Introduction With an annual production of edible fungi exceeding 13  million tons, China is the largest producer of edible fungi  in the world. It accounts for about 75% of the world’s  total edible fungi production [1]. Nonetheless, large  amounts of SMS are not effectively used and are freely  discarded, causing environmental and groundwater pollution. Soil application is an ideal method for sustainable utilization of SMS, owing to its low bulk density and  well-developed porosity and containing a large number  of nutrients (inorganic nitrogen, phosphorus, potassium, etc.) and organic matter [2]. SMS can slowly and  continuously provide nutrients for crops after application in the soil, promote the formation of soil aggregates,  improve the physicochemical properties of the soil, promote soil nutrient activation, and improve the activity  of aerobic microorganisms [3]. The SMS contains many  hard-to-degrade compounds, such as lignocellulose, fat,  wax, tannin, etc., and some phytopathogenic bacteria,  all of which can produce phytotoxic compounds after  degradation, including polyphenols, organic acids, quinones, aldehydes, alcohols, and so on. Therefore, fresh  SMS should not be applied directly and must be composted before being used as a fertilizer or as a medium  for the cultivation of seedlings [4]. In light of the complex physical and chemical composition of lignocellulose,  it degrades slowly, and the high concentration of lignocellulose in SMS frequently results in long composting  times, low fermentation temperatures, and incomplete  decomposition [2]. Adding some exogenous substances  such as inoculating microorganisms [5], bulking agents  (biochar, diatomite, etc.) [6, 7], oxidants (MnO2, etc.) [8]  and molasses amendment [9] can accelerate the aerobic  composting process and improve the quality of compost,  which is an efficient method for treating SMS. Composting is primarily driven by microorganisms, primarily bacterial communities, which decompose organic matter and form new humic compounds  through secondary metabolism [10–12]. The impact of  microorganisms on composting depends on the structure and functional capacity of the microbial community [11, 13]. Microbial community succession has a  direct influence on the degradation process in the composting process, which is also the key factor to promote  the degradation of organic matter [14, 15]. Moreover,  carbon and nitrogen conversion is the most important  biochemical process in composting [16]. Sugars, cellulose, and lignin can be hydrolyzed by microorganisms  secreting hydrolytic enzymes under good aeration conditions, and the end products are CO2, H2O, and inorganic salts. While under anaerobic conditions, organic  acids and reducing gases (CH4, H2, etc.) are formed.  Under the action of microorganisms, nitrogenous  organic compounds degrade to produce amino acids,  polyphenols, peptides, organic acids, alcohols, and  other intermediates and inorganic nitrogen (NH4 ,  NO3 −) by hydrolysis, ammonification, nitrification, and  denitrification. By oxidizing polyphenols to quinones  by microbial oxidase, these intermediates serve as precursors for humic acid synthesis, and then humic acid  is produced by condensing the quinones with amino  acids and peptides, a stable compound able to preserve  carbon and nitrogen in compost [12]. During the composting process, releasing CO2 and CH4 gases causes  carbon nutrient losses; nitrification, denitrification,  and volatilization of ammonium lead to nitrogen losses.  Severe carbon and nitrogen losses significantly affect  microbial activity and reduce the conversion rate of  organic matter and compost product quality. Furthermore, the released greenhouse gases (N2O, CO2, and  CH4, etc.), in turn, cause pollution to the environment  [17]. Therefore, in order to accelerate composting,  enhance compost quality, and protect the environment,  it is essential to study carbon and nitrogen cycles and  reduce greenhouse gas emissions. Corn steep liquor (CSL), a by-product in the industrial wet processing of corn starch, is a dark brown  liquid after concentration [18]. In addition to soluble sugars, soluble proteins, organic acids, vitamins,  enzymes, and inorganic ions, CSL is an excellent source  of nitrogen and carbon for microbial fermentation [19].  Moreover, CSL is widely utilized in many fields due to  its high yield, low price, and abundant supply [18]. For  example, it is used for microbial culture medium [20],  degradation of organic pollutants [21], a raw material  for biological agents [22], and plant disease control  [23]. However, the application of CSL in composting  has not been reported. Our previous study demonstrated that adding 0.5% 

