1. Introduction
In China, the amount of urban kitchen waste (KW) exceeded 50% of that of municipal solid waste (MSW) [1], with an annual production of more than 120 million tons [2], which was a focus of MSW classification and management. On-site and near-site treatment of KW is a potential effective strategy for improving MSW management, with the following advantages: i) reducing of the construction investment and operation costs for MSW classification collection and transportation facilities; ii）facilitating effective regulation of MSW sorting in centralized KW production sites (e.g. supermarket, farmers' markets, and cafeterias, etc.) and in residential communities. KW is characterized by high moisture content, high biodegradable organic matter content, low C/N ratio and low pH, of which the suitable treatment technologies include anaerobic digestion and aerobic composting [3]. Compared with anaerobic digestion, composting is more suitable for the on-site and near-site treatment of KW in centralized production sites and in residential communities, with the technical characteristics of low construction investment and operation costs, and easy operation and maintenance [3-6].
Conditioning of KW composting material is required to shorten composting period, improve quality of compost products, and reduce emissions of greenhouse gases and odors. Biomass wastes are often used as amendments and bulking materials, such as green waste (GW) [7,8], agricultural straw [9,10], mushroom substrate [11] and sawdust [12,13]. For urban KW composting, GW is an available amendment and bulking materials, with low bulk density of 190-980 kg/m3, low moisture content (MC) of 21.1%-67.9% and high C/N ratio of 13.5-79.0 [14]. The main components of GW consist of cellulose at about 40% dry weight, hemicellulose at about 20-30% dry weight, lignin at about 20-30% dry weight [8,14], which are complex, low-biodegradable macromolecular organic substances. In addition, microbial inoculants promote decomposition and humification of complex macromolecular organic substances [8,13], increase nitrogen conservation [15,16], improve available phosphorus content [13] and reduce greenhouse gas and odor emissions [3,17]. However, it is difficult for commercial microbial agents to be widely used in KW composting due to the high price [8,9,13]. Furthermore, supplemental addition of commercial microbial agents is required due to competition with native microorganisms in composting material [18,19]. On the other hand, matured compost is a cheap and readily available inoculant agent and bulking agent, with the characteristics of high porosity, low moisture content, and rich in bacteria and fungi adapted to composting materials and composting processes [18,19]. Therefore, combining GW and mature composting to condition the composting materials of urban KW should be an effective and economical strategy.
Composting reactors are suitable for on-site and near-site composting of urban KW [1,4,5,20-22]. However, the thermophilic stage is often shortened due to low ambient temperatures, high heat loss and small composting material bulk, etc. [5,21], which reduces the hydrolysis and maturation of complex macromolecular organic substances (e.g., hemicellulose, cellulose, and lignin), resulting in the long composting period and the low maturity of compost products. Therefore, assisted-heating methods, such as direct electric heating [1,4,20,22] and thermal oil heating [23], are commonly used in compost reactors, in order to increase the temperature of thermophilic stage or extend the duration of thermophilic stage. However, there were some problems in these heating methods, such as high heat loss, poor heat transfer in composting materials, easy charring of composting materials and high electricity consumption. Therefore, the heating of composting reactors should be improved and optimized. Infrared (IR) heating can provide non-contact heating to the material through thermal radiation, which has technical advantages such as high electrothermal efficiency (over 90%), good heat transfer, avoiding charring of composting materials, and easy installment and maintenance [24]. In addition, mid-infrared rays (2.5-25 μm) are easily absorbed by water, polysaccharides (e.g., cellulose, hemicellulose, and lignin), lipids, nucleic acids, proteins, and amino acids [24], which are abundantly contained in composting materials. Furthermore, IR heating induces transition of the vibrational and rotational energy levels in complex macromolecular organic substances, promoting chemical and biochemical reactions [24]. However, composting reactor with IR heating had not been reported. Therefore, it was necessary to explore the efficiency and technical characteristics of IR heating composting reactors.
