Variation In δ15N Values Research Articles

Isotopic insights on cave bear palaeodiet

More than 300 cave bear bones from all over Europe have carbon and nitrogen isotopic composition that match overwhelmingly a diet based on plants, except for samples from two caves in Romania, for which high nitrogen-15 amounts have been interpreted as reflecting an omnivorous diet. This paper aims at deciphering the various factors influencing the carbon and nitrogen isotopic composition of a potential omnivorous species like cave bear, those linked to trophic levels and variations among plants and those caused by physiological factors. The comparison of European cave bears with coeval Late Pleistocene large mammals with different diets clearly shows that all the cave bear populations, including those from Romania, present isotopic values overlapping with herbivores, not with carnivores. Therefore omnivory is very unlikely for cave bears. Consumption of plants with high δ15N values, such as graminoids, forbs and possibly fungi, could explain in part the observed isotopic pattern. In addition, the variations in δ15N values through ontogeny support the hypothesis of a different hibernation pattern for the Romanian cave bears with high δ15N values. Future investigations using new isotopic approaches, especially nitrogen isotopic composition of collagen amino acids, should contribute to decipher the paleoecology of these Romanian cave bears.

Stable carbon isotopic composition indicates large presence of maize in Brazilian soy sauces (shoyu)

Carbon and nitrogen (N) isotopic compositions were used to test the presence of C4 carbon in soy sauces (shoyu) produced in Brazil. Seventy samples of Brazilian shoyu were analyzed and compared with the same product from Japan, China, Singapore, Taiwan, and United States. The average (±standard-deviation) δ13C of the Brazilian shoyu was −15.0 ± 3.4. The average (±standard-deviation) δ13C of five shoyu samples from Japan was −25.2 ± 0.6‰. The δ13C of shoyu from US (−25.3‰) and Singapore (−25.9‰) were close to Japanese samples. Shoyu δ13C values from China were equal to −20‰ and −15.4‰, and from Taiwan equal to −20.2‰. The average δ15N value of Brazilian shoyu was 0.3 ± 1.2‰. The variability of δ15N values among shoyu from other countries was much lower than Brazilian shoyu, and the average was 1.4 ± 0.3‰. Using a simple isotopic mass balance, we estimated that most of the shoyu in Brazil contains <20% soy, and the dominant cereal is maize, probably because in Brazil maize is cheaper than soybean.

Correction to 'Starvation effects on nitrogen and carbon stable isotopes of animals: an insight from meta-analysis of fasting experiments'.

[This corrects the article DOI: 10.1098/rsos.170633.].

Spatial food‐web structure in the eastern tropical Pacific Ocean based on compound‐specific nitrogen isotope analysis of amino acids

AbstractThe effective evaluation of trophic interactions in pelagic food webs is essential for understanding food web ecology, conservation biology, and management. We tested the applicability of compound‐specific isotope analysis of amino acids (CSIA‐AA) for (1) characterizing trophic positions (TPs) of nine species from four trophic groups (tunas, squids, myctophids, and euphausiids) within a pelagic food web in the eastern tropical Pacific (ETP) Ocean, (2) evaluating trophic discrimination factors (TDFs) of each trophic group, and (3) detecting spatial changes in TPs and food chain length across a region with heterogeneous productivity. Although δ15N values of bulk tissues generally increased from south‐to‐north, CSIA‐AA revealed that trophic positions were uniform throughout our study area. These results suggest that variability in δ15N values were largely driven by nitrogen cycling dynamics in the ETP, which highlights the importance of these processes for the interpretation of δ15N values in food web studies. Absolute TP estimates were unrealistic for higher‐level species, and TDFs (tunas: 4.0‰, squids: 4.6‰, myctophids: 5.0‰, and euphausiids: 7.0‰) were lower than a widely used ecosystem TDF. We used remotely sensed oceanographic data to evaluate the physical oceanography and biological productivity throughout our study area and found significant relationships between δ15N values, nitrate concentrations, and SST across our study area. We did not find a gradient in phytoplankton cell size co‐occurring with an expected productivity gradient across our sampling region, which substantiated our isotope results indicating non‐significant spatial changes in TP and food chain length across the ETP.

