Measuring Gas Exchanges of Soil and Vegetation with Large, Transparent Gas Accumulation Chambers

Direct measurements of greenhouse gas exchange between the land surface and the atmosphere are critical in developing our understanding of the underlying biogeochemical processes and in parameterizing global climate models. Gas accumulation chambers are regularly used to make localized soil gas flux measurements and there are a variety of chamber implementations with a range of operational characteristics. One developing niche of applications is the use of tall transparent chambers with large surface areas, which enclose vegetation while allowing natural light in, facilitating the measurement of soil and vegetation net gas flux (i.e. NEE) in a way that many chamber designs don't allow. This direct measurement can be valuable in characterizing the flow of gases in an ecosystem without needing to resort to indirect measurements.

Despite the benefits of these types of measurements, chambers with a large volume require additional considerations to ensure high quality data is produced. The volume creates difficulties with adequately mixing gas within the chamber headspace to draw an accurate sample. Chamber transparency allows light in but acts as a greenhouse while the chamber is closed thereby increasing the internal temperature. Depending on the implementation, tall soil collars or chamber bases can additionally alter the natural turbulence and heat exchange in the boundary layer causing a buildup of gases and excess heat, which can lead to high humidity and condensation in the chamber headspace. In order to ensure accurate chamber-based measurements, these impacts must be considered and mitigated where appropriate.

Here, we use a variety of commercially available chamber configurations in addition to peripheral environmental measurements in order to understand their impact on flux measurements. We deployed three large footprint (52 cm diameter) chambers, with overall heights of 45 cm, 75cm, and 115cm, each on identical terrain. We present accumulation curves and flux data from these chambers, contextualized with soil and meteorological measurements, including an in-chamber temperature profile within the tallest chamber. Finally, we discuss how prominent the preceding factors are, the effect they have on chamber measurements, and how they can be accounted for to produce high quality data.