Effects of growth and tissue type on the kinetics of <Superscript>13</Superscript>C and <Superscript>15</Superscript>N incorporation in a rapidly growing ectotherm

The use of stable isotopes to investigate animal diets, habitat use, and trophic level requires understanding the rate at which animals incorporate the ¹³C and ¹⁵N from their diets and the factors that determine the magnitude of the difference in isotopic composition between the animal’s diet and that of its tissues. We determined the contribution of growth and catabolic turnover to the rate of ¹³C and ¹⁵N incorporation into several tissues that can be sampled non-invasively (skin, scute, whole blood, red blood cells, and plasma solutes) in two age classes of a rapidly growing ectotherm (loggerhead turtles, Caretta caretta). We found significant differences in C and N incorporation rates and isotopic discrimination factors (Δ¹³C = δ¹³C_tissues − δ¹³C_diet and Δ¹⁵N = δ¹⁵N_tissues − δ¹⁵N_diet) among tissues and between age classes. Growth explained from 26 to 100% of the total rate of incorporation in hatchling turtles and from 15 to 52% of the total rate of incorporation in juvenile turtles. Because growth contributed significantly to the rate of isotopic incorporation, variation in rates among tissues was lower than reported in previous studies. The contribution of growth can homogenize the rate of isotopic incorporation and limit the application of stable isotopes to identify dietary changes at contrasting time scales and to determine the timing of diet shifts. The isotopic discrimination factor of nitrogen ranged from −0.64 to 1.77‰ in the turtles’ tissues. These values are lower than the commonly assumed average 3.4‰ discrimination factors reported for whole body and muscle isotopic analyses. The increasing reliance on non-invasive and non-destructive sampling in animal isotopic ecology requires that we recognize and understand why different tissues differ in isotopic discrimination factors.     

Stable carbon and nitrogen incorporation in blood and fin tissue of the catfish <Emphasis Type="Italic">Pterygoplichthys disjunctivus</Emphasis> (Siluriformes,Loricariidae)

A feeding trial was performed in the laboratory with the catfish species Pterygoplichthys disjunctivus to determine stable carbon (¹³C) and nitrogen (¹⁵ N) turnover rates and discrimination factors in non-lethally sampled tissues (red blood cells, plasma solutes, and fin). A second feeding trial was conducted to determine what P. disjunctivus could assimilate from low-quality wood-detritus—refractory polysaccharides (e.g., cellulose), or soluble wood-degradation products inherent in wood-detritus. This was performed by feeding the fish an artificial wood-detritus diet with fibrous (δ¹³C = −26.36‰; δ¹⁵  N = 2.13‰) and soluble portions (δ¹³C = −11.82‰; δ¹⁵  N = 3.39‰) that had different isotopic signatures and monitoring the dynamics of isotopic incorporation in the different tissues over time. Plasma solutes turned over more quickly than red blood cells for ¹³C and ¹⁵ N. However, in contrast to previous studies of juvenile fishes, C and N incorporation was primarily driven by catabolic tissue turnover as opposed to growth rate. Tissue-diet discrimination factors for ¹⁵ N varied from 4.08 to 5.17‰, whereas they were <2‰ for ¹³C (and less than 0.3‰ for plasma and red blood cells). The results of trial two suggested that P. disjunctivus could not assimilate refractory polysaccharides. Moreover, the δ¹³C and δ¹⁵ N signatures of wild-caught P. disjunctivus from Florida confirmed their detrital trophic standing in Floridian aquatic ecosystems.     

Stable carbon and nitrogen isotopic discrimination in whole blood of red-throated ant tanagers Habia fuscicauda

Analysis of stable isotope ratios is increasingly used to reconstruct diets in passerine birds, but studies of diet–tissue isotopic discrimination for this avian group are scarce. We determined ¹⁵N and ¹³C diet–tissue discrimination factors on whole blood in the red-throated ant tanager (Habia fuscicauda), an insectivorous–frugivorous passerine. Birds were fed an isotopically uniform, semi-synthetic diet of dog puppy dry food, soy protein isolate, wheat germ, and other ingredients, during 92 days. Average (± SD) diet–tissue discrimination was 2.6 ± 0.2‰ for N and 2.2 ± 0.1‰ for C. Nitrogen diet-tissue discrimination was similar to the values found previously in other passerines fed animal protein and it can probably be used to accurately reconstruct protein dietary origin in passerines feeding on animal protein (e.g., insects). In the case of C, diet reconstruction might be affected by metabolic routing of dietary nutrients.     