Aphid‐tending ants on introduced fennel: can resources derived from non‐native plants alter the trophic position of higher‐order consumers?

1. Although plant invasions often reduce insect abundance and diversity, non‐native plants that support phytophagous insects can subsidise higher trophic levels via elevated herbivore abundance.2. Here ant–aphid interactions on non‐native fennel on Santa Cruz Island, California are examined. Fennel hosts abundant, honeydew‐producing fennel aphids. The patchiness of fennel and the relative lack of honeydew‐producing insects on other plants at our study sites suggest that assimilation of fennel‐derived honeydew would increase the abundance and decrease the trophic position of the omnivorous, aphid‐tending Argentine ant.3. To assess the strength of the ant–aphid interaction, a comparison of ant abundance on and adjacent to fennel prior to and 3 weeks after experimental aphid removal was performed. Compared with control plants with aphids, ants declined in abundance on and around fennel plants following aphid removal. At the habitat scale, pitfall traps in fennel‐dominated habitats captured more ants than in fennel‐free scrub habitats.4. To determine if assimilation of aphid‐produced honeydew reduces the ant's trophic position, variation in δ15N values among ants, plants and other arthropods was analysed. Unexpectedly, δ15N values for ants in fennel‐dominated habitats were higher than those of arthropod predators from the same sites and also higher than those of ants from fennel‐free habitats.5. Our results illustrate how introduced plants that support phytophagous insects appear to transfer energy to higher trophic levels via elevated herbivore abundance. Although assimilation of fennel‐derived honeydew did not appear to reduce consumer trophic position, spatial variation in alternative food resources might obscure contributions from honeydew.

Fluid-induced breakdown of white mica controls nitrogen transfer during fluid–rock interaction in subduction zones

ABSTRACTIn order to determine the effects of fluid–rock interaction on nitrogen elemental and isotopic systematics in high-pressure metamorphic rocks, we investigated three different profiles representing three distinct scenarios of metasomatic overprinting. A profile from the Chinese Tianshan (ultra)high-pressure–low-temperature metamorphic belt represents a prograde, fluid-induced blueschist–eclogite transformation. This profile shows a systematic decrease in N concentrations from the host blueschist (~26 μg/g) via a blueschist–eclogite transition zone (19–23 μg/g) and an eclogitic selvage (12–16 μg/g) towards the former fluid pathway. Eclogites and blueschists show only a small variation in δ15Nair (+2.1 ± 0.3‰), but the systematic trend with distance is consistent with a batch devolatilization process. A second profile from the Tianshan represents a retrograde eclogite–blueschist transition. It shows increasing, but more scattered, N concentrations from the eclogite towards the blueschist and an unsystematic variation in δ15N values (δ15N = + 1.0 to +5.4‰). A third profile from the high-P/T metamorphic basement complex of the Southern Armorican Massif (Vendée, France) comprises a sequence from an eclogite lens via retrogressed eclogite and amphibolite into metasedimentary country rock gneisses. Metasedimentary gneisses have high N contents (14–52 μg/g) and positive δ15N values (+2.9 to +5.8‰), and N concentrations become lower away from the contact with 11–24 μg/g for the amphibolites, 10–14 μg/g for the retrogressed eclogite, and 2.1–3.6 μg/g for the pristine eclogite, which also has the lightest N isotopic compositions (δ15N = + 2.1 to +3.6‰).Overall, geochemical correlations demonstrate that phengitic white mica is the major host of N in metamorphosed mafic rocks. During fluid-induced metamorphic overprint, both abundances and isotopic composition of N are controlled by the stability and presence of white mica. Phengite breakdown in high-P/T metamorphic rocks can liberate significant amounts of N into the fluid. Due to the sensitivity of the N isotope system to a sedimentary signature, it can be used to trace the extent of N transport during metasomatic processes. The Vendée profile demonstrates that this process occurs over several tens of metres and affects both N concentrations and N isotopic compositions.

