Volatile Fatty Acid Production from Chicken Manure under Acidic and Alkaline Anaerobic Fermentation: Ammonia Inhibition and Microbial Mechanisms

Abstract

This study investigated the effect of initial pH on volatile fatty acid (VFAs) production during anaerobic fermentation of chicken manure and further elucidated the mechanisms of ammonia inhibition and microbial community dynamics. The results showed that weakly acidic to neutral conditions (pH=5 and 7) were most favorable for VFA accumulation, reaching a maximum of 16264.2 mg COD/L, while strongly acidic (pH=3) or strongly alkaline (pH=11) conditions inhibited VFA production. Under optimal pH conditions, the activation of acidogenic bacteria and the inhibition of methanogens by high ammonia nitrogen concentrations promoted VFA accumulation. Microbial community analysis revealed that Firmicutes and Bacteroidota were the dominant phyla, with Romboutsia and Lactobacillus identified as key genera under acidic conditions, while Bacillus and Bacteroides played crucial roles under neutral conditions. Metagenomic prediction further indicated that carbohydrate and amino acid metabolism were the dominant metabolic pathways, with key functional enzymes including pyruvate ferredoxin oxidoreductase [EC:1.2.7.3], acetyl-CoA carboxylase [EC:6.4.1.2], and phosphoglycerate mutase [EC:5.4.2.12]. This study elucidates the pH-dependent mechanisms of VFA production and identifies the key microbial taxa and enzymatic processes that regulate acidogenic fermentation.