Co-composting of sewage sludge with plant biomass, and analysis of microbiome relevant to plant growth promotion

In this study, a pilot-scale co-compositing process using sewage sludge and plant biomass in Japan was used as a model for a local circular bioeconomy. Heavy metal concentrations in the developed sewage sludge compost (NSSC) were lower than the threshold values of composts reported worldwide, and the fiber content was higher (33.6 %) than that of commercial sewage sludge compost (CSSC) (9.7 %), indicating that co-composting with plant biomass resolves the issues associated with sewage sludge. Komatsuna (Brassica rapa var. peruviridis) showed plant growth-promoting effects after fertilizing with NSSC compared to CSSC. The alpha diversities of NSSC were 4- to 6-fold higher than those of CSSC, and several effective biosynthetic gene clusters, such as non-ribosomal peptide synthases, siderophores, and acetoin metabolism, were detected in the predominant microorganisms, suggesting that NSSC may promote plant growth owing to an increase in the presence of antagonistic microorganisms and promotion of microbial interactions in the soil. [Display omitted] •Heavy metal concentrations in newly developed sewage sludge compost (NSSC) were low.•Aggregate and muddy properties were improved by co-composting with plant biomass.•NSSC has plant growth-promoting (PGP) effect compared with commercial compost (CSSC).•Microbial diversities of NSSC were 4- to 6- fold higher than those of CSSC.•Predominant bacteria in NSSC possess effective biosynthetic gene clusters for PGP.