Improving and combining isotopic approaches to optimize sensitivity and accuracy of N2 and N2O fluxes in the field

Nitrous oxide (N₂O) emissions contribute notably to the greenhouse effect and are driven mainly by agricultural practices, while nitrogen (N) losses as N₂O and dinitrogen (N₂) also impair plant N nutrition. The ¹⁵N gas-flux method (¹⁵NGF) can be used for the direct quantification of N₂ and N₂O from denitrification, while the natural abundance isotopic composition of N₂O provides valuable clues about its microbial sources and its reduction to N₂. However, both methods suffer from limited sensitivity, causing field data sets to have gaps when fluxes are below the detection limit. Soil air sampling can, in principle, overcome these limitations. Accounting for diffusive isotopic effects, admixture with atmospheric N₂O, and changes in produced N₂ and N₂O during transport in the soil remains challenging. To evaluate detection limits and to correct raw data, calibration with standard gases that cover the isotopic range of the experimental samples is required. Until recently, suitable gases were not commercially available. Our aim is to develop and test solutions that overcome these limitations.

To obtain continuous field and lab data, we combined results from conventional N₂O flux studies with isotopic data. ¹⁵NGF was applied in the field under normal atmosphere as well as under artificially N₂-depleted atmosphere (¹⁵NGF+) to improve detection limits. Additionally, under normal atmosphere, chamber accumulation was extended to 20 hours and soil air was analyzed. Precision and bias were evaluated using custom-made gas standards. In parallel treatments, isotopic N₂O fluxes at natural abundance were determined and evaluated using the N₂O isotopocule mapping approach to evaluate N₂O pathways. To enhance our understanding of N₂O processes, a combined approach of ¹⁵NGF and N₂O isotopocules is also promising. All approaches were compared to evaluate how the data can be combined to obtain continuous field flux data.