Industrial aerobic composting and the addition of microbial agents largely reduce the risks of heavy metal and ARG transfer through livestock manure

Composting is an effective and necessary modality in the recycling of agricultural wastes such as livestock manure, furfural, and straw. However, the risks of heavy metals (HMs) and antibiotic resistance genes (ARGs) during industrial-scale composting process have not been adequately assessed, especially with the addition of bacterial agents. In this study, changes in HMs toxicity, ARGs propagation and microbial community structure during industrial-scale aerobic composting of livestock manure were firstly investigated with various substrates addition. Moreover, the effect of the addition of bacterial agents (Bacillus strains) was evaluated. The results showed that industrial aerobic composting process could immobilize various HMs with different extents and significantly reduce the levels of ARGs such as intl1 and oqxB genes. The addition of Bacillus strains could further reduce the levels of most detected ARGs and the bioavailability of Cu and Cr, and the relative abundance of ereA and tetA was undetectable in some materials. After composting, the main bacterial community structures were similar among different substrates irrespective of bacterial agents and indicated that the composting process was the main driver for their change. This study provides a scientific reference for the safe reuse of livestock manure.