Co-composting of cattle manure and wheat straw covered with a semipermeable membrane: organic matter humification and bacterial community succession

Semipermeable membrane-covered composting is one of the most commonly used composting technologies in northeast China, but its humification process is not yet well understood. This study employed a semipermeable membrane-covered composting system to detect the organic matter humification and bacterial community evolution patterns over the course of agricultural waste composting. Variations in physicochemical properties, humus composition, and bacterial communities were studied. The results suggested that membrane covering improved humic acid (HA) content and degree of polymerization (DP) by 9.28% and 21.57%, respectively. Bacterial analysis indicated that membrane covering reduced bacterial richness and increased bacterial diversity. Membrane covering mainly affected the bacterial community structure during thermophilic period of composting. RDA analysis revealed that membrane covering may affect the bacterial community by altering the physicochemical properties such as moisture content. Correlation analysis showed that membrane covering activated the dominant genera Saccharomonospora and Planktosalinus to participate in the formation of HS and HA in composting, thus promoting HS formation and its structural complexity. Membrane covering significantly reduced microbial metabolism during the cooling phase of composting.


Introduction
The annual output of organic agricultural waste in China is extraordinarily enormous. The Chinese annual output of animal manure and crop straw has arrived at about 3.8 billion tons and 900 million tons, respectively (Bluemling and Wang 2018;Qi et al. 2021). As typical organic wastes, cattle manure and crop straw are rich in organic matter and mineral elements, such as nitrogen, phosphorus, and potassium . They can be transformed into nuisance-free, Responsible Editor: Diane Purchase * Zhanjun Cheng zjcheng@tju.edu.cn 1 sanitary, and nutritious products by aerobic composting, which can be applied as fertilizer (Ren et al. 2020). During composting, organic matter is transformed in two different ways: part of the organic matter undergoes mineralization to produce CO 2 , H 2 O, and inorganic salts, while the other part is synthesized by microorganisms into humus through a complex humification process (Zhang and Sun 2017). Humic substances (HS) are not only the most essential compositions of compost but also significant indicators of the fertility of compost (Zhang et al. 2018). Highly humified compost applied to soil can effectively enhance soil fertility and facilitate the growth of crops and plants (Zhang et al. 2015). Besides, increasing the humification degree means reducing the mineralization of organic matter, thus reducing CO 2 emissions to a certain extent, increasing the conversion rate and fixation efficiency of carbon, and thereby improving the resource utilization efficiency of organic wastes (Ren et al. 2020). Currently, a great deal of studies has concentrated on adding exogenous substances or microbial agents to compost to enhance the humification of compost Zhao et al. 2016). There are few studies on organic matter decomposition and HS conversion patterns in different composting technologies. Consequently, it is essential to investigate the coupling relationship between physicochemical properties, humus, and microbial community structure in different composting technologies. On this basis, efficient composting technologies can be combined with other technologies to find a more effective way to promote HS formation in compost and improve organic matter utilization.
In recent years, semipermeable membrane-covered composting has been widely applied for its advantages of high efficiency, strong adaptability, and environmental friendliness . Combined with the ventilation device, the semipermeable membrane covering the upper part of the composting reactor can keep a uniform oxygen concentration and create a slight pressure environment inside the reactor, which improves the utilization rate of oxygen and composting efficiency by prolonging the high-temperature stage (Robledo et al. 2020). Robledo-Mahon et al. (2018) found that membrane covering accelerated the heating rate of sludge composting, increased the abundance of microorganisms capable of decomposing refractory organic matter in the early stages of composting, and shortened the duration of composting. Li et al. (2020) found that covering a semipermeable membrane improved composting nitrogen preservation rate and reduced the emission of NH 3 and H 2 S. In addition, some researches have demonstrated that greenhouse gas emissions from composting process were significantly reduced after covering with a semipermeable membrane (Ma et al. 2018a;Sun et al. 2018). Fang et al. (2021) combined intermittent aeration with membrane covering for cattle manure composting and found that the combined process reduced CH 4 emissions by 99.89% compared to the control. Through the above studies, it was found that membrane covering can effectively improve the efficiency of carbon and nitrogen fixation in compost and reduce gas emissions during the composting process. However, there is a gap in studying the influence of membrane covering on the development of HS in composting. Al-Alawi et al. (2019a) studied the structural properties of organic matter in sewage sludge composting covered with a semipermeable membrane and found that at the end of composting, the number of aromatic chains of organic matter increased and the humification degree increased significantly compared to the initial stage. Based on previous researches, semipermeable membrane covering changes the physicochemical properties of composting, which in turn affects the activities and community composition of microbes in composting . Fang et al. (2022) analyzed the microbial mechanisms that reduce methane production in membrane covering compost. The result showed that the micro-positive pressure environment under the membrane led to oxygen infiltration into the compost pellets inhibiting the activity of anaerobic methanogenic bacteria, thus reducing the relative abundance of methanogenic pathways. Ma et al. (2018a) showed that membrane covering increased the relative abundance of cellulose-degrading bacteria and thus may have accelerated cellulose degradation. It is thus clear that membrane covering can influence the degradation and conversion of organic matter by changing the physicochemical properties of the compost or the activity of specific microorganisms. However, there is little consideration on how changes in physicochemical properties and microbial communities affect HS formation during the semipermeable membrane-covered composting process. Above all, it is essential to investigate the HS formation and the correlation between HS, microbial community, and physicochemical parameters to elucidate the humification process in semipermeable membrane covered composting.
This research carried out comparative tests of composting with or without covering semipermeable membrane. The bacterial community composition was measured adopting high-throughput sequencing. The relation among bacterial community and humic components was explored by correlation analysis, so as to study the impact of semipermeable membrane covering on humus formation and microbial succession during composting.

