## 1 Introduction
### Why Soil Gases?
Waterlogged sites such as peat and wetlands are often net sources of CH\({}_{4}\) and have very low O\({}_{2}\) concentrations and high CH\({}_{4}\) concentrations in the soil profile. Well aerated upland soils in forests are usually CH\({}_{4}\) sinks where methanothropy dominates over methanogenesis [16, 17, 18]. Soils can also be a source of and sink for N\({}_{2}\)O, even though research mainly focused on N\({}_{2}\)O production and emission of soils. However, there is evidence that soils can consume N\({}_{2}\)O [19]. This can also be deduced from sub-atmospheric concentrations that can be found in soils [16]. Net N\({}_{2}\)O uptake rates of soils, however, are usually low compared to reported potential emissions. N\({}_{2}\)O production mainly occurs within soil aggregates in sub- or anoxic zones [15]. N\({}_{2}\)O is a by-product during nitrification and denitrification [20]. N\({}_{2}\)O can be reduced to N\({}_{2}\) during microbial denitrification in soils which is eventually emitted to the atmosphere. Resulting N\({}_{2}\) fluxes and fluctuations in N\({}_{2}\) soil concentration due to production of N\({}_{2}\) are very small compared to the atmospheric background N\({}_{2}\) concentrations (78%) so that they cannot be detected by normal means due to technical restrictions. A laboratory methodology called baPS [21] theoretically allows accounting for the small changes associated with the production of N\({}_{2}\) (and N\({}_{2}\)O, O\({}_{2}\), CO\({}_{2}\)). Yet, large methodological uncertainties remain regarding the dissolution of gases in soil water and the accuracy of the concentration measurements of CO\({}_{2}\) and O\({}_{2}\). The current state of the art methodology to quantify N\({}_{2}\) fluxes uses isotopically labelled nitrate as N source [22], which is only used for short-term studies and not for long-term monitoring. C\({}_{2}\)H\({}_{4}\) is a flammable gas whose atmospheric concentrations are usually negligible outside cities and industrial areas. It has great ecological significance because it is a powerful phyto hormone, which affects ripening, flowering, senescence and root growth [23]. The so-called triple response observed for roots exposed to elevated C\({}_{2}\)H\({}_{4}\) concentrations involves reduction in elongation, swelling of the hypocotyl, and a change in the direction of growth [24]. C\({}_{2}\)H\({}_{4}\) is produced by most plant parts, and can also be produced by microbial activity. In soil even extremely low concentrations of a few ppb can have a strong effect on root growth [25]. In water saturated soil C\({}_{2}\)H\({}_{4}\) can accumulate and reach concentrations \(>10\) ppm [26]. C\({}_{2}\)H\({}_{4}\) can be produced and consumed in the soil, similar to CH\({}_{4}\) and N\({}_{2}\)O. The local concentration of C\({}_{2}\)H\({}_{4}\) in the soil represents the balance of production and consumption and depends on the diffusivity of the aerated soil [25; 27]. Soil-atmosphere gas fluxes and soil gas concentrations change over time. Soil gases are a result of different processes and respond to different drivers. The temporal evolution of concentrations and fluxes can show reactions to single events or underlie different cycles, from diurnal to seasonal cycles. Soil CO\({}_{2}\) fluxes, for example, show strong seasonal cycles in temperate climates. This reflects the dependence on photosynthesis and decomposition of soil organic matter which both peak in summer [8; 28]. Usually, soil CO\({}_{2}\) fluxes also show diurnal cycles as response to diurnal temperature variations [29]. In drier and warmer climates, soil moisture can be the main driver, since decomposition of soil organic matter and photosynthesis by plants requires a minimum of water availability [30]. Similar to soil respiration, CH\({}_{4}\) consumption can underlie diurnal and seasonal cycles [31; 32; 33]. Periods of heavy rain can strongly reduce CH\({}_{4}\) consumption rates.