C and N conversions under the action of microbial metabolism are key links in aerobic composting process, which involves the decomposition and polymerization of organic matter, nitrogen conservation and material maturation , as well as the structure of microbial communities and their interactions, etc., and is an important breakthrough for an in-depth understanding of composting process and its technical characteristics [15]. In this study, a laboratory-scale IR heating sealed composting reactor was established to extend the thermophilic stage and promote KW composting, in which the KW was amended by GW and matured compost, and the sealed composting reactor reduces heat loss and malodorous gas release and promotes assimilation of NH3-N [25]. Composting experiments were conducted using two lab-scale sealed composting reactors, of which the aims were as follows: i) Using one reactor with IR heating as the experiment group (EG) and the other reactor without IR heating as the control group (CK), the C and N transformation characteristics of the 2 reactors were analyzed, in order to understand the efficiency of IR-assisted heating composting system; ii) The bacterial and fungal community structures and the microbial interactions in the 2 reactors were analyzed by 16S rRNA and ITS rRNA sequencing as well as network analysis, to elucidate the characteristics of microbial actions with IR-assisted heating in promoting KW composting.
2. Materials and Methods
2.1. Raw Composting Materials
KW were collected from the campus canteens of Chongqing University of Science and technology (CQUST) on the day of composting startup, including unprocessed kitchen waste (UPKW) and processed kitchen waste (PKW). GW was also collected from the CQUST campus area on Autumn, 2023. Before the startup of composting, the UPKW, PKW, and GW were crushed to a size of 10-20 mm by a toothed roll crusher [16], respectively. Mature compost was previously produced through the same composting material ratio and IR-heating composting operation. The raw composting materials included UPKW, PKW, GW and mature compost with a total wet weight mass of 30.0±0.5 kg and a wet weight mass ratio of 3:3:2:2. The above initial material ratios were mainly based on the following reasons: i）the wet weight productions of UPKW and PKW in the CQUST campus canteens were comparable , while the GW average output was lower and seasonal batch-output; ii）the appropriate range of moisture content of initial composting materials was 55%-65% [9,12,15]; iii) the suitable mass ratios of mature compost used as the inoculant and bulking agents was 10%-20% [4,18,19]. Composting began when the original composting material was thoroughly mixed and was recorded as day 0. The physicochemical properties of raw composting materials and their mixture were shown in Table 1. The main components of UPKW were vegetable leaves, vegetable roots and a small number of scraps, while those of PKW were leftovers. Therefore, the physicochemical properties of UPKW and PKW were significantly different.
2.2. Composting Reactor with IR-Assisted Heating and Its Control System
The 2 lab-scale composting reactors with IR-assisted heating were used, in which the infrared heater of the CK always turned off during the composting process, while the infrared heater of the EG automatically turned on and off from day2 to day17, to control the composting material within a temperature range of 55 ± 2°C. As shown in Fig. 1, the walls of the 2 reactors consisted of double-layered polycarbonate panels to improve thermal insulation. An aerator was installed in the bottom tank and connected with an air pump to control the oxygen concentration at 5 vol%-8 vol% in the upper of the reactor with automatically intermittent aeration. A spiral agitator and its motor were mounted on the axis of the lower semi-cylindrical chamber for stirring composting materials. An infrared heater (DLIRTN220-600-420-23 × 11BG, Nanjing Danlian Technology Co., Ltd., China) combined with a reflector was mounted on the cover. Oxygen content, temperature, and humidity in the air above the composting materials, and the temperature in the middle of composting material were monitored in real-time by a set of oxygen partial pressure sensor, temperature-humidity sensor and temperature sensor. A PLC controller was used to automatically control the operation of the air pump and infrared heater of the compost reactor. In addition, the 2 reactors were respectively placed on electronic scales to show the weight changes of composting materials.
Changes in weight of compost materials, middle temperature of compost materials and ambient temperature were recorded every 12 h, and the total power consumption and infrared heating power consumption were recorded daily. Samples were collected on day 0, 2, 7, 17, 25 and 40, respectively. Before sampling, the composting materials were stirred and mixed. Subsequently, aliquots were sampled from the middle depth of composting materials at 1/4, 1/2, and 3/4 of the mixing axis. The mixed samples were divided into three parts, which were used as fresh samples, air-drying samples, and the stored samples at -65°C for analyzing microbial community structure.
2.3. Physiochemical Analysis
One part of fresh samples was dried at 105°C to determine moisture content (MC), and then calcined at 550°C to determine OM. Other part of fresh samples was mixed with ultrapure water at a ratio of 1:10 (w/v), followed by the shake at 200 rpm for 2 h and the centrifugation at 1.0×104 g for 10 min, to collect centrifugal supernatants. The supernatants were used to measure germination index (GI) [9], E2/E3 (i. e., 250 nm/365 nm) ratio, and the fluorescence excitation-emission matrix (EEM) of dissolved organic matter (DOM) at a concentration of 1 mg/L, in which EEM spectra were processed by the previously described method [26]. Rayleigh scattering in the EEM spectra was deducted by using MATLAB 2021a (MathWorks, USA).
