Relationship between soil organic C degradability and the evolution of the delta13C signature in profiles under permanent grassland

The main objective of this research was to investigate to what extent the potential C dynamics of soil organic matter (SOM) are related to the degree of 13C enrichment with increasing depth in soil profiles under permanent grassland. The evolution of the C content and the 13C natural abundance (delta13C value) of SOM were investigated in three soil profiles (0-40 cm depth) under permanent grassland of varying texture (a loamy sand, a loam and a clay loam soil). The delta13C value of the SOM showed a gradual increase with increasing depth and decreasing C content in the profiles, ranging from 1.9 per thousand (loamy sand soil), 2.9 per thousand (clay loam soil) and 4 per thousand (loam soil) in relation to the delta13C value of SOM at the surface. The relationship between the 13C enrichment and total organic C content at different depths in the profiles (down to 40 cm depth in the loam and clay loam soil, down to 25 cm depth in the loamy sand soil) could be well described by the Rayleigh equation. The enrichment factors epsilon, associated with the Rayleigh approximation of the data, ranged from -1.57 per thousand (clay loam soil) to -1.64 per thousand (loamy sand soil) and -1.91 per thousand (loam soil). The potential C dynamics in four depth intervals from the profiles (0-10, 10-20, 20-30 and 30-40 cm depth) were determined by means of an incubation experiment. The C decomposition rate constants from the four sampling depths in the profiles showed a significant, positive correlation (y = 0.21x + 0.018, R(2) = 0.66, p < 0.005) with the corresponding Deltadelta13C values (change of the delta13C value per depth increment). A better correlation was obtained when only the data from the upper 20 cm in the profiles (y = 0.21x + 0.019, R(2) = 0.78, p < 0.05) were considered. These results suggest that the Deltadelta13C values in the surface layers of profiles under permanent grassland may serve as an indicator of the potential degradability or the stability of the SOM (in terms of C decomposition rate constants).     

Characterization of soil organic matter fractions from grassland and cultivated soils via C content and delta13C signature

Variations in (13)C natural abundance and distribution of total C among five size and density fractions of soil organic matter, water soluble organic C (WSOC) and microbial biomass C (MBC) were investigated in the upper layer (0-20 cm) of a continuous grassland soil (CG, C(3) vegetation), a C(3)-humus soil converted to continuous maize cultivation (CM, C(4) vegetation) and a C(3)-humus soil converted to a rotation of maize cultivation and grassland (R). The amounts of WSOC and MBC were both significantly larger in the CG than in the CM and the R. In the three soils, WSOC was depleted while MBC was enriched in (13)C as compared with whole soil C. The relative contributions to the total C content of C stored in the macro-organic matter and in the size fraction 50-150 microm decreased with decreasing total C contents in the order CG > R > CM, while the relative contribution of C associated with the clay- and silt-sized fraction <50 microm increased. This reflects a greater stability and physical protection against microbial degradation associated with soil disruption (tillage) of the clay- and silt-associated organic C, in relation to the organic C in larger size fractions. The size and density fractions from the CG soil showed significant differences in (13)C enrichment, indicating different degrees of microbial degradation and stability of soil organic C associated with physically different soil organic matter (SOM) fractions. Delta(13)C analysis of the size and density fractions from CM and R soils reflected a decreasing turnover rate of soil organic C with increasing density among the macro-organic matter fractions and with decreasing particle size.     

Short-term dynamics of slurry-derived plant and microbial sugars in a temperate grassland soil as assessed by compound-specific delta13C analyses

