Opportunities for mitigation

One of the main aims of determining the microbial sources of N2O is to provide a scientific basis for more targeted mitigation strategies. Data on the environmental regulation of N2O production during the different microbial processes are scarce, particularly for nitrifier denitrification and nitrate ammonification. More effort needs now to be placed on considering all potential sources of N2O when measuring and interpreting N2O fluxes from different environments and under different management regimes. Richardson et al (2009) argue that understanding the regulation of the denitrifier N2O reductase is central to the development of management options to lower net N2O emissions by enhancing its reduction to N2, rather than trying to eliminate denitrification. They consider the effect on the regulation of the N2O reductase of management options such as application of nitrogenous or copper fertilizer to regulate Cu availability for this Cu-based enzyme, soil organic matter (SOM) management and liming of crops or grasslands with specific characterized carbon and nitrogen traits.

Reducing the application of inorganic N fertilizers has traditionally been considered as the most successful option for lowering net emission of N2O. However, source partitioning by Baggs et al (unpublished data) suggests that nitrifier denitrification may be increased under low-N conditions, and therefore lower N application may not necessarily be the most appropriate strategy in all systems. There are 'new' options being proposed for mitigation of greenhouse gases that may also hold the potential for enhancing C sequestration. One of these is application of biochar, which is a good example of where the ability to determine the N2O source becomes essential. Although the mechanism by which biochar can lower net emission of N2O is still open to debate (Renner, 2007; Gaunt and Lehmann, 2008), its chemical composition and structure suggest that it may be due to uptake of NH4+ into the char structure, and a change in pH, rather than by enhancing the reduction of N2O to N2. If this is the case then the largest effect will be in lowering nitrifier-N2O, with only indirect effects on nitrate-reducing processes. This can be verified by coupling approaches for determining microbial sources of N2O with rigorous investigations of the effect of biochar on the regulation of different enzymatic steps of each process and on microbial community structure. Such studies are required before biochar can be recommended as a reliable management option for lowering greenhouse gas emissions. This is just one example of where the effects on microbial sources of N2O should also be considered in the light of possible interactions with the C cycle, and most pertinently with methane oxidation and methanotroph activity (Philippot et al, 2009b).

Our ability to determine the microbial source of N2O in terrestrial systems, and even to quantify the contribution from each source, is improving with the advent and development of new techniques. This offers us exciting opportunities for targeted management options to optimize mitigation potential, but we still have some way to go before this can become a reality. Better understanding is required of the regulation of all processes, and particularly the uncertainties associated with the conditions conducive to N2O production during nitrate am-monification and nitrifier denitrification. To be able to determine the microbial source of N2O with any degree of accuracy in natural, unfertilized or fragile ecosystems we need to refine methodologies, or combine established and new methodologies, moving us away from reliance on application of 15N-labelled substrates, which may artificially favour one process over others. Any mitigation approach should be grounded in predictions of future emissions from different management scenarios under a continuing changing climate. To achieve this, models require further development to encompass all microbial sources of N2O, and should also consider abiotic sources and plant-mediated emissions. This will be facilitated by advances in techniques to unite source partitioning with up-scaling of emissions from the micro-plot. The benefit in lowering model uncertainty that parameterizing the underpinning microbiology would provide is uncertain, but to ensure sustainability for the future it is essential to ascertain that any management option imposed to lower emissions has no adverse effect on the diversity and functioning of the microbial community.

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