Why do we need to know the microbial source of nitrous oxide

The ultimate aim in determining the microbial source of N2O is to better constrain the global N2O budget and to inform mitigation strategies. This is essential for the formulation of appropriate and more targeted mitigation strategies, which at the time of writing are urgently required if industrialized countries are to reach the Kyoto and post-Kyoto targets for reduction in greenhouse gas emissions, for example in Europe a reduction of up to 80 per cent by 2050 compared to emissions in 1990. Soil is a complex heterogeneous matrix that renders attributing N2O production to different processes a challenge, as different processes may occur simultaneously in different microsites of the same soil (Robertson and Tiedje, 1987). Simply relying on a net emission of N2O from a system does not tell us the responses of the key component groups of the microbial community. However, it is essential to understand these responses, because when imposing strategies for mitigation of N2O it is important that there is no detrimental effect on the long-term functioning of the ecosystem facilitated through changes in key microbial functional groups. To ensure this we should link source partitioning - i.e. determination of the microbial sources of N2O - to the understanding of activity and ecology of the underpinning microbial community. This means that efforts to measure N2O from different environments and under different management regimes should more regularly consider all N2O processes, with the starting point being the identification of conditions under which particular processes predominate. Unless the controls on enzyme regulation associated with these processes, and associated product ratios (primarily N2O and N2) are determined, then it will not be possible to develop more targeted mitigation strategies. The different processes involved in N2O production respond differently to environmental parameters or imposed management, and the enzymatic systems of each process are regulated differently. This means that the down-regulation of N2O production in one process as a result of management practice or change in environmental conditions may well lead to the up-regulation of N2O production in another. Thus appropriate management for one process, may not be appropriate for another, and may well need to be flexible depending on the system, the prevailing environmental conditions, and the management options available.

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