Introduction With an annual production of edible fungi exceeding 13  million tons, China is the largest producer of edible fungi  in the world. It accounts for about 75% of the world’s  total edible fungi production [1]. Nonetheless, large  amounts of SMS are not effectively used and are freely  discarded, causing environmental and groundwater pollution. Soil application is an ideal method for sustainable utilization of SMS, owing to its low bulk density and  well-developed porosity and containing a large number  of nutrients (inorganic nitrogen, phosphorus, potassium, etc.) and organic matter [2]. SMS can slowly and  continuously provide nutrients for crops after application in the soil, promote the formation of soil aggregates,  improve the physicochemical properties of the soil, promote soil nutrient activation, and improve the activity  of aerobic microorganisms [3]. The SMS contains many  hard-to-degrade compounds, such as lignocellulose, fat,  wax, tannin, etc., and some phytopathogenic bacteria,  all of which can produce phytotoxic compounds after  degradation, including polyphenols, organic acids, quinones, aldehydes, alcohols, and so on. Therefore, fresh  SMS should not be applied directly and must be composted before being used as a fertilizer or as a medium  for the cultivation of seedlings [4]. In light of the complex physical and chemical composition of lignocellulose,  it degrades slowly, and the high concentration of lignocellulose in SMS frequently results in long composting  times, low fermentation temperatures, and incomplete  decomposition [2]. Adding some exogenous substances  such as inoculating microorganisms [5], bulking agents  (biochar, diatomite, etc.) [6, 7], oxidants (MnO2, etc.) [8]  and molasses amendment [9] can accelerate the aerobic  composting process and improve the quality of compost,  which is an efficient method for treating SMS. Composting is primarily driven by microorganisms, primarily bacterial communities, which decompose organic matter and form new humic compounds  through secondary metabolism [10–12]. The impact of  microorganisms on composting depends on the structure and functional capacity of the microbial community [11, 13]. Microbial community succession has a  direct influence on the degradation process in the composting process, which is also the key factor to promote  the degradation of organic matter [14, 15]. Moreover,  carbon and nitrogen conversion is the most important  biochemical process in composting [16]. Sugars, cellulose, and lignin can be hydrolyzed by microorganisms  secreting hydrolytic enzymes under good aeration conditions, and the end products are CO2, H2O, and inorganic salts. While under anaerobic conditions, organic  acids and reducing gases (CH4, H2, etc.) are formed.  Under the action of microorganisms, nitrogenous  organic compounds degrade to produce amino acids,  polyphenols, peptides, organic acids, alcohols, and  other intermediates and inorganic nitrogen (NH4 ,  NO3 −) by hydrolysis, ammonification, nitrification, and  denitrification. By oxidizing polyphenols to quinones  by microbial oxidase, these intermediates serve as precursors for humic acid synthesis, and then humic acid  is produced by condensing the quinones with amino  acids and peptides, a stable compound able to preserve  carbon and nitrogen in compost [12]. During the composting process, releasing CO2 and CH4 gases causes  carbon nutrient losses; nitrification, denitrification,  and volatilization of ammonium lead to nitrogen losses.  Severe carbon and nitrogen losses significantly affect  microbial activity and reduce the conversion rate of  organic matter and compost product quality. Furthermore, the released greenhouse gases (N2O, CO2, and  CH4, etc.), in turn, cause pollution to the environment  [17]. Therefore, in order to accelerate composting,  enhance compost quality, and protect the environment,  it is essential to study carbon and nitrogen cycles and  reduce greenhouse gas emissions. Corn steep liquor (CSL), a by-product in the industrial wet processing of corn starch, is a dark brown  liquid after concentration [18]. In addition to soluble sugars, soluble proteins, organic acids, vitamins,  enzymes, and inorganic ions, CSL is an excellent source  of nitrogen and carbon for microbial fermentation [19].  Moreover, CSL is widely utilized in many fields due to  its high yield, low price, and abundant supply [18]. For  example, it is used for microbial culture medium [20],  degradation of organic pollutants [21], a raw material  for biological agents [22], and plant disease control  [23]. However, the application of CSL in composting  has not been reported. Our previous study demonstrated that adding 0.5%