Nitrogen Isotope Fractionation Research Articles

Early planetesimal differentiation and late accretion shaped Earth's nitrogen budget.

The relative roles of protoplanetary differentiation versus late accretion in establishing Earth's life-essential volatile element inventory are being hotly debated. To address this issue, we employ first-principles calculations to investigate nitrogen (N) isotope fractionation during Earth's accretion and differentiation. We find that segregation of an iron core would enrich heavy N isotopes in the residual silicate, while evaporation within a H2-dominated nebular gas produces an enrichment of light N isotope in the planetesimals. The combined effect of early planetesimal evaporation followed by core formation enriches the bulk silicate Earth in light N isotopes. If Earth is comprised primarily of enstatite-chondrite-like material, as indicated by other isotope systems, then late accretion of carbonaceous-chondrite-like material must contribute ~ 30-100% of the N budget in present-day bulk silicate Earth. However, mass balance using N isotope constraints shows that the late veneer contributes only a limited amount of other volatile elements (e.g., H, S, and C) to Earth.

Carbon and Nitrogen Isotope Fractionations During Biotic and Abiotic Transformations of 2,4-Dinitroanisole (DNAN).

2,4-Dinitroanisole (DNAN) is a main constituent in various new insensitive munition formulations. Although DNAN is susceptible to biotic and abiotic transformations, in many environmental instances, transformation mechanisms are difficult to resolve, distinguish, or apportion on the basis solely of analysis of concentrations. We used compound-specific isotope analysis (CSIA) to investigate the characteristic isotope fractionations of the biotic (by three microbial consortia and three pure cultures) and abiotic (by 9,10-anthrahydroquinone-2-sulfonic acid [AHQS]) transformations of DNAN. The correlations of isotope enrichment factors (ΛN/C) for biotic transformations had a range of values from 4.93 ± 0.53 to 12.19 ± 1.23, which is entirely distinct from ΛN/C values reported previously for alkaline hydrolysis, enzymatic hydrolysis, reduction by Fe2+-bearing minerals and iron-oxide-bound Fe2+, and UV-driven phototransformations. The ΛN/C value associated with the abiotic reduction by AHQS was 38.76 ± 2.23, within the range of previously reported values for DNAN reduction by Fe2+-bearing minerals and iron-oxide-bound Fe2+, albeit the mean ΛN/C was lower. These results enhance the database of isotope effects accompanying DNAN transformations under environmentally relevant conditions, allowing better evaluation of the extents of biotic and abiotic transformations of DNAN that occur in soils, groundwaters, surface waters, and the marine environment.

Experimental simulation of thermal evolution of nitrogen content and isotopes in source rocks: Implications for nitrogen cycling characterization and oil-source correlation

Sedimentary nitrogen isotopes have been widely used to reconstruct the nitrogen cycling and redox characteristics of ancient oceans, but they also have important value in the study of oil genesis, such as oil classification. However, the thermal evolution of nitrogen isotopes in the source rock and associated oil is currently unclear. In this study, the thermal evolution (from the oil generation to the dry gas stage) of three types of low-maturity source rock was simulated. And the content and isotope composition of total organic carbon (TOC) and total nitrogen (TN) of the pyrolyzed source rock, expelled oil and retained oil were measured. In combination with weight-quantitative analyses, the following conclusions can be drawn: (1) During the main oil and gas generation stage, the pyrolyzed source rocks lost 15.1–28.3% of TN, which is much lower than the loss of TOC (26.2–46.1%). This difference may be related to the fact that the TOC content of the generated oil and gas is two orders of magnitude higher than the TN. In addition, TOC loss occurred mainly within the oil generation-expulsion stage, whereas TN loss occurred mainly in the dry gas stage and higher maturity stage. (2) The nitrogen isotope compositions of three types of source rocks and associated kerogens became heavier by 0.5–1.5‰ as thermal maturity increased, which is similar to the change of organic carbon isotope compositions. The δ15N values of pyrolyzed source rocks generally have poor relationships with either the amount of expelled oil and gas or the TN loss, which is different from the δ13Corg values. The δ15N fluctuations may be attributed to three processes: oil generation-expulsion, N2 gas generation-expulsion, and inorganic N (like NH4+) expulsion. The bulk nitrogen isotope composition of source rocks is only slightly affected by thermal maturity and can be used to indicate the nitrogen biogeochemical cycle and water redox conditions during the deposition. (3) Similar to δ13Corg values, δ15N values of both expelled oil and retained oil increased gradually with enhanced thermal maturity, implying a control of kinetic isotope effect. Among the three types of source rock, two of them having relatively light nitrogen isotope compositions (δ15N value of ∼2.5‰) generated and expelled oils with similar nitrogen isotope compositions, while the third type, which has heavy nitrogen isotope composition (δ15N value of ∼13.0‰), generated and expelled oils with much lighter nitrogen isotope compositions (by 3–6‰). This finding shows that there may be significant nitrogen isotope fractionation during the generation and primary migration of oil from source rocks. The nitrogen isotopes should be used in oil-source correlation with caution, even though related mechanisms need to be further studied. Nevertheless, using a combination of organic carbon and nitrogen isotopes, the oil generated and expelled by the three types of source rocks could be distinguished. Therefore, the use of nitrogen isotopes for oil-source correlation under geological conditions should be combined with other isotope and molecular indicators.

