## 3 Results and Discussion
### Effect of Different Calculation Approaches for Gas Flux Estimation
#### 3.3.1 Time Series Soil-Atmosphere Fluxes  
Soil-atmosphere fluxes of O\({}_{2}\) and CO\({}_{2}\) showed a strong seasonal behaviour (Figure 9) as observed from the soil gas profiles (Figure 5), with high flux rates during summer where temperatures were highest and soil moisture usually lowest (Figure 9). The time course of O\({}_{2}\) seemed to reflect inversely the time course of CO\({}_{2}\). CO\({}_{2}\) emissions and O\({}_{2}\) consumption reached from values close to zero up to approximately 6 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\), and had an average flux of 1.97 \(\upmu\)mol CO\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\) and of 1.95 \(\upmu\)mol O\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\). This flux corresponds to 746 g C m\({}^{-2}\) yr\({}^{-1}\) which is an average value for temperate forests [116; 117]. Our soil respiration data represent an ideal base for further investigation e.g., the quantification of the effect of the environmental drivers such as soil temperature and soil moisture (Figure 9) [12; 28] or the relationship between soil respiration and tree growth [37]. Working with the time series of all 13 soil gas monitoring plots will also allow comparing the effect of different trees, e.g., beech vs. spruce [117], and forest dynamics like thinning or drought effects. Soil-atmosphere fluxes of CH\({}_{4}\) showed more variability than soil CH\({}_{4}\) concentrations, yet obvious seasonal cycles as observed with CO\({}_{2}\) and O\({}_{2}\) fluxes were not observed. Soil CH\({}_{4}\) consumption ranged between 0.001 and 0.003 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\), which is in the range of CH\({}_{4}\) consumption rates observed in the region [41; 72]. The average yearly consumption of 8.08 kg CH\({}_{4}\) ha\({}_{.1}\) yr\({}^{-1}\) corresponds to a high value (top 25%) compared to temperate forest soils worldwide [18]. Future detailed analysis of the CH\({}_{4}\) profiles and fluxes of all sites will allow studying the temporal evolution and long-term changes of CH\({}_{4}\) consumption. This is of special interest since it is currently under debate that soils might be losing their capacity to consume atmospheric CH\({}_{4}\) as recently observed by [PERSON] and [PERSON] [118]. Figure 9: Time series of soil-atmosphere fluxes of O\({}_{2}\), CO\({}_{2}\), CH\({}_{4}\), soil and air temperature, monthly rainfall and soil water content at 10 cm depth at the ES spruce plot. Uncertainty of the flux estimation is indicated in grey and is derived from the variability of the replications in the concentration profiles and the uncertainty in the estimation of the diffusivity. Please note the different scales on the y axis for O\({}_{2}\), CO\({}_{2}\) and CH\({}_{4}\). The uncertainty in the CO\({}_{2}\), O\({}_{2}\) and CH\({}_{4}\) fluxes estimation changed over time (Figure 9) and resulted from the uncertainty in the gradient estimation and measurement of the soil water content. The uncertainty in CO\({}_{2}\) and O\({}_{2}\) fluxes seemed to be much smaller than in the CH\({}_{4}\) fluxes which is due to the stronger gradients and less scattering in the data (Figures 6 and 7). The estimation of the uncertainty is not fully comparable between the CO\({}_{2}\) and O\({}_{2}\) fluxes since their calculation approach (LR vs. LL) was different and a different number of data points were used for the calculation of the gradient. The relative contribution of the measurement uncertainty of the soil water content was constant over time, so that changes in the total uncertainty of flux estimates originate directly from the estimation of gradients in the gas concentration profiles. If the gradients in the respective soil gas profile become less distinct due to more scattering of the concentrations, the uncertainty in the flux estimation increases, e.g., the summer peak in soil CO\({}_{2}\) efflux in 2013 (Figure 9). The uncertainty in the parameter estimates of the diffusivity model would be an additional source of uncertainty which was not included so far.