An optical method for carbon dioxide isotopes and mole fractions in small gas samples: Tracing microbial respiration from soil, litter, and lignin.

RATIONALE

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

METHODS

Peaks in infrared absorbance following constant volume sample injection to a carrier were used to independently measure [ CO ] and [ CO ] for subsequent calculation of δ C 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 C label at C of the propyl side chain.

RESULTS

Once-daily TDL calibration enabled accurate quantification of δ C values and [CO ] 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 CO . Production of CO from native soil C, added litter, and lignin C varied over four orders of magnitude. Multiple-pool first-order decay models fitted to data (R  > 0.98) indicated substantially slower turnover for lignin C (17 years) than for the dominant pool of litter (1.3 years) and primed soil C (3.9 years).

CONCLUSIONS

Our TDL method provides a flexible, precise, and high-throughput (60 samples h ) 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.