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An optical method for carbon dioxide isotopes and mole fractions in small gas samples: Tracing microbial respiration from soil, litter, and lignin
Author(s) -
Hall Steven J.,
Huang Wenjuan,
Hammel Kenneth E.
Publication year - 2017
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.7973
Subject(s) - chemistry , soil respiration , isotope ratio mass spectrometry , carbon dioxide , lignin , microcosm , environmental chemistry , isotope , stable isotope ratio , isotopes of carbon , soil carbon , dissolved organic carbon , litter , analytical chemistry (journal) , soil water , mass spectrometry , total organic carbon , chromatography , soil science , ecology , environmental science , physics , organic chemistry , quantum mechanics , biology
Rationale Carbon dioxide isotope (δ 13 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 2 isotopes at low cost and high throughput, but are seldom used for small samples (≤5 mL). We developed a TDL method for CO 2 mole fraction ([CO 2 ]) and δ 13 C analysis of soil microcosms. Methods Peaks in infrared absorbance following constant volume sample injection to a carrier were used to independently measure [ 12 CO 2 ] and [ 13 CO 2 ] for subsequent calculation of δ 13 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. Results Once‐daily TDL calibration enabled accurate quantification of δ 13 C values and [CO 2 ] 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 CO 2 . Production of CO 2 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 (R 2 > 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). 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.