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

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^-2 d^-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.