## 4 Discussion
### CO\({}_{2}\) Molar Fraction Dynamics  
It is almost certain that CO\({}_{2}\) concentrations vary depending on physicochemical properties, soil cover types, and climatic conditions. However, the CO\({}_{2}\) change between forest land and grassland is similar to approximately 100 to 1300 mg CO\({}_{2}\) m\({}^{2}\) h\({}^{-1}\) than barren land and cropland [3]. The ranges of CO\({}_{2}\) concentration measured at the grassland in Kyushu Island, Japan was around 1300-8700 ppm (0.1 m depth), 2500-10,800 ppm (0.2 m depth), 4200-13,200 ppm (0.3 m depth), and 5800-16,500 ppm (1.0 m depth) [48]. It has also been reported that the soil CO\({}_{2}\) concentration measured at a depth of 0.3 m in the South-Moravian region was around 5000 to 8000 ppm [49]. The soil CO\({}_{2}\) concentrations in our forest site are more comparable with a study conducted previously in a temperate deciduous forest using NDIR that ranged between 774 ppm and 6521 ppm at 1 m depth [50]. Higher CO\({}_{2}\) concentration at a depth less than 1 m has also been reported in forest site Meurthe et Moselle, France, with around 20,000 ppm at 60 cm depth [51] and a study conducted in a temperate forest located at Shale Hills Critical Zone Observatory, Pennsylvania, US, reported that the CO\({}_{2}\) concentration reached up to 50,000 ppm at a depth of 1.5 m, attributed to the greater root respiration depth partly due to higher macroporosity of sandstone lithologies [52]. The CO\({}_{2}\) concentration above 10% is generally found in the active geothermal area with a significant pressure gradient transport instead of the solely diffusive process [24; 25]. Some studies suggest a strong positive correlation between xc and temperature [3; 49; 53], where temporal CO\({}_{2}\) concentration is affected by soil temperature on a biweekly basis [50]. Our work demonstrates that CO\({}_{2}\) and temperature are positively correlated at 1 and 6 m depth. In a temperate forest, the temperature is considered the primary driver of heterotrophic soil respiration, releasing CO\({}_{2}\) as organisms decompose  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**FTIR Resolution** & **Equation** & **Adjusted R\({}^{2}\)** \\ \hline
1 cm\({}^{-1}\) & Y = 640 \(\times\)\(-\)781 & 0.987 \\
4 cm\({}^{-1}\) & Y = 650 \(\times\)\(-\)1351 & 0.976 \\ \hline \hline \end{tabular}
\end{table}
Table 2: The linear fitting comparison between FTIR resolution of 1 cm\({}^{-1}\) and 4 cm\({}^{-1}\).  
soil organic carbon (SOC) [54; 55; 56]. However, the effects of soil temperature toward heterotrophic respiration are rather complex [57]. It might also indirectly affect the microbial, substrate quantity, and substrate availability on a daily to annual basis [53; 54]. On minutes to an hourly basis, it might also influence the enzyme activity produced by bacteria and fungi [58].  
Furthermore, most of the studies carried on in the forest soil mainly focused at a depth of 0 to 1 m, where the soil CO\({}_{2}\) production is often a correlation to biological activity is more instantaneous. Very little data are available on the evolution of the CO\({}_{2}\) concentration in the deepest layers of the soil. In the present study, the abiotic drivers' relationship, including SWC and u\({}^{\star}\), is further explored and provided in Figure 9.