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

RationaleCarbon dioxide isotope (C-13 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 (5mL). We developed a TDL method for CO2 mole fraction ([CO2]) and C-13 analysis of soil microcosms. MethodsPeaks in infrared absorbance following constant volume sample injection to a carrier were used to independently measure [(CO2)-C-12] and [(CO2)-C-13] for subsequent calculation of C-13 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-13 label at C of the propyl side chain. ResultsOnce-daily TDL calibration enabled accurate quantification of C-13 values and [CO2] compared with isotope ratio mass spectrometry (IRMS), with long-term external precision of 0.17 and 0.31 for 5 and 1mL samples, respectively, and linear response between 400 and 5000mol mol(-1) CO2. Production of CO2 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-2>0.98) indicated substantially slower turnover for lignin C (17years) than for the dominant pool of litter (1.3years) and primed soil C (3.9years). ConclusionsOur 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.