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An anaerobic digestion experiment was investigated to evaluate the impact of increasing amounts of ammonium nitrogen due to poultry manure addition on the reactor performance, especially on the microbiome response. The microbial community structure was assessed by using a 16S rRNA gene approach, which was further correlated with the prevalent environmental conditions by using statistical analyses. The addition of 50% poultry manure led to a process disturbance indicated by a high VFA content (almost 10 g HAc-Eq L −1 ) in combination with elevated concentrations of ammonium nitrogen (5.9 g NH 4 + -N kg FM −1 ) and free ammonia (0.5 g NH 3 kg FM −1 ). Simultaneously the microbiome, changed from a Bacteroidetes -dominated to a Clostridiales -dominated community accompanied by a shift from the acetoclastic to the hydrogenotrophic pathway. The “new” microbial community was functional redundant as the overall process rates were similar to the former one. A further increase of poultry manure resulted in a complete process failure. [ABSTRACT FROM AUTHOR]