Differentiation ofc,d1cytochrome and copper nitrite reductase production in denitrifiers

N2O profiles in the enhanced CANON process via long-term N2H4 addition: minimized N2O production and the influence of exogenous N2H4 on N2O sources

Production of the greenhouse gas nitrous oxide (N₂O) from the completely autotrophic nitrogen removal over nitrite (CANON) process is of growing concern. In this study, the effect of added hydrazine (N₂H₄) on N₂O production during the CANON process was investigated. Long-term trace N₂H₄ addition minimized N₂O production (0.018% ± 0.013% per unit total nitrogen removed) and maintaining high nitrogen removal capacity of CANON process (nitrogen removal rate and TN removal efficiency was 450 ± 60 mg N/L/day and 71 ± 8%, respectively). Ammonium oxidizing bacteria (AOB) was the main N₂O producer. AOB activity inhibition by N₂H₄ decreased N₂O production during aeration, and the N₂H₄ concentration was negatively correlated with N₂O production rate in NH₄⁺ oxidation via AOB, whereas N₂O production was facilitated under anaerobic conditions because hydroxylamine (NH₂OH) production was accelerated due to anammox bacteria (AnAOB) activity strengthen via N₂H₄. Added N₂H₄ completely degraded in the initial aeration phases of the CANON SBR, during which some N₂H₄ intensified anammox for total nitrogen removal to eliminate N₂O production from nitrifier denitrification (ND) by anammox-associated, while the remaining N₂H₄ competed with NH₂OH for hydroxylamine oxidoreductase (HAO) in AOB to inhibit intermediates formation that result in N₂O production via NH₂OH oxidation (HO) pathway, consequently decreasing total N₂O production.

Relative Contribution of nirK- and nirS- Bacterial Denitrifiers as Well as Fungal Denitrifiers to Nitrous Oxide Production from Dairy Manure Compost

The relative contribution of fungi, bacteria, and nirS and nirK denirifiers to nitrous oxide (N₂O) emission with unknown isotopic signature from dairy manure compost was examined by selective inhibition techniques. Chloramphenicol (CHP), cycloheximide (CYH), and diethyl dithiocarbamate (DDTC) were used to suppress the activity of bacteria, fungi, and nirK-possessing denitrifiers, respectively. Produced N₂O were surveyed to isotopocule analysis, and its ¹⁵N site preference (SP) and δ¹⁸O values were compared. Bacteria, fungi, nirS, and nirK gene abundances were compared by qPCR. The results showed that N₂O production was strongly inhibited by CHP addition in surface pile samples (82.2%) as well as in nitrite-amended core samples (98.4%), while CYH addition did not inhibit the N₂O production. N₂O with unknown isotopic signature (SP = 15.3-16.2‰), accompanied by δ¹⁸O (19.0-26.8‰) values which were close to bacterial denitrification, was also suppressed by CHP and DDTC addition (95.3%) indicating that nirK denitrifiers were responsible for this N₂O production despite being less abundant than nirS denitrifiers. Altogether, our results suggest that bacteria are important for N₂O production with different SP values both from compost surface and pile core. However, further work is required to decipher whether N₂O with unknown isotopic signature is mostly due to nirK denitrifiers that are taxonomically different from the SP-characterized strains and therefore have different SP values rather than also being interwoven with the contribution of the NO-detoxifying pathway and/or of co-denitrification.