## 4 Discussion
### Analysis of Wind Turbulence and Soil Water Content  
Several periods with high and low friction velocity were detected during the monitoring period (Figure 9) where the relationship between friction velocity and soil CO\({}_{2}\) concentration at \(-6\) m is very pronounced, especially during dry periods with R\({}^{2}\) of 0.6 where SWC is less than 0.2 m\({}^{3}\)/m\({}^{-3}\) (CO\({}_{2}=-14,\)882u\({}^{\star}+55,\)066). However, this relationship does not extend during wet periods where SWC is above 0.2 m\({}^{3}\)/m\({}^{3}\). This finding acts as empirical evidence of subsoil ventilation and the importance of advection in gas transport processes where the decrease of \(\chi\)c coincides with high u\({}^{\star}\) that fits with the linear model (coefficients: b[0] 55,066, b[1] \(-14,\)882, R\({}^{2}\): 0.68) (Figure 9). Some previous works focusing on soil CO\({}_{2}\) dynamics have reported the influence between \(\chi\)c and u\({}^{\star}\)[26; 31; 59]. The lower soil of our site consists of sandy and clayey passages caused by the complex sedimentary source material. The calcareous bedrock mainly consisted of calcium carbonate and a few amounts of clay minerals surrounded by the weathered layer rich in clays due to decarbonization. It is a widely held view that limestone's solubility in weak acid and limestone, resulting in karst forming processes. It has been previously studied that the karst system stores a large amount of CO\({}_{2}\) underground, which later was emitted during windy days and filled quickly during low wind speed through cracks and fissures [59]. Even though some sinkholes in our site are visible, this hypothesis needs further investigation to understand the rate and magnitude of calcite dissolution. Also, monitoring data suggest that the subsoil's permeability is most likely reduced during wet periods diminishing molecular diffusion and advection. The lower the SWC value, the more enormous the ventilation's potential to remove CO\({}_{2}\) from the soil, as indicated by previous works in a karstic environment [59; 60]. It is widely known that the increase of temperature by 10 \({}^{\circ}\)C leads to a higher respiration rate factor, known as temperature sensitivity of soil respiration, that is often expressed as the Q10 [61; 62]. However, this factor related to biological activity cannot fully explain the variation. Instead, the CO\({}_{2}\) fluctuation is likely due to pressure pumping effects (PPE)  
Figure 9: The relationship between hourly CO\({}_{2}\) molar fraction (\(\chi\)c) at 6m depth and friction velocity (u\({}^{\star}\)) above the canopy for different levels of SWC. promoted by u* fluctuation where the soil acts as a semi-permeable membrane storing and releasing CO2[33; 63; 64]. The continuous wavelet-based analysis (Figure 10) was employed of the dataset from 17 June to 23 July 2020, which was chosen due to the dataset's continuity. The wavelet analysis was done using R software and Waveletcomp Package [44; 65] to test our hypothesis regarding pressure pumping effects generated in a deep soil layer. Friction velocity (u*) and temperature (35 m) recorded from the 17 June to 23 July 2020 ranged from 0 m/s-1 m/s and 10-28 \({}^{\circ}\)C, respectively. Wavelet power analysis of temperature and u* (Figure 10a(1,2)) demonstrates substantial diurnal scale as expected shown by maximum power wavelet at periods of 24 (24 h). Wavelet power levels of u* in periods of 24 were slightly higher with 3.33 compared to temperature with 2.87, indicating more pronounce daily variation. As for the air pressure, even though the pressure above  
Figure 10: Half-hourly wavelet-based analysis from 17 June to 23 July 2020 which include (**a**) wavelet power spectrum of (**a1**) temperature at 35 m, (**a2**) friction velocity (u*), (**a3**) air pressure at 35 m, and (**a4**) air pressure difference (\(\Delta\)p) between 35 m and \(-\)6 m. Moreover, wavelet coherence analysis (**b**) shows the multivariate statistical significance of (**b1**) temperature at 35 m over u* and (**b2**) \(\Delta\)p over u*.