Sex-related variation in δ15N values of ground beetles (Coleoptera, Carabidae): A case study

Abstract Intersexual differences in food preferences and patterns of nutrient allocation are well known for a variety of animals, but this factor is not taken into account in the stable isotope-based studies of soil food webs. We used stable isotope composition of carbon and nitrogen (δ 13 C and δ 15 N values) to address the intersexual differences in the trophic niche of 27 species of ground beetles sampled in Northern Poland and Western Russia. Despite a high level of intra- and interspecies variation, males were, on average, enriched in 15 N relative to females by 0.5 ± 0.1‰. Observed enrichment was most pronounced among species with high δ 15 N values and among spring breeding species collected in June. We did not find significant sex-related differences in δ 13 C values. Our results indicate sex-specific patterns in resource allocation or foraging behavior of predatory Carabidae during the breeding season.

Plasticity of trophic interactions among sharks from the oceanic south-western Indian Ocean revealed by stable isotope and mercury analyses

Sharks are a major component of the top predator guild in oceanic ecosystems, but the trophic relationships of many populations remain poorly understood. We examined chemical tracers of diet and habitat (δ15N and δ13C, respectively) and total mercury (Hg) concentrations in muscle tissue of seven pelagic sharks: blue shark (Prionace glauca), short-fin mako shark (Isurus oxyrinchus), oceanic whitetip shark (Carcharhinus longimanus), scalloped hammerhead shark (Sphyrna lewini), pelagic thresher shark (Alopias pelagicus), crocodile shark (Pseudocarcharias kamoharai) and silky shark (Carcharhinus falciformis), from the data poor south-western tropical Indian Ocean. Minimal interspecific variation in mean δ15N values and a large degree of isotopic niche overlap – driven by high intraspecific variation in δ15N values – was observed among pelagic sharks. Similarly, δ13C values of sharks overlapped considerably for all species with the exception of P. glauca, which had more 13C-depleted values indicating possibly longer residence times in purely pelagic waters. Geographic variation in δ13C, δ15N and Hg were observed for P. glauca and I. oxyrinchus. Mean Hg levels were similar among species with the exception of P. kamoharai which had significantly higher Hg concentrations likely related to mesopelagic feeding. Hg concentrations increased with body size in I. oxyrinchus, P. glauca and C. longimanus. Values of δ15N and δ13C varied with size only in P. glauca, suggesting ontogenetic shifts in diets or habitats. Together, isotopic data indicate that – with few exceptions – variance within species in trophic interactions or foraging habitats is greater than differentiation among pelagic sharks in the south-western Indian Ocean. Therefore, it is possible that this group exhibits some level of trophic redundancy, but further studies of diets and fine-scale habitat use are needed to fully test this hypothesis.

Preservation Effects on Stable Isotope Values of Archived Elasmobranch Fin Tissue: Comparisons between Frozen and Ethanol‐Stored Samples

AbstractElasmobranch fin tissue has been sampled and archived for decades to support genetics research. However, these collections have the potential to provide additional information on the trophic ecology of and habitat use by elasmobranch species. The use of fin tissue is especially attractive considering the threatened status of many elasmobranchs and the call for limiting mortalities. Yet, the use of fin samples for stable isotope analysis requires either that (1) storage methods do not alter tissue isotope values or (2) any alterations in isotope composition that occur during storage are predictable. In this study, paired fin tissues sampled from Smalltooth Sawfish Pristis pectinata and cownose rays Rhinoptera spp. were stored frozen and in ethanol and were subsequently analyzed for carbon (δ13C) and nitrogen (δ15N) isotope ratios. Fin δ13C and δ15N values were highly correlated between treatments for both taxa (r2 ≥ 0.80). For Smalltooth Sawfish, ethanol storage significantly increased fin δ13C values by 0.5 ± 0.1‰ (mean ± SE) and decreased fin δ15N values by 0.1 ± 0.1‰ relative to frozen samples; differences were similar for cownose rays (δ13C: 0.2 ± 0.2‰; δ15N: 0.2 ± 0.1‰) but were not significant. A range of approximately 3‰ for δ13C between treatments could have effects on data interpretation, suggesting the use of regressions for ethanol correction of δ13C values, although trends were comparable between frozen and ethanol‐preserved samples without correction. Given the low variability in δ15N values, a correction is not warranted. For endangered species such as the Smalltooth Sawfish, stable isotope analysis of ethanol‐archived fin samples can provide important information regarding habitat use and trophic ecology, with potential significance for conservation and management strategies. The general uniformity in isotope ratio shifts observed for archived samples between the two taxa suggests that these findings can be generalized across elasmobranch species.Received March 27, 2014; accepted July 30, 2014

