Biotic and abiotic mechanisms affecting N2O production from biological ammonia oxidation in natural and engineered systems

Abstract

Natural and engineered systems are substantial N$_2$O source, potent greenhouse gas. AOB-induced NH$_3$ oxidation is a major biological N$_2$O-genearing pathway and multiple biotic and abiotic mechanisms have been suggested to be responsible for AOB-mediated N$_2$O production. This thesis revealed the novel mechanism of enzymatic and non-enzymatic N$_2$O production from AOB-driven NH$_3$ oxidation at three different eco-systems. The first study revealed the enhancement effect of PO$_4^{3-}^-P abundance on N$_2$O yields from abiotic NH$_2$OH decomposition using biotic and abiotic lab-scale incubation and soil microcosm, which may have substantial implications to N$_2$O emissions from extracellular NH$_2$OH release by AOB-mediated NH$_3$ oxidation in PO$_4^{3-}$-rich soils (e.g. soil exposed by animal excreta). Moreover, three novel effectors were identified to have significant contribution on N$_2$O emission from oxic and anoxic tanks of activated sludge at full-scale WWTPs using random forest, which also indicated the predominant N$_2$O production from oxic unit via aerobic AOB-mediated NH$_2$OH oxidation pathway. Furthermore, considering that AOB and heterotrophic denitrifying bacteria co-thrive in PN-AMX system, we showed that the substantial impacts of interspecies interaction between AOB and heterotrophic denitrifying bacteria on the elevated N$_2$O production, where NH$_2$OH extracellularly secreted from AOB-induced NH$_3$ oxidation stimulated N$_2$O production from aerobic NO$_2^-$-mediated reduction of heterotrophic denitrifying bacteria. Overall, these findings may contribute to resolve the uncertainty unfeasible to track N$_2$O generation pathway using conventional N$_2$O-producing mechanism.