Air-dried samples were mixed with ultrapure water and 2 M KCl solution in the ratio of 1:10 and 1:20 (w/v), respectively, followed by shaking at 200 rpm for 2 h and the centrifugation at 1.0×104 g for 10 min, to collect centrifugal supernatant. The extracted supernatant with ultrapure water was used to determine pH, EC, water-soluble nitrogen content (i.e., NH4+-N, NO3--N, NO2--N) by a pH meter, conductivity meter, and ion chromatograph (CIC-100, Qingdao Shenghan Chromatography Technology Co., Ltd., China), respectively. The total soluble nitrogen content was determined by the Chinese standard method of HJ636-2012. The extracted supernatant extracted with 2 M KCl solution was used to determine the NH4+-N contents in the composting materials through distillation and titration [17]. The analysis of Total Kjeldahl nitrogen (TKN) in the composting materials was carried out according to the previous procedure [17]. Additionally, organic nitrogen (Org-N) content was calculated by subtraction method. The air-dried samples were also used to determine the humic acid (HA), fulvic acid (FA) and humic substances (HS) contents according to the previous literature [3], and to determine the total carbon (TC) and total nitrogen (TN) contents with an elemental analyzer (Vario Micro cube, Elementar, Germany). All tests were measured three times.
2.4. Microbial Community Analysis
The DNA temples of samples were extracted by the TIANamp soil DNA kit (Beijing Tiangen Biochemical Technology Co., Ltd., China). The concentration and purity of DNA temples were detected by a NanoDrop 2000 UV spectrophotometer (Thermo Fisher Scientific, USA), and DNA quality was analyzed using 1% agarose gel electrophoresis. The V3-V4 region of the 16S rRNA gene was amplified with the primers of 338F (5'-ACTCCTACGGGGAGGCAGCAG-3') and 806R (5'-GGACTACHVGGGTWTCTAAT-3') [3,15,27]. The ITS1 region of the ITS gene was amplified by the primers of ITS1F (5 '-CTTGGTCATTTAGAGGAAGTAA-3') and ITS2R (5 'GCTGCG TTCTTCATGC-3') [27,31]. The purified amplicons were sequenced on an Illumina MiSeq PE300 platform. The PCR amplification and sequencing were carried out by Meiji Biomedical Technology Co. LTD. (Shanghai, China), and the data were analyzed with the Majorbio Cloud Platform (https://cloud.majorbio.com). Network analysis was carried out to analyze the interactions among bacterial or fungal genera, as well as the correlations between the bacterial or fungal genera and the C and N transformation.
3. Results and Discussion
3.1. Physiochemical Property Changes of Composting Materials
From Fig.2a, the material temperatures in the 2 reactors increased rapidly until day 2. After day 1, the material temperatures were higher than 50℃. On day 2, the material temperatures in the CK and EG peaked up to 61.4℃ and 64.8℃, respectively. Subsequently, the material temperature of the CK gradually decreased and was below 50℃ on day 7, indicating that the thermophilic stage (≥ 50℃) lasted for about 5 days. After day 11, the differences between the CK material temperature and the ambient temperature remained relatively stable. That is, the cooling stage in the CK was from day 7 to day 11, and the maturation stage was from day 11 to day 40. Due to IR-assisted heating, the thermophilic stage of the EG lasted from day 2 to day 17. After turning off the infrared heater, the material temperature in the EG decreased rapidly, and then maintained a relatively stable difference with ambient temperature, indicating that the materials entered the maturation stage directly. After day 18, the material temperatures in the EG were always slightly lower than those in the CK.
