Can we use the CO2 concentrations determined by continuous-flow isotope ratio mass spectrometry from small samples for the Keeling plot approach?

Occurrence of Priming in the Degradation of Lignocellulose in Marine Sediments

More than 50% of terrestrially-derived organic carbon (terrOC) flux from the continents to the ocean is remineralised in the coastal zone despite its perceived high refractivity. The efficient degradation of terrOC in the marine environment could be fuelled by labile marine-derived material, a phenomenon known as “priming effect”, but experimental data to confirm this mechanism are lacking. We tested this hypothesis by treating coastal sediments with ¹³C-lignocellulose, as a proxy for terrOC, with and without addition of unlabelled diatom detritus that served as the priming inducer. The occurrence of priming was assessed by the difference in lignocellulose mineralisation between diatom-amended treatments and controls in aerobic sediment slurries. Priming of lignocellulose degradation was observed only at the initial stages of the experiment (day 7) and coincided with overall high microbial activity as exemplified by total CO₂ production. Lignocellulose mineralisation did not differ consistently between diatom treatments and control for the remaining experimental time (days 14–28). Based on this pattern, we hypothesize that the faster initiation of lignocellulose mineralisation in diatom-amended treatments is attributed to the decomposition of accessible polysaccharide components within the lignocellulose complex by activated diatom degraders. The fact that diatom-degraders contributed to lignocellulose degradation was also supported by the different patterns in ¹³C-enrichment of phospholipid fatty acids between treatments. Although we did not observe differences between treatments in the total quantity of respired lignocellulose at the end of the experiment, differences in timing could be important in natural ecosystems where the amount of time that a certain compound is subject to aerobic degradation before burial to deeper anoxic sediments may be limited.

Sample matrix effects on measured carbon and oxygen isotope ratios during continuous-flow isotope-ratio mass spectrometry

RATIONALE

Continuous-flow isotope-ratio mass spectrometry (CF-IRMS) is frequently used to analyze CO2 found in media such as air, breath, and soil pore space gas with the aid of a sample preparation and transfer device such as a Gasbench II. This study investigated the effect that matrices other than helium (He) have on the measured δ(13)C and δ(18)O isotope ratios of CO2.

METHODS

Identical CO2 was added to sample vials with matrices of pure He, pure N2, or a 21:79 mixture of O2/N2 and analyzed by a ThermoFinnigan Delta(Plus) XP isotope-ratio mass spectrometer coupled to a ThermoFinnigan Gasbench II. Variables such as CO2 concentration, sample analysis sequence, and sample matrix removal ('blanking') through manipulation of an injection and dilution open split were tested to identify systematic isotope ratio offsets between the different matrix types.

RESULTS

The process of blanking induced a δ(13)C and δ(18)O offset of ≤0.2‰ between otherwise identical populations of CO2 samples in He. The (13)C/(12)C and (18)O/(16)O isotope ratios of CO2 sampled from pure N2 or a mixture of O2/N2 were found to be within 0.1 to 0.2‰ of those of an identical CO2 sampled from a He matrix when N2 or O2/N2 was removed prior to transport to the mass spectrometer. The measured oxygen isotope ratios of CO2 sampled from N2 and O2/N2 varied by as much as 0.6‰ and 4‰, respectively, if matrix gas was not removed prior to ionization.

CONCLUSIONS

Sampling CO2 from matrices similar to air does not significantly affect the measured (13)C/(12)C and (18)O/(16)O isotope ratios of CO2 when a gas-handling procedure that includes the removal of matrix gas is utilized. This procedure is much preferable to introducing matrix gas and potentially isobaric interference to the ion source.

In situ 13CO2 pulse-labeling in a temperate heathland--development of a mobile multi-plot field setup

RATIONALE

Pulse-labeling with (13)CO2 and the subsequent analysis of (13)C-carbon via isotope ratio mass spectrometry (IRMS) have been shown to be an excellent method to investigate the terrestrial carbon cycle. Improving (13)CO2 manipulation experiments will facilitate our understanding of carbon cycling processes.

