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 ¹⁵N gas flux method has been limited by poor sensitivity because current isotope ratio mass spectrometry is not precise enough to detect low ¹⁵N₂ fluxes in the presence of a high atmospheric N₂ background. For laboratory studies, detection limits are improved by incubating soils in closed systems and under N₂‐ depleted atmospheres. Methods: We developed a new procedure to conduct the ¹⁵N gas flux method suitable for field application using an artificially N₂‐depleted atmosphere to improve the detection limit at the given precision of mass spectrometry. Laboratory experiments with and without ¹⁵N‐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 ¹⁵N 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 N₂+N₂O fluxes was 10 ppb, which was 80‐fold better than that of the reference method. Further improvement can be achieved by lowering the N₂ 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.