Biological activity in soil causes fluxes of O into and CO out of the soil with significant global relevance. Hence, the dynamics of CO concentrations in soil can be used as an indicator for biological activity. However, there is an enormous spatial and temporal variability in soil respiration, which has led to the notion of hotspots and hot moments. This variability is attributed to the spatiotemporal heterogeneity of both plant–soil–microbiome interactions and the local conditions governing gas transport. For the characterization of a given soil, the local heterogeneities should be replaced by some meaningful average. To this end, we introduce a line sensor based on tubular gas-selective membranes that is applicable at the field scale for a wide range in water content. It provides the average CO concentration of the ambient soil along its length. The new technique corrects for fluctuating external conditions (i.e., temperature and air pressure) and the impact of water vapor without any further calibration. The new line sensor was tested in a laboratory mesocosm experiment where CO concentrations were monitored at two depths during the growth of barley ( L.). The results could be consistently related to plant development, plant density, and changing conditions for gas diffusion toward the soil surface. The comparison with an independent CO sensor confirmed that the new sensor is actually capable of determining meaningful average CO concentrations in a natural soil for long time periods.