In view of recent discussions about climate change and the anthropogenically enhanced greenhouse effect, the aim of this study was to determine the short-term carbon (C) dynamics in a grassland soil after slurry application. It is known that, depending on cultivation practices, agro-ecosystems can act either as sources or as sinks for atmospheric CO2. C3 and C4 slurries were applied, differing in their stable C isotope signature, to be able to differentiate between native (soil-inherent) and fresh (slurry-applied) C. Samples were taken from 0-2, 2-7.5 and 7.5-15 cm soil depths from 90 days before until 4 weeks after slurry application at various intervals. We carried out compound-specific stable isotope analysis (CSIA) of plant- (arabinose and xylose) and microbial-derived sugars (fucose and rhamnose). Up to 45% of the applied slurry-derived xylose was found in the 0-2 cm soil depth within 24 h after slurry application, with this figure decreasing rapidly and then increasing again towards the end of the experiment. Therefore, during the first phase of slurry incorporation, preferentially the soluble part of slurry entered the first 2 cm of soil while, after about 2 weeks, particulate slurry-derived organic matter was incorporated into the soil. The ratio between plant- and microbial-derived sugars together with delta13C values of individual sugars in the 2-7.5 cm soil depth revealed that the dissipation of sugars from the 0-2 cm soil depth was not only due to leaching, but also was caused by microbial degradation of the fresh C because slurry did not contain significant amounts of rhamnose while the delta13C values of rhamnose became progressively enriched in 13C during the experiment. Stable isotope measurements of bulk soil previously only showed significant differences between C4 and C3 plots at 0-2 cm soil depth. The CSIA of the individual sugars was much more sensitive than bulk isotope measurements, revealing significant differences between C4 and C3 plots even at the 2-7.5 cm soil depth during the first phase of the experiment. Additionally, the dynamics of slurry-derived plant and microbial sugars could be followed specifically.     

Two new organic reference materials for delta13C and delta15N measurements and a new value for the delta13C of NBS 22 oil

Analytical grade L-glutamic acid is chemically stable and has a C/N mole ratio of 5, which is close to that of many of natural biological materials, such as blood and animal tissue. Two L-glutamic acid reference materials with substantially different 13C and 15N abundances have been prepared for use as organic reference materials for C and N isotopic measurements. USGS40 is analytical grade L-glutamic acid and has a delta13C value of -26.24 per thousand relative to VPDB and a delta15N value of -4.52 per thousand relative to N2 in air. USGS41 was prepared by dissolving analytical grade L-glutamic acid with L-glutamic acid enriched in 13C and 15N. USGS41 has a delta13C value of +37.76 per thousand and a delta15N value of +47.57 per thousand. The delta13C and delta15N values of both materials were measured against the international reference materials NBS 19 calcium carbonate (delta13C=+1.95 per thousand ), L-SVEC lithium carbonate (delta13C=-46.48 per thousand ), IAEA-N-1 ammonium sulfate (delta15N=0.43 per thousand ), and USGS32 potassium nitrate (delta15N=180 per thousand ) by on-line combustion continuous-flow and off-line dual-inlet isotope-ratio mass spectrometry. Both USGS40 and USGS41 are isotopically homogeneous; reproducibility of delta13C is better than 0.13 per thousand, and that of delta15N is better than 0.13 per thousand in 100-microg amounts. These two isotopic reference materials can be used for (i) calibrating local laboratory reference materials, and (ii) quantifying drift with time, mass-dependent fractionations, and isotope-ratio-scale contraction in the isotopic analysis of various biological materials. Isotopic results presented in this paper yield a delta13C value for NBS 22 oil of -29.91 per thousand, in contrast to the commonly accepted value of -29.78 per thousand for which off-line blank corrections probably have not been quantified satisfactorily.     

Mechanism and rate constants for the decomposition of 1-pentenyl radicals

This paper is concerned with the mechanisms and rate constants for the decomposition of 1-penten-3-yl, 1-penten-4-yl, and 1-penten-5-yl radicals. They are formed from radical attack on 1-pentene, which is an important decomposition product of normal alkyl radicals with more than 6 carbon atoms in combustion systems. This work is based on related data in the literature. These involve rate constants for the reverse radical addition process under high-pressure conditions, chemical activation experiments, and more recent direct studies. The high-pressure rate constants are based on detailed balance. The energy transfer effects and the pressure dependences of the rate constants are determined through the solution of the master equation and are projected to cover combustion conditions. The low barriers to these reactions make it necessary to treat these thermal reactions as open systems, as in chemical activation studies. The multiple reaction channels make the nature of the pressure effects different from those usually described in standard texts. The order of stability is 1-penten-3-yl approximately 1-penten-4-yl > 1-penten-5-yl and straddles those for the n-alkyl radicals. A key feature in these reactions is the effects traceable to allylic resonance. However, the 50 kJ/mol allylic resonance energy is not fully manifested. The important unsaturated products are 1,3-butadiene, the pentadienes, allyl radicals, and vinyl radicals. The results are compared with the recommendations in the literature, and significant differences are noted. Extensions to larger radicals with similar structures are discussed.     

