Nitrogen isotope analysis of aqueous ammonium and nitrate by membrane inlet isotope ratio mass spectrometry (MIRMS) at natural abundance levels.

RATIONALE: Existing methods for the measurement of the 15 N/14 N isotopic composition of ammonium and nitrate are either only suitable for labelled samples or require considerable sample preparation efforts (or both). Our goal was to modify an existing analytical approach to allow for natural abundance precision levels. METHODS: Published reaction protocols were used to convert ammonium into N2 by NaOBr and nitrate into N2 O by TiCl3 . A membrane inlet system was developed and coupled to an isotope ratio mass spectrometer to allow precise determination of the analytes. RESULTS: Concentrations of ≥35 µmol/L N for both ammonium or nitrate could be analysed for δ15 N values with precisions of better than 0.9 mUr. While ammonium analyses exhibited a small concentration dependency and an offset of 2.7 mUr at high ammonium concentrations irrespective of the standard isotopic composition, nitrate analysis showed no offset but a blank contribution visible at very low concentrations. CONCLUSIONS: The presented method is capable of fast measurement of δ15 N values in ammonium and nitrate from aqueous samples with reasonable accuracy at natural abundance levels. It will thus facilitate the application of isotopic methods to studies of nitrogen cycling in ecosystems.

A closer look into the nitrogen blank in elemental analyser/isotope ratio mass spectrometry measurements.

RATIONALE: One important limitation for the precise measurement of minute amounts of nitrogen (N) in solid samples by elemental analyser/isotope ratio mass spectrometry (EA/IRMS) is the accurate determination of the analyser blank value. This study was performed to identify different sources, amounts and isotopic composition of N blanks in EA/IRMS in order to identify measures for minimising the effect of the N blank on N isotopic data quality. METHODS: Different types of autosamplers, with and without zero-blank functionality, were tested by analysing different amounts of substances of varying isotopic composition by EA/IRMS. RESULTS: Using zero-blank autosamplers reduces the atmospheric N2 blank from 60 nmol to between 4 and 5 nmol depending on the autosampler type. This blank is derived from atmospheric N2 leaking into the elemental analyser, trapped in the sample tin capsules or contained in the oxygen added for combustion. Another source of blank is the reaction tube. As the sources of the blank differ, the isotopic composition of the blank is very variable. In addition, cross-contamination from previous samples may contribute up to 3.3 nmol N. CONCLUSIONS: For precise measurements of minute amounts of N in solid samples, reduction of the N blank is the most promising strategy. Correcting for the remaining N blank is only meaningful if the sample isotopic composition is very different from that of the N blank, because the precise determination of the isotopic composition of the N blank is not possible.

Comparison of methods to determine triple oxygen isotope composition of N2O.

RATIONALE: The oxygen isotope anomaly, Δ(17) O, of N2 O and nitrate is useful to elucidate nitrogen oxide dynamics. A comparison of different methods for Δ(17) O measurement was performed. METHODS: For Δ(17) O measurements, N2 O 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 N2 O) 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 Δ(17) O measurement on N2 O gas samples.

Combined ¹³C and ¹5N isotope analysis on small samples using a near-conventional elemental analyzer/isotope ratio mass spectrometer setup.

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.

The underestimated importance of belowground carbon input for forest soil animal food webs.

The present study investigated the relative importance of leaf and root carbon input for soil invertebrates. Experimental plots were established at the Swiss Canopy Crane (SCC) site where the forest canopy was enriched with (13)C depleted CO(2) at a target CO(2) concentration of c. 540 p.p.m. We exchanged litter between labelled and unlabelled areas resulting in four treatments: (i) leaf litter and roots labelled, (ii) only leaf litter labelled, (iii) only roots labelled and (iv) unlabelled controls. In plots with only (13)C-labelled roots most of the soil invertebrates studied were significantly depleted in (13)C, e.g. earthworms, chilopods, gastropods, diplurans, collembolans, mites and isopods, indicating that these taxa predominantly obtain their carbon from belowground input. In plots with only (13)C-labelled leaf litter only three taxa, including, e.g. juvenile Glomeris spp. (Diplopoda), were significantly depleted in (13)C suggesting that the majority of soil invertebrates obtain its carbon from roots. This is in stark contrast to the view that decomposer food webs are based on litter input from aboveground.

Combined effects of earthworms and vesicular-arbuscular mycorrhizas on plant and aphid performance.

• Vesicular-arbuscular mycorrhiza (VAM) and earthworms are known to affect plant and herbivore performance. However, surprisingly few studies have investigated their interactions. • In a glasshouse experiment we investigated the effects of earthworms (Aporrectodea caliginosa) and VAM (Glomus intraradices) on the growth and chemistry of Plantago lanceolata and the performance of aphids (Myzus persicae). • Earthworms did not affect VAM root colonization. Earthworms enhanced shoot biomass, and VAM reduced root biomass. VAM increased plant phosphorus content, but reduced the total amount of N in leaves. Earthworms led to a preferential uptake of soil N compared with 15 N from the added grass residues in the absence of VAM. Earthworm presence reduced the concentration of catalpol. Earthworms and VAM combined accelerated the development of M. persicae, while the development tended to be delayed when only VAM or earthworms were present. • We suggest that earthworms promote plant growth by enhancing soil N availability and may affect herbivores by influencing concentrations of secondary metabolites. VAM enhances the P uptake of plants, but presumably competes with plant roots for N.

