Mobile, outdoor continuous-flow isotope-ratio mass spectrometer system for automated high-frequency 13C- and 18O-CO2 analysis for Keeling plot applications

Techniques for measuring carbon and oxygen isotope compositions of atmospheric CO2 via isotope ratio mass spectrometry

Measuring the stable isotope compositions of atmospheric CO₂ is common in earth and atmospheric sciences, and various analytical methods have been developed utilizing continuous-flow (CF) or dual-inlet (DI) isotope ratio mass spectrometry (IRMS). Air is typically collected via passive, manual, or automated collection methods and the volume of the air sample ranges from 10 to 300 mL for CF-IRMS to >1 L for DI-IRMS to yield a measurable amount of atmospheric CO₂ gas. It has been determined that the integrity of vials and flasks for air sample storage can be compromised after 3 days of air collection for δ¹³ C values and within 10 hours for δ¹⁸ O values. Air samples must be purified after collection to remove constituents of air, such as Ar, O₂ , N₂ , N₂ O, and water vapor, to avoid isobaric interferences during mass spectrometric measurement. Purification is generally undertaken by utilizing commercial or custom-made preconcentration devices, the blanking method for CF-IRMS, or an offline/online cryogenic separation using a vacuum line for DI-IRMS. Ambient N₂ O is a component of air that may affect analytical results and thus must either be corrected for or be removed using a gas chromatographic column. In some cases, water is removed during air collection by using a common chemical desiccant, magnesium perchlorate (Mg(ClO₄ )₂ ), or by a dry ice/alcohol mixture (-78°C). Lastly, a linearity issue for IRMS due to the low amount of purified CO₂ from a typical ambient air sample must be considered. In general, analytical precisions of 0.02-0.21‰ and 0.04-0.34‰ for CF-IRMS and 0.01-0.02‰ and 0.01-0.02‰ for DI-IRMS are expected for δ¹³ C and δ¹⁸ O measurements, respectively.

Development of a Monitoring System for Semicontinuous Measurements of Stable Carbon Isotope Ratios in Atmospheric Carbonaceous Aerosols: Optimized Methods and Application to Field Measurements

Carbon content constitutes a major fraction of atmospheric particulate matter (PM) and directly influences the earth's climate and human health. The stable carbon isotope ratios (δ¹³C) can be used to track potential sources and atmospheric processes of carbonaceous aerosols. Previously, determination of δ¹³C was always conducted in offline carbonaceous aerosol samples. The poor time-resolution results cannot provide information regarding the temporal evolution of δ¹³C at a short-time scale. In this study, we developed a new system for online measurements of δ¹³C in atmospheric carbonaceous aerosols by combining a semicontinuous organic carbon/elemental carbon (OC/EC) analyzer and online cavity ring-down spectroscopy (CRDS) (OC/EC analyzer-CRDS). To provide better stability in the determination of δ¹³C, a carrier gas with CO₂ (∼200 ppm) in "balance gas" was used, and Keeling analysis was employed to separate the δ¹³C signal of the sample from background CO₂ gas. Our results showed that the accuracy and absolute precision of the δ¹³C measurements by the OC/EC analyzer-CRDS system were better than 0.1‰ and 0.5‰, respectively, for the samples containing carbon content more than 5 μg. Furthermore, we employed this system to monitor δ¹³C (δ¹³C-TC) in particulate total carbon (TC) with a time resolution of 2-4 h over Beijing in late summer and early autumn, 2019. During the sampling period, the TC concentrations varied from 0.1 to 12.0 μg m^-3 with a mean value of 6.0 ± 2.4 μg m^-3. The δ¹³C-TC ranged from -28.2 to -24.2‰ (mean value was -25.9 ± 0.9‰) without significant diurnal variations, suggesting similar contributing sources to TC. Comparing the δ¹³C signatures of different emissions, we found that liquid fuels and primary and secondary C₃ plants were likely the dominant sources of particulate TC. Finally, we found that atmospheric heavy precipitation washed out the aged aerosols from the polluted air, resulting in significant depletion (∼2.4‰) of δ¹³C-TC in the atmosphere. This paper described a novel system for conducting online measurements of δ¹³C in atmospheric carbonaceous aerosols and provided us information to better understand the temporal evolution of emission sources and atmospheric processes of carbonaceous aerosols.

