Prospects and Limitations of an Isotope Tracer Technique for Understanding Sulfur Cycling in Forested and Agro-Ecosystems

Abstract

Applications of isotopically distinct sulfur compounds have recently been used for tracing the fate of added sulfur in whole catchments or sub-compartments therein. Basic principles, the analytical methodology, and data evaluation for this isotope tracer technique are briefly described. We recommend that δ³⁴S-values of applied and natural sulfur compounds in the investigated ecosystem should differ by more than 20‰ in order to successfully ascertain sulfur fluxes. Where possible, a high ratio of applied sulfur loads versus sulfur pool sizes in the ecosystem should also be realized in order to allow the assessment of sulfur transformations in the study area. Prospects and limitations of this isotope tracer technique are critically discussed by reviewing results from recently or currently conducted lysimeter and field experiments.     

Stable isotope terminology

In order to achieve better mutual understanding, a uniform and unambiguous language concerning certain terms, symbols, units, and nomenclature in stable isotope - especially in (15)N tracer technique has been recommended. In particular, some definitions of fundamental terms, some nomenclature rules for stable isotopically labelled compounds and some symbols and units in the formula language of isotope stoichiometry are given. Recent fundamental recommendations of the IUPAC have been accepted as a basis, to which, if necessary, specialities of stable isotope chemistry, especially (15)N application are added or adapted.     

Advances in nitrogen-15 use for environmental studies in the soil-plant system

Abstract

A review is being presented of the latest results in the use of nitrogen-15 in the soil-plant system. The literature covers the period from 1991 to early April 1993. The increase in environmentally related experiments shows that the stable nitrogen isotope technique, applied either as tracer or as the natural isotope tracing approach, is well suited particularly to ecosystem studies. This trend is supported by convenient analytical systems. An important challenge to scientists in the field of environmental research is to contrive ways and means of permitting the plants to utilize the available nitrogen in the soil more efficiently. Examples of research areas needing more attention are pointed out.     

Stable isotope terminology

In order to achieve better mutual understanding, a uniform and unambiguous language concerning certain terms, symbols, units, and nomenclature in stable isotope – especially in ¹⁵N tracer technique has been recommended. In particular, some definitions of fundamental terms, some nomenclature rules for stable isotopically labelled compounds and some symbols and units in the formula language of isotope stoichiometry are given. Recent fundamental recommendations of the IUPAC have been accepted as a basis, to which, if necessary, specialities of stable isotope chemistry, especially ¹⁵N application are added or adapted.     

Use of dual isotope tracers in biomedical research

Biomedical stable isotope studies involve administration of tracer and measurement of isotope enrichment in blood, urine, feces or breath. The aim of the studies is to gather quantitative information about a specific metabolic function. However, the measured isotope enrichment may be affected by other metabolic events than only this function. In this case, a correction is necessary. The best approach is to add a second tracer simultaneously which is known to be metabolised by all interfering metabolic events but not by the function of interest. This dual isotope approach also enables simultaneous measurement of two interrelated functions. A summary of selected applications involving dual isotope tracer studies is presented. The applications deal with energy expenditure (doubly labelled water technique), cholesterol absorption, starch and lactose digestion, fat digestion, bile acid metabolism and the combination of stable and radioactive carbon isotopes in breath testing.     

Current Trends in Laser Separation of Isotopes in Monatomic Vapors

A comparative analysis of the techniques of laser isotope separation in monatomic vapors is presented. The restraints inherent in the conventional isotope separation technique AVLIS (atomic vapor laser isotope separation) are discussed as applied to the large-scale production of various isotopes. The requirements that should be met by the chemical reactions are formulated for the use of these reactions in isotope separation. The photochemical technique was shown to be quite competitive with the AVLIS.     

Sulfur isotopic composition of H2S and SO4(2-) from mineral springs in the Polish Carpathians

A number of springs in Carpathian Mts. contain dissolved H2S and SO4(2-) in concentrations above 10 mg/dm3. In this study we have investigated the sulfur isotope composition (delta34S) of the dissolved sulfur species in the springs from the flysch area in the Carpathian Mts. along the tectonic dislocation. It is believed that some of these springs may carry a major fraction of dissolved sulfur species of extremely deep sulfur (of mantle origin), which is subjected to SO4(2-)-H2S isotope exchange at high temperatures. The original isotopic compositions may be modified by reduction/oxidation at low temperatures and by admixture of sulfur from other sources. In order to distinguish the sulfur of mantle origin we investigated delta34S of dissolved sulfide and sulfate and on the basis of known concentrations we calculated delta34S of total dissolved sulfur. The isotope fractionation between sulfate and sulfide helped to distinguish the sulfur origin. Evaluating the sulfur isotope exchange, we selected 4 springs which likely have only weakly disturbed sulfur of mantle origin.     

