14N Ratios Research Articles

Denitrification and nitrous oxide cycling within the upper oxycline of the eastern tropical South Pacific oxygen minimum zone

One of the shallowest, most intense oxygen minimum zones (OMZs) is found in the eastern tropical South Pacific, off northern Chile and southern Peru. It has a strong oxygen gradient (upper oxycline) and high N2O accumulation. N2O cycling by heterotrophic denitrification along the upper oxycline was studied by measuring N2O production and consumption rates using an improved acetylene blockage method. Dissolved N2O and its isotope (15N:14N ratio in N2O or δ15N) and isotopomer composition (intramolecular distribution of 15N in the N2O or δ15Nα and δ15Nβ), dissolved O2, nutrients, and other oceanographic variables were also measured. Strong N2O accumulation (up to 86 nmol L−1) was observed in the upper oxycline followed by a decline (around 8-12 nmol L−1) toward the OMZ core. N2O production rates by denitrification (NO2− reduction to N2O) were 2.25 to 50.0 nmol L−1 d−1, whereas N2O consumption rates (N2O reduction to N2) were 2.73 and 70.8 nmol L−1 d−1. δ15N in N2O increased from 8.57% in the middle oxycline (50-m depth) to 14.87% toward the OMZ core (100-m depth), indicating the progressive use of N2O as an electron acceptor by denitrifying organisms. Isotopomer signals of N2O (δ15Nα and δ15Nβ) showed an abrupt change at the middle oxycline, indicating different mechanisms of N2O production and consumption in this layer. Thus, partial denitrification along with aerobic ammonium oxidation appears to be responsible for N2O accumulation in the upper oxycline, where O2 levels fluctuate widely; N2O reduction, on the other hand, is an important pathway for N2 production. As a result, the proportion of N2O consumption relative to its production increased as O2 decreased toward the OMZ core. A N2O mass balance in the subsurface layer indicates that only a small amount of the gas could be effluxed into the atmosphere (12.7-30.7 µmol m−2 d−1) and that most N2O is used as an electron acceptor during denitrification (107-468 µmol m−2 d−1).

The flux and isotopic composition of reduced and total nitrogen in Bermuda rain

The concentration and 15N/ 14N ratio of total nitrogen (TN) were measured in precipitation samples collected at Bermuda between January and December 2000. By correcting for nitrate, analyzed previously, the concentration and δ 15N of “reduced” N (RN, i.e., ammonium + organic N) were also determined. The TN precipitation flux (∼ 10–19 mmol N m − 2 yr − 1 ) is twice the NO 3 − precipitation flux, and the mass-weighted annual average δ 15N of TN, − 2.3‰, is higher than the δ 15N of NO 3 − in the same samples (− 4.5‰), indicating that RN has an annual average δ 15N of − 0.6‰. While neither the concentration nor the flux of RN (6.8 µM and 5.2 mmol N m − 2 yr − 1 , respectively) in precipitation shows statistically significant seasonal variation, the δ 15N of RN varies significantly from − 2.7‰ in the cool season to 1.5‰ in the warm season. This seasonality in the δ 15N of RN is similar to that of NO 3 −, implying that RN and NO 3 − in precipitation may have related sources or, more speculatively, mechanisms of inter-conversion. Additionally, the seasonality of the RN δ 15N at Bermuda is similar to that of typical ammonium concentrations in precipitation at Bermuda, both showing maxima in the spring and late summer, raising the possibility that the maxima in the RN δ 15N derives from ammonium at those times. Finally, the low δ 15N of the TN flux will cause it to have an effect on the δ 15N of Sargasso Sea thermocline NO 3 − that is in the same sense as the effect of N 2 fixation, with slightly greater isotopic leverage. If the atmospheric TN flux is not marine-derived, it could explain a substantial fraction of the previously documented upward decrease in NO 3 − δ 15N from deep water into the thermocline of the Sargasso Sea, for example, ∼ 20 to 35‰ of it, assuming a N2 fixation rate of 45 mmol N m-2 yr-1 as estimated by Hansell et al. [Hansell, D.A., Bates, N.R., and Olson, D.B., 2004. Excess nitrate and nitrogen fixation in the North Atlantic Ocean. Mar. Chem., 84:243–265.].

