Underestimation of denitrification rates from field application of the ¹⁵N gas flux method and its correction by gas diffusion modelling

Common 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 closed chambers that are placed on the soil surface. Gas diffusion is considered to be the main transport process in the soil. Because accumulation of gases within the chamber decreases concentration gradients between soil and the chamber over time, the surface efflux of gases decreases as well, and gas production rates are underestimated if calculated from chamber concentrations without consideration of this mechanism. Moreover, concentration gradients to the non-labelled subsoil exist, inevitably causing downward diffusion of ¹⁵N-labelled denitrification products. A numerical 3-D model for simulating gas diffusion in soil was used in order to determine the significance of this source of error. Results show that subsoil diffusion of ¹⁵N-labelled N₂ and N₂O – and thus potential underestimation of denitrification derived from chamber fluxes – increases with chamber deployment time as well as with increasing soil gas diffusivity. Simulations based on the range of typical soil gas diffusivities of unsaturated soils showed that the fraction of N₂ and N₂O evolved from ¹⁵N-labelled NO^-₃ that is not emitted at the soil surface during 1 h chamber closing is always significant, with values up to > 50% of total production. This is due to accumulation in the pore space of the ¹⁵N-labelled soil and diffusive flux to the unlabelled subsoil. Empirical coefficients to calculate denitrification from surface fluxes were derived by modelling multiple scenarios with varying soil water content. Modelling several theoretical experimental set-ups showed that the fraction of produced gases that are retained in soil can be lowered by lowering the depth of ¹⁵N labelling and/or increasing the length of the confining cylinder. Field experiments with arable silt loam soil for measuring denitrification with the ¹⁵N gas flux method were conducted to obtain direct evidence for the incomplete surface emission of gaseous denitrification products. We compared surface fluxes of ¹⁵N₂ and ¹⁵N₂O from ¹⁵N-labelled micro-plots confined by cylinders using the closed-chamber method with cylinders open or closed at the bottom, finding 37% higher surface fluxes with the bottom closed. Modelling fluxes of this experiment confirmed this effect, however with a higher increase in surface flux of 89 %. From our model and experimental results we conclude that field surface fluxes of ¹⁵N-labelled N₂ and N₂O severely underestimate denitrification rates if calculated from chamber accumulation only. The extent of this underestimation increases with closure time. Underestimation also occurs during laboratory incubations in closed systems due to pore space accumulation of ¹⁵N-labelled N₂ and N₂O. Due to this bias in past denitrification measurements, denitrification in soils might be more relevant than assumed to date. Corrected denitrification rates can be obtained by estimating subsurface flux and storage with our model. The observed deviation between experimental and modelled subsurface flux revealed the need for refined model evaluation, which must include assessment of the spatial variability in diffusivity and production and the spatial dimension of the chamber.