Composting process and sample collection
Fresh cattle manure was collected from Mianhe Cattle Farm (Tianjin, China). Wheat straw was purchased from Dezhou, Shandong Province, China. The wheat straw was cut into a length of approximately 3-5 cm. In total, 60 kg of fresh cow manure and 10 kg of wheat straw were mixed evenly to obtain the raw material with a suitable moisture content and C/N ratio . The mixture was divided equally into two parts and loaded into a static aerobic composting bin (about 90 L), respectively. The air distribution duct was placed at the bottom, and the pile was ventilated at a flow rate of about 0.3 m 3 /h. The experiments included two treatments, i.e., composting covered with a semipermeable membrane system (CM) and non-covered composting system (the control, CK). Semipermeable membrane had selective permeability, its core material was expanded polytetrafluoroethylene (e-PTFE), the pore size was 0.2 μm, which can prevent the transmission of large molecules such as dust and aerosols, while allowing the discharge of small molecules such as CO 2 . The average pore diameter of the membrane was about 0.2 μm, the water resistance was greater than 150 kPa, the water vapor permeability resistance was ≤ 9.5 m 2 ·Pa/W, and the air permeability was 4-6.5 m 3 /m 2 ·h. The detailed physicochemical characteristics of the raw materials are described in Supplementary Materials.
The aerobic composting lasted 42 days, with manual turning of the piles five times (7, 14, 21, 28, and 35 days). The temperature of each pile and ambient air were recorded daily: 9:00 AM, 3:00 PM, and 9:00 PM. Solid samples were collected on day 0, 3,6,9,12,15,18,24,30,36, and 42 during the composting using a five-point sampling method. The collected samples were separated into two portions, one was kept in 4 °C refrigerator for the determination of physicochemical properties, and the other was stored in − 20 °C refrigerator for biological analysis.

Physicochemical analyses
The fresh samples as described above were taken to determine moisture content (MC), organic matter (OM), and seed germination index (GI) (Wang et al. 2017). The MC and OM were quantified by the drying-weighing approach (Qin et al. 2021). Fresh samples were blended with deionized water at a ratio of 1:10 (w: v) and shaken for 2 h, and the extracting solution was obtained by filtering. Ten radish seeds were placed on filter paper in Petri dishes and moistened with 5 mL of extracting solution. Then the Petri dishes for each sample were incubated at 25 °C for 48 h. The number of germinating seeds and their root length were determined. Ultrapure water was used as control, and all samples were analyzed in triplicate. GI was subsequently calculated as follows: A portion of fresh samples was air dried naturally at ambient temperature and passed through a 0.25-mm sieve for the determination of total organic carbon (TOC). TOC was determined by the potassium dichromate volumetric method ).