Earthworms as drivers of the competition between grasses and legumes

Grasses and legumes are grown together worldwide to improve total herbage yield and the quality of forage, however, the causes of population oscillations of grasses and legumes are poorly understood. Especially in grasslands, earthworms are among the most important detritivore animals functioning as ecosystem engineers, playing a key role in nutrient cycling and affecting plant nutrition and growth. The objectives of the present greenhouse experiment were to quantify the effects of earthworms on grass–legume competition in model grassland systems at two harvesting dates – simulating the widespread biannual mowing regime in Central European grasslands. The presence of earthworms increased the productivity of grasses and legumes after 6 weeks but only that of grasses after another 10 weeks. In mixed treatments, the presence of grasses and earthworms decreased legume shoot biomass, the amount of nitrogen (N) in shoot tissue and the number of legume flowerheads while the presence of legumes and earthworms increased the amount of N in grass shoots and the infestation of grasses with aphids. Analyses of 15N/ 14N ratios indicate that, compared to legumes, grasses more efficiently exploit soil mineral N and benefit from legume presence through reduced “intra-functional group” competition. In contrast to previous experiments, we found no evidence for N transfer from legumes to grasses. However, legume presence improved total herbage and N yield. Earthworms likely modulate the competition between grasses and legumes by increasing soil N uptake by plants and thereby increasing the competitive strength of grasses. Earthworms function as essential driving agents of grass–legume associations by (I) increasing grass yield, (II) increasing the amount of N in grass hay, (III) increasing the infestation rate of grasses with aphids, and (IV) potentially reducing the attractiveness of grass–legume associations to pollinators.

Effects of N-application on utilization of 15N and 13C and quality in two wheat cultivars

We studied N uptake and distribution in wheat, and the incorporation of nitrogen and carbon into gluten and non-gluten proteins using a double-labelling approach with 15N and 13C. Doses of N-fertilizer were split and applied at emergence, onset of stem elongation, and heading at rates of 280/140/140 mg N pot−1, respectively simulating 90/45/45 kg N ha−1. Five different combinations of N-fertilizations containing no or 10 % 15N were performed. The recovery of 15N added at the stages emergence, stem elongation or heading were 42, 60, and 64 %. Application of 15N at all three stages yielded in 51 % recovery. Remobilisation of straw N was greater for Golia. The 15N concentration in gluten proteins of Golia show higher values than Gönen. The ratio of 15N gluten/15N non-gluten proteins of Golia were higher, which implies a lower non-gluten protein activity during grain filling. The 13C concentration in gluten and non-gluten proteins did not differ between both cultivars.

Natural Abundance of 15N in Nitrogen-Limited Forests and Tundra Can Estimate Nitrogen Cycling Through Mycorrhizal Fungi: A Review

The hyphae of ectomycorrhizal and ericoid mycorrhizal fungi proliferate in nitrogen (N)-limited forests and tundra where the availability of inorganic N is low; under these conditions the most common fungal species are those capable of protein degradation that can supply their host plants with organic N. Although it is widely understood that these symbiotic fungi supply N to their host plants, the transfer is difficult to quantify in the field. A novel approach uses the natural 15N:14N ratios (expressed as δ15N values) in plants, soils, and mycorrhizal fungi to estimate the fraction of N in symbiotic trees and shrubs that enters through mycorrhizal fungi. This calculation is possible because mycorrhizal fungi discriminate against 15N when they create compounds for transfer to plants; host plants are depleted in 15N, whereas mycorrhizal fungi are enriched in 15N. The amount of carbon (C) supplied to these fungi can be stoichiometrically calculated from the fraction of plant N derived from the symbiosis, the N demand of the plants, the fungal C:N ratio, and the fraction of N retained in the fungi. Up to a third of C allocated belowground, or 20% of net primary production, is used to support ectomycorrhizal fungi. As anthropogenic N inputs increase, the C allocation to fungi decreases and plant δ15N increases. Careful analyses of δ15N patterns in systems dominated by ectomycorrhizal and ericoid mycorrhizal symbioses may reveal the ecosystem-scale effects of alterations in the plant–mycorrhizal symbioses caused by shifts in climate and N deposition.

Sub-recent nitrogen-isotope trends in sediments from Skagerrak (North Sea) and Kattegat: Changes in N-budgets and N-sources?

