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PubMed1995

Correlation between stable nitrogen isotope ratios and concentrations of organochlorines in biota from a freshwater food web.

The relationship between total concentrations of hexachlorocyclohexane (sigma HCH), sigma DDT, and chlorinated bornanes (toxaphene, sigma CHB) and the trophic position of biota from a subarctic lake was investigated using stable isotope ratios of nitrogen (15N/14N). Zooplankton, benthic invertebrates, and forage and piscivorous fishes were analysed for 15N/14N and organochlorines using mass spectrometry and high resolution capillary gas chromatography (GC-ECD), respectively. The trophic relationships of the biota were clearly defined, with 15N/14N increasing an average of 3.3/1000 from prey species to predator. Mean concentrations of sigma HCH were lowest in chironomids (subfamily Chironominae, 0.2 ng/g wet wt.) and highest in burbot liver (Lota lota; 30.2 ng/g wet wt.). Mean concentrations of sigma DDT and sigma CHB ranges from 0.5 and 2.0 (ng/g wet wt.), respectively, in snails (Family Limnaeidae), to 3430 and 2820 (ng/g wet wt.) in burbot liver. Regression analyses indicated that both the wet and lipid weight concentrations of sigma HCH, sigma DDT, and sigma CHB in the biota from this food web were significantly related to trophic position, as defined by delta 15 N. Results from this study indicated that delta 15 N can be used to predict concentrations of organochlorines in freshwater biota.

Animals↗
PubMed2005

Primary consumer stable nitrogen isotopes as indicators of nutrient source.

Non-point source loading of nitrogen and phosphorus is a primary cause of eutrophication of inland waters, although the diffuse and variable nature of nutrient inputs makes it difficult to trace and identify nutrient pathways. Stable nitrogen isotope values (delta15N) in aquatic biota are thought to reflect anthropogenic nutrient inputs, and they may be a promising tool fortracing nutrient sources in watersheds. We measured delta15N of aquatic consumers from a suite of 27 Danish lakes spanning a range of trophic states (oligotrophic to eutrophic) and land uses (forest, urban, agriculture). Primary consumer delta15N values (PCdelta15N) varied more than 14% among lakes. Models of PCdelta15N were developed from limnological, nitrogen loading, and nitrogen source variables using an information-theoretic approach (Akaike's Information Criteria, AIC). Models based on land use/ land cover performed best, indicating that elevated delta15N is not only associated with high nitrogen loading, but is also reflective of nitrogen source. Urban and agricultural land use in the watershed, and particularly within the riparian buffer areas, was quantitatively linked to elevated biotic delta15N.

Denmark↗
PubMed2004

Carbon and nitrogen isotopic anomalies in an anhydrous interplanetary dust particle.

Because hydrogen and nitrogen isotopic anomalies in interplanetary dust particles have been associated with carbonaceous material, the lack of similar anomalies in carbon has been a major conundrum. We report here the presence of a 13C depletion associated with a 15N enrichment in an anhydrous interplanetary dust particle. Our observations suggest that the anomalies are carried by heteroatomic organic compounds. Theoretical models indicate that low-temperature formation of organic compounds in cold interstellar molecular clouds can produce carbon and nitrogen fractionations, but it remains to be seen whether the specific effects observed here can be reproduced.

Carbon Isotopes↗
PubMed2000

Evaluation of dietary intake using carbon and nitrogen isotope analysis of human hair of Chinese living in southern part of China.

We examined dietary intakes among 69 people of three ethnic groups (Yao Chinese, Zhung Chinese and Han Chinese) in the autonomous country of Bama in China where many elderly people have a long life span in 1998. Dietary intake was evaluated using carbon and nitrogen isotope ratios of human hair (delta 13C, delta 15N) standardized by standard gas. The distributions of delta 13C in Yao Chinese and Han Chinese were scattered from -21 to -17/1000, which were close to that of C4 plants such as corn and millet. The distribution of delta 13C in Zhung Chinese was located from -25 to -22/1000, which was near that of C3 plants such as rice, wheat and potato. These values showed a much wider range when compared with delta 13C in Chinese living in urban areas. On the other hand, delta 15N in each group was distributed in the area ranged from 8 to 11/1000. The result of delta 13C was consistent with data obtained from a nutritional survey, but that of delta 15N was a little higher than expected. The carbon and nitrogen isotope ratios of human hair among three ethnic groups showed characteristic distributions, although the relation between these isotope ratios and longevity is uncertain.