Methods to adjust for the interference of N2O on delta13C and delta18O measurements of CO2 from soil mineralization

In this paper we present an overview of the present knowledge relating to methods that avoid interference of N2O on delta13C and delta18O measurements of CO2. The main focus of research to date has been on atmospheric samples. However, N2O is predominantly generated by soil processes. Isotope analyses related to soil trace gas emissions are often performed with continuous flow isotope ratio mass spectrometers, which do not necessarily have the high precision needed for atmospheric research. However, it was shown by using laboratory and field samples that a correction to obtain reliable delta13C and delta18O values is also required for a commercial continuous flow isotope ratio mass spectrometer. The capillary gas chromatography column of the original equipment was changed to a packed Porapak Q column. This adaptation resulted in an improved accuracy and precision of delta13C (standard deviation(Ghent): from 0.2 to 0.08 per thousand; standard deviation(Lincoln): from 0.2 to 0.13 per thousand) of CO2 for N2O/CO2 ratios up to 0.1. For delta18O there was an improvement for the standard deviation measured at Ghent University (0.13 to 0.08 per thousand) but not for the measurements at Lincoln University (0.08 to 0.23 per thousand). The benefits of using the packed Porapak Q column compared with the theoretical correction method meant that samples were not limited to small N(2)O concentrations, they did not require an extra N2O concentration measurement, and measurements were independent of the variable isotopic composition of N2O from soil.     

13C-13C spin-spin coupling constants in the spectra of monosubstituted pyridines

The ¹³C- {¹H} NMR spectra of pyridine and a number of monosubstituted pyridines for compounds with the natural percentage of the ¹³C isotope were analyzed. The direct, geminal, and vicinal ¹³C-¹³C spin-spin coupling constants (SSCC) were determined. Linear relationships that link the ¹³C-¹³C SSCC in the spectra of monosubstituted pyridines and benzenes were obtained.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1383–1385, October, 1985.     

Determination of the gas-phase decomposition rate constants of heptyl-1 and heptyl-2 hydroperoxides C7H15OOH

The gas-phase decomposition of n-heptyl-1 and n-heptyl-2 hydroperoxides C₇H₁₅OOH, which split into two radicals C₇H₁₅O and OH, has been investigated in the temperature range of 250–360°C. The decomposition has been carried out in a hydrogen–oxygen mixture (the hydroperoxide represents about 50 ppm) so as to avoid secondary reactions between the formed radicals and the reactants. Although the H₂–O₂ mixture is not spontaneously reactive in our conditions, it operates the transformation, through a fast and well-known process, of the OH radicals into HO₂ radicals and then into H₂O₂. However, C₇H₁₅O radicals are also transformed into HO₂ radicals and then into H₂O₂, but through an unknown process. To avoid heterogeneous reactions, vessel and probe are coated by B₂O₃ and then treated by the slow combustion of hydrogen at 510°C and 250 torr before the experiments are performed. As the reaction scheme is very simple, due to the use of the H₂–O₂ mixture, the determination of the evolutions of the HO₂ concentration (followed by electronic paramagnetic resonance) lead to the determination of the gas-phase decomposition rate constant of hydroperoxides. For the n-heptyl-1 hydroperoxide the rate constant is and for the n-heptyl-2 hydroperoxide it is .     

Direct 13C-13C spin-spin coupling constants in the vinyl group of N-vinylazoles

The unshared electron pair of the dicoordinated nitrogen atom introduces a stereospecific contribution to the ¹³C-¹³C spin-spin coupling constant (SSCC) in the vinyl group of a series of N-vinylazoles, the value of which depends on the energetics of the unshared pair.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 197–200, February, 1989.     