We determined 15N/ 14N ratios of total nitrogen in surface sediments and dated sediment cores to reconstruct the history of N-loading of the North Sea. The isotopic N composition in modern surface sediments is equivalent to and reflects the isotopic mixture of oceanic nitrate on the one hand ( δ 15N = 5‰) and the imprint of river-borne nitrogen input into the SE North Sea ( δ 15N up to 12‰ in estuaries of the SE North Sea) on the other hand. We compare the results with δ 15N records from pre-industrial sediment intervals in cores from the Skagerrak and Kattegat areas, which both constitute significant depositional centres for N in the North Sea and the Baltic Sea/North Sea transition. As expected, isotopically enriched anthropogenic nitrogen was found in the two records from the Kattegat area, which is close to eutrophication sources on land. Enrichment of δ 15N in cores from the Skagerrak – the largest sediment sink for nitrogen in the entire North Sea – was not significant and values were similar to those found in sediment layers representing pre-industrial conditions. We interpret this isotopic uniformity as an indication that most riverine reactive nitrogen with its characteristic isotopic signature is removed by denitrification in shallow shallow-water sediments before reaching the main sedimentary basin of the North Sea.

Comparison of C and N stable isotope ratios between surface particulate organic matter and microphytoplankton in the Gulf of Lions (NW Mediterranean)

Carbon and nitrogen stable isotope ratios of particulate organic matter (POM) in surface water and 63–200 μm-sized microphytoplankton collected at the fluorescence maximum were studied in four sites in the Gulf of Lions (NW Mediterranean), a marine area influenced by the Rhone River inputs, in May and November 2004. Some environmental (temperature, salinity) and biological (POM, Chlorophyll a and phaeopigments contents, phytoplankton biomass and composition) parameters were also analysed. Significantly different C and N isotopic signatures between surface water POM and microphytoplankton were recorded in all sites and seasons. Surface water POM presented systematically lower δ 13C (∼4.2‰) and higher δ 15N (∼2.8‰) values than those of microphytoplankton, due to a higher content of continental and detrital material. Seasonal variations were observed for all environmental and biological parameters, except salinity. Water temperature was lower in May than in November, the fluorescence maximum was located deeper and the Chlorophyll a content and the phytoplankton biomass were higher, along with low PON/Chl a ratio, corresponding to spring bloom conditions. At all sites and seasons, diatoms dominated the phytoplankton community in abundance, whereas dinoflagellate importance increased in autumn particularly in coastal sites. C and N isotopic signatures of phytoplankton did not vary with season. However, the δ 15N of surface water POM was significantly higher in November than in May in all sites likely in relation to an increase in 15N/ 14N ratio of the Rhone River POM which influenced surface water in the Gulf of Lions. As it is important to determine true baseline values of primary producers for analysing marine food webs, this study demonstrated that C and N isotopic values of surface water POM cannot be used as phytoplankton proxy in coastal areas submitted to high river inputs.

Interpretation of the carbon abundance in Saturn measured by Cassini

Spectral observations of Saturn from the far infrared spectrometer aboard the Cassini spacecraft [Flasar, F.M., et al., 2005. Temperatures, winds, and composition in the Saturnian system. Science 307, 1247–1251] have revealed that the C/H ratio in the planet is in fact about twice higher than previously derived from ground based observations and in agreement with the C/H value derived from Voyager IRIS by Courtin et al. [1984. The composition of Saturn's atmosphere at northern temperate latitudes from Voyager IRIS spectra - NH 3, PH 3, C 2H 2, C 2H 6, CH 3D, CH 4, and the Saturnian D/H isotopic ratio. Astrophys. J. 287, 899–916] . The implications of this measurement are reanalyzed in the present report on the basis that volatiles observed in cometary atmospheres, namely CO 2 , CH 4 , NH 3 and H 2 S may have been trapped as solids in the feeding zone of the planet. CH 4 and H 2 S may have been in the form of clathrate hydrates while CO 2 presumably condensed in the cooling solar nebula. Carbon may also have been incorporated in organics. Conditions of temperature and pressure ease the hydratation of NH 3 . Such icy grains were included in planetesimals which subsequently collapsed into the hydrogen envelope of the planet, then resulting in C, N and S enrichments with respect to the solar abundance. Our calculations are consistent, within error bars, with observed elemental abundances on Saturn provided that the carbon trapped in planetesimals was mainly in the form of CH 4 clathrate and CO 2 ice (and maybe as organics) while nitrogen was in the form of NH 3 hydrate. Our approach has implications on the possible pattern of noble gases in Saturn, since we predict that contrary to what is observed in Jupiter, Ar and Kr should be in solar abundance while Xe might be strongly oversolar. The only way to verify this scenario is to send a probe making in situ mass spectrometer measurements. Our scenario also predicts that the 14N/ 15N ratio should be somewhat smaller in Saturn than measured in Jupiter by Galileo.

