Effect of combined cadmium and doxycycline pollution on the nitrogen cycle in constructed ditch wetlands

Rural drainage ditches serve as important sinks for heavy metals and antibiotics. However, the combined effects of heavy metals and antibiotics on ecological processes and functions in ditches remain unclear. We studied the combined effects of cadmium and doxycycline on nitrification, denitrification, anaerobic ammonia oxidation, and nitrous oxide emission rates in constructed ditch wetlands during 84 days. Results show that combined pollution decreased denitrification and anaerobic ammonia oxidation rates, but increased nitrous oxide production rates. The addition of either cadmium or doxycycline decreased the glucokinase and glyceraldehyde-3-phosphate dehydrogenase activities, and their combined pollution had a higher impact on adenosine triphosphate than on nicotinamide adenine dinucleotide. Moreover, the activities of the electron transport system, nitrate reductase, and nitrite reductase were enhanced by short-term pollutant exposure, but inhibited by long-term pollutant exposure. Structural equation modelling further revealed that electron production and transport impacted the rates of denitrification, anaerobic ammonia oxidation, and nitrous oxide production directly and indirectly by changing the activities of nitrate and nitrite reductases.