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.
Nitrous oxide (N2O) emissions from agricultural land are often estimated by measuring changes in N2O concentrations over a given period in the headspace of a gas‐sampling chamber covering a specific soil area. This technique is particularly challenging in tall growing row crops such as maize (Zea mays L.), to which farmers regularly apply fertilizer banded below the seeds to ensure good crop development. Placing chambers in the inter‐row space leads to bias in flux measurements, due to exclusion of fertilized and rhizosphere soil. Chambers for N2O flux measurements should therefore be placed centered over the row. A new split chamber for gas sampling was developed in this study from a closed, rectangular chamber (original chamber: 78 cm × 78 cm, 51 cm height). The new chamber is applicable for use for the complete maize growing cycle until harvest. For each flux measurement, the two parts of the chambers are placed in a gas‐tight seal on a collar previously inserted into soil covering a representative area of land. In a later growth stage, when plant height exceeds chamber height, stalks of developed maize plants can be fixed between the two chamber parts through a rubber‐tightening opening on the top of the chamber. Air tightness of the split chamber was tested in the laboratory and the split chamber was compared with the original chamber in a field experiment with slurry injection under maize seeds. The laboratory test demonstrated similar air tightness of both chamber types. The field test yielded almost identical N2O fluxes for the original chamber (244 µg N2O‐N m−1 h−1) and the split‐chamber (254 µg N2O‐N m−1 h−1). It can be concluded that the split chamber is an adequate gas‐sampling unit, with particular advantages when flux measurements are conducted in tall growing row crops.
<p><strong>Abstract.</strong> Common methods for measuring soil denitrification in situ include monitoring the accumulation of <sup>15</sup>N-labelled N<sub>2</sub> and N<sub>2</sub>O evolved from <sup>15</sup>N-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 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 <sup>15</sup>N 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 <sup>15</sup>N-labelled N<sub>2</sub> and N<sub>2</sub>O &#8211; and thus potential underestimation of denitrification derived from chamber fluxes &#8211; 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 N<sub>2</sub> and N<sub>2</sub>O evolved from <sup>15</sup>N-labelled NO<sub>3</sub> that is not emitted at the soil surface during one hour chamber closing is always significant with values up to >&#8201;50&#8201;% of total production due to accumulation in the pore space of the <sup>15</sup>N-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.</p> <p>Field experiments with arable silt loam soil for measuring denitrification with the <sup>15</sup>N gas flux method were conducted to obtain direct evidence for the incomplete surface emission of gaseous denitrification products. We compared surface fluxes of <sup>15</sup>N<sub>2</sub> and <sup>15</sup>N<sub>2</sub>O from <sup>15</sup>N&#8211;labelled micro-plots confined by cylinders using the closed chamber method with cylinders open or closed at the bottom, finding 37&#8201;% higher surface fluxes with bottom closed. Modeling fluxes of this experiment confirmed this effect, however with a higher increase in surface flux of 89&#8201;%.</p> <p>From our model and experimental results we conclude that field surface fluxes of <sup>15</sup>N-labelled N<sub>2</sub> and N<sub>2</sub>O 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 <sup>15</sup>N-labelled N<sub>2</sub> and N<sub>2</sub>O. 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 modeled 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.</p>
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