Revealing N management intensity on grassland farms based on natural δ15N abundance

High application rates of farmyard manure on grasslands generally results in nitrogen (N) losses and a shift in N isotope composition. The aim of this study was to elucidate to which degree the 15N signatures of miscellaneous N pools at two different levels of management intensity may be used to reproduce the N level of various grassland farms in practise, i.e. beyond the control of experimental plots. We hypothesized that (i) higher δ15N values in soil, plant and animal samples can be found with intensified grassland farming and high N-input management, (ii) that the 15N signature originating from manure application is influenced by application technique and (iii) that it declines with increasing distance from stable to field. To test these hypotheses, we monitored different N pools on nine different farms (dairy, suckler, beef production) in grassland dominated regions of North Rhine-Westphalia, Rheinland-Palatinate (Germany) and Styria (Austria). Samples were taken from 0 to 5cm soil depth and aboveground biomass at each of five sub-sites on farm, as well as from feed components, fertilizers, and cattle product and tissue like milk, hair, faeces and urine.The results indicated a considerable variation in δ15N values of the top soil (1.47‰ to 7.91‰) and of harvested plant material (−2.18‰ to 6.79‰). On average, δ15N values of samples from high N-input grasslands were elevated by 2.8 delta units relative to those of low N-input grasslands. For the soil and plant samples, the δ15N values were thus closely correlated with the overall N balance as well as with stocking rate and fertilizer input (r2=0.71 to 0.85). Respective trends for the isotopic signatures in milk, hair and faeces were also evident but less apparent. Furthermore, low emission application techniques of organic fertilizer and increasing distances from the stable to the field exhibited lower δ15N values of top soils and plants, but only in the low input system. We conclude that high N-input on grassland farms systematically changes the δ15N values of soils and above ground biomass and thus also the N signature in animal tissues. Application of N isotope technique to these N pools thus allows for tracing back intensity of fertilizer management regardless of high natural δ15N variations in terrestrial environments.

Trophic shift in the diet of the pelagic thresher shark based on stomach contents and stable isotope analyses

The trophic ecology of the pelagic thresher shark (Alopias pelagicus) was analysed using stomach contents and carbon (δ13C) and nitrogen (δ15N) stable isotope analysis. The sharks were caught in Ecuadorian waters between June and December 2003. Approximately 24 prey species were found in the stomachs of 111 sharks. The most common species were Dosidicus gigas, Benthosema panamense and Sthenoteuthis oualaniensis. Mean (± SD) muscle isotope values of the pelagic thresher shark were −16.0±0.3‰ for δ13C and 13.7±1.1‰ for δ15N; mean (± SD) vertebrae values were −16.7±2.0 for δ13C and 9.4±1.8 for δ15N. There were no significant sex-related differences in muscle δ13C and δ15N; however, female vertebrae had significantly higher mean (± SD) δ13C values (−16.4±2.0‰) than males (−18.8±0.3‰). Inter-individual variation in δ15N values was observed in both tissues and was likely related to foraging in different ecosystems with distinct isotope values. Both stomach content and isotope data suggest that the pelagic thresher shark is a specialist predator in Ecuadorian waters. In general, little is known about the foraging ecology and movement patterns of this globally distributed but elusive top marine predator, and how these fundamental ecological traits vary among populations. By using two independent but complementary approaches, our study addresses these questions for the pelagic thresher sharks in Ecuadorian waters. Such information is critically important for the design of effective management strategies for these highly exploited but poorly understood predators.

