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The aim of this study was to revealed the core fungal mechanisms for the transformation of carbon (C) and nitrogen (N) during co-composting of spent mushroom substrate(SMS) and corn steep liquor (CSL) process. There were two treatments: CK (fully SMS) and CP (CK + 0.5% CSL). The results indicated that the core fungi, fungal community structure and fungal diversity were the main driving forces for the transformation of C and N during composting. The rich nutrition in CSL changed fungal community structure, increased the abundance and diversity of fungi in the early stage, but decreased in the late stage of composting, promoted the conversion and conservation of C and N in composting. Core fungus in the C and N transformation were identified. The compensating effects of core fungi were encouraged in CP composting. The influence of core fungus on the C and N conversion could be divided into two pathways in CP, One was that core fungi promoted organic C and N degradation by increasing the abundance and diversity of fungi; while the other was promoted the the synthesis of organic C and N by the complementarity effects of core fungal, contribute to the C and N conservation in compost. These findings showed that the addition of CSL to composting was a promising application to treat SMS with lower C/N ratio.

Background Carbon and nitrogen are essential energy and nutrient substances in the composting process. Corn steep liquor (CSL) is rich in soluble carbon and nitrogen nutrients and active substances and is widely used in the biological industry. Nonetheless, limited research has been done on the effect of CSL on composting. This work firstly reveals the effect of adding CSL to bacterial community composition and carbon and nitrogen conversion during composting. This study provides the choice of auxiliary materials for the spent mushroom substrate compost (SMS) and some novel knowledge about the effect of bacterial community on C and N cycling during composting of SMS and CSL. Two treatments were set up in the experiment: 100% spent mushroom substrate (SMS) as CK and SMS + 0.5% CSL (v/v) as CP. Results The results showed that the addition of CSL enhanced the initial carbon and nitrogen content of the compost, altered the bacterial community structure, and increased the bacterial diversity and relative abundance, which might be beneficial to the conversion and retention of carbon and nitrogen in the composting process. In this paper, network analysis was used to screen the core bacteria involved in carbon and nitrogen conversion. In the CP network, the core bacteria were divided into two categories, synthesizing and degrading bacteria, and there were more synthesizing bacteria than degrading bacteria, so the degradation and synthesis of organic matter were carried out simultaneously, while only degrading bacteria were found in the CK network. Functional prediction by Faprotax identified 53 groups of functional bacteria, among which 20 (76.68% abundance) and 14 (13.15% abundance) groups of functional bacteria were related to carbon and nitrogen conversion, respectively. Adding CSL stimulated the compensatory effect of core and functional bacteria, enhanced the carbon and nitrogen transformation ability, stimulated the activity of low-abundance bacteria, and reduced the competitive relationship between the bacterial groups. This may be why the addition of CSL accelerated the organic matter degradation and increased carbon and nitrogen preservation. Conclusions These findings indicate that the addition of CSL promoted the cycling and preservation of carbon and nitrogen in the SMS composts, and the addition of CSL to the compost may be an effective way to dispose of agricultural waste.