Automated quantitative and isotopic (13C) analysis of dissolved inorganic carbon and dissolved organic carbon in continuous-flow using a total organic carbon analyser

A method for the automated (13)C analysis of dissolved inorganic and organic carbon species has been developed to operate on a continuous-flow isotope ratio mass spectrometer (CF-IRMS). For natural and anthropogenic carbon species, the (13)C stable isotope has proven to be an excellent environmental tracer. Analytical performance tests were carried out on various organic compounds from easily oxidisable (sugar) to difficult (humic acid). A set of natural samples was also analysed to confirm the flexibility of the system. Analytical precision (2sigma) is typically <0.20 per thousand with sample reproducibility from 0.10-0.35 per thousand depending on reactivity of material. We believe this to be the first successful use of a total organic carbon (TOC) analyser for both dissolved inorganic and, specifically, dissolved organic species for (13)C stable isotope analysis in an automated CF-IRMS system. Routine analysis is achieved fairly quickly, is relatively simple with little or no sample manipulation, and will allow new and exciting studies for stable isotope research in both natural abundance and organic tracer studies not easily achieved before.     

Compound-specific delta13C analysis of individual amino sugars--a tool to quantify timing and amount of soil microbial residue stabilization

There is strong scientific evidence that microbial residues such as amino sugars may be stabilized in soil. However, up to now, no investigation has been carried out to quantify both the amount and timing of such stabilization. This is primarily due to methodological constraints, because it is not possible to differentiate between stabilized (old) and recently produced (new) amino sugars when these biomarkers are conventionally analyzed, e.g. by means of gas chromatography and flame ionization detection. Therefore, the aim of the present study was to test whether compound-specific isotope analysis (delta13C) of amino sugars extracted from soil could be used to differentiate between old and new microbial residues. For this aim a method for the delta13C analysis of individual amino sugars was developed and optimized. First results of delta13C values of glucosamine, galactosamine, mannosamine, and muramic acid in soil samples from two different ecological studies are presented, clearly indicating that discrimination between soil inherent and newly formed amino sugars is possible in stable isotope labeling experiments. Our results further showed that, in the short term (within 1 month), only few amino sugars were built, thus making highly 13C-enriched substrates necessary for the quantification of new amino sugar production and for the determination of amino sugar turnover rates.     

Large old trees influence patterns of delta13C and delta15N in forests

Large old trees are the dominant primary producers of native pine forest, but their influence on spatial patterns of soil properties and potential feedback to tree regeneration in their neighbourhood is poorly understood. We measured stable isotopes of carbon (delta(13)C) and nitrogen (delta(15)N) in soil and litter taken from three zones of influence (inner, middle and outer zone) around the trunk of freestanding old Scots pine (Pinus sylvestris L.) trees, to determine the trees' influence on below-ground properties. We also measured delta(15)N and delta(13)C in wood cores extracted from the old trees and from regenerating trees growing within their three zones of influence. We found a significant and positive gradient in soil delta(15)N from the inner zone, nearest to the tree centre, to the outer zone beyond the tree crown. This was probably caused by the higher input of (15)N-depleted litter below the tree crown. In contrast, the soil delta(13)C did not change along the gradient of tree influence. Distance-related trends, although weak, were visible in the wood delta(15)N and delta(13)C of regenerating trees. Moreover, the wood delta(15)N of small trees showed a weak negative relationship with soil N content in the relevant zone of influence. Our results indicate that large old trees control below-ground conditions in their immediate surroundings, and that stable isotopes might act as markers for the spatial and temporal extent of these below-ground effects.     

