Effects of Saccharina japonica and spent mushroom compost mixed compost on composting quality and dynamic change of microbial diversity at different C/N ratios

Seaweed as a fertilizer for plant growth has been renowned for centuries. The active substances in seaweed can be effectively exploited by land-based co-composting. The efficiency of co-composting can be improved by adjusting the initial C/N ratios of raw materials or adjusting the initial water content. The purpose of this study is to study how the initial C/N ratio in the composting process of a mixture of Saccharina japonica algae and spent mushroom compost residue affects the composting process, explore the most suitable C/N ratio, and determine the physicochemical and biological parameters in the composting process to evaluate the effects of different initial C/N on the composting process. The dynamic changes in microbial diversity were evaluated using high-throughput sequencing at different C/N ratios: 20:1, 30:1, 40:1 and 50:1. 16S rRNA sequencing showed that at 30:1 C/N after composting, the content of Pseudomonas in the experimental group 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. Spearman’s correlation analysis showed that pH and alginate significantly affected the succession of dominant bacteria during composting. The results showed that when the C/N ratio was 30:1, this ratio would produce high-quality mature compost.