Adolescent↗
PubMed2006

Sedimented organic nitrogen isotopes in freshwater wetlands record long-term changes in watershed nitrogen source and land use.

Although historic land use is widely recognized as an important determinant of watershed N cycling, efforts to examine land use legacy effects are limited by incomplete historical data. This research evaluates N isotopes of sedimented organic matter (delta15N(org)), in a palynological context, as a long-term proxy of changes in N source to wetland biota. N and S isotope measurements of organic sediments, fossil plant fragments, and living plants are used to explore isotope stratigraphies of wetland sediment cores. Processes potentially contributing to isotope stratigraphies are investigated including the following: a change in N source, diagenesis, and denitrification. We document the delta15N(org) stratigraphy of a core from the Smithsonian Environmental Research Center, MD, U.S.A. spans approximately 350 years, during which time delta15N(org) increases from +2 per thousand to +7 per thousand. Reconstructed population density and wastewater inputs to the watershed suggest that the increase in delta15N reflects changing land use from forested conditions to increasing nutrient inputs from human waste. Our results illustrate the importance of hydrologic connectivity in delivering waste-derived N in a watershed characterized by relatively low human population density. These results also demonstrate how this approach can expand the temporal horizon over which we can assess human impacts to watershed N dynamics.

Agriculture↗
PubMed2007

The calibration of the intramolecular nitrogen isotope distribution in nitrous oxide measured by isotope ratio mass spectrometry.

Two alternative approaches for the calibration of the intramolecular nitrogen isotope distribution in nitrous oxide using isotope ratio mass spectrometry have yielded a difference in the 15N site preference (defined as the difference between the delta15N of the central and end position nitrogen in NNO) of tropospheric N2O of almost 30 per thousand. One approach is based on adding small amounts of labeled 15N2O to the N2O reference gas and tracking the subsequent changes in m/z 30, 31, 44, 45 and 46, and this yields a 15N site preference of 46.3 +/- 1.4 per thousand for tropospheric N2O. The other involves the synthesis of N2O by thermal decomposition of isotopically characterized ammonium nitrate and yields a 15N site preference of 18.7 +/- 2.2 per thousand for tropospheric N2O. Both approaches neglect to fully account for isotope effects associated with the formation of NO+ fragment ions from the different isotopic species of N2O in the ion source of a mass spectrometer. These effects vary with conditions in the ion source and make it impossible to reproduce a calibration based on the addition of isotopically enriched N2O on mass spectrometers with different ion source configurations. These effects have a much smaller impact on the comparison of a laboratory reference gas with N2O synthesized from isotopically characterized ammonium nitrate. This second approach was successfully replicated and leads us to advocate the acceptance of the site preference value 18.7 +/- 2.2 per thousand for tropospheric N2O as the provisional community standard until further independent calibrations are developed and validated. We present a technique for evaluating the isotope effects associated with fragment ion formation and revised equations for converting ion signal ratios into isotopomer ratios.

Air Pollutants↗
PubMed1988

Nitrogen isotopic fractionation and 18O exchange in relation to the mechanism of denitrification of nitrite by Pseudomonas stutzeri.

Two types of mechanisms for the enzymatic reduction of NO2- to N2O have been proposed. In one, two NO2- ions are reduced in parallel, with the nitrogen-nitrogen bond formed from reduced intermediates. In the second, the two NO2- ions enter the reaction sequentially, with the nitrogen of at least one of the two having a valence of 3+ when the nitrogen-nitrogen bond is formed. Our objective was to distinguish between these two types of mechanism. Toward that end, the exchange of 18O from H2O to NO2- and the overall nitrogen isotopic fractionation factor (beta obs) were measured. The rate of exchange of oxygen from H2O to NO2-, resulting from a protonation-dehydration step preceding reductive events in both mechanisms, was less than 10% of the rate of denitrification at both low and high [NO2-]. The value of beta obs was 1.010 +/- 0.001 and 1.020 +/- 0.001 at low and high [NO2-], respectively. Expressions for beta obs, as a function of the measured rate of entry of oxygen from H2O into NO2-, were derived for both types of mechanism. The measured dependence of beta obs on substrate concentration, as constrained by the 18O exchange data, is inconsistent with the first type of mechanism, but consistent with the second type. Thus, by combining nitrogen isotopic fractionation and 18O exchange data, we rule out any mechanism in Pseudomonas stutzeri in which NO2- ions are reduced in parallel, with the nitrogen-nitrogen bond being formed from reduced intermediates.

