## 3 Results
### Potential Function Prediction and Metabolic Pathways of the Microbial Communities in S- and N-NXDQ  
Based on commercial databases of KEGG and MetaCyc [26], the metabolic pathway enrichment was analyzed. A total of three hundred and ninety-nine bacterial metabolic pathways and seventy-four fungal metabolic pathways were obtained in the context of MetaCyc analysis. It can be observed from Figure 8a, that bacterial communities mainly presented six metabolic pathways, including metabolic clusters, macromolecule modification, degradation/utilization/assimilation, biosynthesis, cell structure biosynthesis, and generation of precursor metabolites and energy, while fungal community mainly detected four pathways, including metabolic clusters, degradation/utilization/assimilation, biosynthesis, and generation of precursor metabolites and energy (Figure 8b). Furthermore, the metabolic pathways in two grades of NXDQ mainly focused on the bacterial biosynthesis and on the fungal biosynthesis and generation of precursor metabolites and energy. Compared with the bacterial community, the fungal community had a higher abundance of dominant metabolic pathways, especially in carbohydrate degradation, amino acid degradation, fatty acid and lipid biosynthesis and fermentation. However, the differences of the bacterial metabolic pathways between S- and N-NXDQs were more obvious.  
According to the volatile compounds taxa, the metabolic pathways in NXDQ were classified into nine categories: starch saccharification, glycolysis, phenylalanine metabolism, butanoate metabolism, alcohol fermentation, fatty acid biosynthesis, esterification, TCA cycle, protein and amino acid metabolism (Figure 9b). To further reveal the difference of metabolic pathway enrichment, the enzymes metabolized by bacterial communities were carried out through KEGG analysis. A total of two thousand and eighty-three enzymes were detected, and the forty-three enzymes that played a key role in metabolic pathways were selected to analyze their difference between S- and N-NXDQ. As shown in Figure 9a, the abundance of enzymes in S-NXDQ, including Cyclomaltodextrin glucanotransferase (EC2.4.1.19), Cyclomaltodextinase (EC3.2.1.54), Aryl-alcohol dehydrogenase (EC1.1.1.90), Lactate dehydrogenase (EC1.1.1.27), Acetolactone decarboxylase (EC4.1.1.5), Acetate-CoA ligase (EC6.2.1.13) and Acetaldehyde dehydrogenase (EC1.2.1.10), were higher than that in N-NXDQ. These enzymes were mainly involved in the first step of starch degradation, conversion of phenylethyl alcohol and phenylacetaldehyde, lactic acid and acetoin synthesis, acetyl-CoA and acetaldehyde synthesis. However, the abundances of 16 enzymes in N-NXDQ were higher than that in S-NXDQ. Sixteen enzymes mainly included some amylases, phenylalanine and phenylacetaldehyde forming enzymes, pyruvate formation and degradation enzymes, long-chain fatty acid synthase, tryptophan and other amino acid synthase.  
Figure 8: Abundance of microbial metabolic pathways in two grades of NXDQ. (**a**) Bacterial metabolic pathways. (**b**) Fungal metabolic pathways.  
Figure 9: Potential functions of the enzyme-encoded bacterial community in different grades of NXDQ. (**a**) Profiles of the key enzymes related to the dominant metabolic pathway based on 16S rRNA gene sequences. (**b**) Predominant metabolic pathway based on dominant enzyme-encoded bacterial community.