15N Enrichment Research Articles

Trophic structure of fish communities in mangrove systems subject to different levels of anthropogenic intervention, Tropical Eastern Pacific, Colombia.

Mangroves are important ecosystems of tropical and subtropical shorelines. Anthropogenic activity decreases their habitat quality, affecting structural and functional trophic features. We hypothesized that higher levels of anthropogenic intervention generate diversity loss and modify the trophic structure of tropical mangrove fish communities. We compared the taxonomic and isotopic (δ13C/δ15N) composition, abundance, trophic position, and isotopic niche of fish communities from three mangrove systems with different anthropogenic intervention levels in the Colombian Pacific. Non-parametric statistical tests and a Bayesian approach were used to analyze data. A total of 1254 specimens belonging to 23 families, 25 genera, and 30 species were identified, presenting higher abundance (821) in moderate anthropogenic intervention level mangrove (Moderate-AIL), with high dominance of one species (Lilestolifera). The low anthropogenic intervention level mangrove (Low-AIL) was the second in abundance (291) but exhibited a greater number of species (23), while the high anthropogenic intervention level mangrove (High-AIL) presented the least abundance (142) and species number (17). The isotopic composition ​​reveals that Moderate and High-AIL mangroves presented enriched 13C and 15N compared to Low-AIL (~ 2 to 4 ‰). Mean trophic position (TP) of communities was slightly higher in the more intervened systems (~ 1 to 2 orders of magnitude), as well as in specific species (Centropomus spp.). Isotopic niche width (TA and SEAc) was greater in High-AIL (41.1 and 9.2), more than doubled compared to Moderate-AIL (33.0 and 4.1). In High-AIL isotopic niche width increased, indicating lesser availability of prey and basal resources. The results obtained in this study support the proposed hypothesis and, suggest that anthropogenic intervention modifies diversity and food webs dynamics, affecting the transfer of matter and energy from macrotidal tropical mangroves to coastal ecosystems. However, it is recommended to be careful concluding differences based exclusively on the anthropogenic intervention level, since it is widely documented that mangrove settings also influence the analyzed trophic parameters.

Fire increases soil nitrogen retention and alters nitrogen uptake patterns among dominant shrub species in an Arctic dry heath tundra

Climate change increases the frequency and severity of fire in the Arctic tundra regions. We assessed effects of fire in combination with summer warming on soil biogeochemical N- and P cycles with a focus on mineral N over two years following an experimental fire in a dry heath tundra, West Greenland. We applied stable isotopes (15NH4+-N and 15NO3−-N) to trace the post-fire mineral N pools. The partitioning of 15N in the bulk soils, soil dissolved organic N (TDN), microbes and plants (roots and leaves) was established. The fire tended to increase microbial P pools by four-fold at both one and two years after the fire. Two years after the fire, the bulk soil 15N recovery has decreased to 10.4% in unburned plots while relatively high recovery was maintained (30%) in burned plots, suggesting an increase in soil N retention after the fire. The contribution of microbial 15N recovery to bulk soil 15N recovery increased from 11.2% at 21 days to 31.5% two years after the fire, suggesting that higher post-fire N retention was due largely to the increased incorporation of N into microbial biomass. Fire also increased 15N recovery in bulk roots after one and two years, but only under summer warming. This suggests that higher retention of post-fire N can strongly increase the potential for N uptake of recovering plants under a future warmer climate. There was significantly lower 15N enrichment of Betula nana leaves while higher 15N enrichment of Vaccinium uliginosum leaves (after three years) in burned than control plots. This shows that fire can alter the N uptake differently among dominant shrub species in this tundra ecosystem, and implies that wildfires may change plant species composition in the longer term.

N2 in deep subsurface fracture fluids of the Canadian Shield: Source and possible recycling processes

