Working with isotope measurements

Data from the LI-7825 CO2 Isotope/NH3 Trace Gas Analyzer include measurements of CO2 isotopologues. SoilFluxPro Software can evaluate these data using the Slope and Keeling Regression methods. The Isotope tab allows the assignment of a Major Gas and Minor Gas for the regression.

To add a new regression:

  1. Open the Isotope tab and click Create New Regression.

  2. Use the default settings or select a Major Gas and Minor Gas from the available variables, then click Create.

  3. Two new regressions are created - Keeling and Slope.

    Each regression object presents options. Expand the object to view them.

    • Which observations?

      • Selected will display a plot and table of data, along with options to adjust the parameters for the measurement selected in the data display.

      • All will show options that can be adjusted for all measurements.

    • Apply: Click to apply changes.

    • Revert: Click to discard and revert to the prior state.

    You can view a plot and table of the Major Gas/Minor Gas or the Keeling plot when a single observation is selected.

Keeling regression

The Keeling regression method uses C13_DELTA and CO2 as minor and major gases by default. These gases are used to name variables in the data file. If you select different gases when creating the regression, the variable name in the data file indicates the gases. The x-axis of the Keeling plot shows 1/major gas. The source estimate comes from the y-intercept of linear regression of the minor gas over 1/major gas.

Table 4‑1. Keeling regression variables.
Interface
variable		 Units Data file variable Description
Major Gas µmol mol-1 MAJOR_GAS Major gas species used to compute the regression (default is CO2).
Major Gas Source - MAJOR_GAS_SOURCE Instrument that measured the major gas; e.g., LI-7825
Minor Gas µmol mol-1 MINOR_GAS Minor gas species used to compute the regression (default is C13_DELTA).
Minor Gas Source - MINOR_GAS_SOURCE Instrument that measured the minor gas; e.g., LI-7825
Deadband s MAJOR/MINOR (KEELING) DEADBAND Deadband length; e.g., C13_DELTA/CO2 (KEELING) DEADBAND.
Stop Time s STOP_TIME Final data point used for the regression.
Slope ‰ per (µmol mol-1) MAJOR/MINOR (KEELING) SLOPE Slope of linear fit; e.g., C13_DELTA/CO2 (KEELING) Slope
Intercept ‰ MAJOR/MINOR (KEELING) INTERCEPT Y intercept of linear regression; e.g., C13_DELTA/CO2 (KEELING) INTERCEPT
R2 - MAJOR/MINOR (KEELING) r2 Coefficient of determination; e.g., C13_DELTA/CO2 (KEELING) r2
Intercept SE ‰ MAJOR/MINOR (KEELING) int_se Standard error of regression intercept term; e.g., C13_DELTA/CO2 (KEELING) int_se
Slope SE ‰ MAJOR/MINOR (KEELING) slope_se Standard error of regression slope term; e.g., C13_DELTA/CO2 (KEELING) slope_se
Figure 4‑1. Options to compute results using the Keeling method.

Slope regression

The Slope regression method uses C13 and C12 as minor and major gases by default. These gases are used to name variables in the data file. If you select different gases when creating the regression, the variable name in the data file indicates the gases. A new slope regression is automatically set to 0.0112372 (the interface displays 0.01124). The source isotopic value is calculated from the slope of a linear fit of the major gas over minor gas standardized to the isotopic standard value:

4‑1source isotopic value = (slope/standard − 1) × 1000

Table 4‑2. Slope regression variables.
Interface
variable		 Units Data file variable Description
Major Gas ‰ MAJOR_GAS Major gas species used to compute the regression; e.g., C12
Major Gas Source - MAJOR_GAS_SOURCE Instrument that measured the major gas; e.g., LI-7825
Minor Gas ‰ MINOR_GAS Minor gas species used to compute the regression; e.g., C13
Minor Gas Source - MINOR_GAS_SOURCE Instrument that measured the minor gas; e.g., LI-7825
Deadband s MAJOR/MINOR (SLOPE) DEADBAND Deadband length; e.g., C12/C13 (SLOPE) DEADBAND
Stop Time s MAJOR/MINOR (SLOPE) STOP_TIME Final data point used for the regression.
Isotopic Standard - MAJOR/MINOR (SLOPE) STANDARD Isotopic standard value = 0.0.0112372; e.g., C12/C13 (SLOPE) STANDARD
Slope Dimensionless MINOR/MAJOR (SLOPE) SLOPE Slope of linear fit; e.g., C13/C12 (SLOPE) SLOPE
Intercept Same as minor gas MINOR/MAJOR (SLOPE) INTERCEPT Y intercept of linear regression; e.g., C13/C12 (SLOPE) INTERCEPT
Source Isotopic Value ‰ MINOR/MAJOR (SLOPE) ISOTOPIC_VALUE Isotopic standard value; e.g., C13/C12 (SLOPE) ISOTOPIC_VALUE
R2 - MINOR/MAJOR (SLOPE) r2 Coefficient of determination; e.g., C13/C12 (SLOPE) r2
Intercept SE Same as major gas MINOR/MAJOR (SLOPE) int_se Standard error of the slope intercept; e.g., C13/C12 (SLOPE) int_se
Slope SE Same as major gas MINOR/MAJOR (SLOPE) slope_se Standard error of the regression slope term; e.g., C13/C12 (SLOPE) slope_se
Figure 4‑2. Options to compute results using the Slope method.