Regional characterization of N2O isotopic composition emitted from soils in view of land cover, agricultural management and annual cycles based on measurements and modeling

While the global budget of nitrous oxide (N₂O) is rather well constrained from a “top-down” perspective considering the change in the atmospheric burden and stratospheric N₂O destruction, estimates of the various sources such as natural/agricultural soils, coastal areas or fossil fuel burning and industry remain uncertain. The isotopic composition of N₂O, i.e., the relative abundances of the four most abundant isotopic species (¹⁴N¹⁴N¹⁶O, ¹⁵N¹⁴N¹⁶O, ¹⁴N¹⁵N¹⁶O, and ¹⁴N¹⁴N¹⁸O) have been identified as instrumental tools for attributing emissions to the corresponding production-consumption processes and to estimate the global budget. During the past two decades, N₂O isotopic composition of individual sources has been investigated, and temporal trends in the isotopic composition of atmospheric N₂O have been studied using and firn air and archived air samples collected in Antarctica. With regard to ¹⁵N and ¹⁸O in atmospheric N₂O, a decreasing trend was consistently observed across studies, but contradictory results have been obtained for site preference (SP), i.e., the difference in the abundances of ¹⁵N¹⁴N¹⁶O and ¹⁴N¹⁵N¹⁶O relative to ¹⁴N¹⁴N¹⁶O. In addition, N₂O isotopic composition for natural or agricultural soils rely on a limited amount of studies and usually cover only parts of the annual cycle.

Since instruments used for optical isotope ratio spectroscopy (OIRS) can be deployed in the field, OIRS offers the opportunity to better characterize individual sources through long-term data in high temporal resolution. However, application of OIRS is challenging and, thus, remains scarce with regard to spatial resolution. For this reason, model-based regional estimates are pertinent to overcome the lack of regional estimates of N₂O isotopic composition, to analyze trends, and to provide data for a refinement of the global budget.

To obtain regional-scale (Switzerland) model-based estimates of N₂O isotopic composition, we used data sets of measured N₂O isotopic composition of two sites that are based on OIRS, and applied the Stable Isotope MOdel for Nutrient cyclEs, SIMONE in conjunction with the biogeochemical model LandscapeDNDC. Our results show that SIMONE/LandscapeDNDC was capable of reflecting especially SP, but also ¹⁵N-N₂O at sites with different soil properties. For agricultural soils, our simulations revealed an annual cycle in SP, with higher values during the growing season, but not for ¹⁵N-N₂O. We will also discuss effects of agricultural management on N₂O emissions as well as temporal trends.