Extraction, separation, and analysis of humic substance
The extraction and determination of HS was conducted following the reports of Zhou et al. (2014). After being air-dried and sieved through 100 mesh, the samples were

Fig. 1
Changes in physicochemical parameters during composting: a temperature, b MC, c OM, d GI extracted by a mixture of 0.1 M Na 4 P 2 O 7 and 0.1 M NaOH at a solid-liquid ratio of 1:20 and shaken for 16 h at room temperature (25 °C) at a speed of 200 rpm/min. After centrifugation for 20 min at 8,000 rpm, the supernatant was collected, and the precipitate was extracted again. All the extracts of the same samples were blended and filtered through a 0.45-μm Millipore membrane to determine the HS (Jiang et al. 2021). The pH of the extract was adjusted to 1.0 by 6 M HCl and left overnight. Finally, the solution was centrifuged at 8000 rpm for 20 min, the precipitate was HA, and the supernatant was FA. The obtained HA was dissolved with 0.1-M NaOH. Then the obtained solutions were dried at 60 °C in a water bath, and the TOC of HS, HA, and FA was measured by potassium dichromate volumetric method.

Microbial analyses
High-throughput sequencing analysis was applied to determine the bacterial community structure at different stages of composting in CK and CM. The samples of CM and CK were taken at 0, 3, 18, and 42 days, respectively. MoBio Power Soil DNA Isolation Kit (MoBio Laboratory, Carlsbad, CA, USA) was used to extract genomic DNA from samples according to manufacturer's instructions. The V3-V4 hypervariable regions of the bacteria 16S rRNA gene were amplified with primers 338F (5′-ACT CCT ACG GGA GGC AGC AG-3′) and 806R (5′-GGA CTA CHVGGG TWT CTAAT-3′), respectively. Purified amplicons were assembled equimolarly and sequenced on the Illumina MiSeq platform (Illumina, San Diego, USA) according to the standard protocol of Majobio Biopharmaceutical Technology Ltd. in Shanghai, China. Sequences were clustered into operational taxonomic units (OTUs) at 97% similarity level using UPARSE (version 7.1 http:// drive5. com/ uparse/). OTUs were classified according the SILVA database (Quast et al. 2013). A representative sequence from each OTU was chosen for downstream analysis.

Data and statistical analyses
The physicochemical properties and humus of compost samples were carried out in triplicate and presented as mean ± standard deviation. Data were statistically analyzed using Origin 8.0 and SPSS 12.0. The relations among environmental factors, humic substances, and microorganisms were analyzed using redundancy analysis (RDA).