For the 2 reactors, the composting materials were in the thermophilic stage from day 2 to day 7. The pH of composting materials in the CK and EG increased from 6.20±0.06 and 6.32±0.03 to 7.16±0.03 and 7.73±0.19 (Fig.2b), and the GI values increased from 15.75±1.95% and 16.91±4.18% to 17.97±4.45% and 27.39±7.91% (Fig.2d), respectively, with the EC decrease of 0.88±0.17 and 1.07±0.11 mS/cm, (Fig.2b), and the MC and the dry weight decreased slightly (Fig.2b), suggesting that IR-assisted heating could promote thermophilic biochemical reactions. From day 7 to day 17, the pH of composting materials in the CK and EG increased from 7.16±0.03 and 7.73±0.19 to 7.57±0.10 and 7.88±0.34 (Fig.2b), respectively, with the MC decrease of 1.61% and 6.95%, and the dry weight reduce rates of 27.8% and 39.8% (Fig.2c). The GI values of composting materials in the CK and EG increased from 17.97±4.45% and 27.39±7.91% to 42.89±5.72% and 90.95±8.29% (Fig.2d), respectively, indicating that the extension of thermophilic stage in the EG with IR-assisted heating resulted in the strongly organic matter mineralization and thermophilic humification. In addition, The C/N ratio of the EG also decreased more than that of the CK.
After day 17, the composting materials in the 2 reactors were in the maturation stage. From day 17 to day 40, The pH of composting materials in the CK and EG remained relatively stable at about 7.36±0.16-7.57±0.10 and 7.77±0.03-7.88±0.34 (Fig.2b), respectively, with the relatively stable MC at about 54.80±1.65% -55.86±1.81% and 50.23±1.05%-50.32±0.72%, and the relatively stable dry weight at about 11.00-11.35 kg and 10.61-10.83 kg (Fig.2c) . The EC of composting materials in the CK and EG continued to decrease from 2.52±0.07 to 1.96±0.04 mS/cm and from 2.65±0.03 to 1.80±0.05 mS/cm, respectively. Furthermore, the GI increase of composting materials in the EG was less than that in the CK (Fig.2d). On day 25, the GI values of composting materials from the CK and EG were 86.39±7.87% and 109.89±7.32%, respectively. On day 40, the GI values were 96.01±7.23% and 115.20±7.67%. GI was an important parameter to characterize the phytotoxicity and maturity of composting materials [8,9]. When the GI value is greater than 80%, the composting materials can be considered to be in a mature and stable state [9,18]. The GI value of composting materials in the EG at the end of infrared-assisted heating (i.e., day 17) was 90.95±8.29%, whereas that in the CK on day 25 was 86.39±7.87%. The materials in the EG had been matured at the thermophilic stage with infrared-assisted heating, and its composting period could be shortened by at least 8 d compared with that of CK.
3.2. Carbon Transformation of Composting Process
From Fig.3a, the OM contents (on a dry weight basis) in composting materials of the 2 reactors presented a gradual decrease. From day 0 to day 17, the decreases of the OM contents in composting materials of the 2 reactors were close to each other. Subsequently, from day 17 to day 40, the decrease of the OM contents in composting materials of the EG was significantly greater than that of the CK. In the study, IR-assisted heating promoted humification of composting materials. Under the IR-assisted heating during from day 2 to day 17, the HA content and HA/FA ratio of composting materials in the EG increased significantly greater than those in the CK (Fig.3d), corresponding to the GI changes of them (Fig.2d). The HA content and HA/FA ratio in the EG increased from 142.01±13.33 to 322.77±15.82 g/kg DM and from 1.57 to 6.87, respectively, while those in the CK were increased from 96.9±2.71 to 48.25±5.74 g/kg DM and from 1.41 to 4.68, respectively. After day 17, the HA content and HA/FA ratio of the EG increased slowly, in which they remained relatively stable after day 25. In contrast, the HA content and HA/FA ratio in the CK still continued to increase after day 25.
The change trends of DOM content in composting materials in the 2 reactors were similar (Fig.3c). Under IR-assisted heating from day 2 to day 17, the E2/E3 value of DOM in the EG was significantly less than those in the CK (Fig.3d), indicating that the degree of polymerization and molecular weight of dissolved aromatic compounds in composting materials of the EG were obviously greater than those of the CK, corresponding to the more rapid increase of the HA content in the former. From Fig.S1, the EEM spectra of DOM from the composting materials showed significant spectral signals of the V region (Ex>280 nm, Em>380 nm [27]) of the DOM sample from the EG on day 7. However, the V region signals from the CK on day 17 remained weak, and then became obvious until on day 25. These results further illustrated that IR-assisted heating promoted the thermophilic humification of DOM in the composting materials.
