Responses of biogeochemical cycles to polyethylene microplastics exposure during aerobic fermentation of dairy manure

Microplastics (MPs) are emerging contaminants that may disrupt Earth's biogeochemical cycles of elements, yet their effects on multi-element cycling during aerobic fermentation of livestock manure remain unclear. This study evaluated how polyethylene (PE) MPs and fermentation strategy affected multi-element functional potential during aerobic fermentation of dairy manure solids by integrating metagenomic functional profiling, co-occurrence networks, and other complementary approaches. Fermentation time dominated functional gene succession, with treatment separation becoming most evident on day 30. Biomarkers were confined to the C cycling and detected only in the low concentration treatments, with none at the higher concentration. Network analysis revealed predominantly positive associations among genes involved in different elemental cycles. The membrane-covered treatment at the lower concentration formed the most connected network. The pathway profiles and network topology in the membrane-covered treatment at the higher concentration were similar to those of the blank control. This suggests that, under high MPs exposure, the membrane-covered treatment retained a functional profile similar to the blank control, likely in association with the more stable fermentation conditions in the membrane-covered treatment. Physicochemical properties, process variables, and gas emissions jointly explained 78% of functional gene variation. Mantel tests and structural equation modelling further linked fermentation conditions, gas emissions, and multi-element functional potential. Overall, multi-element functional responses to MP exposure varied with fermentation stage, strategy, and PE MP concentration. These findings highlight the importance of fermentation management when assessing multi-element functional responses to MP exposure during manure valorization.