Method optimization and comparison for high-frequency static-chamber measurements of CO2, CH4 and N2O fluxes from soil

Chamber-based in situ measurements of greenhouse gas (GHG) fluxes have been widely applied in numerous studies. However, with the increasing applications of online optical GHG analyzers, the chamber system designs and measurement protocols are quite diverse and also uncertain.

Here, we compared various measurement approaches for soil CO₂, CH₄, and N₂O fluxes, by optimizing the chamber base collar, chamber volume, within-chamber fan, fertilizer treatments and integration time for flux calculation. Additionally, we conducted a flux monitoring campaign over 80 hours with both automated multi-chamber system (M16 multiplexer + LI-COR 7810&7820) and discrete sampling with offline measurement by gas chromatography (GC; Agilent 8890). Fluxes were computed from concentration time series and evaluated using goodness-of-fit and error diagnostics (e.g., R², CV, RMSE, MAE) and by systematically varying the regression time window to quantify window dependence. In parallel, soil water status (WFPS) and rainfall were monitored, and soil inorganic N (NH₄⁺ and NO₃^-) was measured at selected rounds to interpret event-driven responses. 

The results showed that base collar installation and internal fan is highly necessary in assuring good linearity of concentration measurement along time and minimizing disturbance upon chamber closure. Regarding the integration time for flux calculation, the length of datasets markedly affected fitting performance and flux magnitude, which are mostly critical for CH₄ and N₂O. During the 80 h automated deployment, fertilized plots exhibited pronounced, pulse peaks of N₂O emissions (hot moments) superimposed on chamber-cycle oscillations, while CH₄ displayed stronger shared background variability across chambers. These N₂O pulses coincided with wetting-related dynamics indicated by WFPS or rainfall patterns and concurrent shifts in inorganic N. Cross-validation between online (LI-COR) and offline (GC) fluxes showed a general agreement for N₂O but a weaker association for CH₄, indicating systematic bias and limited explanatory power of sparse offline point sampling under rapidly changing conditions. Together, our results provide practical, species-specific guidance for chamber-based flux measurements and highlight the need for harmonized synchronization and computation protocols when integrating online and/or offline approaches, especially during event-driven flux dynamics.   

Keywords: Static Chamber; Greenhouse Gas Flux; Online–offline Comparison; Hot Moments