## 1 Introduction  
### Why Soil Gases? Soils provide many functions as they represent the base of agricultural production and terrestrial ecosystem productivity. Soils can be considered as large bioreactors. They store and provide water, nutrients, and anchorage to plants, which are able to assimilate these nutrients and additionally carbon (C) by photosynthesis. Soils not only provide the essential base for a terrestrial habitat of plants,but also for fauna, fungi, and microbes. The latter contribute, evolve and constitute together with the physico-chemical soil structure the function of a large bioreactor, with its specific processes and properties, and its fluxes of energy and matter [1,2]. Many processes occurring in soils involve the consumption and production of different gas species [3]. Studying soil gas concentrations and soil gas fluxes can help us to understand and quantify the underlying processes and functions. In recent years, studies of carbon dioxide (CO\({}_{2}\)), methane (CH\({}_{4}\)) and nitrous oxide (N\({}_{2}\)O) and the role of soils as sinks or sources of these greenhouse gases (GHG) have become particularly important [3,4]. However, other gas species also play an important role in the soil. For example, oxygen (O\({}_{2}\)) is essential for living roots, this means the soil must be well aerated, or in case of waterlogged sites, plants must develop strategies to supply O\({}_{2}\) to their roots as it is known for reeds (_Phragmites_ spec.) and rice plants (_Orica_ spec.). Soils can also be a source or sink of other gases such as hydrogen (H\({}_{2}\)), ethylene (C\({}_{2}\)H\({}_{4}\)) and nitrous acid [5] which are important precursors of atmospheric chemistry. Sources of gases can be roots, litter and microbes [3], but also geological sources may play a role [6]. Most soil gas research focused on CO\({}_{2}\), CH\({}_{4}\), and N\({}_{2}\)O, which are the most important anthropogenic GHG. They are co-responsible for global climate change [4] and linked into the global C and nitrogen (N) cycles. Atmospheric concentration of CH\({}_{4}\) has risen from preindustrial 702 ppb to 1872 ppb in 2020, and N\({}_{2}\)O concentration increased by a factor of 1.2 to 333 ppb, while atmospheric CO\({}_{2}\) increased from preindustrial 278 ppm to 409 ppm in 2020 [7]. Even though atmospheric concentrations and soil-atmosphere fluxes of N\({}_{2}\)O and CH\({}_{4}\) are much smaller than the concentration and fluxes of CO\({}_{2}\) their relevance is substantial due to the higher radiative forcing of N\({}_{2}\)O and CH\({}_{4}\) molecules [4]. Soil gas concentrations can differ substantially from atmospheric concentrations, and can be both, higher and lower than atmospheric concentrations, depending on the prevailing process in the soil. Soil CO\({}_{2}\) emissions mainly result from heterotrophic and autotrophic respiration in the soil, i.e., from microbes, soil fauna and roots, and amount up to more than 75% of the carbon that is assimilated by plants during photosynthesis [8]. Soil CO\({}_{2}\) emissions can deviate temporarily from soil respiration e.g., after heavy rainfall when soil CO\({}_{2}\) is blocked in the soil pore space for some time [9]. Soil generally acts as a source of CO\({}_{2}\), even if the soil is a net sink for C due to plant growth and C fixation as recalcitrant soil organic matter. Studying soil CO\({}_{2}\) concentrations and fluxes can help to understand how environmental changes affect stability of soil organic matter [10] and how plants respond to such changes [11]. Increasing temperatures due to global climatic change might mobilize carbon stored in the soil, yet, increases of plant-derived carbon inputs to soils might exceed these increases in decomposition [12]. Soils can be a source of and sink for CH\({}_{4}\)[3,13,14]. Methanogenesis is a strictly anaerobic process [5], but can occur as well in aerated soils, where oxygen-deficient zones can exist within water-saturated soil aggregates [15].