Nitrogen fertilization and δ18 O of CO2 have no effect on 18 O-enrichment of leaf water and cellulose in Cleistogenes squarrosa (C4 ) - is VPD the sole control?

The oxygen isotope composition of cellulose (δ¹⁸ O_Cel ) archives hydrological and physiological information. Here, we assess previously unexplored direct and interactive effects of the δ¹⁸ O of CO₂ (δ¹⁸ O_CO2 ), nitrogen (N) fertilizer supply and vapour pressure deficit (VPD) on δ¹⁸ O_Cel , ¹⁸ O-enrichment of leaf water (Δ¹⁸ O_LW ) and cellulose (Δ¹⁸ O_Cel ) relative to source water, and p_ex p_x , the proportion of oxygen in cellulose that exchanged with unenriched water at the site of cellulose synthesis, in a C₄ grass (Cleistogenes squarrosa). δ¹⁸ O_CO2 and N supply, and their interactions with VPD, had no effect on δ¹⁸ O_Cel , Δ¹⁸ O_LW , Δ¹⁸ O_Cel and p_ex p_x . Δ¹⁸ O_Cel and Δ¹⁸ O_LW increased with VPD, while p_ex p_x decreased. That VPD-effect on p_ex p_x was supported by sensitivity tests to variation of Δ¹⁸ O_LW and the equilibrium fractionation factor between carbonyl oxygen and water. N supply altered growth and morphological features, but not ¹⁸ O relations; conversely, VPD had no effect on growth or morphology, but controlled ¹⁸ O relations. The work implies that reconstructions of VPD from Δ¹⁸ O_Cel would overestimate amplitudes of VPD variation, at least in this species, if the VPD-effect on p_ex p_x is ignored. Progress in understanding the relationship between Δ¹⁸ O_LW and Δ¹⁸ O_Cel will require separate investigations of p_ex and p_x and of their responses to environmental conditions.

In situ measurement of CO2 and water vapour isotopic compositions at a forest site using mid-infrared laser absorption spectroscopy

We conducted continuous, high time-resolution measurements of CO2 and water vapour isotopologues ((16)O(12)C(16)O, (16)O(13)C(16)O and (18)O(12)C(16)O for CO2, and H2(18)O for water vapour) in a red pine forest at the foot of Mt. Fuji for 9 days from the end of July 2010 using in situ absorption laser spectroscopy. The δ(18)O values in water vapour were estimated using the δ(2)H-δ(18)O relationship. At a scale of several days, the temporal variations in δ(18)O-CO2 and δ(18)O-H2O are similar. The orders of the daily Keeling plots are almost identical. A possible reason for the similar behaviour of δ(18)O-CO2 and δ(18)O-H2O is considered to be that the air masses with different water vapour isotopic ratios moved into the forest, and changed the atmosphere of the forest. A significant correlation was observed between δ(18)O-CO2 and δ(13)C-CO2 values at nighttime (r(2)≈0.9) due to mixing between soil (and/or leaf) respiration and tropospheric CO2. The ratios of the discrimination coefficients (Δa/Δ) for oxygen (Δa) and carbon (Δ) isotopes during photosynthesis were estimated in the range of 0.7-1.2 from the daytime correlations between δ(18)O-CO2 and δ(13)C-CO2 values.

Carbon dynamics in aboveground biomass of co-dominant plant species in a temperate grassland ecosystem: same or different?