Mechanisms of nitrogen isotope fractionation at an ancient black smoker in the 2.7 Ga Abitibi greenstone belt, Canada

Abstract The biological nitrogen (N) cycle on early Earth is enigmatic because of limited data from Archean (meta-)sediments and the potential alteration of primary biotic signatures. Here we further investigate unusual 15N enrichments reported in 2.7 Ga meta-sediments from the Abitibi greenstone belt, Canada, purportedly related to a 15N-enriched Archean atmosphere. Given that sediments from this region are contemporaneous with large-scale volcanogenic massive sulfide deposits, we utilize Cu and Zn contents to trace the effects of hydrothermal circulation on N isotope fractionation. We show that high δ15Nbulk values as high as +23‰ are associated with Cu-Zn mineralization, whereas unmineralized organic-rich shales exhibit much lower δ15Nbulk and δ15Nkerogen values. Moreover, we find a large offset between δ15Nbulk and δ15Nkerogen of as much as 17‰ and relate this to the addition of organic-bound N during the late-stage emplacement of organic-rich veins. We conclude that the previously reported high δ15N values are most parsimoniously explained by biotic and abiotic mechanisms rather than a 15N-enriched atmosphere. Crucially, both mechanisms require the presence of NH4+ in hydrothermal fluids, supporting the hypothesis that hydrothermal discharge was an important nutrient source for Neoarchean marine life.

Revealing the ammonia oxidation process and shortcut nitrification performance using nitrogen and oxygen isotope fractionation effect

Natural abundance isotope fractionation properties have become the most effective way to explore nitrogen transformations of biological nitrogen removal from wastewater. The migration and transformation characteristics of N and O elements in the shortcut nitrification were analyzed using the N and O dual isotopic fractionation technique. The effects of dissolved oxygen (DO) and temperature changes on the performance of shortcut nitrification and isotopic fractionation were investigated. The fractionation characteristics of N and O elements during shortcut nitrification were explored by adjusting DO concentration (0.2–0.4, 1–1.2 and 3–4 mg/L) and temperature (33 ± 1 °C, 25 ± 1 °C and 18 ± 1 °C). Both δ15NNO2 and δ18ONO2 showed a gradually increasing trend with the accumulation of NO2−-N, and the fractionation effects induced by temperature were significantly higher than those by DO. The higher the temperature, the more significant the increase in δ15NNO2; the higher the DO, the more remarkable the increase in δ18ONO2, while δ15NNO2: δ18ONO2 was maintained at 0.77–6.45. The 18O-labeled H2O was successfully transferred to NO2−-N, and the replacement of O element was as high as 100 %, indicating that DO and H2O simultaneously participated in the shortcut nitrification process. The dynamic changes in isotope fractionation effects can be successfully applied to reveal the performance and mechanism of shortcut nitrification.

