## 1 Introduction  
_Agaricus bisporus_ is a large fungus belonging to the families _Mycota_, _Eumycota_, _Basidiumocytina_, _Eubasiotinycetes_, _Agariales_, _Agriaceae and Agaricus_. It is an edible fungus that is most widely cultivated, with the highest yield and largest consumption worldwide. As a key food source, over 2/3 of edible mushrooms worldwide are grown in mainland China. The mushroom industry in mainland China plays a more important role in food and medicine than in many Western countries, and it provides jobs for more than 25 million farmers [1]. The most important genus of edible fungi commercially produced in China is mushrooms (mainly _Agaricus bisporus_), accounting for approximately 30% of the global market [2]. The nutrients needed for the growth and development of _A. bisporus_ mainly originate from composts, in which the components are mainly in four categories: carbon sources, nitrogen sources, mineral elements and auxins. Therefore, the fermentation quality of composts is closely related to the above four kinds of substances, and it is also one of the key factors affecting the yield of _A. bisporus_. A suitable total nitrogen content and C/N ratio are required for high-quality composts [3, 4]. The humidity, total nitrogen content, pH and C/N ratio of the culture material should be in the ranges of 68-72%, 2.0-2.4%, 7.4-7.6, and 17-20:1, respectively [5, 6, 7]. In the second stage of producing edible fungi, where millet straw is used as compost, the physical and chemical indexes are the decisive factors in determining how much edible fungi will be yielded [8], and they need to be in certain ranges to obtain the highest yield. The fermentation of composts of _A. bisporus_ is a process of decomposition and transformation of various chemical substrates by the action of microorganisms. The research mainly focused on the composition of composts [8; 9; 10; 11; 12], diversity and dynamics and functional characteristics of the microbial community [13; 14], the application of spent mushroom substrate [15; 16], the type of culture medium and supplement and the amount of addition [17]. In the past, studies on the composts of _A. bisporus_ have mainly focused on nitrogen content and the C/N ratio and have not reported on metabolites during the fermentation process of composts. At present, the culture process of _A. bisporus_ in China mainly adopts the secondary fermentation method and the tertiary fermentation method. However, due to the high technical requirements of the secondary fermentation method, it tends to cause low yield and quality fluctuations or even result in fermentation failure [18], but these shortcomings can be addressed by the tertiary fermentation method. However, the difference in metabolites in the fermentation process between these two methods has not been researched thus far. Metabonomics is a science for the qualitative and quantitative analysis of all metabolites in life [19]. This technique is a new discipline that developed rapidly after genomics, transcriptomics and proteomics [20]. It has been reported that the application of the technology to the field of edible fungi has mainly covered the changes in volatile compounds during the ripening process of _Volvariella volvacea_ and _Pleurotus ostreatus_[21], metabolic markers of damage to _A. bisporus_[22], and changes in extracellular metabolites in _P. ostreatus_ mycelia under high temperature [23]. Therefore, it seems that the metabonomics technique can provide a way to identify metabolites [24] and study the differences in metabolites in composts under different fermentation methods. In this study, metabonomic technology was used to study the changes in metabolites based on gas chromatography-mass spectrometry (GC-MS) in the fermentation process of the secondary fermentation method of Shanghai Lianzhong and the tertiary fermentation method of Jiangsu Yuguan.