Sample acidification effects on carbon and nitrogen stable isotope ratios of macrofauna from a Zostera noltii bed

Acidification treatment has been used in most stable isotope studies to eliminate inorganic non-dietary carbon, allowing δ13C measurement of organic carbon needed for trophic studies. Because only limited information about the acidification effects on isotopic ratios exists in the literature, we provide an assessment of such effects for a wide range of benthic macroinvertebrates from estuarine intertidal sediments with sparse Zostera noltii Hornem. vegetation. Our results revealed only few significant δ13C shifts for macrobenthos; however, δ13C decrease after acidification was substantial for several species. We conclude that acidification is not necessary for removing calcium carbonate in a majority of macrobenthos, but we also suggest preliminary testing before deciding it can be omitted. The magnitude of the δ15N shifts was substantial for some species and variability in δ15N values of replicate acidified samples exceeded that of non-acidified samples. Therefore, sample acidification should be avoided for δ15N determination. Moreover, acidification effects vary considerably among species belonging to a single class, rendering generalisations from single-species information potentially spurious.

δ15N values of macroalgae as an indicator of the potential presence of waste disposal from land-based marine fish farms

The nitrogen isotope ratio (δ15N) in tissues of native macroalgae was evaluated as a means of indicating the intensity and spatial extent of organic contamination due to disposal of waste from land-based marine fish farms (LBMFFs). Three species of macroalgae from the genus Fucus and the green macroalgae Codium tomentosum were selected for study. The study was carried out at seven flat marine fish farms located in Galicia (NW Spain). Tests were carried out to determine the intra-annual variation in δ15N values and any differences between selected macroalgae. The δ15N values enrichment was observed close to the disposal point, and δ15N values varied more widely throughout the year (±5.57 ‰) at sites affected by the marine fish farm effluent compared to natural conditions (±2 ‰). No significant differences in the isotopic signals were observed in the different species studied (standard major axis). The δ15N values of macroalgae may be an ideal means of detecting the presence of LBMFFs effluents.

Seasonal and annual variation in intake of traditional marine foods by a Yup'ik Eskimo population: a sequential dietary record from hair stable isotope analysis

There has been considerable research and public health interest in the potentially protective effects of traditional diets on disease risk in Yup'ik Eskimos. Traditional diets include high levels of fish and marine mammal intake, which provide omega‐3 fatty acids, vitamin D, and other protective nutrients. However, self‐report methods of diet assessment do not capture seasonal patterns of intake. Here, we investigate seasonal patterns of traditional marine mammal and fish intake by measuring variations in a biomarker of marine intake, the nitrogen isotope ratio (δ15N), sequentially along hair from a subset of participants in the Center for Alaska Native Health Research (CANHR) study (n = 19). We found pronounced seasonal variations in δ15N values, which we interpret as reflecting seasonal differences in the availability of specific fish and marine mammals. A pronounced summer peak in δ15N values corresponded to increased salmon availability. Furthermore, variations occurred fairly consistently across participants. Thus, participants’ ranking with respect to traditional food intake tended to stay constant. This is particularly relevant for the CANHR study, as participation varied seasonally and participants were recruited primarily during the winter months. This work was funded by a Centers of Biomedical Research Excellence grant from the NIH National Center for Research Resources (P20 RR333065).

Stable isotope biogeochemistry of seabird guano fertilization: results from growth chamber studies with maize (Zea mays).

BackgroundStable isotope analysis is being utilized with increasing regularity to examine a wide range of issues (diet, habitat use, migration) in ecology, geology, archaeology, and related disciplines. A crucial component to these studies is a thorough understanding of the range and causes of baseline isotopic variation, which is relatively poorly understood for nitrogen (δ15N). Animal excrement is known to impact plant δ15N values, but the effects of seabird guano have not been systematically studied from an agricultural or horticultural standpoint.Methodology/Principal FindingsThis paper presents isotopic (δ13C and δ15N) and vital data for maize (Zea mays) fertilized with Peruvian seabird guano under controlled conditions. The level of 15N enrichment in fertilized plants is very large, with δ15N values ranging between 25.5 and 44.7‰ depending on the tissue and amount of fertilizer applied; comparatively, control plant δ15N values ranged between −0.3 and 5.7‰. Intraplant and temporal variability in δ15N values were large, particularly for the guano-fertilized plants, which can be attributed to changes in the availability of guano-derived N over time, and the reliance of stored vs. absorbed N. Plant δ13C values were not significantly impacted by guano fertilization. High concentrations of seabird guano inhibited maize germination and maize growth. Moreover, high levels of seabird guano greatly impacted the N metabolism of the plants, resulting in significantly higher tissue N content, particularly in the stalk.Conclusions/SignificanceThe results presented in this study demonstrate the very large impact of seabird guano on maize δ15N values. The use of seabird guano as a fertilizer can thus be traced using stable isotope analysis in food chemistry applications (certification of organic inputs). Furthermore, the fertilization of maize with seabird guano creates an isotopic signature very similar to a high-trophic level marine resource, which must be considered when interpreting isotopic data from archaeological material.