3.3. Nitrogen Transformation of Composting Process
From Fig.3e, the TKN、Org-N and NH4+-N contents of composting materials in the 2 reactors showed a decreasing trend from day 0 to day 2. From day 2 to day 7, the composting materials in the 2 reactors were in the thermophilic stage, but the Org-N contents increased remarkably. It had been reported that during the thermophilic stage in a sealed composting reactor, a strong convective environmental condition was formed, including evaporation, condensation and reflux of water, resulting in that the NH3-N (g) volatilized from composting materials also quickly was dissolved in condensation droplets and returned to the composting materials with the droplets, which mitigated the loss of N and promoted the assimilation of NH4+-N [25]. From day 2 to day 7, the Org-N content increase in composting materials of the EG was less than that of the CK, mainly ascribed to the higher pH value of the former than that of the latter, resulting in greater NH3-N (g) volatilization from the former materials than that from the latter materials. Subsequently, the composting materials in the EG with IR-assisted heating remained in the thermophilic stage until day 17, resulting in that the Org-N content increase of the EG was significantly higher than that of the CK from day 7 to day 17. From day 17 to day 40, the Org-N contents of composting materials in the 2 reactors increased slightly.
During the composting process, the soluble TN content of composting materials in the 2 reactors decreased and then gradually increased, in which the increases of the soluble TN content and Org-N content in composting materials of the EG were notably greater than those of the CK from day 2 to day 17. The above results were because that the NH4+-N assimilation might be enhanced more by the strong convective environmental conditions with IR-assisted heating than that without IR-assisted heating (Fig.3f). From day 17 to day 40, the soluble Org-N content and NH4+-N content of composting materials in the 2 reactors decreased rapidly, accompanied with the rapid increase of NO3--N content, in which the increase of soluble NO3--N content in the EG was significantly more than that in the CK from day 17 to day 25. The result should be related to that the composting materials in the EG had been maturated on day 17, whereas those in the CK was not maturated until day 25, since nitrification usually occurred after the maturation of organic matter in composting materials [16,19].
3.4. Efficiency Analysis of Composting Process
The composting material in the EG was maturated on day 17 with the GI of 90.95±8.29%, while that in the CK had been maturated on day 25 with the GI of 86.39±7.87% (Fig.2d). The matured level of the composting material in the EG on day 17 was close to that in the CK on day 25. Thus, the efficiency of the composting process with IR-assisted heating was evaluated through comparing the physicochemical properties of the composting material on day 17 in the EG to that on day 25 in the CK, as well as the energy consumptions of the above composting processes. From Table S1, compared with the CK, the duration of the thermophilic stage of the EG was extended by 10 d with IR-assisted heating, of which the main advantages were as follows: i) High-temperature maturation of KW composting materials was achieved with IR-assisted heating, avoiding the need for the maturation stage and shortening the composting period; ii) The TN content of compost products from the EG was slightly increased due to the NH4+-N assimilation enhanced under the strong convective environmental conditions of the sealed reactor, accompanied with the slight reduce of the TC content; iii) The wet and dry weights of compost products in the EG were obviously reduced more than those in the CK, saving the transportation costs.
However, IR-assisted heating significantly increased the energy consumption of composting. In the EG, the IR-assisted heating power consumption was 0.76 kW·h/kg WM, accounting for the major part of total energy consumption, while the other power consumption including aeration and stirring was 0.05 kW·h/kg WM. Thus, the total energy consumption was 0.81 kW·h/kg WM. In the CK, the total energy consumption only included aeration and stirring, which was 0.03 kW·h/kg WM. The aeration and stirring power consumption of the EG was significantly greater than that of the CK, corresponding the stronger decomposition of organic substances and the more material loss in the former than those in the latter. Zhou et al. (2022) reported that a community-scale in-situ rapid biological reduction system of 150 kg/d with an electric heater was used to treat food waste, in which the compost product could be recycled as a bio-solid fuel or a fertilizer, with a total power of about 1.01 kW·h/kg [4]. The total power consumption of the EG was lower than that in the previous report, ascribed to that the effectiveness of infrared heater should be higher than that of electric heater. The results indicated that IR-assisted heating was a promising way to improve composting efficiency. On the other hand, the scale of the reactor with IR-assisted heating (i.e., EG) was small, with an initial materials weight of only about 30 kg, resulting in higher dissipation-heat loss of the reactor wall. If increasing the design scale of IR-assisted heated composting reactor in practice, the reactor wall heat loss and the IR-assisted heating power consumption can be significantly reduced. Therefore, the composting reactor with IR-assisted heating should be a promising strategy for the in-situ and near-situ composting of KW.