Understanding the role of individual organisms in whole-ecosystem carbon (C) fluxes is probably the biggest current challenge in C cycle research. Thus, it is unknown whether different plant community members share the same or different residence times in metabolic (τmetab ) and nonmetabolic (i.e. structural) (τnonmetab ) C pools of aboveground biomass and the fraction of fixed C allocated to aboveground nonmetabolic biomass (Anonmetab ). We assessed τmetab , τnonmetab and Anonmetab of co-dominant species from different functional groups (two bunchgrasses, a stoloniferous legume and a rosette dicot) in a temperate grassland community. Continuous, 14-16-d-long (13) C-labeling experiments were performed in September 2006, May 2007 and September 2007. A two-pool compartmental system, with a well-mixed metabolic and a nonmixed nonmetabolic pool, was the simplest biologically meaningful model that fitted the (13) C tracer kinetics in the whole-shoot biomass of all species. In all experimental periods, the species had similar τmetab (5-8 d), whereas τnonmetab ranged from 20 to 58 d (except for one outlier) and Anonmetab from 7 to 45%. Variations in τnonmetab and Anonmetab were not systematically associated with species or experimental periods, but exhibited relationships with leaf life span, particularly in the grasses. Similar pool kinetics of species suggested similar kinetics at the community level.

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.

Validation and application of cavity-enhanced, near-infrared tunable diode laser absorption spectrometry for measurements of methane carbon isotopes at ambient concentrations

Methane is an effective greenhouse gas but has a short residence time in the atmosphere, and therefore, reductions in emissions can alleviate its greenhouse gas warming effect within a decadal time frame. Continuous and high temporal resolution measurements of methane concentrations and carbon isotopic ratios (δ(13)CH4) can inform on mechanisms of formation, provide constraints on emissions sources, and guide future mitigation efforts. We describe the development, validation, and deployment of a cavity-enhanced, near-infrared tunable diode laser absorption spectrometry system capable of quantifying δ(13)CH4 at ambient methane concentrations. Laboratory validation and testing show that the instrument is capable of operating over a wide dynamic range of methane concentration and provides a measurement precision for δ(13)CH4 of better than ± 0.5 ‰ (1σ) over 1000 s of data averaging at ambient methane concentrations. The analyzer is accurate to better than ± 0.5 ‰, as demonstrated by measurements of characterized methane/air samples with minimal dependence (<1 ‰) of measured carbon isotope ratio on methane concentration. Deployment of the instrument at a marsh over multiple days demonstrated how methane fluxes varied by an order of magnitude over 2 day deployment periods, and showed a 17 ‰ variability in δ(13)CH4 of the emitted methane during the growing season.

Fully automated, high-precision instrumentation for the isotopic analysis of tropospheric N2O using continuous flow isotope ratio mass spectrometry

RATIONALE

Measurements of the isotopic composition of nitrous oxide in the troposphere have the potential to bring new information about the uncertain N2O budget, which mole fraction data alone have not been able to resolve. Characterizing the expected subtle variations in tropospheric N2O isotopic composition demands high-precision and high-frequency measurements. To enable useful observations of N2O isotopic composition in tropospheric air to reduce N2O source and sink uncertainty, it was necessary to develop a high-precision measurement system with fully automated capabilities for autonomous deployment at remote research stations.

METHODS

A fully automated pre-concentration system for high-precision measurements of N2O isotopic composition (δ(15)N(β) , δ(15)N(α), δ(18)O) in tropospheric air has been developed which combines a custom liquid-cryogen-free cryo-trapping system and gas chromatograph interfaced to a continuous flow isotope ratio mass spectrometry (IRMS) system. A quadrupole mass spectrometer was coupled in parallel to the IRMS system during development to evaluate peak interference. Multi-port inlet and fully-automated capabilities allow streamlined analyses between in situ air inlet, air standards, flask air sample, or other gas source in exactly replicated analysis sequences.

