Unveiling the Mechanism Underlying the Effects of Ammonia on n-Caproate Production: Influenced Pathways, Key Enzymes, and Microbiota Functions

•2 g/L ammonia was the optimal dose and inhibitory threshold for MCFAs production.•FNA and NH4+ were confirmed to be involved in the inhibitory effect.•5 g/L ammonia could induce the process of excessive ethanol oxidation.•Ammonia shifted the dominant CE functional bacteria.•Functions of RBO, FAS, K+ transport and ATPase metabolism were inhibited by high ammonia.