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34Aimed: 35 The main aim of this study is to improve the mechanistic understanding of soil CO 2 efflux 36 (F s ), especially its temporal variation at short-time scales, by investigating, through modeling, 37 which underlying process among CO 2 production and its transport up to the atmosphere is 38 responsible for observed intra-day variation of F s and soil CO 2 concentration [CO 2 ]. 39Methods: 40In this study, a measurement campaign of F s and vertical soil [CO 2 ] profiles was conducted in 41 a Scots Pine Forest soil in Hartheim (Germany) and used to develop a model testing several 42 hypotheses. A reference model taking into account a purely diffusive CO 2 transport and a 43 temperature-dependent CO 2 production is compared to models with a more complex 44 description of either CO 2 production or CO 2 transport. For transport, the introduction of 45 advection and the dispersion is investigated. For the production, the emergent hypothesis of 46 the phloem pressure concentration wave (PPCW) influence is tested. 47Results: 48We show that intra-day variation of F s and [CO 2 ] is better represented when the more 49 complex CO 2 production expression is taken into account compared to the more detailed 50 description of CO 2 transport. 51Conclusion: 52We conclude that focus should be placed on the potential factors affecting the CO 2 53 production, rather than on the transport process description 54This is the preprint version of a manuscript that has been published in Plant and Soil 2015