## 3 Results
### Seasonal, Daily, and Hourly Variations
Figure 5d, the daily \(\chi\)c (ppm) at \(-1\) and \(-6\) m and friction velocity (\(\mathbf{u^{*}}\)) above the canopy are provided. The daily average of friction velocity (\(\mathbf{u^{*}}\)) was relatively higher during the winter (0.7 m/s) than in summer (0.4 m/s). The \(\chi\)c generally increases as the depth, from the near-atmospheric layer (\(-1\) m) up to a few meters to the subsoil layer (\(-6\) m). This study demonstrates that subsoil CO\({}_{2}\) at 1 m depth is constantly lower than subsoil CO\({}_{2}\) at 6 m depth ranging from 1000-7000 ppm at \(-1\) and 10,000-60,000 ppm at \(-6\) m. The \(\chi\)c at \(-1\) m was at the lowest in winter and highest in summer in June 2020. Meanwhile, the seasonal variation of \(\chi\)c at \(-6\) m is somewhat weak. However, it can be seen that the rise of \(\chi\)c at \(-6\) m depth occurs together with the fall of \(\mathbf{u^{*}}\) and the decrease in SWC due to the absence of rainfall. The seasonal variation of \(\mathbf{u^{*}}\) is also weak, where it is found to be low during summer and higher during winter, with an exception in early December 2019 to the end of February 2020. To better understand the general relationship between variables statistically, the matrix correlation based on [PERSON]'s is provided in Figure 6.  
The heatmap based on hourly data (Figure 6) between observed variables from 1 June 2019 to 23 July 2020 shows that the influence of temperature m on subsoil CO\({}_{2}\) concentration at \(-6\) m was very pronounced with an R of 0.94 (T\({}_{\text{soil}}\) at \(-1\) m) and 0.83 (T\({}_{\text{soil}}\) at 0 m). The CO\({}_{2}\) concentration at \(-6\) m was also weakly inversely correlated with atmospheric pressure at 0 m (R: \(-0.59\)), soil-air pressure at \(-6\) m (R: \(-0.48\)), SWC (R: \(-0.50\)), and soil temperature at \(-6\) m (R: \(-0.59\)). However, a strong correlation between temperature and soil CO\({}_{2}\) does not extend to the shallower depth of \(-1\) m. Pearson's CO\({}_{2}\) concentration at \(-1\) m with soil temperature at \(-1\) m and 0 m were \(-0.36\) to 0.14 consecutively. This relatively low correlation is most likely due to time window measurement inconsistency. While all temperature data were measured hourly during the monitoring campaign, \(\chi\)c at \(-1\) m was measured only once every visit during midday. Furthermore, the heat map demonstrates a pronounced pattern between soil temperature at various depths and SWC with R more significant than \(-0.7\), except for soil temperature at \(-6\) m. To better understand the driving mechanism of CO\({}_{2}\) concentration at \(-6\) m during dry and wet periods and weather correlation between subsoil CO\({}_{2}\) concentrations extends to seasonal timescales, Principle Component Analysis (PCA) and time series of hourly data corresponding to observed variables are provided in Figure 7.  
Figure 6: Heat map based on Pearson’s correlation test matrix of and hourly data including CO\({}_{2}\) molar fraction (\(-1\) m and \(-6\) m) (xC), soil water content (SWC), u* (friction velocity), temperature(max) (T, at \(-6\) m and \(0\) m), soil air pressure (\(-6\) m), atmospheric pressure (p, at \(35\) m and \(0\) m), and precipitation (P). The correlations were calculated when there was a measurement of CO\({}_{2}\) at \(-6\) m.  
Figure 7: The left side of the figure indicates the principal component analysis (PCA) of hourly data spanning from 19 September 2019 to 23 July 2020, and the right side shows an intraday variation of hourly u* and CO\({}_{2}\) during 48 h. Subfigure (**a**) corresponds to wet periods (SWC \(>0.2\) m\({}^{3}\)/m\({}^{3}\)), and subfigure (**b**) represents dry periods (SWC \(\leq 0.2\) m\({}^{3}\)/m\({}^{3}\)). Figure 7 focuses on a principal component analysis (PCA) and hourly variations of \(\chi\)c at 6 m depth auxiliaries data. The color affected the different parameters refers to the significance of the variables (cos\({}^{2}\)) expressed between 0 (bad defined) and 1 (well defined).