Stoichiometry of the Peripheral Stalk Subunits E and G of Yeast V1-ATPase Determined by Mass Spectrometry

The stoichiometry of yeast V(1)-ATPase peripheral stalk subunits E and G was determined by two independent approaches using mass spectrometry (MS). First, the subunit ratio was inferred from measuring the molecular mass of the intact V(1)-ATPase complex and each of the individual protein components, using native electrospray ionization-MS. The major observed intact complex had a mass of 593,600 Da, with minor components displaying masses of 553,550 and 428,300 Da, respectively. Second, defined amounts of V(1)-ATPase purified from yeast grown on (14)N-containing medium were titrated with defined amounts of (15)N-labeled E and G subunits as internal standards. Following protease digestion of subunit bands, (14)N- and (15)N-containing peptide pairs were used for quantification of subunit stoichiometry using matrix-assisted laser desorption/ionization-time of flight MS. Results from both approaches are in excellent agreement and reveal that the subunit composition of yeast V(1)-ATPase is A(3)B(3)DE(3)FG(3)H.

Parasite-Induced Changes in Nitrogen Isotope Signatures of Host Tissues

To estimate isotopic changes caused by trematode parasites within a host, we investigated changes in the carbon and nitrogen isotope ratios of the freshwater snail Lymnaea stagnalis infected by trematode larvae. We measured carbon and nitrogen stable isotopes within the foot, gonad, and hepatopancreas of both infected and uninfected snails. There was no significant difference in the delta13C and delta15N values of foot and gonad between infected and uninfected snails; thus, trematode parasite infections may not cause changes in snail diets. However, in the hepatopancreas, delta15N values were significantly higher in infected than in uninfected snails. The 15N enrichment in the hepatopancreas of infected snails is caused by the higher 15N ratio in parasite tissues. Using an isotope-mixing model, we roughly estimated that the parasites in the hepatopancreas represented from 0.8 to 3.4% of the total snail biomass, including the shell.

Natural bioremediation of a nitrate-contaminated soil-and-aquifer system in a desert environment

Mining activities are a major source of land degradation in arid regions, and remediation methods developed for mesic sites may not be appropriate for arid sites. In climates where potential evapotranspiration exceeds precipitation, it might be possible to prevent the migration of contaminants away from a mine site by controlling the site water balance through vegetation, and allowing natural attenuation processes to reduce pollutant levels over time. We investigated the feasibility of remediating a nitrate-contaminated source-plume system in a desert environment using biological methods. The study site was a former uranium mill in Monument Valley, Arizona, where NO 3 − used in ore processing had leaked from the soil beneath a tailings pile (referred to as the source area) into an alluvial aquifer (referred to as the plume) spreading away from the source area. We used 15N/ 14N ratios and direct assays of denitrification to show that biological denitrification occurred in both the source area and plume, and could reduce NO 3 − levels over time. Denitrification in the source area could be stimulated by providing additional moisture to the soil through irrigation, whereas providing a carbon source (ethanol) stimulated denitrification in samples from the plume. We used 18O/ 16O and H/deuterium signatures in water and plant samples to show that the native saltbush ( Atriplex canescens) and black greasewood ( Sarcobatus vermiculatus) plant community was extracting water from the plume, offering a possible means of preventing migration of the plume by controlling grazing over the site. We conclude that biological remediation is a feasible alternative to pump/treat solutions at this type of site.

