Improvement of the 15N gas flux method for in situ measurement of soil denitrification and its product stoichiometry

Rationale Field measurement of denitrification in agricultural ecosystems using the N-15 gas flux method has been limited by poor sensitivity because current isotope ratio mass spectrometry is not precise enough to detect low N-15(2) fluxes in the presence of a high atmospheric N-2 background. For laboratory studies, detection limits are improved by incubating soils in closed systems and under N-2-depleted atmospheres. Methods We developed a new procedure to conduct the N-15 gas flux method suitable for field application using an artificially N-2-depleted atmosphere to improve the detection limit at the given precision of mass spectrometry. Laboratory experiments with and without N-15-labelling and using different flushing strategies were conducted to develop a suitable field method. Subsequently, this method was tested in the field and results were compared with those obtained from the conventional N-15 gas flux method. Results Results of the two methods were in close agreement showing that the denitrification rates determined were not biased by the flushing procedure. Best sensitivity for N-2 + N2O fluxes was 10 ppb, which was 80-fold better than that of the reference method. Further improvement can be achieved by lowering the N-2 background concentration below the values established in the present study. Conclusions In view of this progress in sensitivity, the new method will be suitable to measure denitrification dynamics in the field beyond peak events.