This study investigated whether bio

This study investigated whether biogas reactor performance, including microbial community development, in response to a change in substrate composition is influenced by initial inoculum source. For the study, reactors previously operated with the same grass–manure mixture for more than 120 days and started with two different inocula were used. These reactors initially showedgreatdifferencesdependingoninoculumsource,buteventuallyshowedsimilarperformance and overall microbial community structure. At the start of the present experiment, the substrate was complemented with milled feed wheat, added all at once or divided into two portions. The starting hypothesis was that process performance depends on initial inoculum source and microbial diversity, and thus that reactor performance is influenced by the feeding regime. In response to the substrate change, all reactors showed increases and decreases in volumetric and specific methane production, respectively. However, specific methane yield and development of the microbial community showed differences related to the initial inoculum source, confirming the hypothesis. However, the different feeding regimes had only minor effects on process performance and overall community structure, but still induced differences in the cellulose-degrading community and in cellulose degradation.Keywords: anaerobic digestion; co-digestion; continuous stirred-tank reactor (CSTR); bio-methane potential (BMP)-test; next-generation amplicon sequencing; terminal restriction fragment length polymorphism (T-RFLP); qPCR; glycoside hydrolase families 5 and 48