The distribution of nitrate 15N/ 14N in marine sediments and the impact of benthic nitrogen loss on the isotopic composition of oceanic nitrate

We report 15N/ 14N ratios of porewater nitrate in sediments from the Bering Sea basin, where microbial nitrate reduction has been identified as a significant sink for fixed nitrogen (N). Strong 15N enrichment in porewater nitrate is observed as one goes deeper in the sediments and nitrate concentration [ NO 3 - ] decreases ( δ 15N generally reaches 25–35‰). Analysis of profiles with a one-dimensional diffusion-reaction model yields organism-scale isotope effects for dissimilatory nitrate reduction ( ε cell) of 11‰ to 30‰, in the same range as measured in previous studies of cultures and the marine and lacustrine water column. Estimates of ε cell, while uncertain, show a negative correlation with bottom water [O 2]; we propose that this relates to the [ NO 3 - ] at the depth of denitrification. The N isotope effect at the scale of nitrate sediment–water exchange ( ε app) is ∼0‰ in two unreactive deep sites and is typically <3‰ at more reactive sites at various depths. ε app is much lower than ε cell because nitrate consumption is nearly complete at the sediment depth of denitrification, minimizing the escape of 15N-enriched nitrate from the sediments. In reactive sediments, this is due to rapid denitrification, while in less reactive sediments, it is due to greater diffusive distances for nitrate to the depth of denitrification. The data suggest that low bottom water [O 2] tends to yield more complete expression of ε cell at the sediment–water scale, due to higher [ NO 3 - ] at the depth of denitrification. While porewater ammonium-N isotopes were not measured, our porewater model suggests that, in sediments with high organic matter supply and/or low-[O 2] bottom waters, the efflux and subsequent oxidation of ammonium enriched in 15N by incomplete nitrification can significantly enhance the total net isotope effect of sedimentary N loss ( ε sed, equivalent to ε app but including ammonium fluxes). Model analysis of representative sedimentary environments suggests a global mean ε sed of ∼4‰ (∼2‰ if restricted to seafloor below 1 km depth).

Microbially available carbon in buried riparian soils in a glaciated landscape

Buried horizons and lenses in riparian soil profiles harbor large amounts of carbon relative to the surrounding soil horizons. Because these buried soil horizons, as well as deep surface horizons, frequently lie beneath the water table, their impact on nitrogen transport across the terrestrial–aquatic interface depends upon their frequency and spatial distribution, and upon the lability of associated organic matter. We collected samples of 51 soil horizons from 14 riparian zones Rhode Island, USA, where soil profiles are characterized by glacial outwash and alluvial deposits. These soil samples came from as deep as 2 m and ranged in carbon content from <1% to 44% in a buried O horizon 54–74 cm deep. We used these samples to: (1) determine the extent to which carbon in buried horizons, and deep surface horizons, is potentially microbially available; (2) identify spatial patterns of carbon mineralization associated with surface and buried horizons; and (3) evaluate likely relationships between soil horizon types, chemical characteristics and carbon mineralization. Carbon mineralization rates associated with buried horizons during anaerobic incubations ranged from 0.0001 to 0.0175 μmol C kg soil −1 s −1 and correlated positively with microbial biomass ( R=0.89, P<0.0001, n=21). Excluding surface O horizons from the analysis, carbon mineralization varied systematically with horizon type (surface A, buried A, buried O, lenses, A/C, B, C) ( P<0.05) but not with depth or depth x horizon interaction (overall R 2=0.59, P<0.0005, n=47). In contrast to this result and to most published data sets, 13C-to- 12C and 15N-to- 14N ratios of organic matter declined with depth ( 13C−26.9 to −29.3 per mil, 15N+5.6 to −0.8 per mil). The absence of a relationship between horizon depth and C availability suggests that carbon availability in these buried horizons may be determined by the abundance and quality of organic matter at the time of horizon formation or burial, rather than by duration since burial, and implies that subsurface microbial activity is largely disconnected from surface ecosystems. Our results contribute to the emerging view that buried horizons harbor microbially available C in quantities relevant to ecosystem processes, and suggest that buried C-rich soil horizons need to be incorporated into assessments of the depth of the biologically active zone in near-stream subsurface soils.

Effects of elevated atmospheric carbon dioxide on amino acid and NH4+‐N cycling in a temperate pine ecosystem