Use of the nitrogen/ oxygen isotope fractionation technique for tracing nitrogen removal from activated sludge in municipal wastewater treatment plants

The dual isotope fractionation technique of nitrogen and oxygen has been successfully applied to analyze microbially-driven nitrogen transformations and partitioning in aquatic ecosystems, but not in municipal water treatment facilities. Nitrogen removal efficiency in wastewater treatment plants using activated sludge is critical and difficult to monitor. Here, we tested the nitrogen and oxygen isotope fractionation technique for capacity to trace nitrogen removal from municipal wastewater. Long-term monitoring of Anaerobic/anoxic/aerobic (A2/O) process municipal wastewater treatment plants revealed that nitrogen removal performance gradually improved with increasing seasonal temperatures. The δ15N value of activated sludge increased from 5.18‰ (January) to 6.37‰ (June). An increase in temperature promoted the increase in NO3--N from 1.23 mg/L (January) to 6.06–7.02 mg/L (June to August) in the aerobic reaction. The δ15NNO3 decreased from 17.82‰ to 37.37‰ to 4.09–17.68‰, and variability in utilization of light and heavy isotopes gradually decreased. The slope of the ratio δ15NNO3:δ18ONO3 gradually decreased from 21.58 ± 12.15 (January) to 0.57 ± 0.51 (August). The dynamic response of δ15N values in sludge and isotopes in water revealed the migration transformation of nitrogen and realized the differential analysis to denitrification performance with seasonality. It also provided a technical basis for long-term stable nitrogen removal system.

Using PMF model and Bayesian model to quantify NOx sources in non-typhoons and Super Typhoon Lekima (2019) in Hangzhou

Quantifying and comparing nitrate (NO3−) sources in typhoons and non-typhoons is important for controlling and reducing their emissions. In this study, the chemical compositions and stable isotopes (δ15N–NO3- and δ18O–NO3-) of non-typhoon and Typhoon Lekima precipitation in Hangzhou (from May–August during 2019 and 2020) were determined. The results revealed that severe acid rain occurred in Hangzhou, except during Typhoon Lekima. According to the positive matrix factorization (PMF) model results, the ion contributions of sources in non-typhoons were as follows: secondary inorganic aerosols + biomass burning > sea salt aerosols > agricultural sources > terrestrial sources, whereas sea salt aerosols + biomass burning were the primary contributors of ions in Typhoon Lekima. The δ15N–NO3- values of non-typhoon and Typhoon Lekima ranged from −4.0 to −1.3‰ and −2.7 to 5.8‰, respectively. The δ18O–NO3- values of non-typhoon and Typhoon Lekima ranged from 56.3‰ to 74.7‰ and 32.2–55.3‰, respectively. The calculated contribution of N2O5 pathway in NOx oxidation to NO3− during non-typhoon precipitation was 64.7 ± 12.2%. If the contribution of OH· pathway during Typhoon Lekima precipitation was 87%, the Monte Carlo simulation results demonstrated that the estimated oxidation proportions of NO via O3 and RO2 (or HO2) ranged from 48.8 to 100% and 0–51.2%, respectively. Based on the improved Bayesian model combined with nitrogen isotope fractionation, the NOx contributions from four sources (biomass burning > coal combustion > mobile sources > microbial N cycle) in non-typhoon precipitation ranged from 21 to 30% (Ԑ = 3.6). In Typhoon Lekima (Ԑ = 5.2), lightning (23.6 ± 11.2%) was one of the primary NOx contributors and a lower contribution from microbial N cycle (17.9 ± 5.4%) compared with the PMF model (22.8%). Moreover, for Typhoon Lekima, the sum of biomass burning, coal combustion, and mobile sources in the improved Bayesian model (58.5%) was slightly lower than that of secondary inorganic aerosols in the PMF model (62.4%). Therefore, the coupled PMF–improved Bayesian model is a feasible and reliable method for source identification and apportionment of NO3− in precipitation.

Insights into nitrate-reducing Fe(II) oxidation by Diaphorobacter caeni LI3T through kinetic, nitrogen isotope fractionation, and genome analyses

