An optical method for carbon dioxide isotopes and mole fractions in small gas samples: Tracing microbial respiration...

Carbon dioxide isotope (δ¹³ C 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 δ¹³ C analysis of soil microcosms.

Peaks in infrared absorbance following constant volume sample injection to a carrier were used to independently measure [¹² CO₂ ] and [¹³ CO₂ ] for subsequent calculation of δ¹³ 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 ¹³ C label at C_β of the propyl side chain.

Once-daily TDL calibration enabled accurate quantification of δ¹³ C 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 years) than for the dominant pool of litter (1.3 years) and primed soil C (3.9 years).

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.