Measurement at the field scale of soil δ13C and δ15N under improved grassland

Variations in natural abundance of carbon (C) and nitrogen (N) stable isotopes are widely used as tools for many aspects of scientific research. By examining variations in the ratios of heavy to light stable isotopes, information can be obtained as to what physical, chemical and biological processes may be occurring. The spatial heterogeneity of soil δ¹⁵N‐ and δ¹³C‐values across a range of scales and under different land use have been described by a number of researchers and the natural abundances of the C and N stable isotopes in soils have been found to be correlated with many factors including hydrology, topography, land use, vegetation cover and climate. In this study the Latin square sampling +1 (LSS+1) sampling method was compared with a simple grid sampling approach for δ¹³C and δ¹⁵N measurement at the field scale. A set of 144 samples was collected and analysed for δ¹⁵N and δ¹³C from a 12 × 12 grid (in a 1 ha improved grassland field in south‐west England). The dimension of each cell of the grid was approximately 11 × 6 m. The 12 × 12 grid was divided into four 6 × 6 grids and the LSS+1 sampling technique was applied to these and the main 12 × 12 grid for a comparison of sample means and variation. The LSS+1 means from the 12 × 12 grid and the four 6 × 6 grids compared well with the overall grid mean because of the low variation within the field. The LSS+1 strategy (13 samples) generated representative samples from the 12 × 12 grid, and hence would be an acceptable method for sampling similar plots for the measurement of mean isotopic composition. Copyright © 2010 John Wiley & Sons, Ltd.     

Minimization of carbon addition during derivatization of monosaccharides for compound-specific delta13C analysis in environmental research

Little is known about the delta13C composition of monosaccharides representing the largest carbon reservoir in the biosphere. The main reason for this might be that monosaccharides have to be derivatized prior to gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) analyses and that a large isotopic correction is necessary for the carbon that has to be added to the original molecule during derivatization, resulting in large uncertainty of the calculated delta13C values of individual monosaccharides. The amount of added derivatization carbon is twice (alditol acetates) or even three times (trimethylsilyl (TMS) derivatives) as high as the amount of the original monosaccharide carbon. In addition, isotope fractionation occurs during acetylation. Therefore, the objectives of this study were (i) to minimize carbon addition during derivatization for GC/C/IRMS measurements of monosaccharides in soil and sediment samples and (ii) to quantify improvements in accuracy and precision of the final results. Minimization of carbon addition was accomplished by derivatization with methylboronic acid (MBA) and TMS thereafter (MBA method). Monosaccharides derivatized with the MBA method instead of TMS reduced the number of added carbon atoms from 2.2-2.7 to 0.3-0.8 per sugar carbon atom. Although the precision of GC/C/IRMS measurements with both methods is comparable (about 0.3 per thousand), delta13C values of an internal standard indicated that the newly developed MBA method is about 2 per thousand more accurate than the TMS method. delta13C comparison between soil samples that differed only slightly in their bulk carbon isotope signature showed that the MBA method is better in proving these small differences on a significant level. Total precision of the whole MBA method including all analytical and calculation steps is better by a factor of almost three than the TMS method.     

13C, 13C coupling constants and 13C chemical shifts of aromatic carbonyl compounds. Effects of ortho- and peri-interactions involving the carbonyl substituent

Carbon-13 n.m.r. data have been determined for a series of 26 aromatic carbonyl compounds including benzoyl, naphthoyl and pyrenoyl derivatives ¹³C labelled in the carbonyl group. Doubly labelled anthraquinone has also been included. The compounds investigated comprise non-hindered molecules and molecules in which the carbonyl substituent is subject to ortho- or peri-interactions affecting conjugation of the carbonyl group with the aromatic ring. The dependence of long range ¹³C,¹³C coupling constants involving the carbonyl carbon, in particular ²J and ³J, on steric conditions is discussed, as is the possibility of deciding on the orientation of the carbonyl bond. The following results have emerged. ²J(s-t)>²J(s-c) for ketones and aldehydes, and the reverse is valid for acids and acid derivatives. (s-t and s-c refer to the orientation of the C?O group relative to the aromatic bond in question with respect to the connecting single bond). For ketones ³J(t,s-c)<³J(t,s-t), and both of these ³J(t) values decrease with increasing angle of twist, θ, about the single bond, whereas ³J(c,s-c) increases with θ. For acids and acid derivatives no similar regularity was found. (The initial t and c refer to the geometry of the three-bond coupling path). Generally it is found that ³J(t)>³J(c) and ³J(t)>²J, confirming earlier results. Theoretical calculations on a few model compounds are qualitatively in accordance with the experimental results. Some sign determinations for coupling constants are presented. A short discussion is given of substituent effects on chemical shifts. Observed trends are consistent with earlier results.