Food web structure of a shallow eutrophic lake (Lake Taihu,China) assessed by stable isotope analysis

Lake Taihu is a large, shallow, and eutrophic lake in China. It has provided local communities with valuable fisheries for centuries, but little is known of the trophodynamics, or of its faunal communities. Carbon and nitrogen isotopic composition was used to assess its trophic pathways and the food web structure [food sources and trophic levels (TL)]. Basal food sources were distinguishable based on their δ¹³C values, ranging from −27.2 to −15.2‰. Consumers were also well separated in δ¹³C (−26.9 to −17.9‰ for invertebrates and −25.7 to −18.1‰ for fishes), which allowed for an effective discrimination of carbon sources between these fauna. An average trophic enrichment factor of 3.4‰ was used to calculate the TLs based on δ¹⁵N of zooplankton, with results indicating a food web having four TLs. Although δ¹⁵N values overlap and cover a large range within trophic compartments, the isotopic signatures of the species assessed revealed a general trend of ¹⁵N enrichment with increasing TL. Stable isotope signatures were also used to establish a general food web scheme in which five main trophic pathways were analyzed.     

Effect of C3–C4 Vegetation Change on δ13C and δ15N Values of Soil Organic Matter Fractions Separated by Thermal Stability

Carbon isotopic composition of soils subjected to C₃–C₄ vegetation change can be used to estimate C turnover in bulk soil and in soil organic matter (SOM) pools with fast and intermediate turnover rates. We hypothesized that the biological availability of SOM pools is inversely proportional to their thermal stability, so that thermogravimetry can be used to separate SOM pools with contrasting turnover rates. Soil samples from a field plot cultivated for 10.5 years with the perennial C₄ plant Miscanthus×gigantheus were analyzed by thermogravimetry coupled with differential scanning calorimetry (DSC). Three SOM fractions were distinguished according to the differential weight losses and exothermic or endothermic reactions measured by DSC. The δ¹³C and δ¹⁵N values of these three fractions obtained by gradual soil heating were measured by IRMS. The weight losses up to 190 °C mainly reflected water evaporation because no significant C and N losses were detected and δ¹³C and δ¹⁵N values of the residual SOM remained unchanged. The δ¹³C values (−16.4‰) of SOM fraction decomposed between 190 and 390 °C (containing 79% of total soil C) were slightly closer to that of the Miscanthus plant tissues (δ¹³C = −11.8‰) compared to the δ¹³C values (−16.8‰) of SOM fraction decomposed above 390 °C containing the residual 21% of SOM. Thus, the C turnover in the thermally labile fraction was faster than that in thermally stable fractions, but the differences were not very strong. Therefore, in this first study combining TG-DSC with isotopic analysis, we conclude that the thermal stability of SOM was not very strongly related to biological availability of SOM fractions. In contrast to δ¹³C, the δ¹⁵N values strongly differed between SOM fractions, suggesting that N turnover in the soil was different from C turnover. More detailed fractionation of SOM by thermal analysis with subsequent isotopic analysis may improve the resolution for δ¹³C.     

Diet: tissue stable isotope fractionation of carbon and nitrogen in blood plasma and whole blood of male reindeer Rangifer tarandus

Estimates of diet derived from stable isotope analyses are sensitive to the accuracy of corrections made for diet-tissue fractionation. In particular, diet-tissue fractionation in reindeer Rangifer tarandus may be expected to differ significantly from the generic values often used in stable isotope dietary calculations, given the known values obtained from other ungulates and the complex digestive system and nutrient recycling characteristic of the species. We fed domestic reindeer a homogenous artificial diet of known isotopic value in order to directly determine diet-tissue isotopic fractionation of carbon and nitrogen, the main elements used in stable isotope dietary analyses. Diet-tissue fractionation values for blood plasma were +3.5 ± 0.1‰ (δ¹³C) and +4.2 ± 0.3‰ (δ¹⁵N). Diet-tissue fractionation values for whole blood were +3.7 ± 0.2‰ (δ¹³C) and +2.5 ± 0.3‰ (δ¹⁵N). Metabolic turnover rates were clearly sufficient for complete tissue replacement over the period of artificial feeding for blood plasma, but may not have been so for whole blood, especially for δ¹⁵N. Our values, except for whole blood δ¹⁵N, differ considerably from the generic values often used in dietary studies and interspecific comparisons of trophic niche. The results underline the importance of obtaining as specific as possible fractionation values for the species, tissue, and in some cases sex and physiological status of animals under examination, and the potential problems associated with assuming ‘generic’ fractionation values when comparing species, especially where digestive processes are dissimilar.     

