Determination of nitrous oxide processes in soil from depth profiles of natural abundance stable isotope values by diffusion-reaction-fractionation modelling 

<p>Analysing isotopocule values of nitrous oxide (N₂O) produced in soil can be used to distinguish N₂O production pathways and to quantify N₂O reduction to N₂. In the field, this is typically accomplished by analysing gas samples collected from closed chambers and calculating the isotopocule values of soil-emitted N₂O taking into account the fraction of atmospheric N₂O. Accuracy of this approach is often limited when N₂O fluxes are low, leading to small fraction of soil-derived N₂O in the chamber gas. To overcome this limitation, some studies used N₂O isotopocules of soil air, assuming that these reflected N₂O produced in soil (Gallarotti<em> et al.</em>, 2021, Zou<em> et al.</em>, 2014). However, this can lead to inaccurate results because (i) due to bi-directional diffusive gas exchange with the atmosphere, soil air is a mixture of soil-derived and atmospheric N₂O and (ii) isotopic fractionation during diffusive flux to the atmosphere leads to enrichment of residual N₂O in soil air. To evaluate these confounding factors and develop an approach to determine isotopocules of N₂O produced in soil from soil air samples, we compared surface fluxes of N₂O isotopocules determined by the closed chamber method (Lewicka-Szczebak et al. 2020) with gas probe data. Moreover, a diffusion-reaction model (Maier<em> et al.</em>, 2017, Well<em> et al.</em>, 2019) will be extended to include isotopic fractionation in order to determine isotopocule values of produced N₂O from soil air data. Scenarios varying in depth&#8211;dependent N₂O production and diffusivity will be analyzed. Results will show to which extent soil air and production values differ, which bias is obtained by using uncorrected soil air values, how well values can be corrected by modeling, and under which conditions soil air sampling might lead to better performance than closed chamber sampling. We expect that soil air sampling can lead to improved sensitivity for isotopocule values of soil-derived N₂O in certain cases, but correction of data is obligate to obtain useful results.</p><p>&#160;</p><p>&#160;</p><p><strong>End Text</strong></p><p>@END_@</p><p>&#160;</p><p>Gallarotti N, Barthel M, Verhoeven E<em> et al.</em> (2021) In-depth analysis of N₂O fluxes in tropical forest soils of the Congo Basin combining isotope and functional gene analysis. The ISME Journal<em>,</em> <strong>15</strong>, 3357-3374.</p><p>Lewicka-Szczebak D, Lewicki MP, Well R (2020) N₂O isotope approaches for source partitioning of N2O production and estimation of N₂O reduction &#8211; validation with the ¹⁵N gas-flux method in laboratory and field studies. Biogeosciences<em>,</em> <strong>17</strong>, 5513-5537.</p><p>Maier M, Longdoz B, Laemmel T, Schack-Kirchner H, Lang F (2017) 2D profiles of CO₂, CH4, N₂O and gas diffusivity in a well aerated soil: measurement and Finite Element Modeling. Agricultural and Forest Meteorology<em>,</em> <strong>247</strong>, 21-33.</p><p>Well R, Maier M, Lewicka-Szczebak D, K&#246;ster JR, Ruoss N (2019) Underestimation of denitrification rates from field application of the N-15 gas flux method and its correction by gas diffusion modelling. Biogeosciences<em>,</em> <strong>16</strong>, 2233-2246.</p><p>Zou Y, Hirono Y, Yanai Y, Hattori S, Toyoda S, Yoshida N (2014) Isotopomer analysis of nitrous oxide accumulated in soil cultivated with tea (Camellia sinensis) in Shizuoka, central Japan. Soil Biology & Biochemistry<em>,</em> <strong>77</strong>, 276-291.</p>