Determination of 13C Enrichment by Conventional GC-MS and GC-(MS)-C-IRMS

Abstract

Isotopic enrichment of branched-chain L-amino acids (BCAA) and branched-chain 2-oxo acids (BCOA) in standard preparations and in human plasma samples withdrawn after oral loads with 1-¹³C labelled BCAA were measured on a conventional GC-MS system and an on-line GC-C-IRMS employing O-TMS quinoxalinol derivatives. It was concluded that the recently introduced GC-C-IRMS, owing to its high sensitivity, is the adequate analytical tool when tracer doses of stable isotope labelled compounds of low enrichment are to be used in biomedical in vivo studies.     

Sulfur Isotopic Composition of H2S and SO4 2− from Mineral Springs in the Polish Carpathians

Abstract

A number of springs in Carpathian Mts. contain dissolved H₂S and SO₄ ^2- in concentrations above 10 mg/dm³. In this study we have investigated the sulfur isotope composition (δ³⁴S) of the dissolved sulfur species in the springs from the flysch area in the Carpathian Mts. along the tectonic dislocation. It is believed that some of these springs may carry a major fraction of dissolved sulfur species of extremely deep sulfur (of mantle origin), which is subjected to SO₄ ^2-—H₂S isotope exchange at high temperatures. The original isotopic compositions may be modified by reduction/oxidation at low temperatures and by admixture of sulfur from other sources.

In order to distinguish the sulfur of mantle origin we investigated δ³⁴S of dissolved sulfide and sulfate and on the basis of known concentrations we calculated δ³⁴S of total dissolved sulfur. The isotope fractionation between sulfate and sulfide helped to distinguish the sulfur origin. Evaluating the sulfur isotope exchange, we selected 4 springs which likely have only weakly disturbed sulfur of mantle origin.     

Sputtering yield of chromium by argon and xenon ions with energies from 50 to 500 eV

A radioactive tracer technique is described for the quantitative measurement of the sputtering yield of a target material electroplated on a copper substrate. Sputtering yields of chromium by argon and xenon ions with energies from 50 to 500 eV are reported. The ion beams, having a current density ranging from 0.01 to 0.1 mA/cm² at an operating pressure of 2×10^–5 Torr, were produced by a low-energy ion gun. The sputtered atoms were collected on an aluminum foil surrounding the target. ⁵¹Cr was used as the tracer isotope. The results indicate that the radioactive tracer technique is sensitive enough in measuring the extremely small amount of sputtered material at low ion currents and low ion energies.     

Short-term sequestration of slurry-derived carbon into particle size fractions of a temperate grassland soil

Surface application of animal wastes in intensive grassland systems has caused growing environmental problems during the last decade and, therefore, increasing public and scientific concern. In the present study we examined if the natural abundance 13C stable isotope tracer techniques could be used to investigate a poorly defined aspect of waste application, i.e. incorporation of slurry-derived C and its distribution in soil organic matter (SOM) fractions with different turnover times of a pasture soil. C3 and C4 slurries (delta13C(V-PDB) = -30.7/1000 and -21.3/1000, respectively) from cows fed either on a maize (C4) or perennial ryegrass (C3) diet were applied to a C3 soil with a delta13C value of (-30.0+/-0.2)/1000. The cattle slurry was applied at 50 m3 ha(-1). Coarse sand, fine sand, silt, clay and fine clay were isolated from bulk soil samples (0-2 cm depth), freeze-dried and ground prior to total organic C (TOC) using elemental analysis and 13C natural abundance analysis by isotope-ratio mass spectrometry. The stable isotope tracer technique did allow to quantify the short-term sequestration of slurry-derived C in particle-size fractions of the grassland soil. Slurry-derived carbon was sequestered in various amounts in the five particle-size fractions, but most of it was sequestered in the coarse sand fraction during the two week experiment. The preferential input into the coarse sand fraction suggests that only the larger particulate slurry-derived materials were trapped into the soil during the experimental period. Less than 40% of the applied slurry-derived C was sequestered into the soil, suggesting a potential for large losses into the wider environment. The practice of surface spreading of slurry to temperate grassland soils is clearly not efficient, and improvements in slurry application methods, such as incorporation directly into the soil, should therefore be encouraged.     

