Grassland Soil and Fossil CO2 Fluxes Monitored Using Continuous CELS Measurements of [CO2] and δ13C

Over an 8 day period, the concentration and δ13C of above-ground CO2 at an experimental C3 grassland site (near Bozeman, MT) were continuously measured in situ using a cavity enhanced laser spectrometer (LGR, model 908-0003). Air was constantly drawn from 13 inlets and sequentially diverted into the analyzer at 150 sec intervals using an automated inlet switching system. These data were used to monitor and quantify the relative abundances of background air, soil, and experimentally introduced, fossil fuel derived CO2. The latter source was continuously vented from the soil subsurface as part of a larger project (ZERT) to simulate and detect a leak from underground CO2 storage. Keeling plots showed that the variations in [CO2] were dominated by the mixing of CO2 from the three sources: background air (δ13C = -8.2o/oo), soil (-27o/oo), and fossil (-56o/oo), forming a wedge of data points characteristic of three-end-member mixing (Fig.). Prior to fossil CO2 injection, a linear Keeling relationship was evident (Fig.). Photosynthetic uptake and fractionation of CO2 contributed only a small amount to the total variations seen, as evident in day-night data comparisons. Soil respiration was more dominant at night, and there were significant, periodic venting events of both soil and fossil CO2. Horizontal and vertical profiles of [CO2] and δ13C allowed estimates of CO2 fluxes from both soil respiration and fossil CO2. This study demonstrates that CELS can be useful in field studies of multiple CO2 sources requiring relatively high temporal and spatial measurement resolution.