Denitrification and N2O: N2 production in temperate grasslands: Processes, measurements, modelling and mitigating negative impacts

In this review we explore the biotic transformations of nitrogenous compounds that occur during denitrification, and the factors that influence denitrifier populations and enzyme activities, and hence, affect the production of nitrous oxide (N₂O) and dinitrogen (N₂) in soils. Characteristics of the genes related to denitrification are also presented. Denitrification is discussed with particular emphasis on nitrogen (N) inputs and dynamics within grasslands, and their impacts on the key soil variables and processes regulating denitrification and related gaseous N₂O and N₂ emissions. Factors affecting denitrification include soil N, carbon (C), pH, temperature, oxygen supply and water content. We understand that the N₂O:N₂ production ratio responds to the changes in these factors. Increased soil N supply, decreased soil pH, C availability and water content generally increase N₂O:N₂ ratio. The review also covers approaches to identify and quantify denitrification, including acetylene inhibition, ¹⁵N tracer and direct N₂ quantification techniques. We also outline the importance of emerging molecular techniques to assess gene diversity and reveal enzymes that consume N₂O during denitrification and the factors affecting their activities and consider a process-based approach that can be used to quantify the N₂O:N₂ product ratio and N₂O emissions with known levels of uncertainty in soils. Finally, we explore strategies to reduce the N₂O:N₂ product ratio during denitrification to mitigate N₂O emissions. Future research needs to focus on evaluating the N₂O-reducing ability of the denitrifiers to accelerate the conversion of N₂O to N₂ and the reduction of N₂O:N₂ ratio during denitrification.