Is it root- or soil-derived carbon that drives denitrification?

The soil microbial process of denitrification produces nitrous oxide (N2O) and dinitrogen (N2) which are emitted to the atmosphere. Whilst N2 is harmless in the atmosphere, emissions of N2O are of concern due to the involvement of this gas in global warming and the destruction of stratospheric ozone. Since soil is a significant source of N2O it is vital to understand the controls on both its production and its reduction to harmless N2. One such control is carbon (C) availability, but this is the least well understood factor with regards to denitrifier-N2O production and the predominant C source used in denitrification is unknown. C from living roots is thought to select for a more active, abundant and distinct microbial community compared to that associated with soil organic matter (SOM), and is therefore hypothesised to result in different quantities and rates of denitrifier-N2O and -N2 production. However, the significance of this dynamic C source in driving denitrification against different backgrounds of SOM-C, and how this selects for complete denitrification through to N2 is unknown, and so is currently ignored in predictive models. This project will determine the extent to which plant-derived C flow drives denitrification in soils of different SOM contents, and management histories. We will use state-of-the-art stable isotope techniques to quantify N2O and N2 production during denitrification and relate this to C utilisation by the microbial community, the activity of denitrifying bacteria in the rhizosphere and the up-regulation of genes involved in N2O production and reduction. Information obtained will be essential for the development of strategies to mitigate N2O emissions through SOM management, and will feed into predictive models which currently ignore the contribution of root-C flow.