Soil gas flux measurement theory

Soil gases are produced by a variety of biological processes, including root respiration, organic matter decay, and microbial activity. Rainwater interacting with calcareous soils can also create soil gases.

Soil gases primarily diffuse into the atmosphere via air-filled pores and cracks in the soil, but they can also be displaced by rain and are very sensitive to changes in pressure. Pressure gradient changes can be driven by wind, or by mechanical influences, such as chamber placement. The air-filled porosity of the soil varies with soil type and moisture content, so these characteristics can have a significant effect on the movement of gas through soils.

Because soil gas fluxes are dependent on soil temperature, organic content, moisture content, pressure, and precipitation, they exhibit a great deal of temporal and spatial variability.

For soil gas flux measurements to be most accurate, conditions within the chamber must be similar to conditions outside the chamber. These conditions include the concentration gradient driving diffusion, barometric pressure, temperature, and moisture of the soil.

LI-COR long-term chambers are designed to minimize the effect the chamber has on the flux of soil gases and to maintain ambient conditions before, after, and during a measurement. Some of these design considerations include:

  • A patented chamber vent that maintains ambient pressure inside the chamber—even in windy conditions.
  • A uniform chamber shape and air inlet and outlet positioning within the chamber to mix of chamber air without fans.
  • A perforated base to allow the natural exchange of gas, sunlight, and precipitation between the atmosphere and soil.

Soil gas flux varies substantially in both space and time. A long-term soil gas flux system with the LI-8250 Multiplexer allows you to observe both spatial and temporal variability. Up to 36 chambers may be connected at once and configured to take automated measurements at defined intervals year-round. This allows you to examine how periodic events, such as rainfall, influence soil gas flux, and to characterize diurnal, seasonal, and annual flux patterns.

Accounting for an altered diffusion gradient

While the above design considerations can minimize artificial effects, they cannot eliminate them. Upon chamber closing, there is an increase in the mole fraction of the gas being measured inside the chamber—suppressing the efflux. Because of this, the gradient of gas diffusion between the soil surface layer and the air is not the same inside and outside the chamber.

To account for the altered diffusion gradient, the diffusion rate is estimated using an exponential function. This technique reduces errors in flux estimates by taking into account the effect of increasing chamber gas concentration on the diffusion gradient and allows for the estimation of flux at the time of chamber closing—when gas concentration is nearest to ambient levels.

Measurement length

For CO2, CH4, and N2O it is generally recommended to limit measurements to between 90 and 180 seconds. This is an important consideration to ensure that the moisture and temperature of the soil within the measurement collar are similar to the surrounding soil and to minimize the effect of altered diffusion.

For example, a 2-minute measurement made once every 30 minutes leaves the soil fully exposed to sun, wind, and precipitation more than 93% of the time. This keeps gas concentration changes due to the chamber to a minimum. However, some gases, such as H2O, may need measurements up to 15 minutes or longer to reliably calculate their flux.

The measurement cycle

The measurement cycle begins with an optional prepurge period with the chamber open, after which the chamber closes. Upon closing, the deadband starts. The LI-8250 Multiplexer begins logging data and continues to do so for the duration of the observation. When the observation is finished the postpurge begins. Then the cycle repeats as configured.

Figure A‑1. A typical measurement cycle.


Provided is a list of various terms used in a measurement and in the user interface.

Collar height

Defined as the distance from the soil surface and the upper edge of the chamber base plate. The measurement is used to compute the volume of the system. See Measuring the collar height for instructions.


The period, usually between 10 and 30 seconds, from complete chamber closing until steady mixing is achieved and the measurement begins. Deadband requirements change based on chamber geometry, system flow rate, collar height, and site characteristics and should be optimized in post-processing using SoilFluxPro Software.

Tubing volume

The volume of the tubing from the multiplexer to the chamber, including any tubing to an extension manifold. To determine tubing volume, measure the combined length of all tubing (AIR IN and AIR OUT) and use the following information to make your calculations: 1 cm of 1/4" outer-diameter Bev-A-Line® tubing adds approximately 0.079 cm3 of volume or 7.9 cm3 per meter.

To simply calculate extension tube volume, multiply the total length in meters by 7.91 for both the AIR IN and AIR OUT tubing. For example:

A‑115 m × 7.91 × 2 = 237 cm3

Measurement start

The start of a measurement. You can begin a measurement using the Start button on the Home page. This will begin the sampling sequence based on the configuration you outlined in Configuring the Sampling Sequence block.

Observation length

The period from complete chamber closing until just before it opens, including the specified deadband. You will need to optimize the observation length for the conditions of your site. For example, if you are measuring fluxes in poor soil (low organic matter) or dry conditions, you may need to extend the observation length to allow more gas to build-up and improve the signal-to-noise ratio.

Note: The LI-8250 Multiplexer begins logging data when the chamber starts to close and continues throughout the entire observation length. The elapsed time does not begin until the chamber is closed.


In multiplexed systems, gases may accumulate in open chambers. Prepurge turns on the pump to mix the air in an open chamber and bring conditions closer to ambient. You will need to optimize the prepurge time for your site. For example, a slightly longer prepurge may be needed in very sandy or soft soils where the chamber movement may disturb the flux.

After an observation, the chamber will automatically rise off the soil collar. If there is more than one observation specified for that chamber, the prepurge sets the time during which the chamber is open. Under still conditions, it may take 2 minutes or more for the chamber air to return to ambient conditions. Under windy conditions, the chamber air may return to ambient levels in as little as 20 or 30 seconds.


The period of airflow through the chamber after the measurement is complete and as the chamber opens. This is important where environmental factors may influence the amount of greenhouse gases, such as CO2, or moisture present in the gas sampling lines. For example, in hot, moist conditions, you may want to increase the postpurge to ensure that the gas sampling lines are purged of moisture that may condense in the lines. In most cases, a postpurge of about 45 seconds is adequate.

Soil area

The surface area of soil in the chamber. For a 20 cm soil collar, the soil area is 317.8 cm2. For home-made soil collars, you will need to compute surface area.

Total system volume

The total calculated volume for all components of the system, including the chamber, multiplexer, extension manifold, gas analyzer(s), and tubing.