3.5. Characteristics of Microbial Community of Composting Process
3.5.1. Microbial diversity and its changes during composting
The bacterial and fungal coverage indices of the composting samples from the 2 reactors were all greater than 99.7% (Table S2), indicating that the sequencing results could accurately reflect the real situation of microorganisms in composting materials and their changes during the composting process. Furthermore, the Chao and Shannon indices of bacteria in the 2 reactors showed similar fluctuating trends. The Shannon indices of fungi showed a decreasing and then increasing trend. The Chao index of fungi in the CK continued to decrease, while that of fungi in the EG fluctuated. From day 25 to day 40, both the Chao and Shannon indices of fungi in the EG were higher than those in the CK.
3.5.2. Influence of IR-assisted heating on bacterial community structure
The bacterial communities in the composting materials from the 2 reactors were mainly composed of Proteobacteria, Firmicutes, Actinobacteriota, Bacteroidota, and Chloroflexi (Fig.4a). On day 7 and day 17, Proteobacteria and Actinobacteriota were the important dominant phyla in the 2 reactors, in which the relative abundances of Firmicutes in the EG (29.27% and 21.76%) were significantly greater than those in the CK (9.49% and 8.18%). Additionally, the relative abundance of Chloroflexi and Gemmatimonadetes in the EG (3.38% and 4.57%) was significantly greater than that in the CK (1.82% and 1.89%) on day 17. After the IR-assisted heating (i.e., on day 25 and day 40), the relative abundances of Firmicutes, Actinobacteriota, Chloroflexi, and Gemmatimonadetes in the EG were significantly greater than those in the CK.
The dominant bacterial genera in the 2 reactors included Bacillus, Pseudomonas, Pseudoxanthomonas, Enterobacter and Cellvibrio, etc. (Fig.4b). On day 7, the relative abundances of Pseudomonas in the EG and CK accounted for 11.46% and 11.70% (Fig.4c), respectively, which were highest and close to each other, but the relative abundance in the EG (0.26%) was much lower than that in the CK (9.97%) on day 17 (Fig.4d). On day 7 and day 17, Bacillus and Pseudoxanthomonas were important dominant genera of bacteria in the EG, in which the relative abundance of Bacillus (9.06% and 7.69%) was 3.74 and 2.51 times higher than those in the CK, respectively. On day 7, the relative abundances of Lysinibacillus, Geobacillus, Streptomyces, unclassified_f_Rhodothemaceae, and Weissella in the EG were 3.65%, 2.93%, 2.72%, 2.66%, and 2.60% in turn, which were significantly higher than those (0.59%, 1.05%, 1.95%, 1.08%, and 0.70%) in the CK (p<0.001). On day 17, the relative abundances of Thermomonospora, unclassified_c_S0134_ terrestrial_group, and Chelatococcus in the EG were 3.59%, 4.40%, and 2.64% in turn, which were higher than those (1.03%, 1.75%, and 2.00%) in the CK (p<0.001).
It had been reported that Bacillus [22,28-30], Pseudoxanthomonas [31], Geobacillus [32], Weissella [3], Thermomonospora [33], and S0134_terrestrial_group [34] were lignocellulose-degrading bacteria, and Lysinibacillus [28], and Streptomyces [15] were cellulose-degrading bacteria. Additionally, Pseudomonas [16,35], Bacillus [22], Pseudoxanthomonas [31], and Thermomonospora [33] had a nitrogen-fixing function, while Chelatococcus [36] had an aerobic denitrifying function. Thereby, the thermophilic maturation of compost materials and the synthesis of organic nitrogen (Org-N) in the EG were promoted, mainly because the proliferations of the thermophilic lignocellulose-degrading bacteria, cellulose-degrading bacteria, nitrogen-fixing bacteria, and denitrifying bacteria were promoted by IR-assisted heating.
