Comparison of portable devices for sub-ambient concentration measurements of methane (CH_4) and nitrous oxide (N_2O) in soil research

摘要

Production and consumption of methane (CH4) and nitrous oxide (N2O) in soils have a strong influence on global greenhouse gases (GHG) budgets. Therefore, it is crucial to precisely measure GHG fluxes at the soil-atmosphere interface. In upland soils, CH4 and N2O can be consumed by microbiological processes, and the respective concentrations can be lower than in the atmosphere, demanding highly sensitive gas analysing systems. Traditionally, soil air is sampled in vials and analysed in the laboratory by gas chromatography (GC). During the last decade, different technologies have been developed that allowed to build portable gas analysers that are able to measure sub-ambient gas concentration directly in the field. Here, we compared sub-ambient to ambient CH4 and N2O concentration values from four portable devices using different measurement technologies (a portable GHG analyser based on laser absorption spectroscopy [LAS], two portable Fourier transform infrared spectroscopy [FTIR] devices and a field gas analyser using photoacoustic spectroscopy [PAS]) to traditional GC analysis in the laboratory (a GC system equipped with a flame ionisation detector [GC-FID] and an electron capture detector [GC-ECD]). The accuracy and precision of photoacoustic spectroscopy measurements are strongly influenced by the water vapour content and non-target gases in the sampling air. We used an advanced set-up for a widely used PAS analyser enabling N2O measurements at sub-ambient concentrations with similar precision and accuracy as the GC-ECD system. Measurements of CH4 and N2O by FTIR and LAS devices were in good agreement with the GC systems. We conclude that the portable devices are suitable for studies of GHG fluxes in the field. Thanks to their universal and portable character, LAS, PAS and FTIR devices represent useful alternatives to currently used technologies for field studies.