Regional characterization of N2O isotopic composition emitted from soils in view of land cover, agricultural management and annual cycles based on measurements and modeling
Authors: Wolf Benjamin , Haas Edwin , Kraus David , Kiese Ralf , Butterbach-Bahl Klaus 
Published: March 2020

&lt;p&gt;While the global budget of nitrous oxide (N&lt;sub&gt;2&lt;/sub&gt;O) is rather well constrained from a &amp;#8220;top-down&amp;#8221; perspective considering the change in the atmospheric burden and stratospheric N&lt;sub&gt;2&lt;/sub&gt;O 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 N&lt;sub&gt;2&lt;/sub&gt;O, i.e., the relative abundances of the four most abundant isotopic species (&lt;sup&gt;14&lt;/sup&gt;N&lt;sup&gt;14&lt;/sup&gt;N&lt;sup&gt;16&lt;/sup&gt;O, &lt;sup&gt;15&lt;/sup&gt;N&lt;sup&gt;14&lt;/sup&gt;N&lt;sup&gt;16&lt;/sup&gt;O, &lt;sup&gt;14&lt;/sup&gt;N&lt;sup&gt;15&lt;/sup&gt;N&lt;sup&gt;16&lt;/sup&gt;O, and &lt;sup&gt;14&lt;/sup&gt;N&lt;sup&gt;14&lt;/sup&gt;N&lt;sup&gt;18&lt;/sup&gt;O) 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, N&lt;sub&gt;2&lt;/sub&gt;O isotopic composition of individual sources has been investigated, and temporal trends in the isotopic composition of atmospheric N&lt;sub&gt;2&lt;/sub&gt;O have been studied using and firn air and archived air samples collected in Antarctica. With regard to &lt;sup&gt;15&lt;/sup&gt;N and &lt;sup&gt;18&lt;/sup&gt;O in atmospheric N&lt;sub&gt;2&lt;/sub&gt;O, 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 &lt;sup&gt;15&lt;/sup&gt;N&lt;sup&gt;14&lt;/sup&gt;N&lt;sup&gt;16&lt;/sup&gt;O and &lt;sup&gt;14&lt;/sup&gt;N&lt;sup&gt;15&lt;/sup&gt;N&lt;sup&gt;16&lt;/sup&gt;O relative to &lt;sup&gt;14&lt;/sup&gt;N&lt;sup&gt;14&lt;/sup&gt;N&lt;sup&gt;16&lt;/sup&gt;O. In addition, N&lt;sub&gt;2&lt;/sub&gt;O isotopic composition for natural or agricultural soils rely on a limited amount of studies and usually cover only parts of the annual cycle.&lt;/p&gt;&lt;p&gt;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 N&lt;sub&gt;2&lt;/sub&gt;O isotopic composition, to analyze trends, and to provide data for a refinement of the global budget.&lt;/p&gt;&lt;p&gt;To obtain regional-scale (Switzerland) model-based estimates of N&lt;sub&gt;2&lt;/sub&gt;O isotopic composition, we used data sets of measured N&lt;sub&gt;2&lt;/sub&gt;O isotopic composition of two sites that are based on OIRS, and applied the &lt;strong&gt;S&lt;/strong&gt;table &lt;strong&gt;I&lt;/strong&gt;sotope &lt;strong&gt;MO&lt;/strong&gt;del for &lt;strong&gt;N&lt;/strong&gt;utrient cycl&lt;strong&gt;E&lt;/strong&gt;s, SIMONE in conjunction with the biogeochemical model LandscapeDNDC. Our results show that SIMONE/LandscapeDNDC was capable of reflecting especially SP, but also &lt;sup&gt;15&lt;/sup&gt;N-N&lt;sub&gt;2&lt;/sub&gt;O 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 &lt;sup&gt;15&lt;/sup&gt;N-N&lt;sub&gt;2&lt;/sub&gt;O. We will also discuss effects of agricultural management on N&lt;sub&gt;2&lt;/sub&gt;O emissions as well as temporal trends.&lt;/p&gt;