In addition to high concentrations of CH4 and H2, abundant dissolved N2 is found in subsurface fracture fluids in Precambrian cratons around the world. These fracture fluids have hydrogeological isolation times on order of thousands to millions and even billions of years. Assessing the sources and sinks of N2 and related (bio)geochemical processes that drive the nitrogen cycle in these long isolated systems can shed insights into the nitrogen cycle on early Earth with implications for other planets and moons. In this study, we collected dissolved gas samples from deep subsurface fracture fluids at seven sites (Kidd Creek, LaRonde, Nickel Rim, Fraser, Copper Cliff South, Thompson, and Birchtree) in the Canadian Shield. Multiple gas components (e.g., H2, O2 and Ar) were integrated with δ15NN2 values to characterize the N2 signatures. Results show that the dissolved N2 in deep subsurface fracture fluids from the Canadian Shield sites are more 15N-enriched than those from the Fennoscandian Shield and the Witwatersrand Basin in the Kaapvaal Craton. The nitrogen isotopic signatures of the Canadian Shield samples coupled with their hydrogeological framework indicate the N2 was sourced from fixed ammonium in silicate minerals in host rocks and was generated by metamorphic devolatilization. Modeling of nitrogen devolatilization from host rocks supports this interpretation, but also suggests that a second process, likely abiotic N2 reduction, is required to account for the observed 15N enrichment in the N2 samples from the Canadian Shield. A 10-year monitoring study for one of the boreholes, at 2.4 km of the Kidd Creek Observatory, shows a steady decrease in δ15NN2 values with time, which coincides with the temporal isotopic evolution of some other gas components in this borehole. Although it cannot be confirmed at this time, this isotopic shift in N2 may be potentially attributed to microbial processes (e.g., anaerobic oxidation of ammonium). Nevertheless, the large 15N enrichments for the majority of the samples in this study suggest that the nitrogen cycle in the deep saline fracture fluids in the Canadian Shield is dominated by abiotic processes. This is in contrast to the nitrogen cycles in the subsurface fracture fluids in the Fennoscandian Shield and the Witwatersrand Basin, which have been shown to be strongly affected by extant microbial ecosystems discovered in those fracture waters.

Long-term fate of nitrogen fixation in Pleurozium schreberi Brid (Mit.) moss carpets in boreal forests

Feather mosses and associated cyanobacteria serve as an important source of nitrogen (N) in boreal forests; however, few studies have effectively traced the N2 fixation in-situ for more than one year. The purpose of this study was to assess the long-term fate of N2 fixed in feather moss carpets in boreal forests of northern Sweden. We conducted a seven-year 15N2(g) labelling and tracing experiment in a boreal forest with a moss bottom layer dominated by Pleurozium schreberi Brid (Mitt). Mesocosms of forest floor with and without mosses present were exposed to a 10% 15N2(g) atmosphere for five days after which samples of moss, shrub, pine seedling and O horizon were taken at varied intervals and analysed for δ15N. Plots were split and burned in year seven to assess the effect of fire on liberating moss N for uptake by Scots pine (Pinus sylvestris L.) seedlings. Feather mosses treated with 15N2(g) were highly enriched in 15N one month after labelling which subsequently declined after seven years to 13% of the original 15N enrichment in green tissue. No measurable 15N enrichment was observed in shrubs or the O horizon in the presence of labelled mosses until greater than five years after the labelling experiment. Fire can liberate moss N, but we did not observe a direct effect of fire on N uptake by pine seedlings. This long-term study indicates that N2 fixed by cyanobacteria in feather moss carpets is conserved in moss tissue for extended periods and slowly transferred to the forest soil O horizon as moss tissue decomposes suggesting that moss N2 fixation is most important as an ecosystem N source at long time scales.

Small-Mammal Shooting as a Conduit for Lead Exposure in Avian Scavengers.

Lead (Pb) exposure is a widespread wildlife conservation threat. Although commonly associated with Pb-based ammunition from big-game hunting, small mammals (e.g., ground squirrels) shot for recreational or pest-management purposes represent a potentially important Pb vector in agricultural regions. We measured the responses of avian scavengers to pest-shooting events and examined their Pb exposure through consumption of shot mammals. There were 3.4-fold more avian scavengers at shooting fields relative to those at fields with no recent shooting, and avian scavengers spent 1.8-fold more time feeding after recent shooting events. We isotopically labeled shot ground squirrels in the field with an enriched 15N isotope tracer; 6% of avian scavengers sampled within a 39 km radius reflected this tracer in their blood. However, 33% of the avian scavengers within the average foraging dispersal distance of nests (0.6-3.7 km) were labeled, demonstrating the importance of these shooting fields as a source of food for birds nesting in close proximity. Additionally, Pb concentrations in 48% of avian scavengers exceeded subclinical poisoning benchmarks for sensitive species (0.03-0.20 μg/g w/w), and those birds exhibited reduced δ-aminolevulinic acid dehydratase activity, indicating a biochemical effect of Pb. The use of shooting to manage small mammal pests is a common practice globally. Efforts that can reduce the use of Pb-based ammunition may lessen the negative physiological effects of Pb exposure on avian scavengers.

