Decoding the role of fungal functional groups in dissolved organic matter transformation and humification during co-composting.

Fungal communities play a vital role in composting by transforming organic waste into eco-friendly products. However, little is known about how their functional traits relate to the dissolved organic matter (DOM) transformation and humification. This study investigated the fungal taxonomic and functional groups succession during co-composting of green waste and sewage sludge under five raw material ratios ranging from 1:1 to 1:3, comparing their associations with DOM transformation. Fungal community composition varied significantly with the feeding ratios. Saprotrophic fungi predominated, while plant pathogens were effectively suppressed. However, extended composting increased the risk of animal-parasitic fungi, highlighting the need for careful management. Fungal 18S rRNA gene copy number declined during the thermophilic phase but rebounded during cooling, correlating with rising humic acid concentrations. Co-occurrence network analysis showed fungal interaction patterns were strongly linked to their functional traits, with specific modules associated with humification. Notably, functional groups exhibited stronger associations with DOM properties than taxonomic groups. Random forest models identified fungal functional groups as superior predictors of the humification index. A structural equation model further confirmed the essential role of fungal functional groups in driving humification (R = 0.75) and humic acid accumulation (R = 0.50). These findings offer practical insights into optimizing composting strategies by adjusting raw material ratios and suggest that inoculating targeted fungal functional groups can enhance compost quality. Overall, this study deepens our understanding of fungal-driven DOM transformation and underscores the critical role of fungal functional traits in determining composting performance.