Improving soil CO2 efflux estimates from in-situ soil CO2 sensors with gas transport measurements

Abstract:

Correctly estimating soil carbon dioxide (CO₂) fluxes emitted to the atmosphere is essential because they are a large component of the ecosystem carbon balance. Continuous estimates of soil CO₂ flux, especially when paired with eddy covariance measurements of whole-ecosystem CO­₂ exchange, help to disaggregate net ecosystem CO₂ exchange. Most researchers estimate soil CO₂ fluxes by applying the gradient method; however, this is only appropriate in the absence of advective or convective processes. Given the rarity of such static states, we must move toward measurement techniques that will allow us to quantify the dynamic soil efflux system with gas transport by convective, advective and molecular diffusion processes.

Convective processes are mainly relevant in caves, where values of relative humidity, temperature and CO₂ molar fraction determine the buoyancy of the external-internal air masses. These convective processes also are important in large fractures when temperature differences between surface and depth can generate convection, transporting CO₂ from deep layers to the atmosphere. Advective processes occur both in caves and in soils, and the CO₂ exchanges are mainly due to three factors: wind, changes in atmospheric pressure, and changes in the water table. Molecular diffusion processes are being widely applied in the determination of soil-atmosphere gas exchanges by applying the gradient method. However, the use of the gradient method can yield inappropriate flux estimates due to the uncertainties mainly associated with the inappropriate determination of the soil diffusion coefficient. Therefore, in-situ methods to determine diffusion coefficient are necessary to obtain accurate CO₂ fluxes. If this is resolved, the gradient method has great potential to become the most used technique to monitor atmosphere-soil CO₂ exchanges within the next few years. Here we review the state of the science and describe a series of field measurements for significantly improving the accuracy of soil CO₂ fluxes.