Application of calcium nitrate with phosphorus promotes rhizosphere alkalization in acid subsoil

AbstractCombatting subsoil acidity is a challenge as the incorporation of lime in subsurface layers may not be economically feasible. The use of nitrate to facilitate plant excess anion uptake has shown the potential to increase soil pH. A column experiment was carried out to demonstrate this root‐induced alkalization in various depths in an acid soil with pH 4.28 (0.01 M CaCl2). It consisted of two types of nitrogen (N) fertilizers (urea and Ca(NO3)2, both at 5 atom% 15N enrichment) banded at a depth of 0–10 cm, 10–20 cm or 20–30 cm, with phosphorus (P) fertilizer (NaH2PO4) at 150 mg P per column either applied to the N layers (N+P) or evenly mixed into the remaining layers without N addition (N‐P). Aluminium‐tolerant wheat (ET8) was used as test plants, receiving an initial N dose of 60 mg kg−1 soil at sowing and an additional dose of 30 mg kg−1 every 10 days from 40 days after sowing. Plants had a total N supply of 90 mg N kg−1 at 50 days and 180 mg N kg−1 at 80 days after sowing. The N+P treatments increased plant N uptake and increased rhizosphere and bulk soil pH. Nitrate placement increased pH of treated layers by up to 0.4 and 0.2 units in the rhizosphere and bulk soils, respectively, compared with the control at Day 80. Rhizosphere alkalization was greater with deeper N+P treatments as these encouraged the proliferation of deeper roots in the treated layer. The form of N and the application method of P fertilizer had no effect on shoot biomass over 80 days of growth. The N+P treatments had 7% higher 15N recovery of fertilizer in plants than the N‐P treatments. The current study also showed that P fertilizer enhanced rhizosphere alkalization induced by calcium nitrate. The results suggest that the use of nitrate fertilizer with P treatments can facilitate alkalization of acid subsoils.Highlights Phosphorus facilitates amelioration of subsurface soil acidity with nitrate fertilizer Placement of nitrate induces alkalization of acid soil at various depths Combined addition of nitrate and P at depth enhances nitrate uptake by wheat Balanced nutrient supply enhances excess anion uptake and rhizosphere alkalization

Stable Isotope Effects of Biogas Slurry Applied as an Organic Fertilizer to Rice, Straw, and Soil.

Biogas slurry (BS) is now increasingly used for organic rice production in China. However, the isotopic response and fractionation of different BS application rates to characterize organic rice cultivation have not yet been investigated. In this study, different fertilizer treatments were applied to rice paddy soil including urea, BS with five different application rates and a control with no fertilizer added. Multiproxy analyses (% C, % N, δ13C, δ15N, δ2H, and δ18O) of rice, rice straw, and soil were undertaken using elemental analyzer-isotope ratio mass spectrometry. Rice, straw, and soil showed only minor isotopic and elemental variations across all fertilizer treatments except for δ15N. δ15N values of rice and straw became more positive (+6.1 to +11.2‰ and +6.1 to +12.2‰, respectively) with increasing BS application rates and became more negative with urea fertilization (+2.8 and +3.0‰, respectively). The soil had more positive δ15N values after BS application but showed no significant change with different application rates. No obvious δ15N isotopic differences were found between the control soil and soils fertilized with urea. 15N fractionation was observed between rice, straw, and soil (Δrice-soil -2.0 to +4.3‰, Δstraw-soil -1.9 to +5.3‰) and their isotopic values were strongly correlated to each other (r > 0.94, p < 0.01). Results showed that % C, % N, δ13C, δ2H, and δ18O in rice displayed only minor variations for different fertilizers. However, δ15N values increased in response to BS application, confirming that BS leaves an enriched 15N isotopic marker in soil, straw, and rice, indicating its organically cultivated status. Results from this study will enhance the stable isotope δ15N databank for assessing organic practices using different fertilizer sources.

Frontispiece: Synthetic Approaches for 15N‐Labeled Hyperpolarized Heterocyclic Molecular Imaging Agents for 15N NMR Signal Amplification by Reversible Exchange in Microtesla Magnetic Fields

Signal amplification by reversible exchange enables efficient nitrogen-15 hyperpolarization of a wide range of bio-compatible substrates by using microtesla magnetic fields. This process is enabled by simulatenous exchange of parahydrogen (the source of polarization) and the N-containing biocompatible molecule to be hyperpolarized. Selected advances in the 15N enrichment of pyridine and imidazole derivatives that can potentially be employed as novel molecular contrast agents are summarized in the Minireview by N. V. Chukanov, E. Y. Chekmenev et al. on page 9727 ff.