Tissue-specific response of δ15N in adult Pacific herring (Clupea pallasi) following an isotopic shift in diet

The objective of this study was to measure the tissue-specific response of isotope δ¹⁵N to changes in isotopic signature of diet in an adult Pacific herring, Clupea pallasi, and to examine the importance of growth and metabolism in this shift. This was accomplished by placing wild adult Pacific herring in captivity and monitoring isotopic shift in tissues with a corresponding isotopic shift in diet, and the application of a metabolism/growth mixing model. Tissues examined were blood, eye, heart, liver, and white muscle. One group of herring was given a δ¹⁵N diet depleted by approximately 5.4‰, and another given a ¹⁵N-enriched diet labeled with 98 atom% l-phenylalanine. This study showed that (i) isotopic response of individual tissues following an isotopic shift in diet varied in both rate of change and fractionation level, (ii) most of this isotopic shift is due to growth, and (iii) white muscle and liver tissue appeared the most responsive to isotopic shift in diet, reaching isotopic equilibrium with diet in a matter of months (not years). For trophic studies using δ¹⁵N, these results indicate that field measurement of Pacific herring should be done after much of summer growth has occurred.     

Grain 15N of crops applied with organic and chemical fertilizers in a four-year rotation

Variations in crop grain and soil N isotope composition (δ¹⁵N) in relation to liquid hog manure (δ¹⁵N of total N was +5.1‰), solid cattle manure (+7.9‰) and chemical fertilizer (+0.7‰ for urea and −1.9‰ for ammonium phosphate) applications, and control (no fertilizer application) were examined through a 4-year crop rotation under field conditions. Canola (Brassica napus), hull-less barley (Hordeum vulgare), wheat (Triticum aestivum), and canola were grown sequentially from 2000 (year 1) to 2003 (year 4). From year 2, hog manure or chemical fertilizers, but not cattle manure, treatments increased grain N concentrations over the control. Grain δ¹⁵N (+0.3 to +2.5‰) of crops applied with chemical fertilizers was lower than those in the other treatments, reflecting the effects of the N source with a lower δ¹⁵N, while the manure treatments tended to increase grain δ¹⁵N. The higher grain δ¹⁵N of crops applied with hog manure (+5.6 to +8.4‰) than those applied with cattle manure (+2.2 to +4.1‰) reflected the higher N availability of liquid hog manure (up to 70% as NH ₄ ⁺ ) than solid cattle manure (99% organic N) and higher potentials for ammonia volatilization loss in hog manure rather than differences in manure δ¹⁵N signatures. Soil total- and extractable-N concentrations and δ¹⁵N tended to vary with the application of N sources with different N isotope composition and availability. Our study expanded the application of the δ¹⁵N technique for detecting N source (organic vs chemical) effects on N isotopic composition to field conditions and across a 4-year rotation, and revealed that N availability played a greater role than the δ¹⁵N signature of N sources in determining crop δ¹⁵N under the studied conditions. Section Editor: H. Lambers     

Effects of nutrient enrichment on C and N stable isotope ratios of invertebrates,fish and their food resources in boreal streams