Nitrate (NO3−)-reducing Fe(II) oxidation (NRFO) is prevalent in anoxic environments. However, it is uncertain in which step(s) the biological Fe(II) oxidation is coupled with denitrification during NRFO. In this study, a heterotrophic NRFO bacterium, Diaphorobacter caeni LI3T, was isolated from paddy soil and used to investigate the transformation of Fe(II) and nitrogen as well as nitrogen isotopic fractionation (δ15N-N2O) during NRFO. Fe(II) oxidation was observed in the Cell+NO3− +Fe(II), Cell+NO2− + Fe(II), and NO2− + Fe(II) treatments, resulting in precipitation of amorphous Fe(III) minerals and lepidocrocite on the surface and in the periplasm of cells. The presence of Fe(II) slightly accelerated microbial NO3− reduction in the Cell+NO3− + Fe(II) treatment relative to the Cell+NO3− treatment, but slowed down the NO2− reduction in the Cell+NO2− + Fe(II) treatment relative to the Cell+NO2− treatment likely due to cell encrustation that blocking microbial NO2− reduction in the periplasm. The δ15N-N2O results in the Cell+NO3− + Fe(II) treatment were close to those in the Cell+NO3− and Cell+NO2− treatments, indicating that the accumulative N2O is primarily of biological origin during NRFO. The genome analysis found a complete set of denitrification and oxidative phosphorylation genes in strain LI3T, the metabolic pathways of which were closely related with cyc2 and cytc as indicated by protein-protein interactions network analysis. It is proposed that Fe(II) oxidation is catalyzed by the outer membrane protein Cyc2, with the resulting electrons being transferred to the nitrite reductase NirS via CytC in the periplasm, and the CytC can also accept electrons from the oxidative phosphorylation in the cytoplasmic membrane. Overall, our findings provide new insights into the potential pathways of biological Fe(II) oxidation coupled with nitrate reduction in heterotrophic NRFO bacteria.

Assessing Biodegradation Processes of Atrazine in Constructed Wetland Using Compound-Specific Stable Isotope Analysis

To bridge the gap between lab-scale microcosm research and field application in the compound-specific stable isotope analysis (CSIA) of atrazine, we studied the characteristics of carbon and nitrogen isotope fractionation in the atrazine degradation processes within a constructed wetland. In the wetland, we observed multiple element (C, N) isotope fractionation parameters, such as kinetic isotope effects and dual isotope slopes. These parameters are very consistent with those observed in the cultivation of AtzA- or TrzN-harboring strains, suggesting a similarity in the pathway and reaction mechanism of atrazine biodegradation between the two settings. However, we recorded variable carbon (εC: −3.2 ± 0.6‰ to −4.3 ± 0.6‰) and nitrogen isotope fractionation (εN: 1.0 ± 0.3‰ to 2.2 ± 0.3‰) across different phases. This variance could lead to an over- or underestimation of the biodegradation extent of atrazine when employing the large or small enrichment factor of the carbon isotope. Intriguingly, the estimation accuracy improved considerably when using the enrichment factor (−4.6‰) derived from the batch cultivation of the pore water. This study advances the application of CSIA in tracking atrazine biodegradation processes in ecosystems, and it also underlines the importance of the careful selection and application of the enrichment factor in quantifying the intrinsic biodegradation of atrazine in ecosystems.

325 Understanding Variation in Residual Feed Intake of Brahman Steers Fed Two Different Diets

Abstract Improving feed efficiency in cattle is a long-standing aim of the beef industry. However, discrepancies in the ranking of bulls based on the diets used and calculation methods employed for defining feed efficiency have been observed. Hence, the objective of this research was to assess the extent to which different factors influence feed efficiency in Brahman steers. Individual DMI and ADG were measured, and residual feed intake (RFI) calculated using linear regression of DMI on ADG and midtest metabolic body weight. Eight-nine steers were initially fed a low-protein diet (LP, 8.8% CP) for 70 days, followed by a high-protein (HP, 13.5% CP) diet for another 70 days. The genetic variation among steers and the proportion of Bos indicus genes were determined using the GGP Indicus 50K assay (Neogen Corporation, Ipswich, Australia). Rumen bacteriome variation was determined with 16S rRNA gene sequencing of rumen fluid collected before and after feeding. Daily diet refusals were visually scored for the proportion of pellets versus hay and expressed as a diet selection index (1 to 3). Organic matter digestibility (OMD) was determined using total feces collection in metabolism crates. Plasma urea nitrogen (PUN) was measured in blood samples collected at four time points. The diet-animal fractionation of nitrogen isotopes (Δ15Ntail hair) was quantified in tail hair using isotope-ratio mass spectroscopy to indicate differences in nitrogen metabolism. Finally, carcass characteristics were determined at slaughter. Including carcass fat depth improved (P = 0.01) DMI prediction (R2 increased from 78.4 to 80.4%) in the LP diet but had a smaller effect (P = 0.39) in the HP diet (R2 increased from 75.0 to 75.2%). In the LP diet, the variation in carcass fat-correct RFI (RFIc) could be explained by DMI (14.5% of the variation), rumen bacteriome (5.89%), diet selection index (3.08%), PUN (1.88%), genetic distance (0.76%) and OMD (0.50%), accounting for a total of 26.6% of the RFIc variation. In the HP diet, the regression model explained 35.3% of the RFIc variation, with DMI explaining 25.9%, diet selection index 7.19%, rumen bacteriome 1.02%, genetic distance 0.73% and Δ15Ntail hair 0.50%. In conclusion, this study highlights the importance of measuring diet selection to accurately determine RFI in cattle. The results also suggest that variations in the rumen bacteriome and nitrogen metabolism parameters play a role in individual variations of feed efficiency. However, most of the variation in feed efficiency remained unexplained.