Impacts of silver nanoparticles on enzymatic activities, nitrifying bacteria, and nitrogen transformation in soil amended with ammonium and nitrate

Silver nanoparticles (AgNPs) are effective antimicrobial compounds that are used in a myriad of applications. Soil microorganisms play crucial roles in nitrogen cycling, but there is a lack of comprehensive understanding of the effects of AgNPs on enzymatic activity in the nitrogen cycle, nitrifying bacteria, and nitrogen transformation in soil. Herein, enzyme activities were determined following the addition of different forms of nitrogen, ammonium nitrogen ((NH4)2SO4), nitrate nitrogen (KNO3), and amide nitrogen (urea, CO(NH2)2) at 200 mg N kg–1, into the soil amended with AgNPs at 0, 10, 50, and 100 mg kg–1. After 7 d of incubation with 10 mg kg–1 AgNPs, the activities of urease, nitrite reductase (NiR), nitrate reductase (NaR), and hydroxylamine reductase (HyR) were reduced by 12.5%, 25.0%, 12.2%, and 24.2%, respectively. Of particular note, more than 53.5%, 61.7%, and 34.7% of NaR, NiR, and HyR activities, respectively, were inhibited at 100 mg kg–1 AgNPs. The abundance (most probable number) of ammonia- and nitrite-oxidizing bacteria (AOB and NOB, respectively) was measured using real-time quantitative polymerase chain reaction (qPCR) and the Cochran method. The abundance of AOB and NOB decreased when AgNPs were present in the soil. The NH4NO3 amendment increased copy numbers of bacterial and archaeal amoA nitrification functional genes, by 38.3% and 12.4%, respectively, but AgNPs at 50 mg kg–1 decreased these values by 70% and 56.4%, respectively. The results of 15N enrichment (atom% excess) of NH4+ and NO3– experiments illustrated the influence of AgNPs on soil nitrogen transformation. According to the 15N atom% excess detected, the conversion of 15N-labeled NH4+ to NO3– was significantly inhibited by the different levels of AgNPs in soil. The reduced gross nitrification rate further confirmed this finding. Overall, this study revealed considerable evidence that AgNPs inhibited nitrogen cycle enzyme activity, the number of nitrifying bacteria, the abundance of the amoA gene, and the gross nitrification rate. Silver nanoparticles inhibited nitrogen transformation, and the rate of nitrification was also negatively correlated with AgNP levels.

Confrontation of the “Dual Tracer” Indirect Method With Direct Ileal Sampling for Indispensable Amino Acid Digestibility of Sunflower Isolate in Humans

ObjectivesThe direct assessment of ileal samples of amino acid (AA) digestibility is invasive in humans. A less invasive but indirect method, namely « dual tracer » was recently developed. It relies on the plasma isotopic enrichment ratio of two labeled protein, a reference protein or alternately AAs labeled with 13C and the test protein labeled with 15N. This recent method has not yet been challenged against direct measurement of ileal digestibility. MethodsSeven healthy volunteers were intubated with naso-ileal tube. Every 30min for 4h, they ingested sunflower biscuits containing a total of 25g of 15N intrinsically labeled sunflower protein isolate. They also ingested 60g chocolate containing a total of 400mg of a mix of 13C algal individual AAs. Ileal contents were collected continuously for 8h following the first meal and plasma was sampled every 30min for 4h and hourly between 4 and 8h. 15N and 13C indispensable amino acid (IAA) ileal digestibility were determined by measuring 15N and 13C enrichment in AAs by GC-C-IRMS and AA content by UHPLC in ileal effluent. Plasma and meal 15N and 13C IAA enrichment were measured by GC-C-IRMS. Isotopic 15N/13C ratio were determined using area under the curve value for each isotope. ResultsUsing direct ileal sampling, average IAA ileal digestibility was: (i) 88.5 ± 5.0% for sunflower isolate (15N) with values ranged from 85.8 ± 5.1% for threonine to 91.1 ± 5.8% for methionine, and (ii) 97.6 ± 1.7% for free AAs (13C) with values ranged from 95.9 ± 2.3% for lysine to 98.8 ± 0.8% for phenylalanine. With the “dual tracer” method, digestibility of isoleucine, leucine, threonine and valine was significantly lower than with ileal determination (from 7.9% for threonine to 24.3% for leucine), Methionine and phenylalanine values were aberrant (over 100%) For lysine, the difference between the two methods was not statistically different (4.7%, p = 0.49). ConclusionsWith our methodological conditions, the “dual tracer” method provides physiological values for most IAA except methionine and phenylalanine. However, values were low compared to ileal digestibility (about 10%) and interindividual variability was high. This less invasive method is promising but requires methodological improvements. Funding SourcesFrench Research National Agency (ANR), financial support of SOFIPROTEOL under the FASO Project PRODIAL.