Natural 15N/14N isotope composition in C3 leaves: are enzymatic isotope effects informative for predicting the 15N-abundance in key metabolites?

Although nitrogen isotopes are viewed as important tools for understanding plant N acquisition and allocation, the current interpretation of natural 15N-abundances (δ15N values) is often impaired by substantial variability among individuals or between species. Such variability is likely to stem from the fact that 15N-abundance of assimilated N is not preserved during N metabolism and redistribution within the plant; that is, 14N/15N isotope effects associated with N metabolic reactions are certainly responsible for isotopic shifts between organic-N (amino acids) and absorbed inorganic N (nitrate). Therefore, to gain insights into the metabolic origin of 15N-abundance in plants, the present paper reviews enzymatic isotope effects and integrates them into a metabolic model at the leaf level. Using simple steady-state equations which satisfactorily predict the δ15N value of amino acids, it is shown that the sensitivity of δ15N values to both photorespiratory and N-input (reduction by nitrate reductase) rates is quite high. In other words, the variability in δ15N values observed in nature might originate from subtle changes in metabolic fluxes or environment-driven effects, such as stomatal closure that in turn changes v0, the Rubisco-catalysed oxygenation rate.

N2 fixation in cowpea plants grown in farmers’ fields in the Upper West Region of Ghana, measured using15N natural abundance

Few studies have assessed the levels of symbiotic N nutrition in legumes grown by farmers in Africa. In this study, the shoots of cowpea plants were sampled from 63 farms in 12 villages within 5 districts of the Upper West Region of Ghana, and assessed for growth and symbiotic N nutrition. The data revealed considerable differences in cowpea plant density per m2, plant growth,15N natural abundance (δ15N), %Ndfa, and N-fixed among different farms under one village, and between villages under the same district, and between districts in the Upper West Region. In farms where there were fewer cowpea plants per m2, plant growth was better and dry matter yield per plant significantly greater, leading to strong variations in δ15N values. Except for four farms at Bamahu which had cowpea shoot Ndfa values of 12.1%, 30.0%, 36.5% and 46.6%, one farm at Babile with Ndfa value of 58.1%, and three farms at Silbelle with Ndfa values of 56.8%, 57.9% and 68.7%, the remaining 55 out of the 63 farms studied showed high shoot Ndfa values, ranging from 70.6% to 99.7%, which clearly indicates that cowpea cultivated by farmers in the Upper West Region of Ghana meet a large proportion of their N requirements from symbiotic fixation. At the district level, isotopic analysis showed that, on average, the15N natural abundance values (%0) of cowpea shoots were −0.496±0.04 for Jirapa, −0.083±0.06 for Nadowli, 0.368±0.08 for Lawra, J.333±0.29 for Wa and 0.365±0.09 for Sissala district. Estimates of the legume’s N derived from fixation were 66.3% for Wa district, 89.9% for Nadowli, 79.4% for Lawra, 78.9% for Sissala and 80.9% for Jirapa district. The amount of N-fixed ranged from 402.3 mg.plant−1 for Nadowli, 176.5 mg.plant−1 for Wa, 235.4 mg.plant−1 for Sissala, 179.0 mg.plant−1 for Lawra to 249.2 mg.plane−1 for the Jirapa district. Expressed on per-hectare basis using cowpea density per m2, the total amount of N-fixed was around 16.6 kg ha−1 in the Nadowli district, 19.1 kg ha−1 in Wa, 23.0 kg ha−1 in Sissala, 2J.1 kg ha−1 in Lawra and 17.6 kg ha−1 in the Jirapa district. Averaged across all 5 districts, N-fixed by cowpea was about 19.5 kg ha−1 in the Upper West Region of Ghana. These data suggest that, increasing N2 fixation in fanners’ fields in Ghana would require optimization of cowpea plant density rather than biological manipulation of the symbiotic process (as %Ndfa values were generally very high).

