Where does denitrification occur in the rhizosphere?

This project will determine the influence of plant carbon flow on the location of production of nitrous oxide (N2O) and dinitrogen (N2) during denitrification in the rhizosphere. Greater understanding on this is essential for the fomulation of management practices for mitigation of the greenhouse gas N2O, targetted to where denitrifiers are active. Denitrification rates and the denitrifier-N2O-to-N2 product ratio are hypothesised to vary with distance away from a root, primarily as a function of C gradient. Complete denitrification to N2 is hypothesised to be greatest adjacent to where C is released and so denitrifier-N2O-to-N2 will increase with distance from the root due to a decline in reductant for the N2O reductase. This will be tested for the first time using a combined reporter gene, stable isotope (15N) enrichment and molecular ecology approach. A series of microcosm experiments will be established in which the rhizosphere C gradient will be naturally created by C flow from barley roots into the soil. Rates of denitrifier-N2O and -N2 production at incremental distances away from the main root will be determined using a 15N-enrichment approach. Unstable gfp tagged Pseudomonas fluorescens will be used to identify where C is being sensed under varying rhizosphere conditions (N availability, water content, C flow, soil organic matter content) in both soil slurries (homogeneous medium) and intact heterogeneous soil. These will be related to the diversity of the bacterial population and expression of key denitrification genes (nirK, norB, nosZ) at the incremental distances from the root. This molecular analysis will be undertaken at SCRI, who will provide full training in these techniques. The student will also receive training in gas chromatography, mass spectrometry and microbiological techniques at Aberdeen, as well as in a wide range of soil physical and chemical analyses.