Understanding Soil Trace Gas Flux Measurements with Automatic Chambers: System Design, Analyzers, Detection Limits, and Automated Data Analysis

Measurements of soil trace gas fluxes are crucial for understanding their atmospheric budgets and assessing the impacts of land use and management practices on these budgets. Automatic chambers coupled with sensitive gas analyzers offer valuable insights into the temporal and spatial variability of trace gas fluxes, advancing our understanding of the mechanisms governing these emissions.

Recent developments in “plug-and-play” automatic chambers, equipped with built-in programs for flux calculation, have simplified soil flux measurements. However, before these new instruments can be effectively utilized for flux analysis, it is essential to fully understand the data they produce.

In this study, we used an array of seven automatic chambers coupled to two infrared gas analyzers (IRGA) to measure soil fluxes of three major greenhouse gases: carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) in a Negev Desert of Israel. Measuring soil fluxes in this environment poses significant challenges, including extreme temperatures (reaching ~60 °C during dry summers and freezing during winters), dust and rainstorms, and very low fluxes approaching the detection limits of state-of-the-art instruments.

To calculate fluxes, we developed an in-house program for flux analysis capable of handling data from multiple sensors connected to automatic static chambers. We compared this program's performance with the manufacturer-supplied software. Additionally, we created an empirical method to assess the limit of detection (LOD) for measured fluxes and compared these empirical LODs with calculated values.

We found that the measured LOD was ~1000 times larger than the calculated LOD, with the discrepancy primarily stemming from minor pressure fluctuations near the soil surface. After applying appropriate corrections for LOD, we observed the temporal variability of CH₄, N₂O, and CO₂ fluxes from desert soils with varying carbon (C) content.

Surprisingly, while N₂O fluxes were effectively zero and CO₂ emissions exhibited a diurnal cycle peaking around noon, CH₄ fluxes were consistently positive (indicating net emissions to the atmosphere). These CH₄ emissions correlated with soil C content but not with soil moisture. Furthermore, emissions were notably higher during the dry summer compared to the wetter winter season. We attribute these unexpected CH₄ emissions to the photodegradation of soil C, driven by high soil temperatures and intense solar radiation during summer months.