Improvement of the N 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 gas flux method has been limited by poor sensitivity because current isotope ratio mass spectrometry is not precise enough to detect low N fluxes in the presence of a high atmospheric N background. For laboratory studies, detection limits are improved by incubating soils in closed systems and under N -depleted atmospheres.

Methods: We developed a new procedure to conduct the N gas flux method suitable for field application using an artificially N -depleted atmosphere to improve the detection limit at the given precision of mass spectrometry. Laboratory experiments with and without N-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 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  + N O fluxes was 10 ppb, which was 80-fold better than that of the reference method. Further improvement can be achieved by lowering the N 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.