Effects of water availability, nitrogen supply and atmospheric CO2 concentrations on plant nitrogen natural abundance values.

The relative effects of soil N, water supply and elevated atmospheric CO2 on foliar δ15N values were examined. Phalaris arundinacea L. (Holdfast) and Physalis peruviana L. (Cape Gooseberry) were grown for 80 d with three water availability treatments, two atmospheric CO2 concentrations and four N supply rates. Elevated CO2 increased total plant biomass and N for each treatment and decreased allocation to roots, leaf N concentrations and stomatal conductance. Leaves had less negative leaf δ13C values under low water supply associated with decreased stomatal conductance and increased leaf N concentration, which decreased the ratio of intercellular to ambient CO2 concentration. The δ15N value of the supplied nitrate (4.15‰) was similar to the value for Phalaris leaves (4.11‰), but Cape Gooseberry leaves were enriched (6.52‰). The effects of elevated CO2 on leaf δ15N values were small, with Phalaris showing no significant change, while Cape Gooseberry showed a significant (P < 0.05) decline of 0.42 ‰. Variation in δ15N values was unrelated to stomatal conductance, transpiration, differential use of N forms or denitrification. Plants with low foliar N concentrations tended to be depleted in 15N. We suggest that changes in N allocation alter foliar δ15N values under different CO2 and water treatments.

δ15N in riverine food webs: effects of N inputs from agricultural watersheds

We examined the use of the natural abundance of nitrogen stable isotopes (δ15N) as a tracer of anthropo genic perturbations of the nitrogen (N) cycle at the watershed scale in 82 river sites draining 13 watersheds in the St. Lawrence Lowlands in Quebec. Mean δ15N values of aquatic primary consumers varied greatly among sites (+2‰ to +15‰), most of this variation (88%) being attributable to site effects. Variation in δ15N values among functional feeding groups of primary consumers within sites was comparatively lower (&lt;1‰). Within watersheds, δ15N values of primary consumers (and organisms of higher trophic levels) tracked longitudinal changes in the percentage of agricultural area. Overall, the percentage of total watershed area under agriculture explained up to 69% of the variation in mean primary consumer δ15N values. Similar positive correlations were observed for predatory invertebrates and non-piscivorous fish. In general, our results show that δ15N in riverine food webs reacts strongly to spatial patterns in the intensity of N inputs related to agricultural land use.

Problems Associated with the AMS Dating of Small Bone Samples: The Question of the Arrival of Polynesian Rats to New Zealand

We have AMS dated samples of Pacific rat (Rattus exulans) bone “collagen” and filtered gelatin samples from the prehistoric site of Shag River Mouth, New Zealand. The age of occupation of this site has previously been determined based on 50 radiocarbon measurements. The site dates to the late Archaic phase of southern New Zealand prehistory (about 650–500 BP; 14th–15th century AD). The results of rat bones which we have dated produce a range in ages, from about 980–480 BP, a difference we attribute to a combination of effects. Pretreatment appears to be an important variable, with results showing differences in 14C age between the progressive “collagen” and filtered gelatin chemical treatment stages. Amino acid profiles suggest there is a proteinaceous but non-collagenous contaminant which is removed by the more rigorous pretreatment. Stable isotopes vary between pretreatments, supporting the removal of a contaminant, or contaminants. Variation in δ15N values imply a range in uptake of dietary protein, and might suggest a potential influence from the local aquatic environment or the consumption of marine-derived protein. Rats are opportunistic, omnivorous mammals, and, therefore, obtain carbon from a variety of reservoirs and so we ought to expect that in environments where there is a variety of reservoirs, these will be exploited. Taken together, the results show that rat bone AMS 14C determinations vary in comparison with the established age of the site, but are in notably better agreement with non-collagenous data than in previously published determinations (Anderson 1996).