Efficient atrazine removal in bioaugmentation constructed wetland: Insight from stable isotope fractionation analysis

Constructed wetlands (CW) provides a sustainable approach to remove pesticides from agricultural or urban runoff, while the low efficiency of the herbicide atrazine remains a challenge. Focused on improving atrazine removal in CW, a bioaugmentation strategy was implemented using the mixed culture of atrazine-degrading bacteria. The results shown a significant increase in atrazine removal from 26.2 ± 6.9% to 60.9 ± 8.7–90.6 ± 4.1% during the bioaugmentation phases. To investigate in-situ degradation processes of atrazine, compound-specific stable isotope analysis (CSIA) was conducted. The carbon and nitrogen isotope fractionation pattern revealed atrazine was primarily degraded via a hydrolysis pathway mediated by microorganisms. The biodegradation extent of atrazine calculated from δ13C signatures suggested that 90% of atrazine was removed via the hydrolysis pathway, with only a small portion being removed abiotically. This was additionally supported by an increase in microbial diversity and the abundance of specific bacteria (such as Rhizobium sp. and Pseudomonas sp.) capable of degrading atrazine. Altogether, this study demonstrated the effectiveness of atrazine-degrading mixed culture and its feasibility on bioaugmentation atrazine removal in CW. This work highlights the potential of bioaugmentation in sustainable pesticide removal approaches and CSIA can contribute to an understanding of in-situ pesticide transformation processes in wetland systems.

The extent of nitrogen isotopic fractionation in rumen bacteria is associated with changes in rumen nitrogen metabolism.

Nitrogen use efficiency is an important index in ruminants and can be indirectly evaluated through the N isotopic discrimination between the animal and its diet (Δ15Nanimal-diet). The concentration and source of N may determine both the extent of the N isotopic discrimination in bacteria and N use efficiency. We hypothesised that the uptake and release of ammonia by rumen bacteria will affect the natural 15N enrichment of the bacterial biomass over their substrates (Δ15Nbacteria-substrate) and thereby further impacting Δ15Nanimal-diet. To test this hypothesis, two independent in vitro experiments were conducted using two contrasting N sources (organic vs inorganic) at different levels either in pure rumen bacteria culture incubations (Experiment #1) or in mixed rumen cultures (Experiment #2). In Experiment #1, tryptone casein or ammonium chloride were tested at low (1 mM N) and high (11.5 mM N) concentrations on three rumen bacterial strains (Fibrobacter succinogenes, Eubacterium limosum and Xylanibacter ruminicola) incubated in triplicate in anaerobic batch monocultures during 48h. In Experiment #2 mixed rumen cultures were incubated during 120 h with peptone or ammonium chloride at five different levels of N (1.5, 3, 4.5, 6 and 12-mM). In experiment #1, Δ15Nbacteria-substrate was lowest when the ammonia-consumer bacterium Fibrobacter succinogenes was grown on ammonium chloride, and highest when the proteolytic bacterial strain Xylanibacter ruminicola was grown on tryptone. In experiment #2, Δ15Nbacteria-substrate was lower with inorganic (ammonium chloride) vs organic (peptone) N source. A strong negative correlation between Δ15Nbacteria-substrate and Rikenellaceae_RC9_gut_group, a potential fibrolytic rumen bacterium, was detected. Together, our results showed that Δ15Nbacteria-substrate may change according to the balance between synthesis of microbial protein from ammonia versus non-ammonia N sources and confirm the key role of rumen bacteria as modulators of Δ15Nanimal-diet.

