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Mofette fields, i.e. geogenic, cold CO2-exhaling gas vents occurring naturally in regions of tectonic or volcanic disturbances provide an excellent opportunity to investigate long-term responses of the soil biota to increased CO2 concentrations. The upper centimeters of mofette soils present a small-scale mosaic of different CO2 and O2 concentrations: From up to 100% CO2 and 0% O2 around local degassing vents to ambient soil atmosphere (<2% CO2). The present field study investigated the influence of CO2 on the community structure of Collembola as representatives of the air-filled fraction of the pore system and of Nematoda as inhabitants of soil water films. Canonical correspondence analyses revealed strong correlations between soil faunal communities and environmental measures, above all CO2 concentration, organic matter content and plant coverage. An increase in CO2 concentration was followed by a steady decline in collembolan and nematode species richness and collembolan densities, but below a threshold of 62% CO2 had no significant effect on overall nematode densities. Collembolans developed viable populations at up to 20% CO2, where some mofettophilous species had their highest densities and frequencies, but other more general species also occurred (66% of overall collembolan densities). Nematodes, on the other hand, maintained individual-rich populations at up to 62% CO2, but above 20% CO2 nematode communities consisted almost entirely (97.6%) of three mofettophilous species: one feeding on bacteria, one on fungi and one on plant roots. Likely a combination of active and passive life phases together with temporal and micro-scale changes in environmental conditions allows survival of few mofettophilous species under CO2 conditions too extreme for most other species. The finding that mofettophilous species maintained denser populations in high CO2 patches, with species optima between 3% and 40% CO2, indicates that they even profit from CO2 degassing, presumably via changes in food supply or due to the lack of competitors. [ABSTRACT FROM AUTHOR]