Mathematics↗
PubMed2001

Stable nitrogen isotopes in waterfowl feathers reflect agricultural land use in western Canada.

Stable nitrogen and carbon isotope analysis was performed on secondary feathers collected from flightless, mallard ducklings (Anas platyrhynchos) from 17 locations across western Canada. The delta15N values of mallard feathers ranged from +6.1 to +23.7 per thousand (AIR). Mean delta15N feather values from the 17 locations were strongly correlated with the percentage of land under agricultural development. Higher delta15N values in waterfowl feathers collected from agricultural areas possibly reflected the entry of excess fertilizer nitrogen into local water bodies. However, other processes may have also been important. These results provide evidence that nitrogen isotope values in avian feathers may reflect long-term nitrogen additions to surface waters in agricultural areas and may also provide important clues in elucidating the origin of nonpoint source nitrogen inputs.

Agriculture↗
PubMed1983

Variable expression of the nitrogen isotope effect associated with denitrification of nitrite.

The overall isotopic fractionation factor observed for denitrification of NO-2 by resting cultures and cell-free extracts of Pseudomonas stutzeri varied widely with the concentration of NO-2 and reductant. The observed isotope effect (beta obs) increased linearly with velocity when the concentration of nitrite was varied and decreased with velocity when reductant concentration was varied. At any given ratio of velocity to maximum velocity, beta obs was approximately the same in intact cells and cell free extracts. These results indicate the following: (a) neither uptake (whole cells) nor enzyme-substrate association (cell-free extracts) is the sole cause of saturation of the overall rate as [NO-2] is increased; (b) a reductive step lies beyond the initial step and at or before the first unidirectional step; and (c) in intact cells, uptake of NO-2 by the cell and egress of NO-2 from it are very rapid compared to reduction at all concentrations of NO-2. A corollary of the last conclusion is that variation of beta obs in intact cells is due entirely to variation in the relative rates of intracellular steps of the denitrification process. The linear relation between beta obs and velocity imposes constraints on any proposed mechanism of denitrification.

Isotope Labeling↗
PubMed1997

Anthropogenic impacts on natural nitrogen isotope variations in Pinus sylvestris stands in an industrially polluted area.

Natural variations of the nitrogen isotopes 15N/14N (delta15N values) and the N concentrations of one-year-old needles from 7-12-year-old pine trees (Pinus sylvestris L.) were determined on 27 sites in the heavily polluted Leipzig-Halle region (former GDR). At three selected sites measurements were repeated over a period of 2 years. N concentrations and delta15N values in different needle age classes were compared at the three sites. The delta15N values of the N in the humus layer and the potential plant available N in the A(h) horizon of the local soil were determined. The 15N/14N isotope ratios (delta15N values) of one-year-old pine needles in the region of Leipzig-Halle were found to vary depending on their specific location by a factor of up to one order of magnitude (-9.6 per thousand to + 0.4 per thousand ). N concentrations in one-year-old pine needles varied between 0.71 and 1.38 mmol eq N g dw(-1). Pine stands with positive or slightly negative delta15N values and high N concentrations in one-year-old needles were concentrated around the cities of Leipzig and Halle and in the industrial areas. More negative delta15N values and lower N concentrations in one-year-old pine needles were found on sites at greater distances from the industrial agglomerations, mainly in the NE forested part. Site specific differences in the delta15N values of the N in the humus layer from three selected sites were similar to those found for the needles. No site specific differences, however, were found for the delta15N values of the water soluble nitrogen fraction from the mineral soil horizons of the same sites.

Journal Article↗
PubMed2003

Nitrogen isotope analysis of ammonium in aqueous solutions using a perfluorosulfonated ionomer membrane for solid-phase microextraction.