RESULTS

The system has the highest precision to date for (15)N site-specific composition results (δ(15) N(α) ±0.11‰, δ(15)N(β) ±0.14‰ (1σ)), attributed mostly to uniformity of analytical cycles and particular attention to fluorocarbon interference noted for (15)N site-specific measurements by IRMS. Air measurements demonstrated the fully automated capacity and performance.

CONCLUSIONS

The system makes substantial headway in measurement precision, possibly defining the limits of IRMS measurement capabilities in low concentration N2O air samples, with fully automated capabilities to enable high-frequency in situ measurements.

Differential regulation of two sucrose transporters by defoliation and light conditions in perennial ryegrass

Sucrose transport between source and sink tissues is supposed to be a key-step for an efficient regrowth of perennial rye-grass after defoliation and might be altered by light conditions. We assessed the effect of different light regimes (high vs low light applied before or after defoliation) on growth, fructans and sucrose mobilization, as well as on sucrose transporter expression during 14 days of regrowth. Our results reported that defoliation led to a mobilization of C reserves (first sucrose and then fructans), which was parallel to an induction of LpSUT1 sucrose transporter expression in source and sink tissues (i.e. leaf sheaths and elongating leaf bases, respectively) irrespective to light conditions. Light regime (high or low light) had little effects on regrowth and on C reserves mobilization during the first 48 h of regrowth after defoliation. Thereafter, low light conditions, delaying the recovery of photosynthetic capacities, had a negative effect on C reserves re-accumulation (especially sucrose). Surprisingly, high light did not enhance sucrose transporter expression. Indeed, while light conditions had no effect on LpSUT1 expression, LpSUT2 transcripts levels were enhanced for low light grown plants. These results indicate that two sucrose transporter currently identified in Lolium perenne L. are differentially regulated by light and sucrose.

Pulse-labelling trees to study carbon allocation dynamics: a review of methods, current knowledge and future prospects

Pulse-labelling of trees with stable or radioactive carbon (C) isotopes offers the unique opportunity to trace the fate of labelled CO(2) into the tree and its release to the soil and the atmosphere. Thus, pulse-labelling enables the quantification of C partitioning in forests and the assessment of the role of partitioning in tree growth, resource acquisition and C sequestration. However, this is associated with challenges as regards the choice of a tracer, the methods of tracing labelled C in tree and soil compartments and the quantitative analysis of C dynamics. Based on data from 47 studies, the rate of transfer differs between broadleaved and coniferous species and decreases as temperature and soil water content decrease. Labelled C is rapidly transferred belowground-within a few days or less-and this transfer is slowed down by drought. Half-lives of labelled C in phloem sap (transfer pool) and in mature leaves (source organs) are short, while those of sink organs (growing tissues, seasonal storage) are longer. (13)C measurements in respiratory efflux at high temporal resolution provide the best estimate of the mean residence times of C in respiratory substrate pools, and the best basis for compartmental modelling. Seasonal C dynamics and allocation patterns indicate that sink strength variations are important drivers for C fluxes. We propose a conceptual model for temperate and boreal trees, which considers the use of recently assimilated C versus stored C. We recommend best practices for designing and analysing pulse-labelling experiments, and identify several topics which we consider of prime importance for future research on C allocation in trees: (i) whole-tree C source-sink relations, (ii) C allocation to secondary metabolism, (iii) responses to environmental change, (iv) effects of seasonality versus phenology in and across biomes, and (v) carbon-nitrogen interactions. Substantial progress is expected from emerging technologies, but the largest challenge remains to carry out in situ whole-tree labelling experiments on mature trees to improve our understanding of the environmental and physiological controls on C allocation.

