Spatial and temporal changes in microbial communities and greenhouse gas emissions in a denitrifying woodchip bioreactor at low water temperatures

Nitrogen (N) pollution is a major threat to ecosystems and a driver of climate change through emissions of the greenhouse gas nitrous oxide (N2O). Mining activities are increasingly recognized for contributing to N pollution due to undetonated, N-based explosives. A woodchip denitrifying bioreactor, installed to treat nitrate-rich leachate from waste rock dumps in northern Sweden, was monitored for two years to determine the spatial and temporal distribution of microbial communities in pore water and woodchips and their genetic potential for different N transformation processes, and how this affected the N removal capacity and possible production of undesired N species, like ammonium, nitrite and N2O. About 80 and 65 % of the nitrate was removed from the leachate the first and second operational year, respectively, which agreed with a decrease in dissolved organic carbon in the outlet water. There was a succession in the microbial community over time and in space along the reactor length in both pore water and woodchips, which was reflected in the genetic potential for N cycling and ultimately also reactor performance. We conclude that DNRA had minimal impact on the overall N removal efficiency due to the low relative abundance of the key gene nrfA involved in DNRA and the low production of ammonium. However, nitrite, ammonium, and N2O were formed in the bioreactor and released in the effluent water, although direct emissions of N2O from the surface was low. The N2O production in the reactor might be explained by the ratio between the genetic potential for overall denitrification and N2O reduction in the woodchip and pore water communities, as indicated by the low ratio between the abundance of nir and nosZ genes. Altogether, the results indicate that the denitrification pathway was temporally as well as spatially separated along the reactor length, and that unwanted reactive N species were produced at different time points and locations in the reactor. Thus, the succession of microbial communities in woodchip denitrifying bioreactors treating mining impacted water develops slowly at low temperature, which impacts the reactor performance.