural and Animal Husbandry Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Changes of temperature (a), pH (b), electrical conductivity (c) and moisture content (d) during composting.
Figure 1. Changes of temperature (a), pH (b), electrical conductivity (c) and moisture content (d) during composting.
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Figure 2. Changes of hydrogen sulfide (a), accumulated amount of hydrogen sulfide (b), sulfate radical (c) and total sulfur (d) during composting.
Figure 2. Changes of hydrogen sulfide (a), accumulated amount of hydrogen sulfide (b), sulfate radical (c) and total sulfur (d) during composting.
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Figure 3. Changes of cellulose (a), hemicellulose (b) and lignin (c) during composting.
Figure 3. Changes of cellulose (a), hemicellulose (b) and lignin (c) during composting.
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Figure 4. Changes of organic matter content during composting.
Figure 4. Changes of organic matter content during composting.
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Figure 5. Changes of bacterial community composition at phylum level (a) and genus level (b).
Figure 5. Changes of bacterial community composition at phylum level (a) and genus level (b).
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Figure 6. The heatmap of the correlation between environmental factors and the top 20 bacteria in compost. (* p < 0.05, ** p < 0.01, *** p < 0.001).
Figure 6. The heatmap of the correlation between environmental factors and the top 20 bacteria in compost. (* p < 0.05, ** p < 0.01, *** p < 0.001).
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Figure 7. Structural equation model of factors in the composting process. The red line indicates a positive correlation, green line indicates a negative correlation. (Significance levels are followed: * p < 0.05, ** p < 0.01). The black line indicates that there may be a potential effect.
Figure 7. Structural equation model of factors in the composting process. The red line indicates a positive correlation, green line indicates a negative correlation. (Significance levels are followed: * p < 0.05, ** p < 0.01). The black line indicates that there may be a potential effect.
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Table 1. Physical and Chemical Properties of Raw Materials.
Table 1. Physical and Chemical Properties of Raw Materials.
Raw MaterialS (%)C (%)N (%)C/NOrganic Material (%)Cellulose (%)Hemicellulose (%)Lignin (%)Ash (%)
CM1.29 ± 0.0228.19 ± 0.574.04 ± 0.016.98 ± 0.0431.03 ± 7.5911.77 ± 0.020.94 ± 0.008.78 ± 0.0130.14 ± 0.35
PW0.25 ± 0.0040.81 ± 6.190.52 ± 0.0678.10 ± 0.8858.62 ± 0.8849.20 ± 0.012.51 ± 0.0023.83 ± 0.08_
WS0.38 ± 0.0340.00 ± 0.690.41 ± 0.8397.56 ± 4.3556.77 ± 1.6741.05 ± 0.0130.96 ± 0.0121.65 ± 0.00_
The “_” indicates missing data.
Table 2. Changes of total sulfur content during composting.
Table 2. Changes of total sulfur content during composting.
TreatmentInitial Total Sulfur (g)Final Total Sulfur (g)Total Sulfur Mass Loss (g)Total Sulfur Loss Ratio (%)
CK1149.07 ± 115.85 Aa493.78 ± 2.10 Bc655.29 ± 117.95 Aa57.03 ± 0.05 Aa
T11349.71 ± 362.01 Aa745.77 ± 8.93 Ab603.94 ± 354.27 Aa44.75 ± 0.15 Aa
T21399.87 ± 282.82 Aa721.63 ± 33.52 Ab678.24 ± 249.30 Aa48.45 ± 0.08 Aa
T31450.03 ± 347.03 Aa840.70 ± 23.41 Aa609.33 ± 370.45 Aa42.02 ± 0.16 Ab
Different capital letters in the same column indicate highly significant differences between groups (p < 0.01), and the same capital letters in the same column indicate no highly significant differences between groups (p > 0.01). Different lowercase letters in the same column indicate a significant difference between groups (p < 0.05), and the same lowercase letter indicates a nonsignificant difference between groups (p > 0.05).
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MDPI and ACS Style

Cai, S.; Ma, Y.; Bao, Z.; Yang, Z.; Niu, X.; Meng, Q.; Qin, D.; Wang, Y.; Wan, J.; Guo, X. The Impacts of the C/N Ratio on Hydrogen Sulfide Emission and Microbial Community Characteristics during Chicken Manure Composting with Wheat Straw. Agriculture 2024, 14, 948. https://doi.org/10.3390/agriculture14060948

AMA Style

Cai S, Ma Y, Bao Z, Yang Z, Niu X, Meng Q, Qin D, Wang Y, Wan J, Guo X. The Impacts of the C/N Ratio on Hydrogen Sulfide Emission and Microbial Community Characteristics during Chicken Manure Composting with Wheat Straw. Agriculture. 2024; 14(6):948. https://doi.org/10.3390/agriculture14060948

Chicago/Turabian Style

Cai, Shangying, Yi Ma, Zhenkang Bao, Ziying Yang, Xiangyu Niu, Qingzhen Meng, Dongsheng Qin, Yan Wang, Junfeng Wan, and Xiaoying Guo. 2024. "The Impacts of the C/N Ratio on Hydrogen Sulfide Emission and Microbial Community Characteristics during Chicken Manure Composting with Wheat Straw" Agriculture 14, no. 6: 948. https://doi.org/10.3390/agriculture14060948

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