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 (N(2)O) and dinitrogen (N(2)) 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 N(2)O and N(2) emissions. Factors affecting denitrification include soil N, carbon (C), pH, temperature, oxygen supply and water content. We understand that the N(2)O:N(2) production ratio responds to the changes in these factors. Increased soil N supply, decreased soil pH, C availability and water content generally increase N(2)O:N(2) ratio. The review also covers approaches to identify and quantify denitrification, including acetylene inhibition, (15)N tracer and direct N(2) quantification techniques. We also outline the importance of emerging molecular techniques to assess gene diversity and reveal enzymes that consume N(2)O during denitrification and the factors affecting their activities and consider a process-based approach that can be used to quantify the N(2)O:N(2) product ratio and N(2)O emissions with known levels of uncertainty in soils. Finally, we explore strategies to reduce the N(2)O:N(2) product ratio during denitrification to mitigate N(2)O emissions. Future research needs to focus on evaluating the N(2)O-reducing ability of the denitrifiers to accelerate the conversion of N(2)O to N(2) and the reduction of N(2)O:N(2) ratio during denitrification.