Quantifying soil denitrification in situ from depth profiles of 15N labelled denitrification products by diffusion-reaction modelling

Common field methods for measuring soil denitrification in situ include monitoring the accumulation of ¹⁵N labelled N₂ and N₂O evolved from ¹⁵N labelled soil nitrate pool in soil surface chambers. Bias of denitrification rates derived from chamber measurements results from subsoil diffusion of ¹⁵N labelled denitrification products, but this can be corrected by diffusion modeling (Well et al., 2019). Moreover, precision of the conventional ¹⁵N gas flux method is low due to the high N₂ background of the atmosphere. An alternative to the closed chamber method is to use concentration gradients of soil gas to quantify their fluxes (Maier &  Schack-Kirchner, 2014). Advantages compared to the closed  chamber method include the facts that (i) time consuming work with closed chambers is replaced by easier sampling of soil gas probes, (ii) depth profiles yield not only the surface flux but also information on the depth distribution of gas production and (iii) soil gas concentrations are higher than chamber gas concentration, resulting in better detectability of ¹⁵N-labelled denitrification products. Here we use this approach for the first time to evaluate denitrification rates derived from depth profiles of ¹⁵N labelled N₂ and N₂O in the field by closed chamber measurements published previously (Lewicka-Szczebak et al., 2020).

We compared surface fluxes of N₂ and N₂O from ¹⁵N labelled microplots using the closed chamber method. Diffusion–based soil gas probes (Schack-Kirchner et al., 1993) were used to sample soil air at the end of each closed chamber measurement. A diffusion-reaction model (Maier et al., 2017) will be  used to fit measured and modelled concentrations of ¹⁵N labelled N₂ and N₂O. Depth-specific values of denitrification rates and the denitrification product ratio will be obtained from best fits of depth profiles and chamber accumulation, taking into account diffusion of labelled denitrification products to the subsoil (Well et al., 2019).

Depending on the outcome of this evaluation, the gradient method could be used for continuous monitoring of denitrification in the field based on soil gas probe sampling. This could replace or enhance current approaches by improving the detection limit, facilitating sampling and delivering information on depth-specific denitrification.  

References:

Lewicka-Szczebak D, Lewicki MP, Well R (2020) N2O isotope approaches for source partitioning of N2O production and estimation of N2O reduction – validation with the 15N gas-flux method in laboratory and field studies. Biogeosciences, 17, 5513-5537.

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, 247, 21-33.

Maier M, Schack-Kirchner H (2014) Using the gradient method to determine soil gas flux: A review. Agricultural and Forest Meteorology, 192, 78-95.

Schack-Kirchner H, Hildebrand EE, Wilpert KV (1993) Ein konvektionsfreies Sammelsystem für Bodenluft. Zeitschrift Fur Pflanzenernahrung Und Bodenkunde, 156, 307-310.

Well R, Maier M, Lewicka-Szczebak D, Koster 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, 16, 2233-2246.