note that in most wastewater treatment plants in contrast, for example, to soils where denitrification is often the primary source of N2O, autotrophic NH3 oxidation makes a relatively greater contribution than heterotrophic denitrification. It seems unlikely that it will ever be possible to develop farming practices that completely eliminate N2O emissions from soil denitrifiers in agriculture. in the form of pig slurry or CuSO4) or foliar fertilizers (e.g. Hence, N2O production and reduction rates are often positively correlated with total carbon or soluble organic carbon availability [60,61]. The expression of the genes encoding the enzymes that produce and consume N2O is regulated by environmental signals, typically oxygen and NO, acting through regulatory proteins, which, either directly or indirectly, control the frequency of transcription initiation. Although they can show how bottom-up emission inventories can be verified by top-down modelling they conclude that, in spite of the wealth of N2O emission measurements of the past 20 years, there are still not enough long-term datasets to provide the information needed to design EFs for different climate zones or soil types. The cumulative effect over the past century has been an estimated approximately 20 per cent increase in atmospheric N2O concentration that is still increasing at a rate of 0.2–0.3% yr−1 [6]. Thus, they have clearly shown that agriculture is the activity mainly responsible for the additional N2O emissions over the past century and a half. In all cases, farmers need to see for themselves that added complexity and increased efforts can result in substantial net benefits to productivity, but they need also to be assured that increasing production actually leads to increases in income. Recent advances in isotopomer approaches promise the ability to be able to estimate the partition between fungal and bacterial N2O production in situ, and to allow estimates of the significance of fungal denitrification across a range of ecosystems [38]. Bakken et al. It may also be possible through plant breeding to manipulate denitrification through inputs into the plant rhizosphere, thereby changing the composition of plant-derived carbon flow or nitrogen uptake demand, or through crop spacing, tillage or integrated inorganic fertilizer, residue and SOM management. Whether it would have a net positive or negative effect on N2O production remains unclear as the net behaviour of denitrification and its controlling mechanisms are not yet fully understood. Sustainable agricultural intensification is defined as producing more output from the same area of land while reducing the negative environmental impacts and at the same time increasing contributions to natural capital and the flow of environmental services [80,81]. The major contributor to the biological production of N2O in many environments is the respiratory NO reductase (NOR) found in denitrifying bacteria and in some ammonia-oxidizing organisms. The ability to denitrify is phylogenetically diverse, and can even be undertaken by microbes traditionally classified as belonging to a different functional group. The membrane-bound regulator NosR, required for operon expression, appears to contain redox centres, including FeS clusters (perhaps for electron supply). Nitrous oxide levels are now higher today than at any other time during the last 800,000 years. Between 23 and 24 May 2011, a residential scientific meeting, entitled ‘Nitrous oxide (N2O) the forgotten greenhouse gas’, was held at the Kavli Royal Society International Centre, Chicheley Hall, Buckinghamshire, UK. The uptake of nitrous oxide by soils is generally regarded to Although N2O only accounts for around 0.03 per cent of total GHG emissions, it has an almost 300-fold greater potential for global warming effects, based on its radiative capacity, compared with that of carbon dioxide (CO2) [7]. One contribution of 12 to a Theo Murphy Meeting Issue ‘Nitrous oxide: the forgotten greenhouse gas’. One regulator that mediates this de-repression is the NO-binding protein NsrR. Changes in ocean temperature also affect the solubility of N2O. Both the single-gene transformants (nosZ) and the multi-gene transformants (nosFLZDY) produced active recombinant N2OR. This rather detailed understanding of the bacterial nitrogen cycle to date has come from studies of Gram-negative bacteria but evidence is now appearing showing that Gram-positive bacteria, such as bacilli, can also carry out denitrification [50,51]. Systematic studies of complex interactions in such eco-systems that are contributing globally to the release of the potent GHG N2O are now feasible. Three approaches were considered. [73] on wastewater treatment plants. [52] have assessed the influence of different long-term fertilization and cultivation treatments in a 160 year-old field experiment, comparing the potential for denitrification with the size and diversity of the soil denitrifier communities. The availability of mobile monitoring systems, such as MS and CRDS, together with isotopic spiking, and coupling to molecular ecology approaches provide the means to diagnose, distinguish and quantify the pathways operating and, hence, to allow a description of the fate of applied N to be reached. Strategies that might be adopted arise from three quite different approaches: first, by managing soil chemistry and microbiology to ensure that bacterial denitrification runs to completion, generating N2 instead of N2O; second, by reducing dependence on fertilizers through engineering crop plants, for example to fix nitrogen themselves in order to sustain growth and yield, or by capitalizing on C–N interactions in the rhizosphere; third, by promoting sustainable agricultural intensification, that is, producing more output from the same area of land while reducing the negative environmental impacts. First, not all soil bacteria have the same tolerance to free ion metals and microbial communities can adapt during long-term exposure, developing pollution-induced community tolerance; second, it is difficult to assess which bacterial cells are exposed to the free metal ions in the soil matrix. The ammonium ion can be oxidized to the nitrate ion [NO3]− in a three-step process called nitrification, the first of which is catalysed by the enzyme ammonia monooxygenase (AMO). Hence, severe copper depletion can lead to enzyme inactivation [45]. A fuller discussion of all these aspects is given by Richardson et al. The oxygen minimum zones of the intermediate layers (between 300 and 700 m water depth) in various regions of the ocean are expanding and have been losing oxygen during the past 50 years. It has been claimed that, if the coupling of nitrogen supply and carbon metabolism could be achieved, excess nitrogen would not be lost to the environment, thereby resulting in lower N2O emissions. Skiba et al. The combustion of fossil fuels produces CO 2 with a different carbon-isotope signature than the CO 2 present in the atmosphere before the Industrial Revolution. Despite its relatively small contribution to the overall global GHG emissions, N2O emissions from biological nutrient removal wastewater treatment plants can be very significant in terms of the contributions to their overall carbon footprint. Since 1971, the NO reduction activities of the bacterial membrane-bound NORs have been reported for many bacteria. Enzymatic activity was detected using the methyl viologen-linked enzyme assay, showing that extracts from both types of transgenic plants exhibited N2O-reducing activity [79]. Indeed, strains which lack nosZ occur within many genera of denitrifying prokaryotes, and if organisms with such a truncated denitrification apparatus were to dominate in soils, it would lead to high N2O/(N2 + N2O) product ratios of denitrification. In most global nitrous oxide budgets soil uptake is simply included ... Stratospheric sinks of nitrous oxide… Finally, these findings are drawn together in a discussion of strategies to mitigate N2O release. It is concluded that, as halocarbons return toward pre-industrial levels, N2O and CO2 are likely to play the dominant roles in ozone depletion.

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