Solid-supported reagents and catalysts for the preparation of large ring compounds

Parallel combinatorial synthesis in solution using immobilized reagents, catalysts, and scavengers has emerged as a powerful technique for the preparation of diverse libraries of compounds. This technique has only recently been applied to the synthesis of large-ring compounds. In this comprehensive review several strategies are presented and discussed, including Pd-catalyzed allylic alkylation, Stille-coupling, macrolactonization and macrolactamization using solid supported reagents and catalysts. In several cases site isolation has allowed operation of these macrocyclization reactions in concentrated solution (pseudo-dilution effect).     

Sulfur Isotope Fractionation in the Biogeochemical Sulfur Cycle of Marine Sediments

Abstract

The sulfur isotopic record of sedimentary sulfides (mainly pyrite) and sulfates shows considerable variations in time and plays an important role in the biological and geochemical interpretation, e.g., of the evolution of life and the oxygen partial pressure of Earth's atmosphere (e.g. [1]). From a comparison of experimental results with Desulfovibrio spp. it can be inferred that the S isotope fractionation during reduction of sulfur compounds is controlled by the number of electrons transferred (Fig. 1). Sulfur isotope discrimination in the sulfur cycle of marine sediments is dominated by dissimilatory bacterial sulfate reduction (BSR), and [2] used laboratory experiments with mesophilic bacteria to postulate that high sulfate reduction rates with abundant sulfate at enhanced temperature dominated the Ocean water chemistry during early Archean time. Experiments with pure cultures of thermophilic sulfate reducers [3] and natural hydrothermally influenced communities [4], however, demonstrated that isotope discrimination is close to average fractionation by mesophiles and that temperature is not directly influencing isotope discrimination during BSR.     

What are the instrumentation requirements for measuring the isotopic composition of net ecosystem exchange of CO2 using eddy covariance methods?

Better quantification of isotope ratios of atmosphere-ecosystem exchange of CO2 could substantially improve our ability to probe underlying physiological and ecological mechanisms controlling ecosystem carbon exchange, but the ability to make long-term continuous measurements of isotope ratios of exchange fluxes has been limited by measurement difficulties. In particular, direct eddy covariance methods have not yet been used for measuring the isotopic composition of ecosystem fluxes. In this article, we explore the feasibility of such measurements by (a) proposing a general criterion for judging whether a sensor's performance is sufficient for making such measurements (the criterion is met when the contribution of sensor error to the flux measurement error is comparable to or less than the contribution of meteorological noise inherently associated with turbulence flux measurements); (b) using data-based numerical simulations to quantify the level of sensor precision and stability required to meet this criterion for making direct eddy covariance measurements of the 13C/12C ratio of CO2 fluxes above a specific ecosystem (a mid-latitude temperate forest in central Massachusetts, USA); (c) testing whether the performance of a new sensor-a prototype pulsed quantum cascade laser (QCL) based isotope-ratio absorption spectrometer (and plausible improvements thereon)-is sufficient for meeting the criterion in this ecosystem. We found that the error contribution from a prototype sensor (approximately 0.2 per thousand, 1 SD of 10 s integrations) to total isoflux measurement error was comparable to (1.5 to 2x) the irreducible 'meteorological' noise inherently associated with turbulent flux measurements above this ecosystem (daytime measurement error SD of approximately 60% of flux versus meteorological noise of 30-40% for instantaneous half-hour fluxes). Our analysis also shows that plausible instrument improvements (increase of sensor precision to approximately 0.1 per thousand, 1 SD of 10 s integrations, and increased sensor stability during the half-hour needed to integrate eddy covariance measurements) should decrease the contribution of sensor error to the point where it is less than the contribution from meteorological noise. This suggests that new sensors using QCL-based isotope ratio absorption spectroscopy should make continuous long-term observations of the isotopic composition of CO2 fluxes via eddy covariance methods feasible.     

A diffusion-controlled mullite formation reaction model based on tracer diffusivity data for aluminium,silicon and oxygen

This paper compiles all the data from our tracer diffusivity studies in single crystalline 2/1-mullite. As tracers we used the rare stable isotopes ¹⁸O and ³⁰Si and the artificial pseudo-stable isotope ²⁶Al. Secondary-ion mass spectrometry was applied to analyze the depth distribution of the tracer isotopes after the diffusion annealing. An essential result of our tracer diffusivity studies was the very low diffusivity of ³⁰Si compared to the diffusivities of ²⁶Al and ¹⁸O, which are almost equal. Based on this observation, we propose a reaction model for the diffusion-controlled formation of mullite in the solid state, which assumes that the growth kinetics of a mullite layer is mainly controlled by the diffusion of aluminium ions and oxygen ions.     