Physical and chemical properties of compost
Temperature can detect microbial activity and composting performance ). Variations of temperature in two treatments are presented in Fig. 1a. Both showed a trend of elevation in composting temperature followed by a slow reduction. The maximum temperature of CM and CK groups arrived at 57.47 °C and 54.07 °C and the thermophilic phase (composting temperature was > 50 °C) were maintained for 9 days and 5 days, separately, which both complied with the harmless compost and standard sanitation requirements (Liu et al. , 2017. Typically, the average temperature of CM was 7.68% higher than CK, and the thermophilic phase of CM was also longer than CK. This was mainly owing to the coverage of the semipermeable membrane preventing the heat loss of compost (González et al. 2016), which is conducive to the reduction of pathogens, weed seeds, and other harmful substances (Ma et al. 2018a). During composting, the moisture content (MC) of CK and CM declined from 59.67% and 60.6% to 43.27% and 47.57%, respectively (as shown in Fig. 1b). Previous investigations have found that when MC is 50-60%, efficient composting can take place (Liang et al. 2003). The MC of CM was considerably higher than that of CK (P < 0.05). After composting began, the MC of CK declined rapidly and maintained steady, while in CM it rose marginally at the initial phase and gradually reduced. This was likely ascribed to the prompt decomposition of organic substances when composting began and a great quantity of metabolic water was generated. The water in CK evaporated promptly because of no semipermeable membrane covering. In CM group, water vapor condensed into droplets when it encountered the membrane during the upward ascension and fell back into the pile, resulting in no significant decrease in MC, which was in line with prior studies .
OM content evolution in the two treatments was similar (Fig. 1c). In CK, OM declined promptly during the initial 9 days and then remained stable. While in CM, the OM was constantly decreasing, possibly because of the high microbial activity of the treatment maintained by membrane coverage. The OM content of CK and CM declined by 14.77% and 18.33%, separately, indicating that membrane covering improved the efficiency of OM degradation in compost. This could be explained by the higher temperature in CM, where microorganisms were more active and organic matter decomposed more vigorously.
GI is considered as an essential bioindicator for assessing the fertility of composting products (Zucconi et al. 1981). In general, it is considered that a compost product is nontoxic and mature when it has a GI of 80% or more  (Riffaldi et al. 1988). The GI of CK and CM arrived at 86% and 93%, respectively (Fig. 1d), indicating that the composts of CK and CM were both phytotoxic-free and mature. It is noteworthy that the GI of CM reached more than 80% by day 36, while that of the CK achieved more than 80% by day 42, indicating that membrane covering accelerated the maturation of composting. As reported by Al-Alawi et al. (2019b), industrial-scale composting covered with semipermeable membrane produced stable and highly humified compost in 30 days, while traditional static composting takes 90-270 days. Figure 2a-c presents the evolutions of HS, HA, and FA compositions within these two treatments. HS content of both treatments decreased slightly in the heating phase and thermophilic phase (0-10 days) and then progressively rose and reached the highest value at the expiration of experiment. This trend was close to those observations in animal manure compost by Jiang et al. (2021). The decrease in HS was attributed to the degradation of destabilized components in HS (Wu et al. 2017a), and the increase in HS may be related to the addition of lower molecular weight organic substances (e.g., phenol, quinone) into the aromatic structure of HS or a significant growth in HA content (Wu et al. 2017a). The HS content of CM was lower than that of CK, probably as a result of the interconversion of HS fractions in composting. The HA concentration of CM and CK raised by 106.37% and 86.55%, respectively. Numerous researches have also demonstrated that HA concentration gradually tended to rise in composting ). The FA content decreased from 29.81 to 22.29 g/kg and 30.91 to 24.81 g/kg in CM and CK treatments, respectively. Notably, the FA in CK raised slightly in the heating stage of composting. Analogous fluctuations were ascribed to active microbial metabolism during the initial phase of composting affecting the instability of FA decomposition as reported by Wu et al. (2021). Microorganisms can utilize ready-made organic substances (including FA) as energy to produce more stable substances like HA . Therefore, the highest HA level and the lowest FA level in mature composting indicated the stability and maturity of mature composting. When composting finished, the CM had 9.28% higher HA content and 10.16% lower FA content than the CK. Covering semipermeable membrane significantly promoted the formation of HA and facilitated the breakdown of FA (P < 0.05).

Analysis of humic substance
Considering the limitation of HS, HA, and FA, many humification indicators were used to appraise the ripeness of compost products, including humification rate (HR), humification index (HI), degree of polymerization (DP), and percentage of humic acid (PHA) (Sánchez-Monedero et al. 1996). The variations of humification indicators are displayed in Fig. 2d-g. HR and HI represent the ratio of HS and HA to TOC, separately, which are often used to assess humification level in aerobic composting process (Mei et al. 2021). In this experiment, HR decreased first and then increased, reflecting the strong humification of compost with the decomposition of OM. HI gradually increased, which was an indication that HS structure became more complex . PHA and DP denote the ratio of HA to HS and FA, respectively. They can reflect the transition from simple molecules (FA) to complicated molecules (HA). PHA and DP of the two treatments showed a gradual upward trend, which reflected that the proportion of HA in mature compost increased. Compared with CK, DP, PHA, HI, and HR of CM increased by 21.57%, 13.29%, 14.02%, and 0.63%, respectively. The outcomes revealed that membrane covering was able to improve the compost humification degree and HS structure complexity.

