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

Abstract

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 C_β 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 C_β varied over four orders of magnitude. Multiple-pool first-order decay models fitted to data (R² > 0.98) indicated substantially slower turnover for lignin C_β (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.

Citation

Hall, Steven J.; Huang, Wenjuan; Hammel, Kenneth E. 2017. An optical method for carbon dioxide isotopes and mole fractions in small gas samples: tracing microbial respiration from soil, litter, and lignin. Rapid Communications in Mass Spectrometry. 31: 1938-1946. https://doi.org/10.1002/rcm.7973.