Carbon and nitrogen stable isotopes are frequently used to study energy sources and food web structure in ecosystems, and more recently, to study the effects of anthropogenic stress on aquatic ecosystems. We investigated the effect of nutrient enrichment on δ¹³C and δ¹⁵N in fine (FPOM), coarse (CPOM) particulate organic matter, periphyton, invertebrates and fish in nine boreal streams in south-central Sweden. In addition, we analysed the diet of benthic consumers using stable isotope data. Increases in δ¹⁵N of periphyton (R ² = 0.88), CPOM (0.78), invertebrates (0.92) and fish (0.89) were related to nutrient enrichment. In contrast, δ¹³C signatures did not change along the nutrient gradient. Our results show that δ¹⁵N has potential as a sensitive indicator of nutrient enrichment in boreal streams. Carbon and nitrogen isotopes failed to elucidate putative diets of selected aquatic consumers. Indeed, comparison of low- and high-impact sites showed that δ¹³C of many consumers were found outside the ranges of basal resource δ¹³C. Moreover, ranges of basal resource δ¹³C and δ¹⁵N overlapped at both low and high sites, making discrimination between the importance of allochthonous and autochthonous production difficult. Our findings show that a fractionation rate of 3.4‰ is not always be appropriate to assess trophic interactions, suggesting that more studies are needed on fractionation rates along gradients of impairment. Handling editor: M. Power     

Temporal and spatial variability in stable isotope compositions of a freshwater mussel: implications for biomonitoring and ecological studies

Stable isotopes can be used to elucidate ecological relationships in community and trophic studies. Findings are calibrated against baselines, e.g. from a producer or primary consumer, assumed to act as a reference to the isotopic context created by spatio-temporal attributes such as geography, climate, nutrient, and energy sources. The ability of an organism to accurately represent a community base depends on how, and over what time-scale, it assimilates ambient materials. Freshwater mussels have served as references for trophic studies of freshwater communities and as indicators of change in nutrient pollution load or source. Their suitability as reference animals has not yet been fully explored, however. We conducted a series of studies examining the suitability of freshwater mussels as isotopic baselines, using their ability to reflect variation in ambient nutrient loads as a case scenario. (1) We analyzed bivalve foot tissue δ¹⁵N and δ¹³C from 22 stream reaches in the Piedmont region of North Carolina, USA to show that compositions varied substantially among locations. Site mean bivalve δ¹³C values correlated with site ambient particulate organic matter (POM) δ¹³C values, and site mean bivalve δ¹⁵N values correlated with site ambient water dissolved δ¹⁵N-NO₃ values. (2) Similarity of results among sample types demonstrated that the minimally invasive hemolymph sample is a suitable substitute for foot tissue in δ¹⁵N analyses, and that small sample sizes generate means representative of a larger population. Both findings can help minimize the impact of sampling on imperiled freshwater mussel populations. (3) In a bivalve transplantation study we showed that hemolymph δ¹⁵N compositions responded to a shift in ambient dissolved δ¹⁵N-NO₃, although slowly. The tissue turnover time for bivalve hemolymph was 113 days. We conclude that bivalves serve best as biomonitors of chronic, rather than acute, fluctuations in stream nutrient loads, and provide initial evidence of their suitability as time-integrated isotopic baselines for community studies.     

Tissue Turnover Rates and Isotopic Trophic Discrimination Factors in the Endothermic Teleost,Pacific Bluefin Tuna (Thunnus orientalis)

Stable isotope analysis (SIA) of highly migratory marine pelagic animals can improve understanding of their migratory patterns and trophic ecology. However, accurate interpretation of isotopic analyses relies on knowledge of isotope turnover rates and tissue-diet isotope discrimination factors. Laboratory-derived turnover rates and discrimination factors have been difficult to obtain due to the challenges of maintaining these species in captivity. We conducted a study to determine tissue- (white muscle and liver) and isotope- (nitrogen and carbon) specific turnover rates and trophic discrimination factors (TDFs) using archived tissues from captive Pacific bluefin tuna (PBFT), Thunnus orientalis, 1–2914 days after a diet shift in captivity. Half-life values for ¹⁵N turnover in white muscle and liver were 167 and 86 days, and for ¹³C were 255 and 162 days, respectively. TDFs for white muscle and liver were 1.9 and 1.1‰ for δ ¹⁵N and 1.8 and 1.2‰ for δ ¹³C, respectively. Our results demonstrate that turnover of ¹⁵N and ¹³C in bluefin tuna tissues is well described by a single compartment first-order kinetics model. We report variability in turnover rates between tissue types and their isotope dynamics, and hypothesize that metabolic processes play a large role in turnover of nitrogen and carbon in PBFT white muscle and liver tissues. ¹⁵N in white muscle tissue showed the most predictable change with diet over time, suggesting that white muscle δ ¹⁵N data may provide the most reliable inferences for diet and migration studies using stable isotopes in wild fish. These results allow more accurate interpretation of field data and dramatically improve our ability to use stable isotope data from wild tunas to better understand their migration patterns and trophic ecology.     

