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Agricultural & Environmental Letters - Research Letters

Simultaneous Measurements of Soil CO2 and CH4 Fluxes Using Laser Absorption Spectroscopy

 

This article in AEL

  1. Vol. 1 No. 1 150014
    unlockOPEN ACCESS
     
    Received: Dec 09, 2015
    Accepted: Jan 21, 2016
    Published: April 1, 2016


    * Corresponding author(s): rachhpal.jassal@ubc.ca
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doi:10.2134/ael2015.12.0014
  1. Rachhpal S. Jassal *a,
  2. Cameron Websterb,
  3. T. Andrew Blacka,
  4. Iain Hawthornec and
  5. Mark S. Johnsonbc
  1. a Faculty of Land and Food Systems, Univ. of British Columbia, Vancouver, Canada
    b Institute for Resources, Environment and Sustainability, Univ. of British Columbia, Vancouver, Canada
    c Dep. of Earth, Ocean and Atmospheric Sciences, Univ. of British Columbia, Vancouver, Canada
Core Ideas:
  • A method for simultaneous measurements of soil CO2 and CH4 fluxes is presented.
  • A laser-based cavity ring-down spectrometer is coupled to automated chambers.
  • A differential equation is solved for the small flow that is exhausted.
  • The system allowed using linear fit to mixing ratio versus chamber closure time.
  • Veff increased by 7% due to GHG adsorption and 4% due to soil porosity.

Abstract

We present a method of simultaneously measuring soil CO2 and CH4 fluxes using a laser-based cavity ring-down spectrometer (CRDS) coupled to an automated non-steady-state chamber system. The differential equation describing the change in the greenhouse gas (GHG) mixing ratio in the chamber headspace following lid closure is solved for the condition when a small flow rate of chamber headspace air is pulled through the CRDS by an external pump and exhausted to the atmosphere. The small flow rate allows calculation of fluxes assuming linear relationships between the GHG mixing ratios and chamber lid closure times of a few minutes. We also calibrated the chambers for effective volume (Veff) and show that adsorption of the GHGs on the walls of the chamber caused Veff to be 7% higher than the geometric volume, with the near-surface soil porosity causing another 4% increase in Veff.


Worldwide concern about global climate change and its effects on our future environment requires a better understanding of the global carbon cycle and emissions of greenhouse gases (GHGs). The magnitude of soil CO2 and CH4 fluxes and their importance in the global climate system has resulted in a dramatic increase in the number of such studies in recent years. This has uncovered many new problems and opportunities, such as how to make unbiased measurements at the desired spatial and temporal scales, as well as efforts to measure more than one GHG simultaneously.

Automated chamber systems have been used extensively to measure soil CO2 efflux at many research sites around the world (e.g., Jassal et al., 2012). The non-steady-state chamber technique uses the measurement of rate of change in the chamber headspace gas mixing ratio (dc/dt) and uses a model to extrapolate it to the pre-lid-closure time (i.e., time = 0) to enable flux calculations. The chamber headspace mixing ratio does not change linearly with time due to a declining difference in mixing ratios between the soil and the chamber headspace. However, Jassal et al. (2012) showed that over short time scales of a few minutes, a linear model will cause negligible error in the calculated flux. Measurement of soil CH4 fluxes has been generally restricted to using static chamber method, mainly due to nonavailability of robust low-cost portable CH4 analyzers. Lai et al. (2012) and Savage et al. (2014) described simultaneous measurement of soil CO2 and CH4 fluxes with automated chamber systems but by using separate sensors for the two gases. In this study, we developed a method of simultaneously measuring soil CO2 and CH4 fluxes using a laser-based cavity ring-down spectrometer coupled to automated non-steady-state chambers.


Materials and Methods

Chamber System Assembly and Operation

An automatic non-steady-state chamber system was installed in a constant temperature (20°C) laboratory. The system consisted of two 0.75-m-long, 0.52-m-i.d. polyvinyl chloride cylinders and two automated chambers (Fig. 1) with a total headspace volume of 61 L each, a data logger (CR1000, Campbell Scientific Inc.), a cavity ring-down spectrometer (CRDS) (Model G2301-f, Picarro Inc.), and an external vacuum pump (Model MD 4 NT, Vacuubrand GMBH). The cylinders were filled with 10 cm of washed gravel (20 mm crushed rock), topped by 15 cm of 4 to 10 mm soil (saved from laboratory sieving of the air-dry soil) overlain by 50 cm of <4 mm soil at the top. The soil (humo-ferric podzol) was packed to a uniform bulk density of ∼1350 kg m−3.

Fig. 1.
Fig. 1.

A schematic flow diagram of a laser-based cavity ring-down spectrometer (CRDS) coupled to an automated chamber system with two chambers. Two solenoid valves connect one of the two chambers to the CRDS and with an external pump pulling chamber headspace air at 0.25 L min−1 (LPM), which is then exhausted to the atmosphere.

 

Coupling the CRDS to the chamber system (Fig. 1) enabled us to measure fluxes of both CO2 (FCO2) and CH4 (FCH4) simultaneously. The CRDS measures headspace CO2, CH4, and water vapor concentrations, allowing determination of GHG mixing ratios. Each chamber was equipped with two small (40-mm, 12 V, 60 mA, 116 L min−1) internal fans (Model Mini-Kaze SY124010L, Scythe USA Inc.) to ensure thorough mixing of the headspace air within the chamber. A 30-cm-long, 3-mm-i.d. vent tube (Model Synflex 1300, Saint-Gobain Performance Plastics) was installed in the dome to prevent the effects of fluctuations in atmospheric pressure in the laboratory. Soil temperature (Ts) and volumetric water content (θ) were measured at the 5- to 10- and 20- to 25-cm depths using GS3 and 5TE (Decagon Devices Inc.) sensors, respectively.

