Lysimeter studies combining 15N tracing and natural abundance stable isotopes to determine N2 and N2O fluxes and processes in arable soils

Mitigating nitrogen (N)-oxide emissions and optimizing N-use efficiency are important aspects of agricultural soil management. Studies that monitor net production of dinitrogen (N₂) and nitrous oxide (N₂O), including the spatial/temporal heterogeneity of denitrification in soils, provide much-needed data to inform models that support management decisions. However, to model denitrification in agricultural ecosystems more accurately, we need data-sets at lab to field scale including reliable measurement of processes and regulating factors of N₂ and N₂O production.

Analysing natural abundance isotopocule values of N₂O (d¹⁵N, d¹⁸O and ¹⁵N site preference) in gas samples from  closed chambers with  data evaluation using the FRAME model (Lewicki, 2022) can be used to distinguish N₂O production pathways and to quantify N₂O reduction to N₂. However, this approach usually fails to distinguish between N₂O production by heterotrophic bacterial denitrification and nitrifier denitrification. Moreover, the accuracy is limited during periods of low activity due to the small fraction of soil-derived N₂O in the samples. This might be overcome by analysing  N₂O isotopocule values of soil air were the fraction of soil-derived N₂O is always higher compared to closed chamber samples.

While N₂ and N₂O fluxes from denitrification can be determined using the ¹⁵N gas flux method (¹⁵NGF), improvement of N₂ sensitivity is needed to detect emissions beyond peak events which can be achieved by establishing an N₂-depleted atmosphere (¹⁵NGF+ method, (Eckei et al., 2024).

Recently, it has been shown that extending the FRAME modelling with results of the ¹⁵NGF conducted in parallel is suitable to better distinguish different denitrification pathways of N₂O production (Micucci et al., 2025). A further advantage of using both approaches is the fact, that FRAME can be easily used outside the lab and in growing crops, while for ¹⁵NGF+, this is very challenging.

We combined three approaches, i.e. (1.) surface fluxes of N₂O isotopocules using the closed chamber method, (2.) Experiments were established on lysimeters with two undisturbed soils cropped with barley.

We used results for FRAME modeling of natural abundance plots and combined them  with ¹⁵NGF+ results to quantify (i) N₂ and N₂O fluxes from the ¹⁵N-labelled NO₃^- pool, (ii) the fraction of N₂O emitted from other (unlabelled) N sources, and (iii) N₂O pathways distinguished by the extended FRAME modelling including heterotrophic bacterial denitrification, nitrifier denitrification, fungal denitrification, nitrification and N₂O reduction to N₂. The latter will be compared to N₂ fluxes obtained by ¹⁵NGF+. First results will be shown.

References:

Eckei, J., et al., 2024. Determining N2O and N2 fluxes in relation to winter wheat and sugar beet growth and development using the improved 15N gas flux method on the field scale. Biology and Fertility of Soils. DOI: 10.1007/s00374-024-01806-z

Lewicki, M.P.D.L.-S., Grzegorz Skrzypek, 2022. FRAME—Monte Carlo model for evaluation of the stable isotope mixing and fractionation. Plos One 17, e0277204.

Micucci, G., et al.., 2025. Combining the 15N Gas Flux Method and N2O Isotopocule Data for the Determination of Soil Microbial N2O Sources. Rapid Communications in Mass Spectrometry 39, e9971.