## 4 Conclusions  
We introduced an experimental set-up and design for long term soil gas monitoring. Typical soil gas profiles and fluxes of one of 13 monitoring plots were presented to discuss the specifications, advantages, and drawbacks of the system. Several passive gas samplers per depth were used which allows a precise assignment of the sampling locations. This is a prerequisite for the assessment of the homogeneity of the soil gas profile and the quantification and qualification of the estimates of the soil gas fluxes. The samplers do not require electrical power or intensive maintenance and affect the soil surface only minimally which makes them ideal for long term monitoring. The soil gas samplers allow efficient and highly reproducible handling of the gas samples which is essential for a good quality assurance. The subsequent GC analysis was optimized for the purpose and additional gas analysis can be included. These features make the set-up an efficient, versatile, flexible, and reliable tool which is suitable for long-term monitoring of soil gas profiles.  
The monitoring routine is scheduled from biweekly to monthly measurements for practical reasons. Such a schedule is suitable to cover seasonal cycles which are known for soil CO\({}_{2}\), O\({}_{2}\) and CH\({}_{4}\). The routine schedule is a drawback for occasional event-based soil gas dynamics such as N\({}_{2}\)O fluxes that are dominated by peak events. While CO\({}_{2}\), O\({}_{2}\), and CH\({}_{4}\) fluxes can be reliably estimated based on their concentration profile, N\({}_{2}\)O fluxes are more difficult to estimate especially when N\({}_{2}\)O soil concentration gradients are weak and indicating concurrent production and consumption of N\({}_{2}\)O at the same time within the profile. Future campaigns with chamber measurements could provide reference values of gas fluxes that can help to eliminate or minimize sources of uncertainty and add additional value to our data.  
The long-term soil gas profile data of 13 forest plots and the ongoing monitoring and evaluation holds the potential to answer various research questions. It is the ideal data basis to study (a) the linkage between soil respiration, tree growth and forest productivity, (b) the interaction and drivers of soil gas fluxes of CO\({}_{2}\), CH\({}_{4}\) and N\({}_{2}\)O, (c) the relationship of CO\({}_{2}\) and O\({}_{2}\) fluxes and potential interfering processes such as anoxic conditions, (d) the interaction between concurrent production and consumption of N\({}_{2}\)O, and (e) appearance and drivers of C\({}_{2}\)H\({}_{4}\) in forest soils. The continuity of the existing long term records soil gas data in combination with the ICP Forest Level 2 monitoring program are the ideal basis for further ecophysiological studies related to soil processes in which soil gases are produced or consumed.  
[PERSON] conceived and designed the manuscript and presentation of results; [PERSON] and [PERSON] conducted the data analysis. [PERSON] conducted the measurements and contributed in the technical section describing the gas analysis. [PERSON] and [PERSON] wrote the paper. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
We would like to thank our colleagues [PERSON], [PERSON], [PERSON], [PERSON] for the gas sampling and maintenance of the monitoring sites. We would like to thank [PERSON] for his initiative to start the soil gas monitoring program, [PERSON] for the management of the monitoring sites, [PERSON] and [PERSON] for the development of the samplers, [PERSON] for the soil water modelling, and all who contributed to the development and maintenance of the system.  
The authors declare no conflict of interest.