AbstractRising atmospheric carbon dioxide (CO2) is expected to increase forest productivity, resulting in greater carbon (C) storage in forest ecosystems. Because elevated atmospheric CO2 does not increase nitrogen (N) use efficiency in many forest tree species, additional N inputs will be required to sustain increased net primary productivity (NPP) under elevated atmospheric CO2. We investigated the importance of free amino acids (AAs) as a source for forest N uptake at the Duke Forest Free Air CO2 Enrichment (FACE) site, comparing its importance with that of better‐studied inorganic N sources. Potential proteolytic enzyme activity was monitored seasonally, and individual AA concentrations were measured in organic horizon extracts. Potential free AA production in soils ranged from 190 to 690 nmol N g−1 h−1 and was greater than potential rates of soil NH4+ production. Because of this high potential rate of organic N production, we determined (1) whether intact AA uptake occurs by Pinus taeda L., the dominant tree species at the FACE site, (2) if the rate of cycling of AAs is comparable with that of ammonium (NH4+), and (3) if atmospheric CO2 concentration alters the aforementioned N cycling processes. A field experiment using universally labeled ammonium (15NH4+) and alanine (13C3H715NO2) demonstrated that 15N is more readily taken up by plants and heterotrophic microorganisms as NH4+. Pine roots and microbes take up on average 2.4 and two times as much NH4+ 15N compared with alanine 15N 1 week after tracer application. N cycling through soil pools was similar for alanine and NH4+, with the greatest 15N tracer recovery in soil organic matter, followed by microbial biomass, dissolved organic N, extractable NH4+, and fine roots. Stoichiometric analyses of 13C and 15N uptake demonstrated that both plants and soil microorganisms take up alanine directly, with a 13C : 15N ratio of 3.3 : 1 in fine roots and 1.5 : 1 in microbial biomass. Our results suggest that intact AA (alanine) uptake contributes substantially to plant N uptake in loblolly pine forests. However, we found no evidence supporting increased recovery of free AAs in fine roots under elevated CO2, suggesting plants will need to acquire additional N via other mechanisms, such as increased root exploration or increased N use efficiency.

The Titan 14N/ 15N and 12C/ 13C isotopic ratios in HCN from Cassini/CIRS

We report the detection of H 13CN and HC 15N in mid-infrared spectra recorded by the Composite Infrared Spectrometer (CIRS) aboard Cassini, along with the determination of the 12C/ 13C and 14N/ 15N isotopic ratios. We analyzed two sets of limb spectra recorded near 13–15° S (Tb flyby) and 83° N (T4 flyby) at 0.5 cm −1 resolution. The spectral range 1210–1310 cm −1 was used to retrieve the temperature profile in the range 145–490 km at 13° S and 165–300 km at 83° N. These two temperature profiles were then incorporated in the atmospheric model to retrieve the abundance profile of H 12C 14N, H 13CN and HC 15N from their bands at 713, 706 and 711 cm −1, respectively. The HCN abundance profile was retrieved in the range 90–460 km at 15° S and 165–305 km at 83° N. There is no evidence for vertical variations of the isotopic ratios. Constraining the isotopic abundance profiles to be proportional to the HCN one, we find C 12 / C 13 = 89 −18 +22 at 15° S, and 68 −12 +16 at 83° N, two values that are statistically consistent. A combination of these results yields a 12C/ 13C value equal to 75 ± 12 . This global result, as well as the 15° S one, envelop the value in Titan's methane ( 82.3 ± 1 ) [Niemann, H.B., and 17 colleagues, 2005. Nature 438, 779–784] measured at 10° S and is slightly lower than the terrestrial inorganic standard value (89). The 14N/ 15N isotopic ratio is found equal to 56 −13 +16 at 15° S and 56 −9 +10 at 83° N. Combining the two values yields 14N/ 15N = 56 ± 8, which corresponds to an enrichment in 15N of about 4.9 compared with the terrestrial ratio. These results agree with the values obtained from previous ground-based millimeter observations [Hidayat, T., Marten, A., Bézard, B., Gautier, D., Owen, T., Matthews, H.E., Paubert, G., 1997. Icarus 126, 170–182; Marten, A., Hidayat, T., Biraud, Y., Moreno, R., 2002. Icarus 158, 532–544]. The 15N/ 14N ratio found in HCN is ∼3 times higher than in N 2 [Niemann, H.B., and 17 colleagues, 2005. Nature 438, 779–784], which implies a large fractionation process in the HCN photochemistry.

