Laboratory evaluation of a new approach to measure denitrification under N2-depeleted atmosphere in situ using the 15N gas flux method

Direct determination of N2 fluxes from soils is complicated by the huge N2 background of the atmosphere. There are two principles to overcome this problem, i.e. adding highly enriched 15N nitrate and monitoring 15N labelled denitrification products (15N gas flux method) or measuring denitrification gases during incubation of soils in absence of atmospheric N2 using gas tight containers and artificial atmosphere. In the laboratory, both approaches have been combined to incubate soils under N2-depeleted atmosphere to improve the detection limit of N2 fluxes (Spott et al. 2006, Meyer et al. 2010, Lewicka-Szczebak et al. 2017). However, until now this approach was limited to lab conditions allowing extended incubation in fully closed systems and to control the incubation atmosphere. Objectives of this study were to develop a procedure to conduct the 15N gas flux method under N2 depleted atmosphere suitable for field application. Moreover, we wanted to check this new approach with respect to stability and bias and optimize its practicability for future routine application in the field. Finally we aimed to compare its results with independent methods of N2 flux quantification. We established a testing system consisting of a 70-L container filled with soil were an aluminium cylinder (15 cm diameter, 35 cm long) was installed to mimic a typical field micro-plot suitable for conducting the 15N gas flux method using closed chambers (Buchen et al. 2016). Soil and chamber atmospheres were replaced with an artificial N2-depleted gas mixture and stability of N2background concentration monitored by online GC analysis. Subsequently, several experiments with 15N-labelled soil were conducted to check the performance under N2 depleted atmosphere in comparison with incubation under ambient atmosphere. N2 fluxes determined with both approaches will be shown References: Buchen, C., Lewicka-Szczebak, D., Fuss, R., Helfrich, M., Flessa, H. and Well, R. (2016) Fluxes of N-2 and N2O and contributing processes in summer after grassland renewal and grassland conversion to maize cropping on a Plaggic Anthrosol and a Histic Gleysol. Soil Biology & Biochemistry 101, 6-19. Lewicka-Szczebak, D., Augustin, J., Giesemann, A. and Well, R. (2017) Quantifying N2O reduction to N2 based on N2O isotopocules - validation with independent methods (helium incubation and 15N gas flux method). Biogeosciences 14(3), 711-732. Meyer, A., Bergmann, J., Butterbach-Bahl, K. and Bruggemann, N. (2010) A new 15N tracer method to determine N turnover and denitrification of Pseudomonas stutzeri. Isotopes in Environmental and Health Studies 46(4), 409-421. Spott, O., Russow, R., Apelt, B. and Stange, C.F. (2006) A N-15-aided artificial atmosphere gas flow technique for online determination of soil N-2 release using the zeolite Kostrolith SX6 (R). Rapid Communications in Mass Spectrometry 20(22), 3267-3274.