Real Ileal Amino Acid Digestibility of Pea Protein Isolate As Compared to Casein in Healthy Adult Humans

Abstract Objectives In the current context of finding plant alternatives to animal proteins, pea would be a good option regarding its high protein content and its well-balanced amino acid (AA) profile. However, we must examine its digestibility, a main criteria of protein nutritional quality. The aim of this study was to determine the real ileal AA and nitrogen (N) digestibility (RIDAA and RIDN) of pea protein as compared to milk casein in humans. We also evaluated their respective nutritional qualities through the calculation of the digestible indispensable amino acid score (DIAAS) and the net postprandial protein utilization (NPPU). Methods Fifteen healthy adult volunteers completed the study and were equipped with a triple-lumen naso-ileal tube. They were given 9 portions of mashed potatoes containing either pea protein or casein isolates that were intrinsically labelled with 15N. PEG-4000 was perfused in the ileum as a non-absorbable marker to measure the intestinal flow rate. Ileal digesta were collected continuously by aspiration with a syringe over an 8-h postprandial period, while plasma and urine were sampled regularly. N and AA contents of digesta and protein isolates were measured using EA and U-HPLC. PEG-4000 content of the digesta was measured by turbidimetric method. Urea was extracted from plasma and urine samples. 15N enrichment was assessed in digesta, urea and protein isolates by EA-IRMS and in individual AAs by GC-C-IRMS. Results Mean RIDAA was 93.6 ± 2.9% and 96.8 ± 1.0% for pea protein and casein, respectively, with no significant difference between groups (P = 0.22). RIDN was 92.0 ± 2.7% and 94.0 ± 1.7% for pea protein and casein, respectively, and were not different (P = 0.11). The DIAAS was 1.00 for pea protein and 1.45 for casein. The NPPU was 67.0 ± 6.2% for pea protein and 70.7 ± 1.9% for casein and the difference was not significant (P = 0.15). Conclusions Overall, the bioavailability was not different between pea protein and casein in healthy adults. The DIAAS of pea protein reached 1, revealing the absence of limiting AA in regard to the requirements. Considering its AA composition and the digestibility results we obtained, pea protein can be qualified as a high-quality protein. Funding Sources Roquette

Comparison of Dual Isotope and Ileal Sampling Methods for the Determination of the Amino Acid Digestibility of Pea Protein and Casein in Adult Humans

ObjectivesThe measurement of amino acid (AA) digestibility of protein through direct ileal sampling is highly invasive and inappropriate for vulnerable populations, such as children or elderly. The new dual tracer method relies on comparing meal and plasma isotopic ratios of 1/a test protein 2/a reference protein (or AA mix) of known digestibility, each one being labelled with a different isotope. The aim of this study was to compare this new indirect dual tracer method to standard ileal method, for the determination of AA digestibility of pea protein and milk casein. MethodsFifteen healthy adult volunteers completed the study and were equipped with a naso-ileal tube. They were given 9 portions of mashed potatoes containing either pea protein or casein isolates that were intrinsically labelled with 15N and 2H. A 13C algal free AA mix was added in the meals as the reference for dual tracer method. Plasma samples were collected regularly from before the first ingestion to 8-h later, while ileal digesta were collected continuously. For ileal sampling method, the AA digestibility (RIDAA) was determined using the recovery a non-absorbable marker (PEG-4000) perfused in the ileum, and the measurement of 15N enrichment of the digesta. For the dual tracer method, the amount of AA absorbed (AbAA) was calculated by the ratio of 2H/13C enrichments in plasma and in meals. The isotopic enrichments were evaluated in digesta, plasma samples and meals by GC-C-IRMS. The AA content was measured in digesta and meals by U-HPLC. ResultsMean AbAA and RIDAA of pea protein were 102.2 ± 3.1% and 94.3 ± 1.5%, respectively. Mean AbAA and RIDAA of casein were 91.9 ± 2.0% and 97.1 ± 0.8%, respectively. The dual tracer method overestimated by 10% and 5% the AA digestibility of pea protein and casein, respectively, and the variability was high. The mean ileal AA digestibility of the 13C free AA mix was high (98.1 ± 1.1%), which validated our choice to use it as the reference ‘protein’ in the dual tracer method. ConclusionsSeveral AA digestibilities obtained with dual tracer method were in the same range as the digestibilities from ileal sampling method. The variability was high and the effect of the protein source was inconsistent. After further research and validation, the dual tracer method could lead to notable advances in the determination of protein quality in humans. Funding SourcesRoquette.