Trophic ecology of the invasive argentine ant: spatio-temporal variation in resource assimilation and isotopic enrichment

Studies of food webs often employ stable isotopic approaches to infer trophic position and interaction strength without consideration of spatio-temporal variation in resource assimilation by constituent species. Using results from laboratory diet manipulations and monthly sampling of field populations, we illustrate how nitrogen isotopes may be used to quantify spatio-temporal variation in resource assimilation in ants. First, we determined nitrogen enrichment using a controlled laboratory experiment with the invasive Argentine ant (Linepithema humile). After 12 weeks, worker δ¹⁵N values from colonies fed an animal-based diet had δ¹⁵N values that were 5.51% greater compared to colonies fed a plant-based diet. The shift in δ¹⁵N values in response to the experimental diet occurred within 10 weeks. We next reared Argentine ant colonies with or without access to honeydew-producing aphids and found that after 8 weeks workers from colonies without access to aphids had δ¹⁵N values that were 6.31% larger compared to colonies with access to honeydew. Second, we sampled field populations over a 1-year period to quantify spatio-temporal variability in isotopic ratios of L. humile and those of a common native ant (Solenopsis xyloni). Samples from free-living colonies revealed that fluctuations in δ¹⁵N were 1.6–2.4‰ for L. humile and 1.8–2.9‰ for S. xyloni. Variation was also detected among L. humile castes: time averaged means of δ¹⁵N varied from 1.2 to 2.5‰ depending on the site, with δ¹⁵N values for queens ≥ workers > brood. The estimated trophic positions of L. humile and S. xyloni were similar within a site; however, trophic position for each species differed significantly at larger spatial scales. While stable isotopes are clearly useful for examining the trophic ecology of arthropod communities, our results suggest that caution is warranted when making ecological interpretations when stable isotope collections come from single time periods or life stages.     

Nitrogen and carbon isotope responses of Chinese cabbage and chrysanthemum to the application of liquid pig manure

The effects of the liquid pig manure (LM) used in organic farming on the natural abundance of ¹⁵N and ¹³C signatures in plant tissues have not been studied. We hypothesized that application of LM will (1) increase δ¹⁵N of plant tissues due to the high δ¹⁵N of N in LM as compared with soil N or inorganic fertilizer N, and (2) increase δ¹³C of plant tissues as a result of high salt concentration in LM that decreases stomatal conductance of plants. To test these hypotheses, variations in the δ¹⁵N and δ¹³C of Chinese cabbage (Brassica campestris L.) and chrysanthemum (Chrysanthemum morifolium Ramatuelle) with two different LMs (with δ¹⁵N of +15.6 and +18.2‰) applied at two rates (323 and 646 kg N ha^-1 for cabbage and 150 and 300 kg N ha^-1 for chrysanthemum), or urea (δ¹⁵N = -2.7‰) applied at the lower rate above for the respective species, in addition to the control (no N input) were investigated through a 60-day pot experiment. Application of LM significantly increased plant tissue δ¹⁵N (range +9.4 to +14.9‰) over the urea (+3.2 to +3.3‰) or control (+6.8 to 7.7‰) treatments regardless of plant species, strongly reflecting the δ¹⁵N of the N source. Plant tissue δ¹³C were not affected by the treatments for cabbage (range −30.8 to −30.2‰) or chrysanthemum (−27.3 to −26.8‰). However, cabbage dry matter production decreased while its δ¹³C increased with increasing rate of LM application or increasing soil salinity (P < 0.05), suggesting that salinity stress caused by high rate of LM application likely decreased stomatal conductance and limited growth of cabbage. Our study expanded the use of the δ¹⁵N technique in N source (organic vs. synthetic fertilizer) identification and suggested that plant tissue δ¹³C maybe a sensitive indicator of plant response to salinity stress caused by high LM application rates.     

