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Rationale The oxygen isotope anomaly, Δ17O, of N2O and nitrate is useful to elucidate nitrogen oxide dynamics. A comparison of different methods for Δ17O measurement was performed. Methods For Δ17O measurements, N2O was converted into O2 and N2 using microwave-induced plasma in a quartz or corundum tube reactor, respectively, or conversion was carried out in a gold wire oven. In each case, isotope ratios were measured by isotope ratio mass spectrometry. Results All the tested methods showed acceptable precision (coefficient of variation <2.4 % at 160 nmol N2O) with high sample size but the sample size dependence was lowest when using microwave-induced plasma in a corundum tube reactor. Conclusions The use of microwave-induced plasma in a corundum tube yields best results for Δ17O measurement on N2O gas samples. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

RATIONALE A high sensitivity elemental analyzer/isotope ratio mass spectrometer setup was developed to allow analysis of 13C and 15N isotopic composition on microgram amounts of C and N, respectively. METHODS Increased sensitivity of a conventional elemental analyzer equipped with a low blank autosampler was obtained by decreased carrier gas flow of 35 mL/min. The diameters of the oxidation and reduction reactors and water trap were reduced to 7.8, 7.8 and 4 mm i.d., respectively, to obtain sharp sample peaks in the mass spectrometer. To increase the lifetime of the reduction reactor, a 1:1 He/O2 mixture was used as oxidizing agent in the elemental analyzer. RESULTS Sample amounts of 0.6 µg N and 1 µg C were sufficient for accurate isotopic analysis with <1 ‰ standard error after blank correction. One major advantage of the setup is the easy switching between conventional EA and μEA as only consumable parts need to be exchanged. CONCLUSIONS The proposed setup proved to be suitable to analyze minute amounts of C and N in one analytical run simultaneously. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: The soil animal food web has become a focus of recent ecological research but trophic relationships still remain enigmatic for many taxa. Analysis of stable isotope ratios of N and C provides a powerful tool for disentangling food web structure. In this study, animals, roots, soil and litter material from a temperate deciduous forest were analysed. The combined measurement of δ 15N and δ 13C provided insights into the compartmentalization of the soil animal food web. Leaf litter feeders were separated from animals relying mainly on recent belowground carbon resources and from animals feeding on older carbon. The trophic pathway of leaf litter-feeding species appears to be a dead end, presumably because leaf litter feeders (mainly diplopods and oribatid mites) are unavailable to predators due to large size and/or strong sclerotization. Endogeic earthworms that rely on older carbon also appear to exist in predator-free space. The data suggest that the largest trophic compartment constitutes of ectomycorrhizal feeders and their predators. Additionally, there is a smaller trophic compartment consisting of predators likely feeding on enchytraeids and potentially nematodes. [Copyright &y& Elsevier]

The aim of the present study was to identify food sources of bark-living oribatid mites to investigate if trophic niche differentiation contributes to the diversity of bark living Oribatida. We measured the natural variation in stable isotope ratios (15N/14N, 13C/12C) in oribatid mites from the bark of oak (Quercus robur), beech (Fagus sylvatica), spruce (Picea abies) and pine (Pinus sylvestris) trees and their potential food sources, i.e., the covering vegetation of the bark (bryophytes, lichens, algae, fungi). As a baseline for calibration the stable isotope signatures of the bark of the four tree species were measured and set to zero. Oribatid mite stable isotope ratios spanned over a range of about 13 δ units for 15N and about 7 δ units for 13C suggesting that they span over about three trophic levels. Different stable isotope signatures indicate that bark living oribatid mites feed on different food sources, i.e., occupy distinct trophic niches. After calibration stable isotope signatures of respective oribatid mite species of the four tree species were similar indicating close association of oribatid mites with the corticolous cover as food source. Overall, the results support the hypothesis that trophic niche differentiation of bark living oribatid mites contributes to the high diversity of the group. [ABSTRACT FROM AUTHOR]

Abstract: Endogeic earthworms ingest large amounts of organic matter enclosed in mineral soil. Part of the soil organic matter is mobilised during the gut passage, but the overall effect of earthworms on the dynamics of soil organic carbon (SOC) is poorly understood because the origin and age of the mobilised SOC pool are unknown. To determine whether endogeic earthworms mobilise old SOC pools, we studied the effect of Octolasion tyrtaeum (Savigny) on 13C signatures of CO2 evolved from soil of a maize field incubated in microcosms with and without earthworms for 150days. Cultivation on this field had changed from wheat (C3 plant) to maize (C4 plant) 23years ago. Earthworms generally decreased in body mass during the experiment but increased CO2 production. Compared to the initial signature (−22.8±0.1‰PDB), δ13C signatures of O. tyrtaeum were more depleted in the wheat (−23.6±0.3‰PDB) and more enriched in the maize soil (−21.0±0.4‰PDB) at the end of the experiment. The δ13CO2-C signatures in the wheat soil were not affected by earthworms, but earthworms decreased δ13CO2-C values in the maize soil during the second half of the incubation period. This indicates enhanced mineralization of old wheat-derived carbon. The results suggest that lumbricid endogeic earthworms contribute to the mobilisation of old carbon pools in soils. [Copyright &y& Elsevier]

The large number of animals that coexist in soil without any clear niche differentiation has puzzled biologists for a long time. We investigated stable isotope ratios (15N/14N) in a diverse group of soil microarthropods, oribatid mites, to evaluate trophic niche differentiation. The natural variation of the stable isotopes 15N/14N was measured in 36 species/taxa from four beech and beech-oak forests. Signatures of δ15N formed a gradient spanning over 12 δ units suggesting that (a) different species occupy different trophic niches and (b) oribatid mites span three to four trophic levels. This study for the first time documented strong trophic niche differentiation in decomposer microarthropods. The results suggest that trophic niche differentiation within taxonomic groups significantly contributes to the high diversity of soil animal taxa. [Copyright &y& Elsevier]

Rationale: A high sensitivity elemental analyzer/isotope ratio mass spectrometer setup was developed to allow analysis of (13)C and (15)N isotopic composition on microgram amounts of C and N, respectively., Methods: Increased sensitivity of a conventional elemental analyzer equipped with a low blank autosampler was obtained by decreased carrier gas flow of 35 mL/min. The diameters of the oxidation and reduction reactors and water trap were reduced to 7.8, 7.8 and 4 mm i.d., respectively, to obtain sharp sample peaks in the mass spectrometer. To increase the lifetime of the reduction reactor, a 1:1 He/O2 mixture was used as oxidizing agent in the elemental analyzer., Results: Sample amounts of 0.6 µg N and 1 µg C were sufficient for accurate isotopic analysis with <1 ‰ standard error after blank correction. One major advantage of the setup is the easy switching between conventional EA and μEA as only consumable parts need to be exchanged., Conclusions: The proposed setup proved to be suitable to analyze minute amounts of C and N in one analytical run simultaneously., (Copyright © 2014 John Wiley & Sons, Ltd.)