Compost production is a critical component of organic waste management. One of the most important properties of compost is its ability to suppress soil-borne pathogens such as Phytophthora nicotianae in pepper plants. Both the physico-chemical and biological properties of composts can be responsible for the suppression of pathogens, although biological properties are the main driver. In this study, we analyzed composts with various levels of suppressiveness against P. nicotianae. We analyzed both physico-chemical properties like pH and electrical conductivity and biological properties like microbial activity, amplicon sequencing and metaproteomics. We believed that the link between community structures and proteins could provide deep insights into the mechanism of compost suppressiveness. Our results indicate that there are differences between suppressive and non-suppressive composts at the phylogenetic level (sequencing) and at the functional level (based on analysis of the cluster of orthologous groups, COGs). The proteins identified were assigned to the carbohydrate process, cell wall structure and inorganic ion transport and metabolism. Proteobacteria could also be new indicators of P. nicotianae suppression.