Technical note: Development and field validation of SMART-ASF, a low-cost automated soil gas flux monitoring system for long-term field observations

Fu, Ching-Chou; Hu, Sheng-Fu; Lin, Li-Hung; Chen, Kuo-Hang; Ku, Chin-Shang; Kuo-Chen, Hao; Wu, Kuo-Wei; Lin, Chin-Jen

doi:10.5194/egusphere-2026-4934

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https://doi.org/10.5194/egusphere-2026-4934

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21 Sep 2026

| 21 Sep 2026

Status: this preprint is open for discussion and under review for Biogeosciences (BG).

Technical note: Development and field validation of SMART-ASF, a low-cost automated soil gas flux monitoring system for long-term field observations

Ching-Chou Fu, Sheng-Fu Hu, Li-Hung Lin, Kuo-Hang Chen, Chin-Shang Ku, Hao Kuo-Chen, Kuo-Wei Wu, and Chin-Jen Lin

Abstract. Continuous, high temporal resolution measurements of soil gas fluxes are essential for understanding terrestrial carbon cycling and subsurface fluid processes, yet long-term autonomous monitoring remains limited by the high cost and operational complexity of conventional systems. This study presents the development and field validation of the SMART Automated Soil Flux (SMART-ASF) system, a low-cost and modular platform that integrates automated chamber-based flux measurements, low-cost gas sensing, wireless telemetry, and solar powered autonomous operation for continuous soil CO₂ flux monitoring.

Laboratory calibration against a commercial LGR M-GGA-918 analyzer demonstrated excellent linearity (R² ≈ 1) over a CO₂ concentration range of approximately 100 to 8500 ppm, with relative deviations generally within 3 to 5 % over the concentration range most relevant to soil gas flux measurements. Independent comparisons with conventional manual closed chamber measurements showed good agreement under field conditions. Long-term field deployment for more than five months demonstrated stable unattended operation without routine maintenance or component replacement under natural environmental conditions. Continuous observations successfully captured both short-term and seasonal variations in soil CO₂ flux, including a marked suppression of emissions during intense rainfall associated with Typhoon Fengshen and the subsequent recovery of background fluxes. The system also consistently resolved low background fluxes below 0.1 g m^-²d^-1, demonstrating sufficient sensitivity for monitoring low emission environments.

These results support the applicability of the SMART-ASF system for long-term automated soil gas flux monitoring and its potential use in terrestrial carbon cycle research and environmental geochemical monitoring.

Received: 15 Aug 2026 – Discussion started: 21 Sep 2026

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Ching-Chou Fu, Sheng-Fu Hu, Li-Hung Lin, Kuo-Hang Chen, Chin-Shang Ku, Hao Kuo-Chen, Kuo-Wei Wu, and Chin-Jen Lin

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RC1:
'Comment on egusphere-2026-4934', Anonymous Referee #1, 31 Oct 2026 reply
This study aims to develop an automated soil gas flux monitoring system for CO₂, with potential for other gases. This was done using several sub-units and it is based on the K30 CO₂ sensor. The technical manuscript is a relevant effort that is within the scope of Biogeosciences. Unfortunately, I argue that this is not an “automated soil gas flux monitoring system” and recommend rejecting the current version for publication due to the concerns detailed below.
Thank you for the opportunity to read your work.
Major comments
Unfortunately, this is the main reason for my recommendation to reject. The sentence “This flushing procedure reproduced the gas-renewal function of an automated open–close chamber without requiring a mechanically actuated chamber lid” (L.211-213) shows a lack of understanding (in my opinion) of using the method of flux chambers in soils. The fact that the chamber is never opened to the atmosphere and only the flushing via tube is active means that a microclimate will form within the chamber, and the soil surface within the chamber will never see light or the real atmosphere above. This makes the current chamber design irrelevant for the majority of soil applications. For example, I assume the soil surface within the chamber was completely different after a few days compared to the grassy area surrounding the chamber (Figure 2). Also, the authors report additional significant problems with the flushing method, including a long stabilization delay lasting a few months after the end of flushing (L.249-261). There are papers that use this technique to flush the chamber air, but they are scarce, and as far as I know, all of them are from chambers over water bodies, in which this issue is different.