METHODS

A mobile field setup for in situ (13)CO2 pulse-labeling was developed for low vegetation field experiments. Two pulse-labeling experiments were conducted in a Danish heathland in September 2010 (Exp1) and May 2011 (Exp2). A flow-through system was developed where labeling chambers were supplied with (13)CO2-enriched air from a gas reservoir. Reservoir and chamber air was sampled over the course of the experiments and analyzed for CO2 concentration and isotopic composition on a GasBench II interfaced with an isotope ratio mass spectrometer. The soil CO2 efflux and the atom% excess in soil respiration were assessed after the (13)CO2-pulse to verify the setup performance.

RESULTS

The carbon dioxide concentrations and (13)CO2 enrichments were stable during the experiments. The CO2 concentrations conformed to the aimed values, whereas the (13)CO2 enrichments were lower than expected. The sources of error for the deviation in observed atom% (13)CO2 values are discussed, and a measurement procedure is suggested for samples highly enriched in (13)C by using adjusted resistor settings of the mass spectrometer. However, more work has to be done. Enrichment patterns in soil respiration agree with published observations indicating satisfactory performance of the developed system.

CONCLUSIONS

A mobile flow-through system suitable for continuous in situ (13)CO2 pulse-labeling was successfully developed that is easily applicable in remote natural ecosystems.

Negative priming effect on organic matter mineralisation in NE Atlantic slope sediments

The priming effect (PE) is a complex phenomenon which describes a modification (acceleration or retardation) in the mineralisation rate of refractory organic matter (OM) following inputs of labile material. PEs are well-studied in terrestrial ecosystems owing to their potential importance in the evolution of soil carbon stocks but have been largely ignored in aquatic systems despite the fact that the prerequisite for their occurrence, i.e. the co-existence of labile and refractory OM, is also true for sediments. We conducted stable isotope tracer experiments in continental margin sediments from the NE Atlantic (550–950 m) to study PE occurrence and intensity in relation to labile OM input. Sediment slurries were treated with increasing quantities of the ¹³C-labelled diatom Thalassiosira rotula and PE was quantified after 7, 14 and 21 days. There was a stepwise effect of diatom quantity on its mineralisation although mineralisation efficiency dropped with increasing substrate amounts. The addition of diatomaceous OM yielded a negative PE (i.e. retardation of existing sediment OM mineralisation) at the end of the experiment regardless of diatom quantity. Negative PE is often the result of preferential utilisation of the newly deposited labile material by the microbial community (“preferential substrate utilization”, PSU) which is usually observed at excessive substrate additions. The fact that PSU and the associated negative PE occurred even at low substrate levels in this study could be attributed to limited amounts of OM subject to priming in our study area (∼0.2% organic carbon [OC]) which seems to be an exception among continental slopes (typically >0.5%OC). We postulate that PEs will normally be positive in continental slope sediments and that their intensity will be a direct function of sediment OC content. More experiments with varying supply of substrate targeting C-poor vs. C-rich sediments are needed to confirm these hypotheses.

