A comparison of linear and exponential regression for estimating diffusive CH4 fluxes by closed-chambers in peatlands

The closed-chamber method is the most common approach to determine CH₄ fluxes in peatlands. The concentration change in the chamber is monitored over time, and the flux is usually calculated by the slope of a linear regression function. Theoretically, the gas exchange cannot be constant over time but has to decrease, when the concentration gradient between chamber headspace and soil air decreases. In this study, we test whether we can detect this non-linearity in the concentration change during the chamber closure with six air samples. We expect generally a low concentration gradient on dry sites (hummocks) and thus the occurrence of exponential concentration changes in the chamber due to a quick equilibrium of gas concentrations between peat and chamber headspace. On wet (flarks) and sedge-covered sites (lawns), we expect a high gradient and near-linear concentration changes in the chamber. To evaluate these model assumptions, we calculate both linear and exponential regressions for a test data set (n = 597) from a Finnish mire. We use the Akaike Information Criterion with small sample second order bias correction to select the best-fitted model. 13.6%, 19.2% and 9.8% of measurements on hummocks, lawns and flarks, respectively, were best fitted with an exponential regression model. A flux estimation derived from the slope of the exponential function at the beginning of the chamber closure can be significantly higher than using the slope of the linear regression function. Non-linear concentration-over-time curves occurred mostly during periods of changing water table. This could be due to either natural processes or chamber artefacts, e.g. initial pressure fluctuations during chamber deployment. To be able to exclude either natural processes or artefacts as cause of non-linearity, further information, e.g. CH₄ concentration profile measurements in the peat, would be needed. If this is not available, the range of uncertainty can be substantial. We suggest to use the range between the slopes of the exponential regression at the beginning and at the end of the closure time as an estimate of the overall uncertainty.     

Cross-evaluation of measurements of peatland methane emissions on microform and ecosystem scales using high-resolution landcover classification and source weight modelling

The methane exchange in an oligotrophic mire complex was measured on the ecosystem and microform scale with the eddy covariance (EC) and the closed chamber technique, respectively. Information about the distribution of three distinct microform types in the area of interest and in each 30 min EC flux source area was derived from a high-resolution (1 m²) landcover map in combination with an analytical source weight model (Kormann and Meixner, 2001). The mean weighted coverage of flark, lawn and hummock microforms in the EC source area (0.3% : 57% : 43%) closely mirrors the overall distribution in the area of interest (0.5% : 50.1% : 49.4%), despite great differences in microform coverage between individual 30 min EC source areas. The measured ecosystem flux was fitted to the sum of three microform flux models based on environmental variables and weighted by their fractional coverage in the EC source area. This method resulted in a better representation of the ecosystem flux compared to an approach based on only one flux model for the whole ecosystem (R² = 0.87, RMSE = 0.44 vs. R² = 0.74, RMSE = 0.61, n = 5181) and thus constitutes a successful down-scaling of measured ecosystem scale flux to the microform scale. A comparison of down-scaled and measured microform fluxes reveals a good agreement for lawn microforms and systematic differences for flark and hummock microforms. Reasons for the differences are thought to be the limited resolution of the landcover classification and the systematic underestimation of hummock fluxes by the closed chamber technique. As a result, hummock fluxes derived by down-scaling of EC fluxes are considered to be more dependable than closed chamber fluxes. The seasonal ecosystem methane budget from gap-filled EC measurements was 9.4 ± 0.2 g CH₄ m⁻²; the budget derived from up-scaled microform measurements was 8.0 ± 0.8 g CH₄ m⁻². The lower value of the latter budget is attributed to the underestimation of flark and hummock fluxes by closed chamber measurements and to the microform gap-filling procedure. Generally, estimates from up-scaled microform measurements are found to be less certain than estimates from EC measurements.