Resolving Event-Driven Soil Gas Fluxes by Coupling High-Frequency Chamber Measurements with Advection–Diffusion Modeling

Soil greenhouse gas (GHG) emissions in agricultural, forestry, and other land uses are driven by coupled biological and physical processes. To monitor these fluxes, automatic chamber systems are now widely used as point-scale measurement techniques. Their high-frequency records provide richer observational coverage across meteorological and hydrogeological conditions, thereby improving the accuracy of annual soil carbon budgets.

Despite advances in monitoring, long-term soil carbon models usually focus solely on simulating soil carbon turnover and decomposition, omitting mechanisms of soil gas transport. Although this simplification may be reasonable in the topsoil, sharp changes in soil saturation or other meteorological factors are not necessarily captured, which can lead to underestimating short-term emissions and biasing annual GHG budgets.

We investigated this issue in a pilot site in the agricultural region (Seeland region, Switzerland) where the water table depth was controlled. We simulated a short flooding event and continuously monitored soil gas flux at high frequency. And our results showed a dampening in CO2 soil gas flux for the flooded plot compared to our control plot, which persisted after it was drained. While this decrease in CO2 flux can be partly attributed to a reduction in aerobic microbial activity, the timescale to recovery to background CO2 fluxes can be attributed to other mechanisms, including advection-diffusion gas transport in the unsaturated zone.

To interpret these dynamics, we employed a 1-D model to assess the role of advection-diffusion, including pressure-driven gas transport, during short-term events. Our model couples water, heat, and gas transport with microbially driven CO2 production. We conducted a sensitivity analysis evaluating different soil conditions and event intensities.

Finally, the integration between high-frequency soil gas flux monitoring systems and gas transport in the unsaturated zone helps deconvolute the soil gas flux signal, while improving the accuracy of the soil GHG budget. This will enhance the process understanding, which can support agricultural management strategies to minimize GHG emissions.