gcb2721-sup-0001-FigureS1.JPGimage/JPG619KFigure S1. Data for daytime 10 June 2008 sampling, are shown below as an example of a typical Keeling plot constructed to determine the δ13C of soil-respired CO₂ during this study at any given time. Data points show the relationship between the inverse of the concentration of CO₂ in the headspace of the soil respiration chamber (x-axis) vs. the δ13C of the air collected from the headspace chamber (y-axis) at a given CO₂ concentration. Each headspace air sample was collected from a different PVC collar, when the CO₂ concentration inside the soil respiration chamber was 400-700 µL L-1 and the CO2 concentration differed by at least 50 µL L-1 from that for the previous sample. The data points were fitted to a linear regression Model I. The extrapolated y-intercept represents the δ13C of soil-respired CO₂ (δ13CRsoil). For this linear regression, the slope was 7194 ± 308, the y-intercept was −28.25 ± 0.1 and the r= 0.9950.

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