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 Cβ 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 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 (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.

Enrichment of Lignin-Derived Carbon in Mineral-Associated Soil Organic Matter

“…Headspace gas was initially measured every 2 days for soils with litter + lignin and every 4 days for control soils (to ensure sufficient accumulation of CO 2 for precise analyses in the latter treatment). 32 After 47 days, samples were measured weekly, or after 189 days, every 2 weeks. We measured CO 2 concentrations and their δ 13 C values immediately prior to flushing the headspace.…”

“…Gas was sampled via a 5 mL syringe with stopcock and immediately injected into a carrier gas (CO 2 -free air) flowing into a tunable diode laser absorption spectrometer (TDLAS, TGA200A, Campbell Scientific, Logan, UT). Concentrations of CO 2 and δ 13 C values were calibrated following Hall et al 32 The production of CO 2 derived from soil, litter, and 13 C β -labeled lignin was calculated by mixing models 32 described in the SI. Additional headspace gas samples (20 mL) were collected for CH 4 measurements during the first 105 days, after which CH 4 production was negligible.…”

Mechanisms underlying limited soil carbon gains in perennial and cover‐cropped bioenergy systems revealed by stable isotopes

“…For dissolved organic C and Na 2 SO 4 ‐extracted C measurements, 0.25 g subsamples were sequentially extracted by nanopure water and 0.5 M Na 2 SO 4 in a 1:50 soil:solution mass ratio, shaken for 2 hr, and then centrifuged for 15 min at 10,000 g. Then, for dithionite‐extracted C measurement, the subsamples were combined with 0.4 g sodium dithionite and 45 ml nanopure water, shaken for 16 hr, and then centrifuged for 15 min at 10,000 g. Concentrations and δ 13 C values of extracted C were measured by boiling acidified samples in persulfate solution within sealed vials filled with CO 2 ‐free air (Huang & Hall, ). The CO 2 oxidized from extracted C and their δ 13 C values were measured on a tunable diode laser absorption spectrometer (TGA200A; Campbell Scientific) by injection (Hall, Huang, & Hammel, ). Dithionite‐extracted C was corrected for a reagent blank and δ 13 C values were not reported due to blank δ 13 C variability; reagent C blanks for the other samples were negligible.…”

Large impacts of small methane fluxes on carbon isotope values of soil respiration

“…This method has also been applied to direct measurements of  13 C values of CH4 (Bergamaschi et al, 1994). We recently developed a high-throughput method for measuring  13 C values of CO2 in small gas samples (Hall et al, 2017). Here, we applied a variation of this method to analyze  13 C values of CH4 by adding an in-line CO2 trap and furnace to combust CH4 to CO2, which enabled  13 C measurements of both gases on the same TDLAS instrument using separate replicate samples.…”

“…The  13 C values of soil respiration are often thought to reflect  13 C of the substrate from which the CO2 was derived (Ehleringer et al, 2000;Breecker et al, 2015;Hall et al, 2017). However, production of methane (CH4) impacts the interpretation of  13 C values of CO2.…”