Using radiocarbon to investigate soil respiration impacts on atmospheric CO2

While soil respiration is believed to represent the largest single source of CO₂ emissions on a global scale, there are few tools available to measure soil emissions at large spatial scales. We investigated whether radiocarbon (¹⁴C) abundance in CO₂ could be used to detect and characterize soil emissions in the atmosphere, taking advantage of the fact that ¹⁴C abundance in soil carbon is elevated compared to the background atmosphere, a result of thermonuclear weapons testing during the mid-20th Century (i.e. bomb-C). Working in a temperate hardwood forest in Northern Wisconsin during 2011-12, we made semi-high-frequency measurements of CO₂ at nested spatial scales from the soil subsurface to 150 m above ground level. These measurements were used to investigate seasonal patterns in respired C sources, and to evaluate whether variability in soil-respired &Delta¹⁴C could also be detected in atmospheric measurements. In our ground-level measurements we found large seasonal variation in soil-respired ¹⁴CO₂ that correlated with soil moisture, which was likely related to root activity. Atmospheric measurements of 14CO₂ in the forest canopy (2 to 30 m) were used to construct Keeling plots, and these provided larger spatial-scale estimates of respired ¹⁴CO₂ that largely agreed with the soil-level measurements. In collaboration with the NOAA we also examined temporal patterns of ¹⁴CO₂ at the Park Falls tall-tower (150 m), and found elevated 14CO₂ levels during summer months that likely resulted from increased respiration from heterotrophic sources. These results demonstrate that a fingerprint from soil-respired CO₂ can be detected in the seasonal patterns of atmospheric ¹⁴CO₂, even at a regionally-integrating spatial scale far from the soil surface.