The introduction of nitrogen from coal into the surface watershed nitrogen cycle due to coal mining activity

Human activity has doubled the turnover rate of the terrestrial nitrogen cycle, leading to a series of environmental problems. A little-studied nitrogen source in terrestrial and aquatic environments is the nitrogen release associated with rock strata. Southwest China features the largest continuous karsts in the world, featuring a fragile ecological environment but abundant coal resources. The current study selected a typical coal mining area to evaluate the migration and transformation of nitrogen related to coal mining in surface watershed. The findings reveal that the total nitrogen in coal seams was as high as 10,162.3 mg/kg, mainly in the form of organic nitrogen, followed by NH4+-N, while the content of NO3−-N was negligible. Based on the isotope fractionation and the co-evolution between Δ15NNO3-NH4 and δ15N-NO3−/δ15N-NH4+, coal mining changed the coal seams' oxidation-reduction state, resulting in the mineralization of organic nitrogen to NH4+-N. Next, NH4+-N gradually oxidized to NO3−-N. Various forms of coal-origin nitrogen may be leached out by acid mine drainage (AMD), potentially contributing >10 % of NO3−-N and 90 % of NH4+-N to the surface river. Another nitrogen source that requires serious consideration is the wide use of ammonium nitrate explosives in coal mining, as blasting residues may contribute about another 10 % to NO3−-N in surface water. Since organic nitrogen accounts for >90 % of extractable nitrogen, the release of coal-origin nitrogen may contribute much more to the total nitrogen in surface water than to NO3−-N. Based on the fractionation of nitrogen and oxygen isotopes of nitrate, low-pH AMD promotes the volatilization of nitrate in the form of nitric acid. The conversion of different forms of nitrogen in AMD will be the focus of future attention.

Equilibrium partitioning and isotopic fractionation of nitrogen between biotite, plagioclase, and K-feldspar during magmatic differentiation

A significant portion of the continental crust is composed of plutonic igneous rocks. However, little is known about the geochemical behaviour of N between the different minerals during magmatic differentiation. To provide new constraints for the behaviour of N during crust formation, we have characterised the geochemistry of nitrogen (N) in the compositionally zoned calc-alkaline pluton at Loch Doon, SW Scotland. We present N concentration and N isotope values for whole-rock data alongside biotite, plagioclase and K-feldspar mineral separates and assess the degree to which these data preserve equilibrium partitioning during magmatic differentiation. We show that whole rock likely inherited its N contents and δ15N signatures from the initial source composition and that this signature is homogenous at a pluton scale. Whilst the whole-rock data are best explained as crust-derived N in the source, the degree of homogenisation across a pluton scale is inconsistent with empirical N diffusivities, ruling out syn-emplacement crustal assimilation as the source of N. Instead, our data suggest a crustal signature inherited from depth associated with the Iapetus subduction zone. At a mineral scale, we find that N preferentially partitions into the feldspars over mica in this system in the order K-feldspar > plagioclase ≈ biotite > quartz, with average mineral–mineral distribution coefficients of DN plagioclase-biotite = 1.3 ± 0.6 and DN Kspar-biotite = 2.8 ± 0.6. Partitioning is accompanied by a large and near constant equilibrium isotope fractionation factor between biotite and both feldspars (averages are Δ15NPlag-Biotite = +7.8 ± 1.2‰ and Δ15NKspar-Biotite = +7.9 ± 1.0‰). In contrast, Δ15NKspar-Plagioclase closely approximates 0‰, where both minerals show δ15N values overlapping with the bulk rock δ15N values. These results show that mica crystallisation generates a 15N-depleted reservoir within plutonic rocks. Moreover, our dataset suggests that feldspars might be a more significant host of N in the igneous portion of Earth’s continental and oceanic crust than previously thought.

