Source determination of nitrous oxide based on nitrogen and oxygen isotope tracing dealing with oxygen exchange.

Source determination of nitrous oxide (N(2)O) from soils has so far been complicated by methodological constraints: the frequently used (15)N tracer method could not differentiate between pathways related to nitrification, that is, nitrifier nitrification (NN), nitrifier denitrification (ND), and nitrification-coupled denitrification (NCD). To overcome this problem, a dual isotope method using both (15)N and (18)O was proposed. However, O exchange between nitrogen oxides and water has been found to disturb such a method. We here explain in detail a novel dual isotope method that allows to quantify O exchange in denitrification and to differentiate N(2)O production from NN, ND, NCD, and fertilizer denitrification (FD). The method has already been applied to a range of soils with good success. Potential of and scope for further improvement of the method are discussed.