Underestimation of denitrification rates from field application of the 15N gas flux method and its correction by gas diffusion modelling
Authors: Well Reinhard , Maier Martin , Lewicka-Szczebak Dominika , Köster Jan-Reent , Ruoss Nicolas 
Published: May 2019

Abstract. 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 NO3- 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.