Estimation of groundwater recharge via deuterium labelling in the semi-arid Cuvelai-Etosha Basin,Namibia

The stable water isotope deuterium (²H) was applied as an artificial tracer (²H₂O) in order to estimate groundwater recharge through the unsaturated zone and describe soil water movement in a semi-arid region of northern central Namibia. A particular focus of this study was to assess the spatiotemporal persistence of the tracer when applied in the field on a small scale under extreme climatic conditions and to propose a method to obtain estimates of recharge in data-scarce regions. At two natural sites that differ in vegetation cover, soil and geology, 500?ml of a 70?% ²H₂O solution was irrigated onto water saturated plots. The displacement of the ²H peak was analyzed 1 and 10 days after an artificial rain event of 20 mm as well as after the rainy season. Results show that it is possible to apply the peak displacement method for the estimation of groundwater recharge rates in semi-arid environments via deuterium labelling. Potential recharge for the rainy season 2013/2014 was calculated as 45 mm a^?1 at 5.6 m depth and 40 mm a^?1 at 0.9 m depth at the two studied sites, respectively. Under saturated conditions, the artificial rain events moved 2.1 and 0.5 m downwards, respectively. The tracer at the deep sand site (site 1) was found after the rainy season at 5.6 m depth, corresponding to a displacement of 3.2 m. This equals in an average travel velocity of 2.8 cm d^?1 during the rainy season at the first site. At the second location, the tracer peak was discovered at 0.9 m depth; displacement was found to be only 0.4 m equalling an average movement of 0.2 cm d^?1 through the unsaturated zone due to an underlying calcrete formation. Tracer recovery after one rainy season was found to be as low as 3.6?% at site 1 and 1.9?% at site 2. With an in situ measuring technique, a three-dimensional distribution of ²H after the rainy season could be measured and visualized. This study comprises the first application of the peak displacement method using a deuterium labelling technique for the estimation of groundwater recharge in semi-arid regions. Deuterium proved to be a suitable tracer for studies within the soil–vegetation–atmosphere interface. The results of this study are relevant for the design of labelling experiments in the unsaturated zone of dry areas using ²H₂O as a tracer and obtaining estimations of groundwater recharge on a local scale. The presented methodology is particularly beneficial in data-scarce environments, where recharge pathways and mechanisms are poorly understood.     

Simultaneous tracer diffusion and interdiffusion in a sandwich-type configuration to provide the composition dependence of the tracer diffusion coefficients

In this paper, a new formalism of a combined tracer and interdiffusion experiment for a binary interdiffusion couple is developed. The analysis requires an interdiffusion couple that initially contains a thin layer of tracers of one or both of the constituent elements at the original interface of the couple (sandwich interdiffusion experiment). This type of interdiffusion experiment was first performed in 1958 by J.R. Manning. The theoretical analysis presented in this paper is based on a newly developed phenomenological theory of isotopic interdiffusion combined with the Boltzmann–Matano formalism. This new analysis now provides the means to obtain the composition dependent interdiffusion coefficient and tracer diffusion coefficients simultaneously from analysis of the interdiffusion and tracer profiles in a single sandwich interdiffusion experiment. The new analysis is successfully applied to the results of Manning’s original ‘sandwich interdiffusion’ experiment in the Ag–Cd system (six couples in total) and is validated with an independent determination of the Ag and Cd tracer diffusion coefficients by Schoen using the conventional thin film technique. Suggestions for further development of the sandwich interdiffusion experiment and analysis to the case of multicomponent alloys are provided.     

Isotope-Ratio-Monitoring Liquid Chromatography Mass Spectrometry (IRM-LCMS): First Results from a Moving Wire Interface System

Abstract A Liquid Chromatography-Combustion (LC-C) Interface, based on a moving wire technique, has been built and tested. The LC effluent is deposited onto a transport wire, which carries the sample through solvent evaporation and combustion ovens. CO(2) from the combustion step is analysed in an isotope ratio mass spectrometer. Performance of the interface was tested by loop injections of sucrose and glucose into a liquid flow of methanol/water (80/20). Accuracy and precision of δ(13)C(PDB) < 1‰ was achieved for sample concentrations > 500 ng/ul (5μl loop), sufficient for studies at natural isotope ratios. In case of (13)C tracer applications the detection limit was determined to be about 20 pg carbon tracer (on wire).