The novelty is unclear. There are at least ten relevant papers on the development of DIY low-cost chambers that are not cited here and need to be referenced, which will reduce the novelty, as they report similar motivations. For example, maybe one of the first ones to present an actual field-working system: “New low-cost automated system of closed chambers to measure greenhouse gas emissions from the tundra” (Gagnon et al., 2016), or one of the newer ones: “FluxMate: An off-grid, cost-effective and dual-mode automated chamber system to monitor carbon dioxide fluxes in grassland ecosystems” (Shirvan et al., 2026). More specific issues related to the novelty:
L.82-85: Please elaborate and refer to these studies, as they are similar to this manuscript, and the similarity vs. novelty should be clearly stated.

L.90: In the motivation, it is written that there is a need for low-cost systems. However, there are no references or information on low-cost systems/sensors in the introduction. I recommend adding 1-2 paragraphs on this approach of using low-cost, open-source systems for environmental and ecological research.

Additional scientific editing is needed. For example:
Sentences that are in the wrong sections. For example:
L.200-206: This section should be moved to the Results section.

l. 450-452: Should be moved to the Methods section.

Figures without subsection notation (e.g., Fig. 2).

Additional comments
L.140: Why “incorporates a Nafion-based gas drying” and not just correct for RH, which is in any case measured within the chamber?

The low-cost statement, in most cases, is also combined with a DIY approach. In most low-cost papers, there are details that can allow you to duplicate the system and sometimes also a full DIY guide with building instructions. In this manuscript, it looks like a company built major components (XY ELEECONTROL TECHNOLOGY), which is also the 2nd affiliation on the title page. I found it less common for a manuscript to report a low-cost system but not provide details on how to replicate it. Consider adding this information.

L.189-191: Why circulate the air to the sensing unit? The main advantage of this type of sensor is its low cost (as stated in the manuscript), which makes it feasible to install it within the chamber without additional components, such as pumps.

L.264-265: Why use such a long duration to calculate the flux (~30 minutes or more)? When using gas analyzers, the chamber is closed for only a few minutes unless the fluxes are very small. A long analysis timeframe, such as 30-60 minutes, is used only when manual gas measurements are taken at 3-5 points to calculate the slope.

L.274: In my opinion, this is incorrect. The temperature used in equation 1 should be the air temperature within the chamber, not the soil temperature. A temperature at 20 cm soil depth is completely different than the air within the chamber and most likely will bias the results significantly.

L.487-489: The lack of a formal minimum detectable flux and periodic drift evaluation is problematic in manuscripts that develop this type of system. To increase the robustness of this work, I recommend conducting this evaluation. There are several papers that use the K30, and using their drift results is also possible.

Figure 8: What is the sen.chamber humidity parameter? Is it the RH? If so, how come it is not close to 100% during/after rain events?

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Citation: https://doi.org/10.5194/egusphere-2026-4934-RC1

Ching-Chou Fu, Sheng-Fu Hu, Li-Hung Lin, Kuo-Hang Chen, Chin-Shang Ku, Hao Kuo-Chen, Kuo-Wei Wu, and Chin-Jen Lin

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https://doi.org/10.5194/egusphere-2026-4934-supplement

Ching-Chou Fu, Sheng-Fu Hu, Li-Hung Lin, Kuo-Hang Chen, Chin-Shang Ku, Hao Kuo-Chen, Kuo-Wei Wu, and Chin-Jen Lin

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Short summary

We developed a low-cost automated system for continuous monitoring of carbon dioxide released from soils. Laboratory and field tests showed good agreement with reference methods. During more than five months of field operation, the system captured both background emissions and changes caused by heavy rainfall. The system provides an accessible approach for long-term monitoring of soil carbon emissions and environmental change.


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