Variations and controls of nitrogen stable isotopes in particulate organic matter of lakes

Nitrogen stable isotope (δ¹⁵N) data of particulate organic matter (POM) from the literature were analyzed to provide an understanding of the variations and controls of δ¹⁵N_POM in lakes at the global scale. The δ¹⁵N_POM variability characterized by seasonal mean, minimum, maximum, and amplitude (defined as δ¹⁵N_POM maximum − δ¹⁵N_POM minimum) from 36 lakes with seasonal data did not change systematically with latitude, but was significantly lower in small lakes than in large lakes. The seasonal mean δ¹⁵N_POM increased from oligotrophic lakes to eutrophic lakes despite large variations that are attributed to the occurrences of nitrogen fixation across the trophic gradient and the differences in δ¹⁵N of dissolved inorganic nitrogen (DIN) in individual lakes. Seasonal mean δ¹⁵N_POM was significantly correlated with DIN concentration and δ¹⁵N_DIN in two subsets of lakes. Seasonal minimum δ¹⁵N_POM in individual lakes is influenced by nitrogen fixation and δ¹⁵N_DIN while seasonal maximum δ¹⁵N_POM is influenced by lake trophic state and δ¹⁵N_DIN. As a result of the dominance of non-living POM in the unproductive surface waters, seasonal δ¹⁵N_POM amplitude was small (mean = 4.2‰) in oligotrophic lakes of all latitudes. On the other hand, seasonal δ¹⁵N_POM amplitude in eutrophic lakes was large (mean = 10.3‰), and increased from low to high latitudes, suggesting that the seasonal variability of δ¹⁵N in the phytoplankton-dominated POM pool was elevated by the greater spans of solar radiation and thermal regimes at high latitudes. The δ¹⁵N_POM from 42 lakes with no seasonal data revealed no consistent patterns along latitude, lake area, and trophic gradients, and a greater than 2‰ depletion compared to the lakes with seasonal data. Along with the large seasonal variability of δ¹⁵N_POM within lakes, these results provide insightful information on sampling design for the studies of food web baseline in lakes. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Disentangling effects of growth and nutritional status on seabird stable isotope ratios

A growing number of studies suggest that an individual’s physiology affects its carbon and nitrogen stable isotope signatures, obscuring a signal often assumed to be only a reflection of diet and foraging location. We examined effects of growth and moderate food restriction on red blood cell (RBC) and feather δ¹⁵N and δ¹³C in rhinoceros auklet chicks (Cerorhinca monocerata), a piscivorous seabird. Chicks were reared in captivity and fed either control (75 g/day; n = 7) or ~40% restricted (40 g/day; n = 6) amounts of high quality forage fish. We quantified effects of growth on isotopic fractionation by comparing δ¹⁵N and δ¹³C in control chicks to those of captive, non-growing subadult auklets (n = 11) fed the same diet. To estimate natural levels of isotopic variation, we also collected blood from a random sample of free-living rhinoceros auklet adults and chicks in the Gulf of Alaska (n = 15 for each), as well as adult feather samples (n = 13). In the captive experiment, moderate food restriction caused significant depletion in δ¹⁵N of both RBCs and feathers in treatment chicks compared to control chicks. Growth also induced depletion in RBC δ¹⁵N, with chicks exhibiting lower δ¹⁵N when they were growing the fastest. As growth slowed, δ¹⁵N increased, resulting in an overall pattern of enrichment over the course of the nestling period. Combined effects of growth and restriction depleted δ¹⁵N in chick RBCs by 0.92‰. We propose that increased nitrogen-use efficiency is responsible for ¹⁵N depletion in both growing and food-restricted chicks. δ¹⁵N values in RBCs of free-ranging auklets fell within a range of only 1.03‰, while feather δ¹⁵N varied widely. Together, our captive and field results suggest that both growth and moderate food restriction can affect stable isotope ratios in an ecologically meaningful way in RBCs although not feathers due to greater natural variability in this tissue.     

