Comparison of static chambers to measure CH₄ emissions from soils

Abstract

The static chamber method (non-flow-through-non-steady-state chambers) is the most common method to measure fluxes of methane (CH₄) from soils. Laboratory comparisons to quantify errors resulting from chamber design, operation and flux calculation methods are rare. We tested fifteen chambers against four flux levels (FL) ranging from 200 to 2300 μg CH₄ m⁻² h⁻¹. The measurements were conducted on a calibration tank using three quartz sand types with soil porosities of 53% (dry fine sand, S1), 47% (dry coarse sand, S2), and 33% (wetted fine sand, S3). The chambers tested ranged from 0.06 to 1.8 m in height, and 0.02 to 0.195 m³ in volume, 7 of them were equipped with a fan, and 1 with a vent-tube. We applied linear and exponential flux calculation methods to the chamber data and compared these chamber fluxes to the reference fluxes from the calibration tank.

The chambers underestimated the reference fluxes by on average 33% by the linear flux calculation method (R_lin), whereas the chamber fluxes calculated by the exponential flux calculation method (R_exp) did not significantly differ from the reference fluxes (p < 0.05). The flux under- or overestimations were chamber specific and independent of flux level. Increasing chamber height, area and volume significantly reduced the flux underestimation (p < 0.05). Also, the use of non-linear flux calculation method significantly improved the flux estimation; however, simultaneously the uncertainty in the fluxes was increased. We provide correction factors, which can be used to correct the under- or overestimation of the fluxes by the chambers in the experiment.

Introduction

The static chamber method (non-flow-through-non-steady-state chamber, Livingston and Hutchinson, 1995) is the most commonly used method to measure non-reactive greenhouse gas (GHG) fluxes, especially methane (CH₄) and nitrous oxide (N₂O), from soils. The basic principle of this technique is to cover a known area of soil with a closed chamber that allows the gas exchange between the soil below the chamber and the chamber headspace. The gas concentration change over time inside the chamber headspace is quantified and translated into a flux rate, representing the flux into or out of the soil.

Debates on how to design an optimal chamber and how to calculate the gas fluxes from soils have been going on for more than 30 years (e.g. Anthony et al., 1995, Conen and Smith, 2000, Forbrich et al., 2010, Hutchinson and Mosier, 1981, Kroon et al., 2008, Kutzbach et al., 2007, Livingston et al., 2005, Livingston et al., 2006, Matthias et al., 1978, Pedersen et al., 2010). Recommendations of using a fan to mix the chamber headspace (Christiansen et al., 2011, Pumpanen et al., 2004), a vent tube to minimize pressure changes in the chamber (Hutchinson and Livingston, 2001, Hutchinson and Mosier, 1981, Xu et al., 2006), and a proper insulation or construction to avoid uncontrolled leakage from the chamber (Hutchinson and Livingston, 2001) are still being discussed and are not widely adopted. The effect of chamber size and geometry on GHG fluxes has not been as widely discussed or tested, although they are key issues in assessing how well the chamber is able to detect the GHG fluxes. In addition, linear regression is the most common method to calculate chamber based CH₄ and N₂O fluxes from soils, though it has been documented to lead to systematic underestimation of the fluxes (Anthony et al., 1995, Gao and Yates, 1998a, Livingston et al., 2005, Kroon et al., 2008, Kutzbach et al., 2007, Pedersen et al., 2010).

Emission measurements of greenhouse gases with closed static chambers imply that the concentration of the target gas increases in the headspace. This gas accumulation decreases the natural concentration gradient between the soil and the chamber headspace and may significantly reduce the gas efflux (Davidson et al., 2002, Kutzbach et al., 2007, Livingston and Hutchinson, 1995, Nay et al., 1994). The purpose of the flux measurement is to obtain an estimate of the undisturbed flux, the flux prior to the chamber deployment. When applying linear regression, one assumes that the gas concentration gradient between the source and the atmosphere does not change, and that the flux is constant during the entire enclosure. A non-linear function (e.g. exponential function) implicitly accounts for the decreasing efflux during the enclosure and estimates the flux at time zero of the chamber closure.

Inter-comparisons of different chamber designs in controlled conditions in combination with different flux calculation methods are scarce and the focus has been on CO₂ (Butnor and Johnsen, 2004, Gao and Yates, 1998b, Nay et al., 1994, Widen and Lindroth, 2003). Pumpanen et al. (2004) performed a chamber calibration campaign for 20 different CO₂ efflux chambers representing static chambers (non-flow-through-non-steady-state chamber), closed dynamic chambers (flowed-through-non-steady-state) and open dynamic chambers (flow-through-steady-state). They found that the bias of the CO₂ fluxes was greatest with static chambers, which underestimated or overestimated the fluxes between −35 and +6% depending on the type of chamber, gas sampling and analysis, and the method of mixing the chamber headspace air. The largest underestimations were observed with static chambers based on syringe gas sampling, which is the most common method in the flux measurements of CH₄ and N₂O fluxes.

Even though the studies with CO₂ chambers have identified critical issues regarding chamber design and sampling, the results are not directly applicable to chambers used for non-CO₂ greenhouse gases, such as CH₄ and N₂O. First of all, chamber designs and sampling protocols are often different. CH₄ and N₂O are most often sampled manually in the field and subsequently analyzed off-site using gas chromatographic methods. In contrast, CO₂ fluxes are typically determined in situ using online analyzers connected to dynamic chambers with a constant headspace mixing. Furthermore, CO₂ fluxes can be several degrees of magnitude larger than CH₄ and N₂O fluxes, leading to higher concentration change within chamber headspace over an enclosure, and allowing for a lower sensitivity of the gas analyzers and shorter enclosure times.

In order to minimize the errors related to the measurements of non-CO₂ greenhouse gas exchange, such as CH₄ and N₂O, there is an urgent need to perform similar evaluation of the chambers in controlled laboratory conditions. We organized a static chamber comparison campaign to gain new knowledge on the differences between static chambers typically used to measure CH₄ and N₂O fluxes from soils. Both CH₄ and N₂O were measured in the experiment; however, here we report the results of CH₄ only. The tested chambers differed in size, shape and material, and were originally operated in different ecosystems (peatlands, forests, agricultural fields). Christiansen et al. (2011) report the effects of chamber placement, manual sampling and headspace mixing on CH₄ fluxes for two static chambers. Here we report the results of a comparison of 15 chambers, and provide general guidelines for chamber designs and flux calculation procedures.

The overall aims of the campaign were (1) to quantitatively assess the uncertainties and errors related to static chamber measurements, (2) explain uncertainties and errors by chamber design and flux calculation methods, and (3) to provide guidelines for static chamber designs, sampling procedures, and flux calculation methods.