## 1 Introduction
### How to Measure Gas Fluxes? During the past decades many studies including the measurements of soil-gas exchange were conducted. Soil gas fluxes were measured in focused individual studies, but also in large monitoring networks (e.g., CarboEurope, Ameriflux, FluxNet, ICOS). The most important field methods used in this context are the micro-meteorological eddy-covariance (EC) method [48] and chamber methods [49]. The gradient-flux method (GM) represents an alternative method [48; 50] that is used less often. EC and chamber techniques can only be used to study net fluxes between soil and atmosphere. Applying the GM provides additional insights into ongoing processes in the soil that do not necessarily lead to net fluxes at the surface. The measuring concept of the EC method postulates turbulent gas transport as dominating gas exchange mechanism in the atmosphere [48; 51]. In most cases, EC measurements are conducted in the free atmosphere above the plant cover, which can be a few meters in height in the case of grassland or crop, but also higher than 40 m in case of tall forests, requiring expensive meteorological towers. The application of EC is restricted to situations and locations with sufficient turbulence. Thus, measurements focusing on soil gas fluxes can be challenging within forest stands due to reduced wind speed and turbulence below the canopy. The application of the EC method requires high frequency measurements of 3D air flow (usually by sonic anemometers) and precise gas concentration measurements (usually by laser gas analysers). Twenty years ago, there were only instruments that could measure H\({}_{2}\)O and CO\({}_{2}\) at the required precision and frequency. During the last years, also instruments for CH\({}_{4}\) and N\({}_{2}\)O measurements have become available, but are still very expensive. The quality of the EC data is very high and includes the gas exchange of the entire ecosystem, i.e., soil, plants and animals within the footprint which is always depending on the wind direction. The costs and demand for infrastructure per site (devices, tower, power, and maintenance) is substantial. Chamber measurements represent probably the oldest and most intuitive method to measure soil-atmosphere gas exchange [52]. Different approaches have been used and many issues about the best methodology have been discussed [47; 53]. The most common application nowadays is the non-steady-state approach [54], which is basically a temporarily closed chamber that covers an area of soil surface so that the gas flux can be derived from the temporal changes in gas concentration within the closed chamber. One prerequisite of this method is that the measurement itself does not interfere with the gas fluxes, or, that this effect is negligible or taken into account in the flux calculation approach. Research showed that there are many pitfalls in the practical application, and that differences in the fluxes estimation of different state of the art chamber systems can be surprising [55; 56]. Manual chamber measurements are laborious, but their easy application allows covering the study area before permanent sampling locations are fixed. Modifying the setting, chamber measurements can also be used to quantify gas fluxes from different sources, e.g., by using transparent and opaque chambers to separate soil and plant respiration and photosynthesis. In combination with treatments like trenching or girdling of trees chamber measurements can be used to separate soil CO\({}_{2}\) fluxes and assign different sources of soil respiration [57]. Practical experience shows that small variations in the manual operation of the chambers may lead to substantial differences, so that a retrospective evaluation of the measurement quality is challenging. Automatic chambers can provide high quality data at high temporal resolution. Yet, they are expensive and require good maintenance and power supply. Additionally, the permanent use of the same measurement position might lead to exclusion of rain which would lead to biased measurements [47; 58].