Bacterial community diversity
Bacteria have a considerable contribution to compost humification degree and compost quality (Zhang et al. 2021b). Chao 1 and Shannon indices were separately used to calculate the abundance and diversity of bacterial communities, with higher values representing higher abundance and diversity. Changes in Chao 1 and Shannon indices showed a reduction in abundance and an increase in diversity of microorganisms in CM compared with CK ( Fig. 3a-b), which is in agreement with preceding research (Ma et al. 2018a). It may be because the membrane covering reduced the impact of the external environment on composting, and oxygen was more uniformly distributed beneath the coverage of membrane (Ma et al. 2018a).
The variance of the bacterial community of each sample during composting process was identified by applying principal coordinate analysis (PCoA) during composting. As illustrated in Fig. 3c, PC1 and PC2 contributed 64.33% and 21.82% of the gross variance, respectively. Thus, the two dimensions could reflect the actual situation of the samples. At the beginning of composting, the two points of CK and CM were almost coincident because they had the same raw materials. During the thermophilic phase, the distance between CK and CM was far, revealing that the bacterial communities were clearly distinct among the two treatments. Membrane covering greatly impacted bacterial community structure during thermophilic period. Cui et al. (2020) indicated that membrane covering had an effect on bacterial community structure during the maturation stage, not during the thermophilic stage. Ma et al. (2020) have proved that during membrane-covered aerobic composting, different ventilation methods substantially influenced bacterial diversity during the thermophilic phase. Analysis of similarities (ANOSIM) further unveiled that the composting phase was a major contributor to microbial community diversity, in similarity with the observations of Li et al. (2020). Composting stage can contribute significantly to the community variance matrix (R = 0.79, P value = 0.028), while the composting type had no significant impact on the community difference matrix (R = − 0.18, P value = 0.736) (see supplementary materials). Figure 4a compares the composition and relative abundance of bacteria at the phylum level in each main stage of composting. Actinobacteria, Proteobacteria, Firmicutes, Bacteroidetes, Chloroflexi, Gemmatimonadetes, and Desulfobacteria were the dominant bacteria during the whole composting process, accounting for 97.99% to 99.53% of the 16S rRNA gene sequences of all samples. These major classification phyla also existed in other composts . When composting started, the percentage of Actinomycetes was the highest. Actinobacteria was able to secrete antibiotics to suppress and eliminate pathogenic microorganisms (Partanen et al. 2010;Tian et al. 2013). The percentage of Actinomycetes in CM on the 3rd day of composting was 17.04% lower than CK. This may be because the relative abundance of other bacteria increased under membrane covering, resulting in a decrease in the percentage of Actinomycetes. Proteobacteria varied similarly in CK and CM, and the relative abundance increased from 6.54% and 5.72% to 37.12% and 32.43%, respectively. Previous research has shown that Proteobacteria contributed a lot to the compost carbon-nitrogen cycle, and the enhancement in its abundance was conducive to reducing the biotoxicity of compost products (Bello et al. 2020). Firmicutes widely existed in high temperature and extreme environments and were considered to be instrumental in lignocellulose decomposition . The percentage of Firmicutes in CK and CM displayed remarkable distinction (P < 0.05) during thermophilic period, which was 11.30% in CK, but reached 47.01% in CM, indicating that membrane covering strengthened the transformation process of lignocellulose to HS (Qin et al. 2021). The percentage of Bacteroidetes in CK and CM dropped from 7.98% and 13.42% to 5.06% and 12.15%, respectively. Previous studies found that Bacteroidetes can break down lignocellulose into short-chain fatty acids (Zhong et al. 2018). Collectively, membrane covering changed the bacterial community of composting, especially during high-temperature periods. Figure 4b displays the relative abundance of bacteria at genus level of each of the samples. Saccharomonospora, Corynebacterium, Halomonas, Bacillus, and f-Fodinicurvataceae were the dominant genera in the whole composting process. Saccharomonospora belongs to Actinobacteria, which can survive in thermal environments and transform lignin and cellulose into HS . It can also produce antibiotic compounds which are beneficial for diminishing the pathogenic load (Topp et al. 2016). Therefore, at a maturing stage, Saccharomonospora became the dominant genus, as formerly observed by Li et al. (2021). The percentage of Saccharomonospora in CM was 4.32% lower than that of CK. Corynebacterium is thought to be widespread in raw cattle manure and is a causative agent of various diseases (Nasim et al. 2021). The percentage of Corynebacterium declined significantly in the late stage of composting. The same results were also observed in the study by Jiang et al. (2019). This revealed that pathogenic microorganisms in livestock manure can be eliminated by composting . Halomonas is considered to be the vital microorganism in charge of nitrification and denitrification in composting, which can reduce NO 2 and NO 3 to N 2 under aerobic circumstances (Zainudin et al., 2020). In the thermophilic stage, the percentage of Halomonas in CK was higher than CM, indicating that the nitrification/denitrification might be more intense in CK. Bacillus can degrade cellulose, hemicellulose, and lignin at about 50 °C to provide material and energy for growth and metabolism ). In the thermophilic stage, the proportion of Bacillus of CM was 31.69%, which was 20.58 times higher than that of CK. Membrane covering increased the percentage of Bacillus during the thermophilic phase remarkably. Except for Bacillus, the percentage of several other dominant genera in CM was lower than that of CK, which was mainly attributed to the large proportion of Bacillus in CM. The change of bacterial community composition in the maturing stage was not as intensive as that in the control treatment. This trend was formerly monitored in other composting piles constructed under comparable circumstances . The possible reason for this was that membrane covering reduced the extraneous environment on microbial composition.

