## 3 Results and Discussion
### Effect of Different Calculation Approaches for Gas Flux Estimation
#### 3.2.2 Vertical Partitioning of Soil Gas Fluxes  
We used the LL method to derive concentration gradients at different depth for CO\({}_{2}\) and CH\({}_{4}\) (Figure 6), which allows calculating vertical profiles of soil gas fluxes (Figure 8) and allocating the production and consumption of gases to a certain depth [44,60]. The increase in the CO\({}_{2}\) flux between the 10 to 5 cm depth layer and the humus layer was extrapolated to the soil-atmosphere interface (Figure 8 left) as proposed by [PERSON] et al. [104]. CO\({}_{2}\) flux between 10 to 5 cm depth was usually \(<\)20% of the estimated efflux. Soil CO\({}_{2}\) fluxes always increased towards the soil surface. The increase in the CO\({}_{2}\) flux between the 5 to 0 cm depth layer and the humus layer was lower during the colder seasons (fall to spring) than in summer when the topsoil was warm. This higher soil respiration rate can be explained at least partly by the enhanced decomposition of organic material in the humus layer during summer. The top mineral soil and humus layer contributed the largest fraction of the total soil respiration as observed by others [11,16,60,84]. Future detailed analysis of the soil gas monitoring data will allow studying the temporal evolution of soil respiration in the mineral soil and the humus layer and their contribution to the total respiration. Such analysis holds the potential to better quantify the dynamics of the mineralization of the humus layer and better understand the C cycle in the soil. The soil-atmosphere flux of CH\({}_{4}\) was set to the estimated CH\({}_{4}\) flux in the humus layer (Figure 8, right) since it can be expected that CH\({}_{4}\) consumption in the top cm of the spruce litter does not contribute substantially to CH\({}_{4}\) consumption [16,105]. The CH\({}_{4}\) flux between 5 and 10 cm depth was very low, which corresponds to the very weak gradient compared to the gradient observed above (Figures 6 and 7 left). The negative soil CH\({}_{4}\) fluxes increased almost linearly from the 10 to 5 cm depth to the humus layer during all seasons (Figure 8 right). More than 90% of the CH\({}_{4}\) was consumed in the top 10 cm of the mineral soil. This corresponds to the observation that the topsoil is the most active CH\({}_{4}\) consumer [16,105]. The CH\({}_{4}\) flux profile showed an apparent seasonality with higher consumption rates in summer and fall than in winter, which resulted from the seasonal changes in soil moisture and soil gas diffusivity (Figures 6 and 7). Figure 7: Comparison of different calculation approaches for the estimation of the soil-atmosphere gas flux. Left: Example of a typical soil CH\({}_{4}\) profile. The lines of the Linear Regression (LR, grey, dashed), Exponential Function (EF, black, dashed) and the functions of the Local Linear approach (LL, green dotted) and the Linear Spline approach (LS, orange) yield different slopes at the soil surface. (Middle) CH\({}_{4}\) consumption rates calculated by the LR and EF approach for our whole data set are much lower than the flux estimates of the LL and LS approach. Right: CO\({}_{2}\) fluxes calculated by the LR and EF are slightly lower than fluxes calculated by the LL and LS approach. The soil O\({}_{2}\) profiles did not allow using the LL or LS approach since they randomly yielded unrealistic flux estimates from time to time where O\({}_{2}\) fluxes in the mineral horizon seemed to be higher than in the humus layer above. Reducing the degree of freedom by using the LR approach instead allowed stabilizing the estimation of the O\({}_{2}\) gradient. This approach is suitable if the observed concentrations gradients are weak due to limited precision of the O\({}_{2}\) measurements, and if the assumption of more or less linear gradients is justified based on prior knowledge. The comparison of the calculation approaches could show that the LR and LS approach yield very similar results for the estimation of the soil-atmosphere flux of CO\({}_{2}\).