Amoxicillin and copper affect humification during compost: Insight into dissolved organic matter and bacterial community succession.

Amoxicillin (AMX) and copper (Cu) often coexist in manure, while their combined effects at varying concentrations and the associated mechanisms influencing compost humification remain unclear. The present study combined multiple spectroscopic characterization and microbial analysis methods to elucidate the effects of AMX and Cu on the humification during aerobic composting of dairy manure. The results revealed that both low and high levels of combined AMX and Cu impeded dissolved organic matter degradation and led to a reduction in humic acid (HA) content by 3.8%-5.0% relative to the control treatment. Notably, the high concentration of combined pollution significantly suppressed the conversion of fulvic acid and hindered overall compost humification. The inhibition of HA formation was due to the inhibition of protein-like material degradation and polysaccharide conversion, which in turn inhibited the Maillard pathway. On the other hand, the combined pollution suppressed the abundance of the main HA formation core bacteria, such as A4b, JG30-KF-CM45, and Microscillaceae. During the cooling period, the complex pollution inhibited amino acid metabolism and carbohydrate metabolism, and suppressed the expression of GHs and GTs. This study provides data support for regulating HA formation during composting under combined pollution.