Adaptation of continuous-flow cavity ring-down spectroscopy for batch analysis of δ13C of CO2 and comparison with isotope ratio mass spectrometry

Measurements of δ(13)C in CO(2) have traditionally relied on samples stored in sealed vessels and subsequently analyzed using magnetic sector isotope ratio mass spectrometry (IRMS), an accurate but expensive and high-maintenance analytical method. Recent developments in optical spectroscopy have yielded instruments that can measure δ(13)CO(2) in continuous streams of air with precision and accuracy approaching those of IRMS, but at a fraction of the cost. However, continuous sampling is unsuited for certain applications, creating a need for conversion of these instruments for batch operation. Here, we present a flask (syringe) adaptor that allows the collection and storage of small aliquots (20-30 mL air) for injection into the cavity ring-down spectroscopy (CRDS) instrument. We demonstrate that the adaptor's precision is similar to that of traditional IRMS (standard deviation of 0.3‰ for 385 ppm CO(2) standard gas). In addition, the concentration precision (±0.3% of sample concentration) was higher for CRDS than for IRMS (±7% of sample concentration). Using the adaptor in conjunction with CRDS, we sampled soil chambers and found that soil-respired δ(13)C varied between two different locations in a piñon-juniper woodland. In a second experiment, we found no significant discrimination between the respiration of a small beetle (~5 mm) and its diet. Our work shows that the CRDS system is flexible enough to be used for the analysis of batch samples as well as for continuous sampling. This flexibility broadens the range of applications for which CRDS has the potential to replace magnetic sector IRMS.

Disentangling drought-induced variation in ecosystem and soil respiration using stable carbon isotopes

Combining C flux measurements with information on their isotopic composition can yield a process-based understanding of ecosystem C dynamics. We studied the variations in both respiratory fluxes and their stable C isotopic compositions (delta(13)C) for all major components (trees, understory, roots and soil microorganisms) in a Mediterranean oak savannah during a period with increasing drought. We found large drought-induced and diurnal dynamics in isotopic compositions of soil, root and foliage respiration (delta(13)C(res)). Soil respiration was the largest contributor to ecosystem respiration (R (eco)), exhibiting a depleted isotopic signature and no marked variations with increasing drought, similar to ecosystem respired delta(13)CO(2), providing evidence for a stable C-source and minor influence of recent photosynthate from plants. Short-term and diurnal variations in delta(13)C(res) of foliage and roots (up to 8 and 4 per thousand, respectively) were in agreement with: (1) recent hypotheses on post-photosynthetic fractionation processes, (2) substrate changes with decreasing assimilation rates in combination with increased respiratory demand, and (3) decreased phosphoenolpyruvate carboxylase activity in drying roots, while altered photosynthetic discrimination was not responsible for the observed changes in delta(13)C(res). We applied a flux-based and an isotopic flux-based mass balance, yielding good agreement at the soil scale, while the isotopic mass balance at the ecosystem scale was not conserved. This was mainly caused by uncertainties in Keeling plot intercepts at the ecosystem scale due to small CO(2) gradients and large differences in delta(13)C(res) of the different component fluxes. Overall, stable isotopes provided valuable new insights into the drought-related variations of ecosystem C dynamics, encouraging future studies but also highlighting the need of improved methodology to disentangle short-term dynamics of isotopic composition of R (eco).

Observing 13C labelling kinetics in CO2 respired by a temperate grassland ecosystem

* The kinetic characteristics of the main sources of ecosystem respiration are quite unknown, partly because of methodological constraints. Here, we present a new open-top chamber (OTC) apparatus for continuous 13C/12C labelling and measurement of ecosystem CO2 fluxes, and report the tracer kinetics of nighttime respiration of a temperate grassland. * The apparatus includes four dynamic flow-through OTCs, a unit mixing CO2-free air with 13C-depleted CO2, and a CO2 analyser and an online isotope ratio mass spectrometer. * The concentration (367 +/- 6.5 micromol mol(-1)) and carbon isotopic composition, delta13C, (-46.9 +/- 0.4 per thousand) of CO2 in the OTCs were stable during photosynthesis as a result of high air through flux and minimal incursion through the buffered vent. Soil CO2 efflux was not affected by pressure effects during respiration measurements. The labelling kinetics of respiratory CO2 measured in the field agreed with that of excised soil + vegetation blocks measured in a laboratory-based system. The kinetics fitted a two-source system (r(2) = 0.97), with a rapidly labelled source (half-life 2.6 d) supplying 48% of respiration, and the other source (52%) releasing no tracer during 14 d of labelling. * Of the two sources supplying ecosystem respiration, one was closely connected to current photosynthesis (approximately autotrophic respiration) and the other was provided by decomposition of structural plant biomass (approximately heterotrophic respiration).