3.5.3. Influence of IR-assisted heating on fungal community structure
The dominant fungi phyla in the composting materials from the 2 reactors was Ascomycota, followed by Basidiomycota (Fig. 5a). It was well known that some ascomycetes were hyperthermophiles and could secrete heat-resistant cellulases and hemicellulases to decompose complex macromolecular organic compounds, such as lignocellulose, cellulose, and hemicellulose [27,28,31], which played an important role in composting. On day 2, Candida, unclassified_ f_Dipodascaceae, and Kodamaea were the main dominant fungi genera in the 2 reactors. From day 7 to day 40, Subsequently, Thermomyces was the dominant fungi genera in the EG and CK, of which the relative abundance ranges were 55.56%-87.45% and 66.77%-86.32%, respectively. In addition, the relative abundances of Aspergillus and Candida in the 2 reactors were second only to that of Thermomyces. On day 7 and day 17, the relative abundances of Aspergillus, Candida, and unclassified_f_Dipodascaceae in the EG (26.70% and 6.40%, 11.36% and 3.24%, and 3.44% and 1.16%, respectively) were significantly higher than those in the CK (i.e., 16.75% and 5.90%, 5.36% and 1.74%, 1.29% and 0.14%, respectively). In addition, the relative abundance of Mycothermus in the EG was remarkably lower than that in the CK. It had been widely accepted that Aspergillus and Candida were important fungi in the hemicellulose, cellulose and lignocellulose decomposing [27,28]. These results suggested that the proliferation of the fungi decomposing hemicellulose, cellulose, and lignocellulose was facilitated by IR-assisted heating during the thermophilic stage.
3.5.4. Influence of IR-assisted heating on microbial interactions
Microbial interactions at genus level in the 2 reactors during the thermophilic stage (i.e., from day 2 to day 17) were analyzed by a single-factor correlation network (|R|> 0.6, p<0.05). From Table S3, the number of bacterial genera was more than 150, while that of fungi genera was less than 100. For both the top 10 and top 15 genera of bacterial relative abundance, the positive correlations in the EG were greater than those in the CK. Meanwhile, the positive correlations were lower than the negative correlations in the 2 reactors, suggesting that the antagonism among dominant bacterial genera was greater than the synergism. For the top 30, 50, 100 and 150 genera, the positive correlations in the EG were lower than those in the CK, in which the positive correlations in the CK were greater than the negative correlation, and the positive correlation of the top 30 genera in the EG was still less than the negative correlation. The results indicated that the synergistic effect among the dominant bacterial genera in the EG was enhanced by a selective pressure from the IR-assisted heating, namely, the temperature increase and the extend of the thermophilic stage, but the synergistic effect among all bacterial genera synergy were weaken.
On the other hand, for the top 10, 15 and 30 genera of the fungal relative abundance, the positive correlations in the EG were lower than those in the CK. In addition, the positive correlations in the 2 reactors were lower than the negative correlations. For the top 50, 100 and 150 genera of the fungal relative abundance, the positive correlations in the EG were greater than those in the CK, and the positive correlations in the 2 reactors were also greater than the negative correlations. The results indicated that the synergistic effect the dominant fungal genera in the EG were weaken with IR-assisted heating, but the synergistic effect among all bacterial genera synergy were enhanced.
