Nitrous oxide (N 2 O) is a powerful atmospheric greenhouse gas and cause of ozone layer depletion. Global emissions continue to rise. More than two-thirds of these emissions arise from bacterial and fungal denitrification and nitrification processes in soils, largely as a result of the application of nitrogenous fertilizers. This article summarizes the outcomes of an interdisciplinary meeting, ‘Nitrous oxide (N 2 O) the forgotten greenhouse gas’, held at the Kavli Royal Society International Centre, from 23 to 24 May 2011. It provides an introduction and background to the nature of the problem, and summarizes the conclusions reached regarding the biological sources and sinks of N 2 O in oceans, soils and wastewaters, and discusses the genetic regulation and molecular details of the enzymes responsible. Techniques for providing global and local N 2 O budgets are discussed. The findings of the meeting are drawn together in a review of strategies for mitigating N 2 O emissions, under three headings, namely: (i) managing soil chemistry and microbiology, (ii) engineering crop plants to fix nitrogen, and (iii) sustainable agricultural intensification.

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SummaryDenitrifying bacteria convert nitrate (NO3 -) to dinitrogen (N2) gas through an anaerobic respiratory process in which the potent greenhouse gas nitrous oxide (N2O) is a free intermediate. These bacteria can be grouped into classes that synthesize a nitrite (NO2 -) reductase (Nir) that is solely dependent on haem-iron as a cofactor (e.g. Paracoccus denitrificans) or a Nir that is solely dependent on copper (Cu) as a cofactor (e.g. Achromobacter xylosoxidans). Regardless of which form of Nir these groups synthesize, they are both dependent on a Cu-containing nitrous oxide reductase (NosZ) for the conversion of N2O to N2. Agriculture makes a major contribution to N2O release and it is recognized that a number of agricultural lands are becoming Cu-limited but are N-rich because of fertilizer addition. Here we utilize continuous cultures to explore the denitrification phenotypes of P. denitrificans and A. xylosoxidans at a range of extracellular NO3