Sources of spatial variation in methane emission from mires in northern Sweden: A mechanistic approach in statistical modeling

Abstract

Methane emissions from six mires in northern Sweden were measured using a closed chamber technique during the frost free season in 1992 The average methane flux over the measurement period, calculated either for each mire or for different plant communities within one mire, ranged from 9 to 83 mg CRs m"2 d"1. The emission rate on each occasion was related to physical and chemical environmental variables, both in a general data set for all mires (n=836) and in subdata sets for individual mires, using multiple linear regression The variables with significant contributions to the models were water table, standing water above the vegetation surface, peat temperatures, and principal components of the near infrared reflectance spectra of peat samples reflecting variations in organic chemical composition. To account for the actual contribution of methane production and methane oxidation, variables desenbing the active pans of the vertically distributed potentials of methane production or oxidation were constructed. The interaction terms between these variables, respectively, describing the active proportion of methanogens and methanotrophs. and the temperature values representing the anoxic and oxic parts of the profile were significantly correlated to the methane emission rate; positively for the production zone and negatively for the consumption zone. By using this mechanistic approach, a significant temperature effect in both the methane production and consumption zone was detected. These constructed temperature variables explain 21 of the variance in the logarithmically transformed methane fluxes using the entire data set (n=836) but only 5 of the variance using peat temperatures from fixed depths. Adding variables describing the organic chemical composition of the peat to the models improved the predictability in 10 of the 11 model sets tested, decreasing the unexplained variance by maximally 50 for a poor fen community model and increasing R2 from 0.40 to 0.68. In the general model, R2 increased from 0.42 to 0.49 with the inclusion of organic chemical composition. The explained variance in the final models, including the substrate variables, ranged from R2=0.49 to R2=0.75, with one exception. Torsmyran (R2=0.36) which is a mire that most closely relates to an occcntric ombrotrophic bog among the mires studied.