The important microorganism genera related with C and N transformation in the 2 reactors at the thermophilic stage were analyzed by the central coefficient values of the two-factor correlation network analysis (|R|> 0.6, p<0.05, top 100 species in abundance), in which the C and N transformation indices included TC, E2/E3, humic substances (HS), TKN, NH4+-N, NO3--N. Specifically, the negative relationship between microorganism genus and E2/E3 suggested that the genus could have a function to polymerize dissolved aromatic compounds. The positive relationship between microorganism genus and HS suggested that the genus could have a function of the maturation of composting materials. The positive relationship between microorganism genus and TKN suggested that the genus could have a function of the NH4+-N assimilation or Org-N synthesization. From Fig.6a and Fig.6b, the correlation networks between the bacterial genera and the C and N transformations in the EG were more complex than those in the CK, and the more bacterial genera were involved in the maturation of composting materials and the Org-N synthesization in the former than those in the latter. In the CK, the important bacterial genera that had the 3 functions, which were the polymerization of dissolved aromatic compounds, the maturation of composting materials and the Org-N synthesization, were involved to the 7 genera in Proteobacteria, such as Chelatococcus, Cellvibrion and unclassified_ f__Methylococcaceae, etc., 6 genera in Bacteroidota, such as unclassified_f__Rhodothermaceae, Parapedobacter and unclassified_ o__Kapabacteriales, etc., 5 genera in Actinobacteriota, such as Thermopolyspora, Actinomadura and unclassified_f__Micromonosporaceae, etc., unclassified_f__Sandaracinaceae and Vulgatibacter in Myxococcota, FFCH7168 in Chloroflexi and TM7a in Patescibacteria, respectively. In the EG, the important bacterial genera that had the 3 functions exhibited more diversity, were involved to the 11 genera in Actinobacteria, such as Thermostaphylospora, Thermopolyspora and Thermomonospora, etc., 8 genera in Proteobacteria, such as Chelatococcus, Chelativorans and Luteimonas, 4 genera in Firmicutes, such as Thermobacillus, Laceyella and Tepidimicrobium, etc., 4 genera in Chloroflexi, such as unclassified_f__A4b, unclassified_ o__SBR1031 and FFCH7168, etc., Anseongella, Chryseolinea and Taibaiella in Bacteroidota , Truepera in Deinococcota, unclassified_ c__S0134_terrestrial_group in Gemmatimonadota and Vulgatibacter in Myxococcota. Bacillus and Pseudoxanthomonas were the dominant genera with high abundance in the EG, which were important lignocellulose-degrading bacteria and cellulose-degrading bacteria[22,28-30], but the roles of them in the thermophilic maturation and the Org-N synthesization were inconspicuous. Additionally, many low-abundance bacterial genera were involved in the processes of the thermophilic maturation and the Org-N synthesization, which should play an important role in the C and N transformations during the thermophilic stage. These results indicated that the diversity of bacterial genera involved in the thermophilic maturation of composting materials and the Org-N synthesization was significantly increased by the IR-assisted heating. Meanwhile, the synergistic effect among the above important bacterial genera in Actinobacteria, Proteobacteria, Firmicutes, Chlorolexi, Bacteroidota, Deinococcota, Gemmatimonadota and Myxococcota might be further enhanced. Thus, the thermophilic maturation and the higher N reservation were achieved in the EG at the thermophilic stage.
From Fig. 6c and Fig. 6d, the important fungal genera with the 3 functions in the CK were involved to 10 genera in Ascomycota, such as Thermomyces, Myceliophthora and Pseudallescheria, etc., and Ganoderma in Basidiomycota. In the EG, the important fungal genera having the 3 kinds of functions were involved to 5 genera in Ascomycota, such as Thermomyces, Pseudallescheria and Scytalidium, etc., Cystofilobasidium and Leucosporidium in Basidiomycota and unclassified_k__Fungi. Except for Thermomyces, the important fungal genera with the 3 functions were low-abundance fungal genera. Aspergillus and Candida were also the dominant genera with high abundance in the EG, which were important lignocellulose-degrading fungi and cellulose-degrading fungi [22,28-30], but the roles of them in the thermophilic maturation and the Org-N synthesization were also inconspicuous. These results indicated that the diversity of the important fungal genera with the 3 functions were slightly reduced in the EG. In addition, the diversity and the synergistic interactions among the important fungal genera with the 3 functions in the EG were much less than those among the important bacterial genera. Therefore, the thermophilic maturation and the Org-N synthesization were promoted by IR-assisted heating during the thermophilic stage, mainly attributed to the diversity increase and the synergism enhancement among the important bacterial genera with the 3 functions of the polymerization of dissolved aromatic compounds, the maturation of composting materials and the Org-N synthesization.
4. Conclusions
IR-assisted heating promoted the KW composting, in which the composting material was adjusted by GW and matured compost. When the thermophilic stage in the reactor with IR-assisted heating was ended, the thermophilic maturation and the higher N reservation of composting materials was realized with the GI of 90.95 ± 8.29% and the total energy consumption of 0.81 kW·h/kg WM, reducing the composting period by more than 8 d. Infrared-assisted heating not only promoted the proliferation of Bacillus and Pseudoxanthomonas in bacteria and Aspergillus and Candida in fungi, which have cellulose and lignocellulose degrading functions, but also increased the diversity of the important bacterial genera with the 3 functions of polymerization of dissolved aromatic compounds, maturation of composting materials and synthesization of Org-N, and enhanced the synergistic effects among these genera. In addition, Thermomyces was a dominant fungal genus with the 3 functions.
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