Alleviating nitrous oxide emission in partial denitrification via transition to a partial denitrification-anammox...

While the partial denitrification (PD)-anammox (PDA) process is acclaimed for its high-efficiency nitrogen removal, its impact on nitrous oxide (N₂O) emissions and the underlying microbial mechanisms driving these emissions remain poorly understood. Here, this study proposed and validated a strategy to mitigate N₂O emissions by transitioning the nitrogen removal pathway from a PD to a PDA pattern through the inoculation of mature anammox sludge. This intervention successfully led to a drastic mitigation of N₂O emissions (emission factor decreased from 5.920% to 0.037%). The transition to PDA catalyzed a profound restructuring within the microbial community, inducing the decline of previously dominant species (e.g., MAG.447, MAG.26 and MAG.118). Crucially, this community shift limited the expression of nitric oxide reductase (nor) genes, especially from the species MAG.447 and MAG.408, thereby inhibiting the primary pathway of N₂O production. Moreover, the decreased expression ratio of nor and N₂O reductase (nosZ) indicated that the reshaped community revealed a potent N₂O reducing capacity, largely driven by the emergence of key atypical denitrifying bacteria (e.g., MAG.80 and MAG.408) with high expression of nosZ gene. The overall findings demonstrate that the substantial reduction of N₂O under the PDA pattern was achieved through a synergistic interplay between upstream source control (suppressing N₂O production) and downstream consumption (enhancing N₂O reduction). This study provides a successful strategy and deep mechanistic insights into N₂O mitigation in PDA systems.