Nitrate production beneath a High Arctic glacier, Svalbard

Natural environmental isotopes of nitrate and ammonium are used in conjunction with major ion chemistry and hydrological data to establish controls upon the biogeochemical cycling of nitrogen beneath a High Arctic polythermal glacier (Midtre Lovénbreen). Here, high nitrate concentrations in subglacial meltwaters suggest that the subglacial environment may be furnishing nitrate in excess of that released from the snowpack and glacier ice. Isotopic values of δ 18O NO 3 suggest the provenance of such excess nitrate to be microbial in origin and δ 15N NO 3 indicates the source nitrogen compounds to have high δ 15N values relative to supraglacial runoff. We address the nitrification of supraglacial ammonium, the mineralization of organic nitrogen and the oxidation of geologic ammonium as potential sources of this additional nitrate. Mass fluxes of N compounds in a subglacial river and their δ 15N ratios indicate that the nitrification of supraglacial ammonium delivered to the glacier bed can account for much, but not all, of the excess nitrate. The additional source most likely involves the mineralization of organic nitrogen, although δ 15N values in rock samples suggest that the dissolution of rock-derived ammonium cannot be discounted if large fractionation effects occur during dissolution. Our results therefore agree with previous catchment scale mass balance studies at the site, which report a major internal loss of NH 4 + from the snowpack following melt. However, at the catchment scale, the NH 4 + loss is greater than the excess of NO 3 − observed in runoff, indicating that microbial assimilation of ammonia into organic matter in a range of other habitats is also likely. The identification of NH 4 +assimilation and nitrification further highlights the non-conservative behaviour of nitrogen in glacial environments and testifies to the importance of microbially-mediated reactions in the biogeochemical cycling of nitrogen in an environment that has, until recently, been regarded as biologically inert.

Nitrogen and Carbon Isotopic Composition of the Sun Inferred from a High-Temperature Solar Nebular Condensate

We report high-precision measurements of nitrogen and carbon isotopic compositions of a carbon-bearing titanium-nitride (osbornite) in a calcium-aluminum-rich inclusion (CAI) from the CH/CB-like carbonaceous chondrite Isheyevo. The mineralogy and petrography of the CAI and thermodynamic calculations indicate that the osbornite formed by gas-solid condensation in a high-temperature (~2000 K) region of the solar nebula. Because isotopic fractionation at high temperature is small, the measured nitrogen [15N/14N = (2.356 ± 0.018) × 10 -3] and carbon [13C/12C = 0.01125 ± 0.00008; 1 σ] isotopic compositions of the Isheyevo osbornite are representative of the solar nebula and, hence, of the Sun. This conclusion is supported by the observations that (1) the measured 13C/ 12C ratio is indistinguishable from the spectroscopic determination of the 13C/12C ratio of the solar photosphere and (2) the measured 15N/14N ratio of osbornite is in excellent agreement with the Galileo spacecraft measurement of the nitrogen isotopic composition of the Jovian atmosphere, the second largest reservoir of nitrogen in the solar system. The inferred 15N/ 14N ratio of the solar nebula is also similar to the nitrogen isotopic composition of the vast majority of chondritic nanodiamonds, suggesting their solar nebula origin.

Translocation and conservation of organic nitrogen within the coral-zooxanthella symbiotic system of Acropora pulchra, as demonstrated by dual isotope-labeling techniques

Carbon (C) and nitrogen (N) metabolism of the hermatypic coral Acropora pulchra and its symbiotic algae (zooxanthellae) was investigated using 13C and 15N isotope tracers. A. pulchra was incubated in seawater containing 13C-labeled bicarbonate and 15N-labeled nitrate (NO 3 −) for 24 h (pulse period), and subsequently 13C and 15N isotopic ratios of the host coral and the zooxanthellae were followed in 13C- and 15N-free seawater for 2 weeks (chase period). Under our experimental condition of NO 3 − (12 μM), C and N were absorbed by the coral–algal symbiotic system with the C:N ratio of 23 during the pulse period. Taking account of concentration dependence of NO 3 − uptake rates determined by a separate experiment, C:N uptake ratios under supposed in situ NO 3 − conditions (< 1.0 μM) would be > 3.0 times higher, if the photosynthetic rate did not change. During the pulse period, more than half of the absorbed 13C and 15N appeared in the host fraction in organic forms. 13C: 15N ratio at the end of the pulse period was similar between the host and the algal fraction, suggesting that algal photosynthetic products were translocated to the host. It is also implied that C:N ratios of the translocated products change depending on N availability for the zooxanthellae. During the chase period, atom % excess (APE) 15N of the zooxanthellae constantly declined, while that of the host slightly increased. Consequently, APE 15N of the both fractions appeared to approach a common steady state value, suggesting that 15N was recycled within the coral–algal symbiotic system. As for C, > 86% of C photosynthetically fixed by the zooxanthellae accumulated in the host at the end of the pulse period, and had a turnover time of ca. 20 days for the host C pool during the following chase period. C:N ratios of organic matter newly synthesized with NO 3 − exponentially declined and converged into 5.7 and 4.5 for the host and the zooxanthellae, respectively. This suggests that organic compounds of high C:N ratios such as lipids and carbohydrates were selectively consumed more rapidly than those of low C:N ratios such as proteins and nucleic acids.

