Abstract: Seaweed as a fertilizer has been renowned for centuries. By co-composting with organic solid waste, the active substances in seaweed, such as alginate, can be effectively utilized. Therefore, the effects of initial C/N ratio (20∶1, 30∶1, 40∶1 and 50∶1) on the composting of Saccharina japonica algae and mushroom compost residue were investigated, and the most suitable C/N ratio was determined. The effects of different initial C/N ratio on composting were evaluated by determining the physical and chemical properties during composting. The results showed that the initial temperature of composting increased rapidly to about 60 ℃. After composting, the pH of all treatment groups was lower than 5.5, and the product had the potential to improve alkaline soil. The germination index (GI) of compost products in all treatment groups was higher than 80%, and there was no biological toxicity. The increase rate of alginate content was the smallest (45.3%) when the initial C/N ratio was 30∶1, which was more conducive to the decomposition of alginate. In addition, 16S rRNA high-throughput sequencing analysis showed that at 30∶1 C/N ratio after composting, the content of Pseudomonas was significantly higher than that in other groups. Pseudomonas is one of the main strains producing alginate lyase, which is beneficial to the degradation of alginate and the release of active substances. In conclusion, when the C/N ratio was 30∶1, high-quality mature compost could be produced, and the appropriate C/N ratio promoted the degradation of organic matter and improved the quality of compost. 

Key words: composting, C/N ratio, macroalgae, bacterial diversity