Process performance and microbial community dynamics in semi-dry mesophilic co-digestion of crop residues and livestock manure.

Semi-dry anaerobic co-digestion represents a promising strategy for efficient resource utilization of diverse agricultural wastes; however, its operational performance under mesophilic conditions and the underlying microbial mechanisms remain insufficiently understood. In this study, a 35-day semi-dry anaerobic co-digestion experiment was conducted using corn stover, wheat straw, cattle manure, and chicken manure as co-substrates, under 12% total solids and 37 °C, to elucidate biogas product dynamics and microbial community succession. The methane production exhibited a distinct biphasic pattern, marked by an initial decline, a prolonged lag phase, and a single, sharp peak that culminated between days 21 and 23. The peak methanogenic potential estimated using the modified Gompertz model reached 137 mL·g-1 VS, indicating incomplete substrate degradation within 35 days. Acetic acid, total volatile fatty acids, and chemical oxygen demand all reached their maximum values on day 15, reflecting intensive hydrolysis and acidogenesis during the first 15 days. In contrast, the lowest pH value was observed on day 15, while ammonia nitrogen concentration fluctuates between 3.5 and 4.0 mg/L. Microbial diversity analysis showed higher bacterial diversity on day 20, and increased archaeal diversity on day 30. The dominant bacterial phyla included Firmicutes, Bacteroidota, Proteobacteria, Actinobacteriota and Fusobacteriota with Caldicoprobacter, Fastidiosipila, Tissierella, and Alkaliphilus identified as the most abundant genera, playing key roles in organic matter degradation and acetate metabolism. Among archaea, Crenarchaeota was the predominant phylum, followed by Euryarchaeota and Korarchaeota. In summary, the semi-dry co-digestion system of the four agricultural wastes was characterized by the dominance of hydrogenotrophic methanogens (Methanobrevibacter) coupled with the syntrophic acetate oxidation pathway under conditions of high ammonia nitrogen. These findings provide mechanistic insights into microbial regulation of semi-dry multi-substrate anaerobic digestion and support its potential application in agricultural waste management.