<Superscript>15</Superscript>N/<Superscript>14</Superscript>N ratios of amino acids as a tool for studying terrestrial food webs: a case study of terrestrial insects (bees,wasps, and hornets)

Compound-specific stable isotope analysis (CSIA) of amino acids is a new method that enables estimates of trophic position for consumers in food webs. We examined the nitrogen isotopic composition (δ¹⁵N) of amino acids of Japanese social insects (three bee, three wasp, and four hornet species) to evaluate the potential of CSIA of amino acids in studies of terrestrial food webs. For wasps, we also examined samples at different growth stages (ranging from egg to adult) to assess the effect of metamorphosis on CSIA estimates of trophic position. The δ¹⁵N values of bulk tissues for Japanese social insects are only weakly correlated with the biologically expected trophic positions. In contrast, the trophic positions estimated from the δ¹⁵N values of amino acids (yielding values of between 2.0 and 2.3 for bees, between 2.8 and 3.3 for wasps, and between 3.5 and 4.1 for hornets) are consistent with the biologically expected trophic positions for these insects (i.e., 2.0 for bees, 3.0 for wasps, and 3.0–4.0 for hornets). Although large variability is observed among the δ¹⁵N values of individual amino acids (e.g., ranging from 3.0 to 14.9‰ for phenylalanine), no significant change is observed in the trophic position during wasp metamorphosis. Thus, the CSIA of amino acids is a powerful tool for investigating not only aquatic food webs but also terrestrial food webs with predatory insects.     

Variations in 15N abundance in a forest fertilization trial: Critical loads of N,N saturation,contamination and effects of revitalization fertilization

¹⁵N abundances of current needles of Norway spruce collected during 23 yrs of a forest fertilization experiment were studied in order to follow ecosystem gains and losses of N. Unlabelled ammonium nitrate at four rates (N0–N3), phosphorus at three rates (P0–P2), and potassium plus other elements including micronutrients at two rates (K0–K1), had been applied to plots in a complete factorial design. Nitrogen had been applied annually at average rates of 0, 34, 68 and 102 kg N ha^-1 yr^-1. Tree growth had responded positively to additions of N, but the response was remarkably more positive to the N2P2K1 treatment. In N1 treatments, δ¹⁵N (‰) declined over time. This was consistent with an earlier study, and should reflect a change in ¹⁵N abundance towards that of fertilizer N (minus discrimination during uptake), which in turn means accretion of most of the N added. As in the earlier study, in which N3 plots lost most of the N added, the present N3 plots showed an increasing δ¹⁵N (‰). This pattern was not significantly affected by additions of P and K plus other elements, although a weak negative effect of P on N accretion was indicated, i.e. there was a tendency δ¹⁵N (‰) to be higher when P was added. This, and another recent result based on an N budget, shows that so-called revitalization fertilization may well increase growth of trees, but also promotes losses of N from the ecosystem. As in the previous study, a decline in δ¹⁵N (‰) on control plots provided evidence of contamination. Given a removal of 100 kg N ha^-1 at stem harvest and a leaching of 2 kg N ha^-1 yr^-1, our data on ¹⁵N suggest that a load of 9 kg N ha^-1 yr^-1 would saturate the ecosystem after 100 years. This load is only about twice the annual deposition at the site.     

‘Are fish what they eat’ all year round?

Isotope turnover in muscle of ectotherms depends primarily on growth rather than on metabolic replacement. Ectotherms, such as fish, have a discontinuous pattern of growth over the year, so the isotopic signature of muscle (δ¹³C and δ¹⁵N) may only reflect food consumed during periods of growth. In contrast, the liver is a regulatory tissue, with a continuous protein turnover. Therefore, the isotopic composition of liver should respond year round to changes in the isotopic signature of food sources. Therefore, we predicted that (1) Whitefish in Lake Geneva would have larger seasonal variation in the isotopic variation of the liver compared to that of the muscle tissue, and (2) the isotope composition of fish muscle would reflect a long-term image of the isotope composition of the food consumed only throughout the growth period. To test these expectations, we compared the isotope compositions of Whitefish muscle, liver and food in a 20-month study. We found that the seasonal amplitude of isotope variation was two to three times higher in liver compared to muscle tissue. During the autumn and winter, when growth was limited, only the isotopic signature of liver responded to changes in the isotope composition of the food sources. The δ¹³C and δ¹⁵N of muscle tissue only reflected the food consumed during the spring and summer growth period.     