The use of custom-made solid-phase microextraction (SPME) fibers coated with a perfluorosulfonated ionomer, Nafion, was investigated for nitrogen isotopic analysis of ammonium in aqueous solutions. Aqueous ammonium was converted to ammonia by addition of a base, followed by absorption from the headspace, desorption in the injection port of a gas chromatograph, and analysis by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). Fibers coated with a Nafion tubing were chosen due to a higher fiber-gas distribution constant and a higher Nafion thickness than fibers coated with Nafion solution, both leading to a higher amount of ammonia absorbed at equilibrium. The Nafion membrane-coated fiber absorbed approximately 20 times more than a commercial polydimethylsiloxane (PDMS) fiber. The isotopic fractionation between fiber and gas was 1.0117 +/- 0.0009 (standard deviation, SD, of all measurements) at an initial ammonia gas concentration of 21-210 microM. At 390 microM initial gas concentration it was slightly lower. When sampling from liquid samples, an ammonium concentration of 10 mM was needed to obtain a sufficient amount of ammonia absorbed. Modeling of the absorption at different temperatures showed that the absorption was approximately constant in the temperature range suitable for SPME experiments. Absorption at room temperature was therefore used for simplicity. A pilot study was conducted in which absorption was achieved from a single 9 microL droplet of sample. The preliminary results showed that delta(15)N analysis was possible for only 0.4-0.5 micromol of ammonium with a SD of 0.8 per thousand (n = 5).

Algorithms↗
PubMed1983

Stable nitrogen isotope ratios of bone collagen reflect marine and terrestrial components of prehistoric human diet.

The delta 15N values of bone collagen from Eskimos and from Northwest Coast Indians dependent on salmon fishing are about 10 per mil more positive than those from agriculturalists in historic times. Among prehistoric humans, two groups dependent on marine food sources show bone collagen delta 15N values that are 4 to 6 per mil more positive than those from two agricultural groups. The nitrogen isotope ratios of bone collagen from prehistoric inhabitants of the Bahamas are anomalously low for reasons that relate to the biogeochemical cycle of nitrogen in coral reefs.

Bone and Bones↗
PubMed2003

Analyzing trophic transfer of metals in stream food webs using nitrogen isotopes.

This study examines detrimental effects of acid mine drainage (AMD) on stream invertebrate communities and tests for a direct relationship between trophic position and accumulation of three metals (Fe, Cu, Zn) by stream invertebrates in situ. On two dates in each of seven stream sites, we measured food chain length, mean trophic level, taxa richness, and trophic position of stream macroinvertebrates comprising the food webs using stable nitrogen isotope ratios. Metals in tissue of representatives of 35 taxa were measured by ICP-OES. Our results are the first direct comparison of uptake of these metals in stream invertebrate taxa according to trophic position as identified by delta15N. As predicted, metal concentrations were generally greater in water and insects from sites adjacent to mining activity and invertebrate taxa richness was significantly lower. Taxa richness increased with distance away from contaminated headwaters. Despite reductions in diversity at sites nearest AMD, food chain length and mean trophic level did not differ between streams. The relationship between trophic position and metal accumulation differed considerably among metals. Specifically, Fe declined (biodilution) and Zn increased (biomagnification) with trophic level, but trophic position had no effect on Cu levels in these insects. Our results highlight fundamental differences in trophic transfer of specific metals through aquatic food webs and identify ecologically important impacts of AMD on stream invertebrates.

Animals↗
PubMed1993

Age differences in stable carbon and nitrogen isotope ratios in a population of prehistoric maize horticulturists.

Stable carbon isotope ratios in prehistoric human bone collagen have been used extensively to document the introduction and intensification of maize horticulture in notheastern North America. Most previous studies are based on small samples of adults who are assumed to characterize the diet of the population. In this study, all 29 individuals buried within an Ontario Iroquoian village site dated A.D. 1530-1580 were analysed for stable isotopes of carbon and nitrogen. Age distribution of the sample ranges from preterm to elderly. Significant negative correlations between age and delta 13C, and age and delta 15N values were found. High delta 13C values in infants and young children (delta 13C = -6.8 to -12.3) suggest a weaning diet high in maize. High delta 15N values in infants relative to adults suggest a trophic level effect during breast-feeding which has been reported in a modern sample by Tuross et al. (Am. J. Phys. Anthropol., 1993). In addition to the isotopic evidence for extremely high carbohydrate (maize) intake, the MacPherson sample includes two juveniles aged 3-4 years, exhibiting circular caries. No other cases of this condition are known in the extensively studied southern Ontario skeletal collections. Together the evidence from dentition and stable carbon isotopes indicates a very high carbohydrate diet in subadults. Circular caries result from developmental stress during enamel formation with subsequent caries formation in areas of thinner enamel. These findings are relevant to studies of infant and early childhood morbidity and mortality among prehistoric maize horticulturists.

