In the era of global warming, solutions to reduce  greenhouse gases emissions  are urgently needed. Nitrous oxide (N2O) has a global warming potential almost 300 times the one of CO2, and accounts for 8% of total anthropogenic GHG emissions. The majority of N2O emissions result from microbially mediated reactions in wastewater treatment plants (WWTP) and agricultural soils; N2O can account for up to 80% of a WWTP carbon footprint. Effective N2O control strategies rely on our knowledge of the mechanisms underlaying microbial N2O production and consumption. This project aims at advancing our fundamental understanding of the long-overlooked capacity of denitrifying microbial communities to reduce N2O to innocuous N2 as a mean to reduce emissions from wastewater treatment. The mechanisms selecting for different denitrifying communities will be studied from a theoretical standpoint and in parallel lab-scale bioreactors. Advanced microbiological methods will be used to characterize the communities under both steady-state and perturbed conditions. The generated knowledge will be included in mathematical models to simulate the impact of community composition and process operation on N2O emissions, and derive optimal control strategies. The project is inherently interdisciplinary and highly innovative; and its success and impact are guaranteed by the solid experience of the candidate and the complementary and outstanding expertise of the promoter and international collaborators.