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

Abstract

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 (≤5mL). 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 Cβ 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 1mL samples, respectively, and linear response between 400 and 5000μmol 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 (17years) than for the dominant pool of litter (1.3years) and primed soil C (3.9years). 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.