## 3 Results and Discussion
### Discussion
Enrichment analysis of KEGG pathways between the first stage and second stage in the secondary fermentation method of Shanghai Lanzhong and the tertiary fermentation method of Jiangsu Yuguan shows that the two methods have the same pathway of significant or extremely significant enrichment: synthesis and degradation of ketone bodies. In addition, the pathways of significant or extremely significant enrichment, including differential metabolites between the two stages of the secondary fermentation model of Shanghai Lanzhong, are also amino acid metabolic pathways, such as arginine, alanine, aspartate and glutamate metabolism; phenylalanine, tyrosine and tryptophan synthesis and metabolism; and vitamin B5 (pantothenate) and CoA biosynthesis, in addition to the pathway of synthesis and degradation of ketone bodies. Enrichment in metabolites along the two stages of the secondary fermentation method was higher than that of the Jiangsu Yuguan tertiary fermentation method. The enriched pathways that were significant or extremely significant in phase II and phase III of the Jiangsu Yuguan tertiary fermentation method were the synthesis and degradation of ketone bodies, which was the same as that in phase I and phase II of the Shanghai Lanzhong fermentation method. The enriched pathways that were significant or extremely significant were also vitamin B6 metabolism and ABC transporters in phase II and phase III of the Jiangsu Yuguan tertiary fermentation method. In both fermentation methods, the significantly enriched pathways, including differential metabolites between the NT stage and phase II were not only the synthesis and degradation of ketone bodies and amino acid metabolism, but also vitamin B6 metabolism, biotin metabolism, and pantothenic and CoA biosynthesis. Bidin is one of the few necessary sulfur cofactors involved in carbon dioxide transfer in carboxylation, decarboxylation and transcarboxylation in all organisms according to [PERSON] et al. [28; 29; 30]. The essential water-soluble vitamin B6 consists of six different vitamins, the alcohol pyridoxine (PN), theamine pyridoxamine (PM), thelealdehyde pyridoxal (PL) and their respective 5\({}^{\prime}\) phosphorylated forms (PNP, PMP, PLP) [31], in which the enzyme cofactor PLP is necessary for more than 140 enzyme reactions [32]. The main metabolic pathways for vitamin B6 to exert key coenzyme activities include (1) synthesis and catabolism of amino acids [33; 34] and (2) conversion of amino acids to bioactive amines [35; 36]. In the pathway of significant enrichment between the N1F and TS stages of the secondary fermentation method in Shanghai Lanzhong, differential metabolites with higher contents in the TS stage compared to those in the N1F stage were (R)-3-hydroxybutyric acid (\(p<\) 0. 01), L-aspartic acid (\(p<\) 0.01) and indole (\(p<\) 0.01). The results showed that the compost participated in carbon metabolism and amino acid metabolism in the process of fermentation, which accumulated carbon and nitrogen sources for the growth of _P. ostreatus_. In the secondary fermentation method, the content of L-aspartic acid in the TS stage was more than 10 times higher than that in the N1F stage. Therefore, it is speculated that L-aspartic acid can be used as a potential marker to identify the fermentation quality of composts, but we think the marker for the evaluation of the fermentation quality needs to be further verified. The results may suggest that with the fermentation of the culture substrate, carbon and nitrogen metabolites accumulated, which provided nutrients for the cultivation of _P. _ostreatus_. The content of 5-aminolevulinic acid was relatively low in phase I and phase II of the tertiary fermentation process but was extremely significantly increased in phase III (\(p<0.01\)). It is speculated that 5-aminolevulinic acid can be used as a potential marker to measure the fermentation quality of composts under the tertiary fermentation method, but we think this compound needs to be further verified as a marker to evaluate fermentation quality.