Predicted potential functions of bacterial community
Based on the KGEE database, PICRUST was used to predict the functions of bacterial compartments in compost samples. As shown in Fig. 5a, most of the predicted protein sequences in the eight samples were annotated using the KEGG pathway, which can be classified into six functional groups, including metabolism (54.14-54.66%), genetic information processing (14.47-18.62%), environmental information processing (13.48-15.44%), cellular processes (2.35-3%), organismal systems (0.8-0.9%), and human diseases (0.82-0.9%). Among the pathways counted for KEGG abundance, there are 12 pathways for metabolism, 4 pathways for genetic information processing, 3 pathways for environmental information processing, and 4 pathways for cellular processes (Fig. 5b). The percentage of genes for membrane transport was the highest, reaching 11.74-13.53%. This was followed by amino acid metabolism and carbohydrate metabolism, which reached 11.3-11.69% and 10.22-11.2%, respectively. Amino acids are a source of carbon and energy for bacterial growth and metabolism, and their presence enhances microbial activity (Bello et al. 2020). The percentage of genes for amino acid metabolism reached its highest level during the cooldown period and declined slightly when composting finished. Amino acids are precursors for the synthesis of HS ). According to Wu et al. (2017b), the higher the abundance of amino acid metabolizing bacteria, the more HS was produced during composting. A same trend was also observed for sequences related to carbohydrate metabolism, which was primarily the result of the intense decomposition of carbohydrates and amino acids in the thermophilic period of composting ). In the maturation process, the carbohydrate metabolism of CM was lower than that of CK, demonstrating that the carbohydrate metabolism of CM became weaker and the composting system was more stable at the end. The gene abundance of enzyme families, nucleotide metabolism and glycan biosynthesis, and metabolism in CM was higher than that in CK, indicating that membrane covering can promote the metabolism of enzymes and accelerate the decomposition of complex organic matter and the biosynthesis of polysaccharides in composting. Furthermore, the abundance of xenobiotics biodegradation and metabolism and terpenoids and polyketone metabolism increased as composting proceeded, which suggested that in the later stages of composting, organic matter with complex structure was slowly degraded and more complex substances such as lignin-humus complexes were synthesized (Zhong et al. 2020).