Utilization of prey from the decomposer system by generalist predators of grassland.

We investigated the linkage between the detrital subsystem and generalist predators of meadow ecosystems by manipulating prey availability in two different ways: we increased resource availability for the decomposer subsystem and thereby decomposer prey by adding mulch materials (detritus enhancement), and we added fruitflies (Drosophila melanogaster, Diptera; prey enhancement) to fenced plots. Both supplemented materials significantly differed in their (13)C/(12)C and (15)N/(14)N ratios from those of the natural litter. We measured density responses of detritivorous, herbivorous and predaceous arthropods to the increased resource supply. We used ratios of natural stable isotopes of N and C in arthropod tissues to trace the flux from the added resources to consumers and to relate density responses of consumers to changes in resource supply. Effects of resource enhancement propagated through at least two trophic levels, resulting in higher densities of major decomposer and predator taxa. Effects of detritus enhancement were much stronger than those of prey enhancement. Signatures of delta(13)C proved density responses of Collembola taxa to be related to the added mulch materials. Among generalist predators, densities of juvenile wolf spiders (Lycosidae) responded more to detritus-enhancement than to prey-enhancement treatments. In contrast, the density of the web-building linyphiid and the non-web gnaphosid spiders remained unaffected. Each spider taxon, including those which did not respond numerically, was significantly enriched in (13)C in detritus-enhancement treatments, suggesting that they gain energy from the decomposer system. Numbers of herbivores-cicadellids and aphids-were similar in each of the treatments, indicating that they were unaffected by changes in predator density. Our results indicate that the lack of a numerical response to resource supplementation is not necessarily due to the absence of a trophic linkage, but may be caused by compensatory changes in mortality factors such as cannibalism and intraguild predation.

The trophic structure of bark-living oribatid mite communities analysed with stable isotopes ((15)N, (13)C) indicates strong niche differentiation.

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 ((15)N/(14)N, (13)C/(12)C) 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 delta units for (15)N and about 7 delta units for (13)C 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.

Nitrogen isotope ratios and fatty acid composition as indicators of animal diets in belowground systems.

This study analyses trophic interactions between soil fungi, micro- and mesofauna in microcosm experiments. The trophic shift of 15N and fatty acids (FAs) was investigated in different food chains, which comprised either two (fungi and grazers) or three (fungi, nematodes and Collembola) levels. Contrary to the widely accepted assumption of 15N enrichment in trophic cascades the experiments revealed enrichment, depletion or no change in 15N of consumers compared to their diet. Factors responsible for this pattern were suggested to be: (1) the main metabolic pathway used for N excretion in ammonotelic nematodes to be similar or depleted in the heavier isotope, and uricotelic Collembola mostly enriched in the heavier isotope; (2) a higher shift in 15N with a high-protein diet (e.g. for predators); (3) compensation due to low-quality food altering the fractionation of 15N. Analysis of the lipid composition showed phospholipids to be generally unaffected and neutral lipids closely related to the FA pattern of the food source. Dietary routing of FAs into neutral lipids occurred, as evidenced by corresponding frequencies of FAs in host and consumer profiles. Additionally, several FAs were only detected in the grazer when present in the food source. Oleic acid showed a shift over three trophic levels, from fungi to nematodes to Collembola. The assimilation of dietary FAs resulted in a more diverse neutral lipid profile, i.e. animals higher in the food chain contained more individual FAs compared to animals lower in the food chain. The results indicate that monoenoic C18 and monoenoic C20 FAs have the potential to act as tools for the bioindication of feeding strategies in belowground systems. We suggest that primary consumers will have no or only trace amounts of monoenoic C20 acids in their neutral lipid profile, whereas consumers feeding on a eukaryote diet will show a considerably higher frequency.

Effects of earthworms and organic litter distribution on plant performance and aphid reproduction.

Human management practices and large detritivores such as earthworms incorporate plant litter into the soil, thereby forming a heterogeneous soil environment from which plant roots extract nutrients. In a greenhouse experiment we investigated effects of earthworms and spatial distribution of (15)N-labelled grass litter on plants of different functional groups [ Lolium perenne (grass), Plantago lanceolata (forb), Trifolium repens (legume)]. Earthworms enhanced shoot and root growth in L. perenne and P. lanceolata and N uptake from organic litter and soil in all plant species. Litter concentrated in a patch (compared with litter mixed homogeneously into the soil) increased shoot biomass and (15)N uptake from the litter in L. perenne and enhanced root proliferation in P. lanceolata when earthworms were present. Growth of clover (T. repens) was rather independent of the presence of earthworms and organic litter distribution: nevertheless, clover took up more nitrogen in the presence of earthworms and exploited more (15)N from the added litter than the other plant species. The magnitude of the effects of earthworms and organic litter distribution differed between the plant species, indicating different responses of plants with contrasting root morphology. Aphid (Myzus persicae) reproduction was reduced on P. lanceolata in the presence of earthworms. We suggest that earthworm activity may indirectly alter plant chemistry and hence defence mechanisms against herbivores.