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Abstract
Abstract. We developed a mass spectrometric soil-gas flux measurement system using a portable high-resolution multi-turn time-of-flight mass spectrometer, called MULTUM, and we combined it with an automated soil-gas flux chamber for the continuous field measurement of multiple gas concentrations with a high temporal resolution. The developed system continuously measures the concentrations of four different atmospheric gases (NO2, CH4, CO2, and field soil–atmosphere flux measurements of greenhouse gases (NO2, O2) ranging over 6 orders of magnitude at one time using a single gas sample. The measurements are performed every 2.5 min with an analytical precision (2 standard deviations) of ±34 ppbv for NO2; ±170 ppbv, CH4; ±16 ppmv, CO2; and ±0.60 vol %, O2 at their atmospheric concentrations. The developed system was used for the continuous field soil–atmosphere flux measurements of greenhouse gases (NO2, CH4, and CO2) and O2 with a 1 h resolution. The minimum quantitative fluxes (2 standard deviations) were estimated via a simulation as 70.2 µgNm-2h-1 for NO2; 139 µgCm-2h-1, CH4; 11.7 mg C m−2 h−1, CO2; and 9.8 g O2 m−2 h−1, O2. The estimated minimum detectable fluxes (2 standard deviations) were 17.2 µgNm-2h-1 for NO2; 35.4 µgCm-2h-1, CH4; 2.6 mg C m−2 h−1, CO2; and 2.9 g O2 m−2 h−1, O2. The developed system was deployed at the university farm of the Ehime University (Matsuyama, Ehime, Japan) for a field observation over 5 d. An abrupt increase in NO2 flux from 70 to 682 µgNm-2h-1 was observed a few hours after the first rainfall, whereas no obvious increase was observed in CO2 flux. No abrupt NO2 flux change was observed in succeeding rainfall events, and the observed temporal responses at the first rainfall were different from those observed in a laboratory experiment. The observed differences in temporal flux variation for each gas component show that gas production processes and their responses for each gas component in the soil are different. The results of this study indicate that continuous multiple gas concentration and flux measurements can be employed as a powerful tool for tracking and understanding underlying biological and physicochemical processes in the soil by measuring more tracer gases such as volatile organic carbon, reactive nitrogen, and noble gases, and by exploiting the broad versatility of mass spectrometry in detecting a broad range of gas species.
Highlights
Soil acts either as a source or a sink for various atmospheric gases such as greenhouse gases (GHGs; NO2, CO2, and CH4) (Oertel et al, 2016; Ito et al, 2018), oxygen (O2 (Turcu et al, 2005; Huang et al, 2018) and biogenic volatile organic compounds (BVOCs) (Insam and Seewald, 2010; Peñuelas et al, 2014; Szog et al, 2017; Mäki et al, 2019)
The room temperature was maintained at 23 ± 1 ◦C and the relative humidity was around 15 % at the beginning of the measurement; it increased to %–33 % after the midnight of January 2019
These variations may be subject to the natural variabilities of atmospheric concentrations; we consider that they are instrumental variations because their distributions demonstrated good agreement with Gaussian distributions (Fig. 7) and the analytical precision obtained from the measurements of standard no. 1 and O2 standard in the laboratory (±34 ppbv for NO2; ±170 ppbv, CH4; ±16 ppmv, CO2; and ±0.60 vol %, O2, 2σ ) almost corresponded to those obtained from atmospheric air
Summary
Soil acts either as a source or a sink for various atmospheric gases such as greenhouse gases (GHGs; NO2, CO2, and CH4) (Oertel et al, 2016; Ito et al, 2018), oxygen (O2 (Turcu et al, 2005; Huang et al, 2018) and biogenic volatile organic compounds (BVOCs) (Insam and Seewald, 2010; Peñuelas et al, 2014; Szog et al, 2017; Mäki et al, 2019). The BVOCs are produced by soil microorganisms, soil fungi, and even plant roots (Peñuelas et al, 2014), and there does not seem to be a simple intermediate/final product of the metabolic cycles and microbial decomposition of organic matter. The simultaneous measurement of multiple soil gases with a higher time resolution is expected to be considerably advantageous to gain a better understanding of soil biological and physicochemical processes and to gauge their environmental effects. Such simultaneous measurements of multiple soil gases remain challenging because of the lack of suitable measurement technology
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