Influence of microbial augmentation on contaminated manure composting: metal immobilization, matter transformation, and bacterial response

Heavy metals (HMs) in husbandry waste have become a serious concern. To understand the impact of bio-augmentation on HMs-influenced composting, HMs-influenced swine manure was composted with nitrogen -retaining microbial agents (NRMA). Compared with control (CG), compost with NRMA (EG) presented a significantly lower HOAc extractable Cd (23.10%), Cu (48.15%), Cr (82.79%), Pb (4.49%), and Zn (29.15%) (P < 0.05). EG also showed 5 days longer high-temperature period during composting. After composting, EG had a 4.14% higher TN (27.93 g/kg) than CG (26.82 g/kg) but a 32.26% lower NH4+-N. The change in pH and NH4+-N driven by microbial activity was found the main reason for HMs bioavailability decrease. NRMA greatly strengthened the enrichment of HMs resistant bacteria such as Actinobacteriota and Chloroflexi in EG, whose abundance increased by 21.23% and 2473.75% compared with day 0 after composting, respectively. HMs resistance genes, such as copA (increased by 936.84%), czcA (62.95%), cadC (63.06%), and pbrT (684.08%), and chrB (16.89%), also elevated in EG than CG. Eventually, NRMA was able to regulate the microbial composition of manure composting under HMs impact, reduce HMs toxicity, and enhance composting efficiency, which should be considered for the safe disposal of such polluted waste.