Rain and wind affect chamber measurements

Abstract

Chamber methods are widely used to measure soil-atmosphere fluxes of different gases. To obtain reliable flux estimates it is important to avoid changes in the natural soil-atmosphere system as far as possible. While issues like chamber dimensions, closure time and calculation approaches have been intensively discussed, there is still uncertainty about other possible biases like wind and rain affecting the measurement.To study the effect of local airflow and rain on chamber flux measurements, we used inert tracer gas (Tetrafluoromethane, CF4) that was continuously injected into the subsoil and concurrently measured during the automatic soil respiration measurements. While the variability of the CO2 fluxes included the changing signal of the biological source and the physical effect of the changing environment, the CF4 measurements allowed isolating the latter. Finite element modeling of gas transport within the soil-chamber-atmosphere system was used to further analyze the effect of rain and soil moisture.Rain events had a strong effect on the measured CF4 fluxes with a decrease shortly after rainfall which was then super-compensated by elevated fluxes for 1–2 days. This pattern could be addressed as an artefact of automatic chamber measurements that has been overlooked so far. Since the chamber closed automatically, it excluded a part of the rain amount. This resulted in a temporarily preferential venting of soil gas through the chamber surface that remained drier than the surrounding soil outside the chamber. Thus, we recommend leaving automatic chambers open during rain to strictly avoid rain exclusion.Local wind speed near the chamber was generally low since we worked in a forest. Yet, CF4 flux and CO2 flux decreased with increasing wind speed, what can be interpreted as an artefact due to the chamber modifying the natural airflow conditions. The wind effect, however, was weaker than the effect of rain.