Linking nitrous oxide emissions to population dynamics of nitrifying and denitrifying prokaryotes in four full-scale wastewater treatment plants.

Ammonia-oxidizing bacteria (AOB), ammonia oxidizing archaea (AOA) and N2 O-reducing denitrifiers were measured by quantitative real-time PCR (qPCR) in activated sludge samples from four full-scale wastewater treatment plants (WWTPs) in South Spain, and their abundances were linked to the generation of N2 O in the samples using multivariate analysis (Non-metric multidimensional scaling, MDS, and BIO-ENV). The average abundances of AOA remained in similar orders of magnitude in all WWTPs (106 copies amoA/L activated sludge mixed liquor), while significant differences were detected for AOB (105 -109 copies amoA/L) and N2 O-reducers (107 -1010 copies nosZ/L). Average N2 O emissions measured in activated sludge samples ranged from 0.10 ± 0.05 to 6.49 ± 8.89 mg N2 O-N/h/L activated sludge, and were strongly correlated with increased abundances of AOB and lower counts of N2 O-reducers. A significant contribution of AOA to N2 O generation was unlikely, since their abundance correlated negatively to N2 O emissions. AOB abundance was favoured by higher NO3 - and NO2 - concentrations in the activated sludge.