Evaluating high time-resolved changes in carbon isotope ratio of respired CO2 by a rapid in-tube incubation technique

Recent insights into fractionation during dark respiration and rapid dynamics in isotope signatures of leaf- and ecosystem-respired CO(2) indicate the need for new methods for high time-resolved measurements of the isotopic signature of respired CO(2) (delta(13)C(res)). We present a rapid and simple method to analyse delta(13)C(res) using an in-tube incubation technique and an autosampler for small septum-capped vials. The effect of storage on the delta(18)O and delta(13)C ratios of ambient CO(2) concentrations was tested with different humidity and temperatures. delta(13)C ratios remained stable over 72 h, whereas delta(18)O ratios decreased after 24 h. Storage at 4 degrees C improved the storage time for delta(18)O. Leaves or leaf discs were incubated in the vials, flushed with CO(2)-free air and respired CO(2) was automatically sampled within 5 min on a microGas autosampler interfaced to a GV-Isoprime isotope ratio mass spectrometer. Results were validated by simultaneous on-line gas-exchange measurements of delta(13)C(res) of attached leaves. This method was used to evaluate the short-term (5-60 min) and diurnal dynamics of delta(13)C(res) in an evergreen oak (Quercus ilex) and a herb (Tolpis barbata). An immediate depletion of 2-4 per thousand from the initial delta(13)C(res) value occurred during the first 30 min of darkening. Q. ilex exhibited further a substantial diurnal enrichment in delta(13)C(res) of 8 per thousand, followed by a progressive depletion during the night. In contrast, T. barbata did not exhibit a distinct diurnal pattern. This is in accordance with recent theory on fractionation in metabolic pathways and may be related to the different utilisation of the respiratory substrate in the fast-growing herb and the evergreen oak. These data indicate substantial and rapid dynamics (within minutes to hours) in delta(13)C(res), which differed between species and probably the growth status of the plant. The in-tube incubation method enables both high time-resolved analysis and extensive sampling across different organs, species and functional types.

Importance of short-term dynamics in carbon isotope ratios of ecosystem respiration (delta13C(R)) in a Mediterranean oak woodland and linkage to environmental factors

Temporal dynamics in carbon isotope ratios of ecosystem respiration (delta13C(R)) were evaluated on hourly, daily and annual timescales in a Mediterranean woodland. Emphasis was given to the periods of transition from wet to dry season and vice versa, when the system turns from a net carbon sink to a source. The constancy of nocturnal delta13C(R) was tested. The relationship between delta13C(R) (determined through Keeling plots) and environmental factors was evaluated through time-lag analysis. Delta13C(R) exhibited high annual variation (> 7). During the transition periods, delta13C(R) correlated significantly with factors influencing photosynthetic discrimination, soil respiration, and whole-canopy conductance. Time-lags differed between below- and above-ground variables, and between seasons. A shift in regression parameters with environmental factors indicated seasonal differences in ecosystem responsiveness (e.g. temperature acclimation). Delta13C(R) exhibited substantial nocturnal enrichment (> 4) from dusk to dawn. These data indicate pronounced short-term dynamics in delta13C(R) at hourly to daily timescales and a modulated response to environmental drivers. Substantial short-term changes in nocturnal delta13C(R) may have important implications for the sampling protocols of nocturnal Keeling plots.