Using stable nitrogen isotope to indicate soil nitrogen dynamics under agricultural soil erosion in the Mun River basin, Northeast Thailand

Soil erosion has threatened food security by decreasing the area of arable lands and disturbing soil nutrient cycling, especially nitrogen (N) cycling. However, there is still a challenge to indicate soil N dynamics by N stable isotope composition (δ15N) under an erodible environment. In the present study, the six soil sites from forest lands and agricultural lands were selected to collect soil samples in the Mun River basin, the largest agricultural region in Thailand. The contents of soil organic nitrogen (SON), C/N ratios, and δ15N values of SON in soil profiles were analyzed to identify soil N transformation processes under agricultural soil erosion. Moreover, the relationships of SON and δ15N with soil erodibility K factor were determined by linear regression analysis. Fine particles in the paddy land were largely lost under intensive soil erosion during a short-term abandonment (1 ~ 5 years). Agricultural soil erosion has reduced SON content by 3.8 ~ 4.9 times. The δ15N values of SON in the paddy soils ranged from –5‰ to 5‰, which was attributed to the application of manure and synthetic fertilizer and the δ15N fractionations in the alteration processes of the redox environment. In the abandoned paddy lands, δ15N values of SON in the soils above the illuvial horizon were 4‰ higher than those in the soils below the horizon. The results indicated the discrepant soil N processes at the two soil layers, i.e., soil N mineralization in the upper layer produces 15N-depleted inorganic N and leads to 15N enrichment in organic residues; the inorganic N was leached into the lower layer and re-assimilated by microbes, resulting in 15N-depleted SON. The results suggest that agricultural soil erosion significantly reduces soil N level. However, microbial re-assimilation of inorganic N in the deep soils can mitigate soil N loss.

Quantifying nitrogen uptake and translocation for mature trees: an in situ whole-tree paired 15N labeling method.

Nitrogen (N) is one of the major nutrients limiting plant growth in terrestrial ecosystems. To avoid plant-microbe competition, previous studies on plant N uptake preference often used hydroponic experiments on fine roots of seedlings and demonstrated ammonium preference for conifer species; however, we lack information about N uptake and translocation in the field. In this paper, we described a method of in situ paired 15N labeling and reported the rates and time course of N uptake and translocation by mature trees in situ. We added 15N-enriched ammonium or nitrate, together with the nitrification inhibitor dicyandiamide, to paired Larix kaempferi (Lamb.) Carr (larch) trees from 30-, 40- and 50-year-old plantations. Fine roots, coarse roots, leaves and small branches were collected 2, 4, 7, 14 and 30days after labeling. Nitrate uptake and translocation averaged 1.59±0.16μg 15N g-1 day-1, which is slightly higher than ammonium (1.08±0.10μg 15N g-1 day-1), in all tree organs. Nitrate contributed 50-78% to N uptake and translocation, indicating efficient nitrate use by larch in situ. We observed no age effect. We suggest that sampling leaves after 4days of 15N labeling is sufficient to detect mature tree N uptake preference in situ. Whole-tree 15N-ammonium recovery equaled that of 15N-nitrate 30days after 15N addition, implying the importance of both ammonium and nitrate to mature larch N use in the long run. We conclude that our method is promising for studying mature tree N uptake preference in situ and can be applied to other conifer and broadleaf species. We suggest using highly enriched 15N tracer to overcome soil dilution and a nitrification inhibitor to minimize ammonium transformation to nitrate. Our study revealed mature tree N preference in situ and demonstrated the strong contribution of nitrate toward mature larch growth on soils rich in nitrate.

Dual labelling by 2H and 15N revealed differences in uptake potential by deep roots of chicory

AimsDeep-rooted crops have been widely used in agricultural systems to access deep resources such as water and nitrogen (N). However, the potential of deep roots to take water and N at various depths have not been well studied. Here we used chicory (Cichorium intybus L.) to study the potential and dynamics of water and nitrogen uptake in deep soil layers (below 1 m). MethodsChicory plants grown in outdoor rhizotrons were labelled by injecting a 2H2O and Ca(15NO3)2 mixture into the soil column at 1.1, 2.3 and 3.5 m depth. Five, ten and twenty days after injection, 2H and 15N were traced in transpiration water and leaves. ResultsWe found enriched 2H and 15N in water and plant samples, and both water and N uptake were observed down to 3.5 m. The 2H enrichment after injection at 1.1 m depth was 1552‰, almost 10 times higher than after injection at 2.3 m depth, which was 156‰. In contrast, injection at 1.1 and 2.3 m depth resulted in similar 15N enrichment of leaf samples. ConclusionDeep water uptake was found to be more sensitive to increased depth and reduced root intensity than N uptake, and labelled N was used more rapidly than labelled water. We propose several possible explanations for the discrepancies between deep water and N uptake, and further discuss the challenges of using isotopes and models in deep root studies.