Evidence for molecular size dependent gas fractionation in firn air derived from noble gases, oxygen, and nitrogen measurements

We present elemental and isotopic measurements of noble gases (He, Ne, Ar, Kr, and Xe), oxygen and nitrogen of firn air from two sites. The first set of samples was taken in 1998 at the summit of the Devon Ice Cap in the eastern part of Devon Island. The second set was taken in 2001 at NGRIP location (North Greenland). He and Ne are heavily enriched relative to Ar with respect to the atmosphere in the air near the close-off depth at around 50–70 m. The enrichment increases with depth and reaches the maximum value in the deepest samples just above the zone of impermeable ice where no free air could be extracted anymore. Similarly, elemental ratios of O 2 / N 2, O 2 / Ar and Ar / N 2 are increasing with depth. In contrast but in line with expectations, isotopic ratios of 15N / 14N, 18O / 16O, and 36Ar / 40Ar show no significant enrichment near the close-off depth. The observed isotopic ratios in the firn air column can be explained within the uncertainty ranges by the well-known processes of gravitational enrichment and thermal diffusion. To explain the elemental ratios, however, an additional fractionation process during bubble inclusion has to be considered. We implemented this additional process into our firn air model. The fractionation factors were found by fitting model profiles to the data. We found a very similar close-off fractionation behavior for the different molecules at both sites. For smaller gas species (mainly He and Ne) the fractionation factors are linearly correlated to the molecule size, whereas for diameters greater than about 3.6 Å the fractionation seems to be significantly smaller or even negligible. An explanation for this size dependent fractionation process could be gas diffusion through the ice lattice. At Devon Island the enrichment at the bottom of the firn air column is about four times higher compared to NGRIP. We explain this by lower firn diffusivity at Devon Island, most probably due to melt layers, resulting in significantly reduced back diffusion of the excess gas near the close-off depth. The results of this study considerably increase the understanding of the processes occurring during air bubble inclusion near the close-off depth in firn and can help to improve the interpretation of direct firn air measurements, as well as air bubble measurements in ice cores, which are used in numerous studies as paleo proxies.

Spatial and temporal variations in the 13C / 12C and 15N / 14N ratios of mammoth hairs: Palaeodiet and palaeoclimatic implications

Four sets of C and N stable isotope analyses carried out on mammoth hairs of different radiocarbon age from four different sites in northern Siberia are reported here. Another set of analyses from the same region, published elsewhere, is reported for comparison. The analyses were performed on small increments along the length of the hairs providing information about short-(seasonal) and long-term (before and after the Last Glacial maximum) diet variation and, hence, about the environment in which the mammoths lived. The mean δ13C and δ15N values for the four sites range between −24.1‰ and −20.9‰ and between 5.8‰ and 9.6‰, respectively. Both the δ13C and δ15N signals increment towards east when samples from the same age are considered. Hair δ15N correlates well with the current amount of precipitation independently of the radiocarbon age of the sample. This could mean that during the periods considered this environmental parameter did not change significantly in the studied sites. Hair δ13C appears to be affected by a different environmental factor that, in this case, varies in function of the radiocarbon age of the samples. This factor could be the CO2 atmospheric content that, as is well know, varies across a glacial cycle depending on temperature. The isotope patterns obtained from each set of samples exhibit roughly sinusoidal oscillations of the δ13C and δ15N values. These oscillations have been attributed to seasonal changes in the isotope values of the mammoth's diet due to plant adaptation to the different seasonal environmental conditions or to the different seasonal plant type availability. A direct physiological effect on the hair δ15N values can also be considered. This study strongly supports the possibility of using hair δ13C and δ15N signals as indicators of geographical and temporal variation in plant isotope composition and, ultimately, as indicators of palaeoclimate changes.