Redundancy analysis and correlation heatmaps analysis of bacterial community
RDA method was adopted for the detection of the dynamic correlation between physicochemical indexes, HS, and its components and bacterial communities in composting (Fig. 6). RDA1 and RDA2 accounted for 60.81% and 25.76% of the variation, respectively. In the thermophilic phase (3 days) of composting, the RDA values of these two groups were different. The temperature was significantly and positively correlated with CM, indicating that temperature had a huge effect on bacterial diversity in membrane covering treatment. OM (P = 0.002), MC (P = 0.008), and HA (P = 0.046) were notably associated with the dominant bacteria (the six genera with the highest relative abundance during composting). Saccharomonospora, f_Fodinicurvataceae, f_JG30-KF-CM45, and Halomonas showed a remarkably negative correlation with OM and FA and a positive association with HA, which suggested that these genera were likely Fig. 6 Redundancy of the correlation between bacterial community, HS fractions, and environmental factors to participate in carbon compounds decomposition and HA formation. Additionally, these bacteria had a significant negative relationship with MC, indicating that they are able to adapt to low moisture environments. Bacillus and Halomonas positively correlated with temperature, indicating that they can resist the thermophilic environment of compost, which has been confirmed in earlier investigations (Zhang et al. 2021a). In addition, RDA also revealed the correlation among physicochemical parameters. Changes in HS and HA were negatively associated with temperature and MC and positively associated with GI, indicating that the formation of HS and HA predominantly occurred during the mature stage of composting. The significant correlation between environmental factors (MC and temperature) and HS and its fractions may explain the HA content in CM was remarkably higher than CK. The better water retention of CM promoted the dissolution of FA and facilitated the utilization of FA by microorganisms. Meanwhile, the thermophilic period of CM was longer. According to Yu et al. (2019), the conversion of FA to HA can be completed only when exposed to high temperature for a long time. In addition, the change of physicochemical properties in CM increased the relative abundance of Bacillus and the gene abundance of enzyme families. This may promote the degradation of lignocellulose, which plays a crucial role in the formation of HA.
The correlation between HS formation and the top 10 dominant genera during composting was investigated through Spearman correlation analysis (Fig. 7). The top 10 dominant genera in CM were more correlated with HS conversion in comparison to CK. In CM, Saccharomonospora and f_Fodinicurvataceae showed significant positive correlations with HS and HR. It is known that Saccharomonospora is a dominant genus in mature composting (Wang et al. 2022). Meanwhile, Planktosalinus showed significant positive correlations with HS and HR in CM. While in CK treatment, only f_JG30-KF-CM45 and Streptomyces showed significant positive correlations with HA, but not with HS. After covering with a membrane, the promotion of HA formation by Saccharomonospora was significantly enhanced. Similarly, the membrane covering promoted the increase of compost humification rate by Planktosalinus. The above results demonstrated that membrane covering will alter the bacterial community structure among composting, which in turn will affect the formation of HS and HA differently. To summarize, the membrane covering promoted the formation and structure complexity of HS by activating dominant bacteria to take part in HA formation in composting.

Conclusion
This study indicated that semipermeable membrane covering promoted the transformation of HS to HA (9.28%) and affected the bacterial community structure. Bacterial abundance was reduced and diversity was increased in CM compared to the control. Membrane covering mainly influenced bacterial community structure in the thermophilic phase of composting. The percentage of Firmicutes showed a significant increase in thermophilic period. Membrane covering significantly reduced microbial metabolism during the cooling phase of composting. RDA analysis showed that the bacterial community Fig. 7 The correlation between HS formation and top 10 dominant genera during composting. Significance level: ***P < 0.001 structure was affected by environmental factors in the compost. Moisture content was the main environmental factor leading to microbial community succession. Saccharomonospora, norank_Fodinicurvataceae, norank_ JG30-KF-CM45, and Halomonas played important roles in HA production. Correlation analysis showed that Saccharomonospora, norank_Fodinicurvataceae, and Planktosalinus had a significant positive correlation with HS transformation in CM. Overall, semipermeable membrane covering can increase composting temperature, shorten composting cycle, and enhance humification degree. The membrane covering might promote the production of HS and its structural complexity by activating the dominant bacterial genus in composting to participate in the formation of HA. Future research will focus on the combination of the semipermeable membrane covering and other technologies like microbial inoculation to improve composting humification degree.