| Summary: | Denitrification is the sequential reduction of NO3- to N2, through a number of intermediary steps, one of which is N2O, a potent green house gas. N2O has a global warming potential over 300 times greater than that of CO2 over a 100 a year period (IPCC, 2007). Soils are a significant source of N2O through the microbially mediated process of denitrification. The rhizosphere is a potentially important source of N2O as rhizodeposited carbon from plant roots can support a larger and more active microbial biomass. Despite this little is known about the effects of low molecular weight carbon (LMWC) on the production of N2O or N2 or how the production of these gases can vary with the small scale variation in carbon quantity found in the rhizosphere. Studies using a soil microcosm revealed that not all LMWC compounds were able to stimulate the production of N2O. Of the three compounds studied the addition of glucose or glutamine to soil resulted in a greater production of N2O than occurred in the control, while the addition of citric acid did not. An experimental system was developed that would allow the creation of a carbon gradient over 6 cm and along which N2O and N2 could be quantified. This allowed an insight into the potential for small scale variation in denitrification. Similar spatial variation in N2O and N2 concentrations were found in both glucose and glutamine treated soil, while no spatial variation in these gasses was found in citric acid treated soil. Peak N2 concentrations occurred closer to the carbon source than peak N2O concentrations, potentially as a result of the higher C : N ratio. This was associated with a shift in the bacterial community, and in glucose treated soil with an increase in the proportion of bacteria containing nosZ or nirK. Despite this spatial patterns in N2O production remained similar even in experiments where the community did not change. The bacterial community did however exert an influence by affecting the magnitude of N2O and N2 produced. The results from this thesis suggest that denitrification has the potential to vary in the rhizosphere as a result of changes in carbon concentrations, carbon compounds and the associated changes in the microbial community.
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