Co-stress of sulfamethoxazole and copper reshapes microbial interaction networks and nitrogen transformation during...

The individual existence of antibiotics or heavy metals in livestock manure poses complex challenges to nitrogen cycling during composting. However, the mechanisms underlying their combined effects remain poorly understood. Here, we investigated the synergistic impacts of sulfamethoxazole (SMZ) and copper (Cu) on nitrogen transformation pathways, gaseous emissions, and microbial succession during swine manure composting. SMZ and Cu individually increased NH₃ volatilization by 17.99% and 27.82%, while their combination further enhanced emissions by 41.71% during the heating and thermophilic phases, mainly by suppressing organic matter degradation and ammonium oxidation. Two major N₂O emission peaks were observed in thermophilic and mature phase, respectively. The first peak, associated with denitrification, was enhanced by SMZ and Cu via increased narG and nosZ abundance. The second peak, associated with nitrifier recovery, reflected contrasting antimicrobial persistence: SMZ induced a transient effect due to rapid degradation, promoting partial recovery of ammonia oxidizing bacteria and nitrification derived N₂O production, whereas Cu exerted continuous suppression throughout composting owing to its high bioavailability fraction, thereby limiting nitrification recovery. Consequently, co-stress produced an intermediate response during the mature phase, resulting from the interaction between SMZ-facilitated nitrification recovery and Cu-induced inhibition. Module preservation analysis of co-occurrence networks revealed no preserved modules across treatments. This suggests substantial restructuring of the microbial community under SMZ and Cu co-stress. Topological role analysis further revealed that SMZ and Cu co-stress intensified microbial competition and reorganized keystone taxa, ultimately redirecting nitrogen transformation pathways during composting. Overall, antibiotic and metal co-stress hindered ammonium utilization, reshaped nitrification and denitrification process, and destabilized microbial networks, highlighting their compounded risks during organic waste composting.