Data acquisition and scheduling of chamber lid opening and closing were facilitated through the use of the data logger. Sample tubes connected each chamber to the CRDS with airflow controlled by a solenoid valve, allowing alternation between chambers. The vacuum pump pulled chamber headspace air through the CRDS at 0.25 L min−1, which was then exhausted, and the loss of volume replaced by ambient air via the vent tube, which slightly dilutes the chamber headspace air. The exhaust air, which was heated up by as much as 20°C above ambient due to the operation of the CRDS and the external pump, was not recirculated to the chamber, thereby avoiding a potential heating effect on the measurements. Each chamber was closed for 4 min at half-hour intervals. The data from the CRDS and the CR1000 were combined and the fluxes were estimated for each half-hour interval. Following chamber closure, the first 20 s of measurements was disregarded to allow any disturbance effect to dissipate (Jassal et al., 2012), and the fluxes were estimated using linear regression between the mixing ratios and time using the 20 to 120 s data.

Flux Calculation

The differential equation describing the change in GHG mixing ratio in the chamber headspace following lid closure (Jassal et al., 2012) can be written aswhere c is the GHG mixing ratio (mol mol−1 dry air) in the chamber headspace at time t, ca is the ambient mixing ratio, ρ is the molar density of dry air (mol m−3), V is the chamber volume (m3), Q is the flow rate (m3 s−1) through the CRDS, F is the GHG flux (mol m−2 s−1), A is the soil surface area (m2), and t is time (s). Integrating Eq. [1] for ca at t = 0 to c at t gives the following:

For small values of Qt/V, that is, small flow rates and small chamber lid closure times but larger V, it can be shown, by using Taylor series expansion, that 1 – e-Qt/V is well approximated by Qt/V. Substituting this for 1 – e-Qt/V in Equation (2) giveswhich shows that for time-invariant F, c changes nearly linearly with t. Calculations of the relative error in approximating 1 e-Qt/V by Qt/V show that for Q of 0.25 L min−1, flux calculation time of 2 min, and V of 61 L, neglecting the error due to the small flow rate, and the associated small dilution effect, would result in a flux error of 0.4% (4% if V were 10 times smaller), which is far lower than the uncertainty in chamber measurements (Jassal et al., 2012).

Testing and Calibration of the System

It has been shown that due to the air-filled pore spaces within the near-surface soil (Jassal et al., 2012) and adsorption of gases of interest onto chamber walls (Bloom et al., 1980), the effective volume (Veff) of the chamber during flux measurements is generally higher than the geometric volume (Vg) of the chamber (Jassal et al., 2012). We measured Veff of our chambers for both CO2 and CH4 following the technique proposed by Goulden and Crill (1997) and using 20.07% v/v CO2 and 0.10% v/v CH4 as the calibration gases. Furthermore, we attempted to determine the contributions from adsorption of gases on the plastic walls of the chambers and that from the air-filled pore spaces in the near-surface soil by measuring Veff with an aluminum plate attached to the bottom of the chambers, and by installing the chambers on the soil column, respectively. Three replicate measurements were made, and in the case of the chambers on the soil column, measurements were made with θ of 0.21 m3 m−3 at the 5-cm depth. We also studied the effect of θ on soil CO2 and CH4 fluxes by simulating wetting and drying events.


Results and Discussion

The coupling of a CRDS to an automated non-steady-state chamber system proved successful in the simultaneous measurement of both soil CO2 and CH4 fluxes. Typical traces of the chamber headspace mixing ratios of CO2 and CH4 shown in Fig. 2a,b demonstrate that the changes in the mixing ratios with time were very well represented by linear relationships (R2 of 0.99 and 0.97, respectively). This validated our assumption that for small flow rates and chamber lid closure times of 3 to 4 min, along with our chamber dimensions, a linear relationship between GHG mixing ratio and chamber lid closure time can be adequately used for simultaneously estimating CO2 and CH4 fluxes. Figure 2c shows the relationship between CH4 uptake and CO2 efflux measured simultaneously at different values of θ. We found that both soil CO2 efflux and CH4 uptake attained their maxima around θ of ∼0.24 m3 m−3 and then decreased with a further increase in θ, suggesting lack of aeration for the microbial community hindering CO2 production and limited diffusion of atmospheric CH4 into the soil as soil-air porosity decreased.

Fig. 2.
Fig. 2.

Typical time series of measured (a) CO2 and (b) CH4 mixing ratios in the chamber headspace following chamber lid closure, with panel (c) showing the relationship between simultaneously measured CO2 efflux and CH4 uptake as a function of soil volumetric water content.

 

Results on the chamber effective volume showed that the average Veff with the aluminum base was higher than Vg by 8 and 6% for CO2 and CH4, respectively. However, when measurements were made on the soil column, these increased to 12% for CO2 and 10% for CH4, indicating that on average, 7% was due to adsorption of the GHGs on to the walls of the chamber and 4% due to the pore space in the near-surface soil. The results further suggested that adsorption of CO2 was slightly higher than CH4.

Acknowledgments

This research was funded by an NRAS grant from BC Innovation Council, and NSERC Discovery grants to MSJ and TAB.

 

References

Footnotes


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