The Tarda Meteorite: A Window into the Formation of D-type Asteroids

Abstract Dynamic models of solar system evolution suggest that D-type asteroids formed beyond Saturn's orbit and represent invaluable witnesses of the prevailing conditions in the outer solar system. Here, we report a comprehensive petrographic and isotopic characterization of the carbonaceous chondrite Tarda, a recent fall recovered in the Moroccan Sahara. We show that Tarda shares strong similarities with the D-type-derived chondrite Tagish Lake, implying that Tarda represents a rare sample of D-type asteroids. Both Tarda and Tagish Lake are characterized by the presence of rare 16O-rich chondrules and chondrule fragments, high C/H ratios, and enrichments in deuterium, 15N, and 13C. By combining our results with literature data on carbonaceous chondrites related to C-type asteroids, we show that the outer solar system 4.56 Gy ago was characterized by large-scale oxygen isotopic homogeneities in (i) the water–ice grains accreted by asteroids and (ii) the gas controlling the formation of FeO-poor chondrules. Conversely, the zone in which D-type asteroids accreted was significantly enriched in deuterium relative to the formation regions of C-type asteroids, features likely inherited from unprocessed, D-rich, molecular-cloud materials.

Nitrous Oxide Formation by Corona Discharge: Isotopic Composition Measurements and Atmospheric Applications

AbstractThe expanding use of nitrogen‐containing fertilizers in agriculture has led to an increase in N2O, an important greenhouse gas and ozone‐depleting substance, since preindustrial times. Isotopic measurements are a valuable tool to distinguish the contribution of different sources of N2O, but the isotopic composition of N2O formed in the low temperature plasma regimes in lightning has not previously been measured. Here, a corona discharge was generated in flowing or static zero air, and the N2O formed at discharge cell pressures from ∼0.1 to 10 Torr (∼13–1,300 Pa) at a discharge voltage of 5 kV and discharge current of 1,500 μA was collected and measured by isotope ratio mass spectrometry. Experiments were also conducted by varying the discharge current from 75 to 1,500 μA at ∼0.5 Torr (∼67 Pa) and 5 kV. The results show enrichments in 15N in product N2O of up to 32‰ relative to the reactant N2 and even larger enrichments in 15N of up to 77‰ at the central nitrogen atom. Depletions in 18O as large as −71‰ relative to reactant O2 were also measured. The isotope‐isotope relationships of the N2O produced in the corona discharge are distinct from those of other sources of N2O, suggesting that isotope measurements can be used to determine whether local or regional variations in the atmospheric concentration of N2O – such as the enhanced N2O levels measured in the upper tropical and subtropical troposphere during the HIPPO mission – can be attributed to lightning activity, soil emissions, or biomass burning.

A Deep-Sea Sponge Loop? Sponges Transfer Dissolved and Particulate Organic Carbon and Nitrogen to Associated Fauna

Cold-water coral reefs and sponge grounds are deep-sea biological hotspots, equivalent to shallow-water tropical coral reefs. In tropical ecosystems, biodiversity and productivity are maintained through efficient recycling pathways, such as the sponge loop. In this pathway, encrusting sponges recycle dissolved organic matter (DOM) into particulate detritus. Subsequently, the sponge-produced detritus serves as a food source for other organisms on the reef. Alternatively, the DOM stored in massive sponges was recently hypothesized to be transferred to higher trophic levels through predation of these sponges, instead of detritus production. However, for deep-sea sponges, the existence of all prerequisite, consecutive steps of the sponge loop have not yet been established. Here, we tested whether cold-water deep-sea sponges, similar to their tropical shallow-water counterparts, take up DOM and transfer assimilated DOM to associated fauna via either detritus production or predation. We traced the fate of 13carbon (C)- and 15nitrogen (N)-enriched DOM and particulate organic matter (POM) in time using a pulse-chase approach. During the 24-h pulse, the uptake of 13C/15N-enriched DOM and POM by two deep-sea sponge species, the massive species Geodia barretti and the encrusting species Hymedesmia sp., was assessed. During the subsequent 9-day chase in label-free seawater, we investigated the transfer of the consumed food by sponges into brittle stars via two possible scenarios: (1) the production and subsequent consumption of detrital waste or (2) direct feeding on sponge tissue. We found that particulate detritus released by both sponge species contained C from the previously consumed tracer DOM and POM, and, after 9-day exposure to the labeled sponges and detritus, enrichment of 13C and 15N was also detected in the tissue of the brittle stars. These results therefore provide the first evidence of all consecutive steps of a sponge loop pathway via deep-sea sponges. We cannot distinguish at present whether the deep-sea sponge loop is acting through a detrital or predatory pathway, but conclude that both scenarios are feasible. We conclude that sponges could play an important role in the recycling of DOM in the many deep-sea ecosystems where they are abundant, although in situ measurements are needed to confirm this hypothesis.

