Understanding the magnitude and controls of soil respiration, the largest outgoing flux of CO2 from terrestrial ecosystems to the atmosphere, is a not-yet resolved but important challenge that requires, among other things, of a vast and solid network of experimental sites and of a robust, accurate and harmonized methodology to continuously monitor soil CO2 effluxes from remote areas, as is the case for measures of net ecosystem exchange (NEE) with eddy covariance systems.

Solid-state CO2 sensors have appeared as an emerging methodology through continuous measurements of CO2 profiles in soils, together with environmental, soil characteristics and diffusion-based models allow tracking soil CO2 fluxes at appropriated spatial and temporal scales. This sensors type might be a cheaper, yet reliable, substitute in the near future for the very expensive and energetically costly automatic chambers-based systems (either close or open dynamic systems), which, so far, dominate the market. On top of that, this approach has the potential to deepen the mechanisms behind soil respiration by: 1 . Exploring the vertical partitioning of biological and geological sources of CO2 production within the soil profile; and 2. Helping us to get a deeper understanding of the abiotic (e.g. pressure pumping, wind, temperature, soil moisture) and biotic (e.g. autotrophic and heterotrophic respiration) controls of this large flux of CO2 which, so far, remains largely unknown due to the complex mechanisms behind the variability of both soil CO2 production and CO2 effluxes. Moreover, and even if solid-state CO2 sensors are currently producing a large amount of data on CO2 production in soils from all over the world, we still lack an understanding of the mechanisms and controls of gas transport through the soil matrix which, therefore, prevents us from having a reliable translation of CO2 production into soil CO2 effluxes.

We, hence, need to design experiments and methodologies that may help us get a better understanding of the relation of CO2 measured across the soil profile with it sources and drivers, in order to obtain reliable estimates of soil CO2 production and efflux.

We here, therefore, would like to call for studies designed to deepen the understanding the origin and fate of soil CO2 and/or processes controlling soil CO2 transport and efflux.