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    Author(s): Steven J. Hall; Wenjuan Huang; Kenneth Hammel
    Date: 2017
    Source: Communications in Mass Spectrometry.
    Publication Series: Scientific Journal (JRNL)
    Station: Forest Products Laboratory
    PDF: Download Publication  (1.0 MB)


    RATIONALE: Carbon dioxide isotope (Δ13C value) measurements enable quantification of the sources of soil microbial respiration, thus informing ecosystem C dynamics. Tunable diode lasers (TDLs) can precisely measure CO2 isotopes at low cost and high throughput, but are seldom used for small samples (≤5 mL). We developed a TDL method for CO2 mole fraction ([CO2]) and Δ13C analysis of soil microcosms.

    METHODS: Peaks in infrared absorbance following constant volume sample injection to a carrier were used to independently measure [12CO2] and [13CO2] for subsequent calculation of Δ13C values. Using parallel soil incubations receiving differing C substrates, we partitioned respiration from three sources using mixing models: native soil organic matter (SOM), added litter, and synthetic lignin containing a 13C label at Cβ of the propyl side chain.

    RESULTS: Once-daily TDL calibration enabled accurate quantification of Δ13C values and [CO2] compared with isotope ratio mass spectrometry (IRMS), with long-term external precision of 0.17 and 0.31‰ for 5 and 1 mL samples, respectively, and linear response between 400 and 5000 μmol mol-1 CO2. Production of CO2 from native soil C, added litter, and lignin Cβ varied over four orders of magnitude. Multiple-pool first-order decay models fitted to data (R2 > 0.98) indicated substantially slower turnover for lignin Cβ (17 y) than for the dominant pool of litter (1.3 y) and primed soil C (3.9 y).

    CONCLUSIONS: Our TDL method provides a flexible, precise, and high-throughput (60 samples h-1) alternative to IRMS for small samples. This enables the use of C isotopes in increasingly sophisticated experiments to test biogeochemical controversies, such as the fate of lignins in soil.

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    Hall, Steven J.; Huang, Wenjuan; Hammel, Kenneth E. [In press]. An optical method for carbon dioxide isotopes and mole fractions in small gas samples: tracing microbial respiration from soil, litter, and lignin. Communications in Mass Spectrometry.


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    Carbon isotope, decomposition, kinetic fractionation, lignin, recalcitrance, soil respiration

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