A mass spectrometric multiple soil-gas flux measurement system with portable high-resolution mass spectrometer MULTUM coupled to automatic chamber for continuous field observation

We developed a mass spectrometric soil-gas flux measurement system using a portable high-resolution multi-turn time-of-flight mass spectrometer, called MULTUM, combined with an automated soil-gas flux chamber for continuous field measurement of multiple gas concentrations. The developed system continuously measures concentrations of four different atmospheric gases (i.e., N2O, CH4, CO2, and O2), of which the concentrations range over six orders of magnitude at a time within a single gas sample. The measurements were performed every 2.5 min with analytical precisions (two standard deviations) of ±34 ppbv for N2O, ±170 ppbv for CH4, ±16 ppmv for CO2, and ±0.60 vol% for O2 at their atmospheric concentrations. The developed system was used for continuous field soil–atmosphere flux measurements of greenhouse gases (GHGs: N2O, CH4, and CO2) and O2 with 1 h resolution. The minimum quantitative fluxes (two standard deviations) were estimated through simulation as 70.2 µg N m−2 h−1 for N2O, 139 µg C m−2 h−1 for CH4, 11.7 mg C m−2 h−1 for CO2, and 9.8 g O2 m−2 h−1 (negative) for O2, whereas the estimated minimum detectable fluxes (two standard deviations) were 17.2 μg N m−2 h −1 for N2O, 35.4 μg C m−2 h−1 for CH4, 2.6 mg C m−2 h−1 for CO2, and 2.9 g O2 m−2 h−1 for O2. The developed system was deployed in the University Farm of the Ehime University (Matsuyama, Ehime, Japan) for a field observation over five days. Interestingly, an abrupt increase in N2O flux from 70 to 682 µg N m−2 h−1 was observed a few hours after the first rainfall, whereas no obvious increase in the CO2 flux was observed, although the temporal responses were different from those observed in a laboratory experiment. No abrupt N2O flux change was observed in succeeding rainfalls. Continuous multiple-gas flux and concentration measurements can be a powerful tool for tracking and understanding of underlying biological and physicochemical processes in the soil through measuring more tracer gases, such as volatile organic carbon gases, reactive-nitrogen gases, and noble gases by taking advantage of the broad versatility of mass spectrometry in detecting broad range of gas species.