The dynamics of non-methane hydrocarbons and other trace gas fluxes at a subarctic mire in northern Sweden

Abstract: In the context of climate change, it is important to understand how the terrestrial carbon cycle is interacting with the anthropogenic increase in atmospheric carbon dioxide (CO2) concentration. Boreal and subarctic regions in the northern hemisphere are great carbon pools, as well as they are subject to predicted warming. These facts place them in the absolute frontline of ecosystems that are to be studied in the context of coupled climate models, where the terrestrial carbon cycle is included. In addition to CO2 and methane (CH4), which have been intensively studied regarding carbon cycle and climate, there are other biogenic volatile organic compounds (BVOC) emitted by vegetation that have shown to be of great importance. One group is called non-methane hydrocarbons (NMHCs). Their emissions is a part of the carbon flux in ecosystems, and have an indirect role in determining atmospheric concentration of some greenhouse gases as well as biogenic aerosols. A study of CO2, CH4 and NMHC flux dynamics has been conducted on the subarctic mire Stordalen in northern Sweden. The objective is to contribute to the existing knowledge about exchanges of CO2 and CH4, and primarily to add new knowledge of NMHC emissions from a subarctic ecosystem, which has not earlier been studied. An automatic multichamber system was used to collect high temporal resolution data of CO2 and total hydrocarbon (THC) fluxes from three different sub-ecosystems on the mire: a wet minerotrophic site, a wet intermediate ombro-minerotrophic site and a semiwet ombrotrophic site. Further, manual sampling of CH4 fluxes was conducted approximately three times a week, from mid-June to late August. This gave the possibility to estimate the amount of NMHCs (THC flux -CH4 flux). A temperature dependent respiration model was developed from night time CO2 flux data, and gross primary production (GPP) could be estimated for each sub-ecosystem. Environmental variables as light, temperature, moisture and thaw depth are included in the correlations. The results show a certain degree of temperature, light and GPP dependency for NMHC emissions from all of the different plant communities, however it is a great distinction between different plant specie. The largest emissions come from wet minerotrophic sub-ecosystems with vascular plants, compared to a mostly Sphagnum moss vegetated semiwet site, 2.8 mg/m2/hr and 0.9 mg/m2/hr respectively. The NMHC flux rates are however, likely to be underestimated. They are based on the molar weight of CH4 (16 g/mol), while isoprene (C5H8), which is one of the lightest NMHCs, have a molar weight that is more then four times greater. The output of NMHC-carbon from the mire ecosystem was found to be close to 2 % from the wet minerotrophic site, in relationship to CO2-C and CH4-C. CH4 emissions are higher from wet, compared to semiwet microsites. The water table position at a depth of 15-25 cm play an important role at the semiwet site, while water table variations within a depth of 0-10 cm do not affect the CH4 emissions. Moreover, the presence of vascular plants and their ability to give qualitative substrates for CH4 production through photosynthesis, as well as their capacity for plant mediated CH4 transport, are most likely explaining the difference between the two sites. This is in addition to the actual moisture content, explained as being the main cause for high CH4 emissions. Conclusively, research of BVOCs from northern latitude ecosystem like the subarctic mire Stordalen, should be a part in future studies of the terrestrial carbon cycle. Their part in the carbon budget at this site is shown to be significant.