Isotopic signature (<Superscript>15</Superscript>N/<Superscript>14</Superscript>N and <Superscript>13</Superscript>C/<Superscript>12</Superscript>C) confirms similarity of trophic niches of millipedes (Myriapoda,diplopoda) in a temperate deciduous forest

The species composition, abundance, and isotopic signature of millipedes (Myriapoda, Diplopoda) were investigated in seven biotopes of Kaluzhskie Zaseki State Nature Reserve. Nine Diplopoda species were found in total, and the local species diversity (within a sampling plot) reached seven species. The Diplopoda tissues were similar to the plant litter in the isotopic composition of nitrogen (δ¹⁵N was by 0.4‰ higher, on average), but were strongly enriched in heavy carbon (δ¹³C was by 4‰ higher, on average). Removal of mineral carbon from the cuticle reduced δ¹³C of Diplopoda by about 1.4‰ on average. Differences in the δ¹⁵N and δ¹³C values between the species did not exceed 2.5‰. Differences in the isotopic compositions of the considered species were small, and, it is impossible to distinguish particular trophic guilds in the Diplopoda community. Analysis of the published data confirmed that isotopic differentiation of millipedes was much less pronounced than in other investigated groups of soil animals. Hence, millipedes of the deciduous forest form a uniform trophic group.     

Isotope ratios and concentrations of sulfur and nitrogen in needles and soils of Picea abies stands as influenced by atmospheric deposition of sulfur and nitrogen compounds

Concentrations and natural isotope abundance of total sulfur and nitrogen as well as sulfate and nitrate concentrations were measured in needles of different age classes and in soil samples of different horizons from a healthy and a declining Norway spruce (Picea abies (L.) Karst.) forest in the Fichtelgebirge (NE Bavaria, Germany), in order to study the fate of atmospheric depositions of sulfur and nitrogen compounds. The mean δ¹⁵N of the needles ranged between −3.7 and −2.1 ‰ and for δ³⁴S a range between −0.4 and +0.9 ‰ was observed. δ³⁴S and sulfur concentrations in the needles of both stands increased continuously with needle age and thus, were closely correlated. The δ¹⁵N values of the needles showed an initial decrease followed by an increase with needle age. The healthy stand showed more negative δ¹⁵N values in old needles than the declining stand. Nitrogen concentrations decreased with needle age. For soil samples at both sites the mean δ¹⁵N and δ³⁴S values increased from −3 ‰ (δ¹⁵N) or +0.9 ‰ (δ³⁴S) in the uppermost organic layer to about +4 ‰ (δ¹⁵N) or +4.5 ‰ (δ³⁴S) in the mineral soil. This depth-dependent increase in abundance of ¹⁵N and ³⁴S was accompanied by a decrease in total nitrogen and sulfur concentrations in the soil. δ¹⁵N values and nitrogen concentrations were closely correlated (slope −0.0061 ‰ δ¹⁵N per μmol eq N g_dw ⁻¹), and δ³⁴S values were linearly correlated with sulfur concentrations (slope −0.0576 ‰ δ³⁴S per μmol eq S g_dw ⁻¹). It follows that in the same soil samples sulfur concentrations were linearly correlated with the nitrogen concentrations (slope 0.0527), and δ³⁴S values were linearly correlated with δ¹⁵N values (slope 0.459). A correlation of the sulfur and nitrogen isotope abundances on a Δ basis (which considers the different relative frequencies of ¹⁵N and ³⁴S), however, revealed an isotope fractionation that was higher by a factor of 5 for sulfur than for nitrogen (slope 5.292). These correlations indicate a long term synchronous mineralization of organic nitrogen and sulfur compounds in the soil accompanied by element-specific isotope fractionations. Based on different sulfur isotope abundance of the soil (δ³⁴S=0.9 ‰ for total sulfur of the organic layer was assumed to be equivalent to about −1.0 ‰ for soil sulfate) and of the atmospheric SO₂ deposition (δ³⁴S=2.0 ‰ at the healthy site and 2.3 ‰ at the declining site) the contribution of atmospheric SO₂ to total sulfur of the needles was estimated. This contribution increased from about 20 % in current-year needles to more than 50 % in 3-year-old needles. The proportion of sulfur from atmospheric deposition was equivalent to the age dependent sulfate accumulation in the needles. In contrast to the accumulation of atmospheric sulfur compounds nitrogen compounds from atmospheric deposition were metabolized and were used for growth. The implications of both responses to atmospheric deposition are discussed.