Presolar Silicate and Oxide Grains Found in Lithic Clasts from Isheyevo and the Fine-grained Matrix of Northwest Africa 801

Abstract We report on the discovery of 33 oxygen-anomalous grains from the CH3/CBb3 chondrite Isheyevo and the CR2 chondrite Northwest Africa (NWA) 801. Oxygen isotopic compositions indicate the origin of the majority grains in stellar outflows of low-mass (∼1.2 to ∼2.2 M ⊙), solar-metallicity red giant or asymptotic giant branch stars, while highly 17O-enriched grains probably have nova origins. Isotopic compositions of the eight 18O-rich grains, including an extremely 18O-rich grain (∼16 times solar 18O/16O ratio), are reproduced by zone mixing of SNe II ejecta. Close-to-normal silicon, magnesium, and calcium isotopic compositions of grains are consistent with the isotope exchange in the interstellar medium or the meteorite parent body, while two grains with Si isotopic anomalies and one grain with Mg isotopic anomalies reflect the Galactic chemical evolution. An Isheyevo clast showed several hot spots with moderate to high 15N enrichments, including a hot spot with an extreme 15N excess of (7225 ± 316)‰. However, no correlation between 15N enrichment and presolar oxygen-rich grain abundance is found. Grains with elliptical shapes probably indicate primary condensation feature. Two complex grains possibly display decoupling of the isotopic and elemental compositions in the grain formation environments. The low silicate-to-oxide abundance ratio for the fine-grained chondrule rims in NWA 801 likely reflects the preferential destruction of silicates due to terrestrial weathering. In NWA 801, the presolar O-rich grain abundance in fine-grained chondrule rims is higher than in the interchondrule matrix, similar to the trend observed for some aqueously altered chondrites of petrologic type 2.

Effect of increasing dietary protein with constant lysine:methionine ratio on production and omasal flow of nonammonia nitrogen in lactating dairy cows

Eight lactating cows were fed 4 diets in which dietary crude protein (CP) was increased in steps of approximately 2 percentage units from 11 to 17% of DM by replacing high-moisture corn with soybean meal supplemented with rumen-protected Met to maintain a Lys:Met ratio of 3:1 in metabolizable protein. Trial design was a replicated 4 × 4 Latin square; experimental periods lasted 28 d, with data and sample collection being performed during wk 3 and 4 of each period. Digesta samples were collected from the rumen as well as the omasum to measure metabolite concentrations and ruminal outflow of N fractions using infusion of 15N-enriched ammonia to quantify microbial nonammonia N (NAN) and nonmicrobial NAN. Data were analyzed using the MIXED procedure of SAS (SAS Institute Inc.). There were linear increases in the yields of milk and true protein and concentration of milk urea N, and a linear decrease in N efficiency, with increasing dietary CP. Apparent ruminal and total-tract N digestibility increased linearly with increasing dietary CP, but estimated true total-tract N digestibility was not affected. Apparent digestibility of the other macronutrients was not influenced by diet. Ruminal ammonia, total AA and peptides, and branched-chain VFA also increased linearly with dietary CP. The 15N enrichment of liquid- and particle-associated microbes linearly declined with increasing dietary CP due to decreasing 15N enrichment of the ammonia pool. Although no effect of dietary CP on nonmicrobial NAN flow was detected, total NAN flow increased linearly from 525 g/d at 11% CP to 637 g/d at 17% CP due to the linear increase in microbial NAN flow from 406 g/d at 11% CP to 482 g/d at 17% CP. Under the conditions of this study, when dietary CP was increased by adding soybean meal supplemented with rumen-protected Met, improved milk and protein yields were driven not by RUP supply but by increased ruminal outflow of microbial protein.