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

Abstract

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.

Field Application of Organic Fertilizers Triggers N2O Emissions From the Soil N Pool as Indicated by 15N-Labeled Digestates

“…Regular soil N min and 15 N min analysis at the sampling dates could have given a hint for respective microbial processes, but would not have completely identified them. Thus, allocating the specific N 2 O pathways after digestate application in the field is still challenging and needs further research and suitable methods to provide accurate measurements (Well et al, 2019).…”

N<sub>2</sub>O isotope approaches for source partitioning of N<sub>2</sub>O production and estimation of N<sub>2</sub>O reduction – validation with the <sup>15</sup>N gas-flux method in laboratory and field studies

“…Chamber closing time was 120 min, and for one chosen field study (F3) the linearity of N 2 increase over 120 min was checked and confirmed. The flux correction for underestimation due to subsoil flux and gas soil storage (Well et al, 2019b) was not performed because the focus of this paper was to determine r N 2 O , while subsoil diffusion of N 2 and N 2 O is almost identical. This correction would thus not significantly impact r N 2 O .…”

“…But the fluxes shown in Fig. S2 in the Supplement are measured fluxes and include the underestimation of 15 N-based estimates (Well et al, 2019b). For laboratory incubations with the constant flow through, N 2 flux was determined in the same manner as for N 2 O (Eq.…”

“…Due to technical limitations, so far only the 15 N gas-flux method had been applied in field conditions to determine r N 2 O (Aulakh et al, 1991;Baily et al, 2012;Bergsma et al, 2001;Buchen et al, 2016;Decock and Six, 2013;Kulkarni et al, 2013;Mosier et al, 1986). Moreover, the first attempt to apply the 15 N gas-flux method under N 2reduced atmosphere in the field has been presented recently (Well et al, 2019a). This new approach increases the sensitivity of the 15 N gas-flux method (80-fold better sensitivity for N 2 + N 2 O flux measurements; Well et al, 2019a), which was so far very limiting for successful application in field studies (Buchen et al, 2016).…”

“…Moreover, the first attempt to apply the 15 N gas-flux method under N 2reduced atmosphere in the field has been presented recently (Well et al, 2019a). This new approach increases the sensitivity of the 15 N gas-flux method (80-fold better sensitivity for N 2 + N 2 O flux measurements; Well et al, 2019a), which was so far very limiting for successful application in field studies (Buchen et al, 2016). But still, application of this approach is technically very demanding and applicable only with a low temporal and spatial resolution.…”