Field-based cavity ring-down spectrometry of δ¹³C in soil-respired CO₂

Measurement of soil-respired CO₂ at high temporal resolution and sample density is necessary to accurately identify sources and quantify effluxes of soil-respired CO₂. A portable sampling device for the analysis of δ(13)C values in the field is described herein. CO₂ accumulated in a soil chamber was batch sampled sequentially in four gas bags and analysed by Wavelength-Scanned Cavity Ring-down Spectrometry (WS-CRDS). A Keeling plot (1/[CO₂] versus δ(13)C) was used to derive δ(13)C values of soil-respired CO₂. Calibration to the δ(13)C Vienna Peedee Belemnite scale was by analysis of cylinder CO₂ and CO₂ derived from dissolved carbonate standards. The performance of gas-bag analysis was compared to continuous analysis where the WS-CRDS analyser was connected directly to the soil chamber. Although there are inherent difficulties in obtaining absolute accuracy data for δ(13)C values in soil-respired CO₂, the similarity of δ(13)C values obtained for the same test soil with different analytical configurations indicated that an acceptable accuracy of the δ(13)C data were obtained by the WS-CRDS techniques presented here. Field testing of a variety of tropical soil/vegetation types, using the batch sampling technique yielded δ(13)C values for soil-respired CO₂ related to the dominance of either C₃ (tree, δ(13)C=-27.8 to-31.9 ‰) or C₄ (tropical grass, δ(13)C=-9.8 to-13.6 ‰) photosynthetic pathways in vegetation at the sampling sites. Standard errors of the Keeling plot intercept δ(13)C values of soil-respired CO₂ were typically<0.4 ‰ for analysis of soils with high CO₂ efflux (>7-9 μmol m(-2) s(-1)).

Can gas chromatography combustion isotope ratio mass spectrometry be used to quantify organic compound abundance?

Quantifying the concentrations of organics such as phospholipid fatty acids (PLFAs) and n-alkanes and measuring their corresponding (13)C/(12)C isotope ratios often involves two separate analyses; (1) quantification by gas chromatography flame ionisation detection (GC-FID) or gas chromatography/mass spectrometry (GC/MS), and (2) (13) C-isotope abundance analysis by gas chromatography/combustion/isotope ratio mass spectrometry (GC-C-IRMS). This requirement for two separate analyses has obvious disadvantages in terms of cost and time. However, there is a history of using the data output of isotope ratio mass spectrometers to quantify various components; including the N and C concentrations of solid materials and CO(2) concentrations in gaseous samples. Here we explore the possibility of quantifying n-alkanes extracted from sheeps' faeces and fatty acid methyl esters (FAMEs) derivatised from PLFAs extracted from grassland soil, using GC-C-IRMS. The results were compared with those from GC-FID analysis of the same extracts. For GC-C-IRMS the combined area of the masses for all the ions (m/z 44, 45 and 46) was collected, referred to as 'area all', while for the GC-FID analysis the peak area data were collected. Following normalisation to a common value for added internal standards, the GC-C-IRMS 'area all' values and the GC-FID peak area data were directly compared. Strong linear relationships were found for both n-alkanes and FAMEs. For the n-alkanes the relationships were 1:1 while, for the FAMEs, GC-C-IRMS overestimated the areas relative to the GC-FID results. However, with suitable reference material 1:1 relationships were established. The output of a GC-C-IRMS system can form the basis for the quantification of certain organics including FAMEs and n-alkanes.

Testing the use of septum-capped vials for 13C-isotope abundance analysis of carbon dioxide

Studying ecosystem processes in the context of carbon cycling and climate change has never been more important. Stable carbon isotope studies of gas exchange within terrestrial ecosystems are commonly undertaken to determine sources and rates of carbon cycling. To this end, septum-capped vials ('Exetainers') are often used to store samples of CO(2) prior to mass spectrometric analysis. To evaluate the performance of such vials for preserving the isotopic integrity (delta(13)C) and concentration of stored CO(2) we performed a rigorous suite of tests. Septum-capped vials were filled with standard gases of varying CO(2) concentrations (approximately 700 to 4000 ppm), delta(13)C values (approx. -26.5 to +1.8 per thousand(V-PDB)) and pressures (33 and 67% above ambient), and analysed after a storage period of between 7 and 28 days. The vials performed well, with the vast majority of both isotope and CO(2) concentration results falling within the analytical uncertainty of chamber standard gas values. Although the study supports the use of septum-capped vials for storing samples prior to mass spectrometric analysis, it does highlight the need to ensure that sampling chamber construction is robust (air-tight).