Adult↗
PubMed1996

Stable carbon and nitrogen isotopic composition of diet and hair of Gidra-speaking Papuans.

The carbon and nitrogen stable isotopic composition of the scalp hair and diet of Gidra-speaking people in four villages in Papua New Guinea is presented. The isotopic composition of hair was measured, while that of the diet was estimated from food consumption survey data and the measured isotopic composition and protein and carbohydrate contents of food items. The average isotopic ratios of the hair samples and of the diet varied among the four study villages, which were selected because of their diverse ecological settings. Comparison was made between hair and calculated dietary isotopic compositions. Two of the four diet-hair enrichment values obtained for 13C (+1.8 and 2.2%) were similar to those previously reported (1.4-2.0%), but the other two values (3.7 and 4.8%) were greater than in earlier reports. 15N enrichment was systematically greater (by 1.0%) than reported values (4.3%) except for one village, where a much greater enrichment (6.9%) was found. The factors potentially relevant to these deviations are discussed. Possible errors in estimating the dietary isotopic composition and minor modifications of dietary habits revealed by food consumption surveys could explain most of the discrepancies. However, the great enrichment of 15N found in one of the villages remains unexplained.

Adult↗
PubMed2003

Nitrogen isotope ratios shift with plant size in tropical bromeliads.

We describe an ontogenetic shift in nitrogen (N) isotopic values in two rosette-forming epiphytic bromeliads. Leaf tissue N isotope values of small individuals of two bromeliad species (mean -6.2 per thousand ) differed from those of large individuals within each species (mean -0.5 per thousand ). Using references for potential N sources, we calculated the relative contribution of autochthonous (soil-derived through leaf litter) and allochthonous (atmospheric deposition) N with a two-member mixing model. Atmospheric sources contributed as much as 77-80% of the N in small individuals, whereas soil-derived N contributed 64-72% (conservative reference value) to 100% (less conservative reference value) of leaf tissue N in large plants. Shifts in N source with increasing plant size may be important aspects of rainforest complexity, an understudied aspect of ecosystem diversity.

Bromeliaceae↗
PubMed1999

Steady-state nitrogen isotope effects of N2 and N2O production in Paracoccus denitrificans.

Nitrogen stable-isotope compositions (delta15N) can help track denitrification and N2O production in the environment, as can knowledge of the isotopic discrimination, or isotope effect, inherent to denitrification. However, the isotope effects associated with denitrification as a function of dissolved-oxygen concentration and their influence on the isotopic composition of N2O are not known. We developed a simple steady-state reactor to allow the measurement of denitrification isotope effects in Paracoccus denitrificans. With [dO2] between 0 and 1.2 microM, the N stable-isotope effects of NO3- and N2O reduction were constant at 28.6 per thousand +/- 1.9 per thousand and 12.9 per thousand +/- 2.6 per thousand, respectively (mean +/- standard error, n = 5). This estimate of the isotope effect of N2O reduction is the first in an axenic denitrifying culture and places the delta15N of denitrification-produced N2O midway between those of the nitrogenous oxide substrates and the product N2 in steady-state systems. Application of both isotope effects to N2O cycling studies is discussed.

Bacteriological Techniques↗
PubMed1995

Nitrogen isotope abundances in the recent solar wind.

Although lunar crystalline rocks are essentially devoid of nitrogen, the same is not true of the lunar regolith. The nitrogen contents of individual regolith samples (which can be as high as 0.012% by mass) correlate strongly with abundances of noble gases known to be implanted in the lunar surface by solar radiation, indicating that lunar regolith nitrogen is also predominantly of solar origin. The large variability in 15N/14N ratios measured in different regolith samples may thus reflect long-term changes in the isotopic composition of the solar radiation. But attempts to explain these variations have been hampered by the lack of any firm constraint on 15N/14N in the present solar wind. Here we report measurements of nitrogen isotopes from two lunar samples that have had simple (and relatively recent) exposure histories. We find that nitrogen implanted in the lunar surface during the past 10(5) to 5 x 10(7) years has a 15N/14N ratio approximately 40% higher than that in the terrestrial atmosphere, which is substantially lower than most previous estimates. This isotopic signature probably represents the best measure of 15N/14N in the present-day solar wind.

Extraterrestrial Environment↗