Ruminant versus non-ruminant manure composting: carbon-to-nitrogen ratio and associated physicochemical traits impact divergent pollutant emissions.

Composting can convert livestock manure into valuable fertilizer, but the relationships between manure type, which essentially reflects differences in carbon-to-nitrogen ratio (C/N) and other associated physical properties, and pollutant gas emissions remain poorly quantified. We monitored temperature, pH, carbon, nitrogen, sulphur contents, bacterial succession, and emissions of methane (CH4), nitrous oxide (N2O), ammonia (NH3) and hydrogen sulfide (H2S) during aerobic composting of non-ruminant manure (chicken and pig) and ruminant manure (sheep and cattle). Sheep and cattle manure (characterized by higher C/N) reached maturity earlier and emitted 30-93% less greenhouse gases and odor than chicken and pig manure (characterized by lower C/N), although cattle manure produced slightly more CH4. High emissions from chicken and pig manure coincided with dominance of Bacillus and Defluviitalea, which declined as readily available carbon was depleted. In contrast, sheep and cattle manure was associated with Actinomadura and Corynebacterium, genera that correlated with the immobilization of nitrogen and sulphur into stable forms during the degradation of recalcitrant carbon. Ammonium concentration was strongly correlated with the overall emission rate, yet the microbial taxa associated with the process shifted predictably with animal diet. The observed associations provide a basis for future hypothesis-driven experiments. The findings provide a microbiome-based guide for controlling and minimizing atmospheric pollution during composting.