## 4 Discussion  
### Field Performances and Processing Infrastructure  
Even though this work aims to develop a continuous flow system to continuously measure the soil gas concentration and the variability of physical parameters, it can also be used as preliminary guidelines to better understand the CO\({}_{2}\) variations in the forest ecosystem. Figure 8 shows a typical infrared profile of gas coming from the borehole at 6 m depth, where only the absorption bands of CO\({}_{2}\) and H\({}_{2}\)O are clearly visible. Even though CH\({}_{4}\) concentration was always below our IR sensor's sensibility limit, it was still detectable with an ex-situ GC analysis at a range of 0.9-2.7 ppm with minor depth-dependence. Moreover, these monitoring stations have shown that soil CO\({}_{2}\) concentration monitoring can be maintained long term without grid power. After deployment, there were follow-up visits to change the batteries every month before December 2019 and every week after December 2019, but cumulatively these equated to only a few fieldwork days. During the deployments, the average power consumption for two days of continuous and simultaneous measurements was roughly 300 A. This includes the power demand from a laptop, FTIR, auxiliary sensors, and circulation module (pumps). Nonetheless, the advantages outweighed the drawback of its power consumption, where the other actively IR gas interfinger can be identified individually.  
The FTIR calibration was deemed very simple considering CO\({}_{2}\) vibrational mode at the wavelength of 2400 cm\({}^{-1}\) (v\({}_{3}\)). The high-frequency data hold many exciting features that are not captured in the synoptic data sets and will be useful for tuning our understanding  
Figure 8: Typical infrared profile of soil gas from the 6 m borehole (Time: 20:03 on 16/Oct/2019). Interferences weak bands of H\({}_{2}\)O\({}_{\rm vapour}\) are also visible on the spectrum at the low and high-frequency range.  
of soil carbon. With this study, this monitoring infrastructure was considerably successful in terms of survivability and management. In the case of continuously accessible power, the system could record and store high-frequency measurements (<1 min). There is also an opportunity to reduce power consumption by eliminating an FTIR Bruker alpha and a laptop while relying only on low power consumption, an IR sensor, and a data logger. Since the FTIR measurement with a resolution of 1 cm\({}^{-1}\) and 4 cm\({}^{-1}\) is visually identical with a polynomial model, the linear model fitting was applied for both resolutions provided in Table 2. Therefore, it is important to understand how FTIR's lower resolution at 4 cm\({}^{-1}\) performs in conjunction with the Beer-Lambert law.  
The resolution of 1 cm\({}^{-1}\) has a slightly higher correlation index using a linear model than 4 cm\({}^{-1}\) at 0.987 and 0.976, respectively. It has been reported that the equation between the partial pressure of CO\({}_{2}\) and CH\({}_{4}\) does not follow the beer-lambert law, especially if it is measured using the spectral resolution of fewer than 1 cm\({}^{-1}\)[43; 47]. Unlike solid or liquid, which does not possess a rotational mode, the gas state involves vibrational and rotational transitions. In the case of gases calibration, the rotation of the bands is often thin (FWHM [Full width at half maximum] < 0.2 cm\({}^{-1}\)), and all the spectral lines that make them are only detectable if a very high resolution (of the order of 0.1 cm\({}^{-1}\)) is used. The area derived from lower resolution FTIR does not precisely match the sum of all the theoretical spectral lines of vibration-rotation. Hence, it is almost certain the calibration curves fit with a three-degree polynomial equation rather than a linear equation result from its low-resolution measurement. However, considering the longer scanning time resulting in lower temporal resolution, the 3-degree polynomial is deemed acceptable for producing a calibration curve.