Transformation mechanisms of ammonium and nitrate in subsurface wastewater infiltration system: Implication for reducing greenhouse gas emissions.

Subsurface wastewater infiltration system (SWIS) has been recognized as a cost-effective and environmentally friendly tool for wastewater treatment. However, there is a lack of knowledge on the transformation processes of nitrogen (N), hindering the improvement of the N removal efficiency in SWIS. Here, the migration and transformation mechanisms of ammonium (NH-N) and nitrate (NO-N) over 10 days were explored by N labeling technique. Over the study period, 49% of the added NH-N remained in the soil, 29% was removed via gaseous N emissions, and 14% was leaked with the effluent in the SWIS. In contrast, only 11% of the added NO-N remained in the soil while 65% of the added NO-N was removed via gaseous N emissions, and 12% with the effluent in the SWIS. The main pathway for NO emission was denitrification (52-70%) followed by nitrification (15-28%) and co-denitrification (9-20%). Denitrification was also the predominant pathway for N loss as N, accounting for 88-96% of the N emission. The dominant biological transformation processes were different at divergent soil depths, corresponding to nitrification zone and denitrification zone along the longitudinal continuum in SWIS, which was confirmed by the expression patterns of microbial gene abundance. Overall, our findings reveal the mechanism of N transformation in SWIS and provide a theoretical basis for establishing a pollutant management strategy and reducing greenhouse gas emissions from domestic wastewater treatment.