Vents and seals in non‐steady‐state chambers used for measuring gas exchange between soil and the atmosphere

Abstract

Summary Despite decades of research to define optimal chamber design and deployment protocol for measuring gas exchange between the Earth's surface and the atmosphere, controversy still surrounds the procedures for applying this method. Using a numerical simulation model we demonstrated that (i) all non‐steady‐state chambers should include a properly sized and properly located vent tube; (ii) even seemingly trivial leakiness of the seals between elements of a multiple‐component chamber results in significant risk of measurement error; (iii) a leaking seal is a poor substitute for a properly designed vent tube, because the shorter path length through the seal supports much greater diffusive gas loss per unit of conductance to mass flow; (iv) the depth to which chamber walls must be inserted to minimize gas loss by lateral diffusion is smaller than is customary in fine‐textured, wet or compact soil, but much larger than is customary in highly porous soils, and (v) repetitive sampling at the same location is not a major source of error when using non‐steady‐state chambers. Finally, we discuss problems associated with computing the flux of a gas from the non‐linear increase in its concentration in the headspace of a non‐steady‐state chamber.

Carbon dioxide flux and net primary production of a boreal treed bog: Responses to warming and water-table-lowering simulations of climate change

“…The NE ff was calculated using the exponential change in chamber headspace CO 2 concentration (Kutzbach et al, 2007) regressed with time, as a function of volume, pressure and air temperature inside the chamber, according to the ideal gas law. The exponential regression was applied following because covering the soil and vegetation can manipulate the sponta-neous CO 2 fluxes across the soil-vegetation-air continuum Davidson et al, 2002;Denmead and Reicosky, 2003;Kutzbach et al, 2007), likely due to suppression of natural pressure fluctuations (Hutchinson and Livingston, 2001) and possible alterations in turbulence between measured intervals (Hutchinson et al, 1993). Therefore, the CO 2 fluxes determined using linear regression likely result in an underestimation of fluxes in a closed-chamber environment (Kutzbach et al, 2007).…”

Spatiotemporal variation of event related N2O and CH4 emissions during fertigation in a California almond orchard

“…All these changes can lead to biased measurements of net gas flux. The minimum collar insertion recommended by Rochette and Eriksen-Hamel (2008) is 5 cm and according to Hutchinson and Livingston (2001), the error due to lateral gas diffusion in the soil for a collar insertion of 5 cm and a deployment time of 80 min can be around 10%. During the season we took soil cores to determine wfps, and extractable NH 4 þ and NO 3 À contents as explained in section 3.2 below.…”

Challenges in measuring the δ13C of the soil surface CO2 efflux

“…[72] Pressure differences within the chamber can also cause large errors in the efflux measurements. [38][39][40]71,73] An increase in the pressure inside a chamber has the effect of slowing CO 2 diffusion, which in turn lowers the measured efflux rate. In contrast, a reduction in pressure draws CO 2 from the soil and leads to an overestimation of the true rate.…”

“…In the field the true value is not known so that the best error estimates are derived from modelled data. Hutchinson and Livingston [39] predicted errors of between 1 and 5% in rate measurements in relation to chamber wall insertion depth (ranging between 0 and 46 cm) for a number of different deployment times and soil air-filled porosity values. They concluded that an insertion depth of 10 cm was completely inadequate for dry, porous or coarse-textured soils, and less than 5 cm was adequate for fine-textured, wet or compacted soil.…”