A pilot study of stable isotope fractionation in Bombyx mori rearing

Hydrogen, oxygen, carbon, and nitrogen isotopes derived from three different strains of silkworms at different life stages involved in silkworm rearing, were measured to understand the fractionation characteristics of stable isotopes at different stages of silkworm development, and to trace the movement of these isotopes from food to larva to excrement and finally to silk. We found that silkworm strain had little effect on δ2H, δ18O and δ13C values. However, a large difference was found in the δ15N levels of newly-hatched silkworms between Jingsong Haoyue and Hua Kang No. 3 orthogonal strains, suggesting that the mating and egg laying differences may result in an inconsistent kinetic nitrogen isotope fractionation. The δ13C values of silkworm pupae and silkworm cocoon also displayed significant differences, suggesting that heavy carbon isotopes are greatly fractionated from the larva to the silk during cocoon formation. Overall, these results may be used to clarify the relationship between isotope fractionation and the ecological process of the Bombyx mori and expand our ability to resolve stable isotope anomalies at a small regional-scale level.

Nitrogen isotope geochemistry of ion adsorption-type REE mineralization: Insights from the weathered granitoid rocks in the Sancheong district, South Korea

Ion adsorption-type rare earth element (REE) deposits formed by the weathering of igneous rocks have drawn much attention in recent years as an important source of heavy REEs including yttrium (Y). Our study investigated the mechanism of REE enrichment on clay fractions formed in weathered crusts of granitic orthogneiss, gabbroic anorthosite, and syenite in the Sancheong district, South Korea, based on trace element and ammonium-nitrogen behavior. Ion adsorption-type REE mineralization commonly occurs with kaolinite and halloysite in the B-horizon layer, which reflects a series of pH changes from wholly weathered surface soil (A-horizon) to slightly weathered rock (C-horizon). The ∑REEs of clay fractions extracted from weathered crusts under weakly acidic conditions (pH 5–7) were higher than those of the bulk rocks. In contrast, the ∑REEs of the clay fractions decreased sharply under more acidic conditions (pH <4). The enrichment of REEs in both the bulk rocks and clay fractions correlates negatively with Eu/Eu* but positively with Th(n) and U(n), which indicates that the REEs were gradually concentrated during igneous and weathering processes. Ammonium-nitrogen concentrations correlate with the enrichment of REEs as well as the increasing δ15N values in the weathered crusts of each rock type. The δ15N values of the clay fractions (av. 5.1‰, av. 3.5‰, av. 1.1‰), which are heavier than those of the bulk rocks (av. 4.0‰, av. 1.4‰, av. –0.7‰) for granitic orthogneiss, gabbroic orthogneiss, and syenite, respectively, are mostly attributed to the influence of sedimentary organic matter, in addition to nitrogen isotope fractionation. The geochemical characteristics and the features of nitrogen behavior in the bulk rock and its clay fraction contribute to our understanding of the genesis of ion adsorption-type REEs mineralization on the weathered crusts of granitoid rocks.

Nitrogen isotopes suggest agricultural and non-agricultural sources contribute equally to NH3 and NH4+ in urban Beijing during December 2018

Agricultural and non-agricultural sources emission contribute to atmospheric ammonia (NH3) and particulate ammonium (NH4+). However, our understanding on the sources of NH3 and NH4+ in PM2.5 (particles smaller than 2.5 μm) during the winter period in the urban atmosphere is limited. Here, we measured the concentrations and stable nitrogen isotopic composition (δ15N) of NH3 and NH4+ in parallel during December 2018 in urban Beijing to assess the non-agricultural and agricultural sources contributions to NH3 and NH4+ in ambient air based on the Chemical Transport Model (CTM), a Bayesian isotope mixing model (SIMMR), and the δ15N signatures that we developed. Our study found weekly NH4+ and NH3 concentrations were on average 2.5 ± 1.4 μg m−3 and 3.8 ± 1.7 μg m−3, respectively during December 2018. Weekly concentration weighted δ15N(NH4+) values ranged from 4.5‰ to 13.7‰ with an average value of 8.2 ± 3.9‰ during December 2018. After accounting for nitrogen isotopic fractionation from NH3 gas to NH4+ conversion, initial δ15N(NH3) values ranged from −22.5‰ to −12.8‰ with an average value of −17.4 ± 3.5‰. Further, weekly measured δ15N(NH3) values ranged from −22.2‰ to −10.2‰ (after correction) with an average value of −15.6 ± 5.3‰ during December 2018. Results from two different isotope-based method showed non-agricultural sources contributed 31.2%–63.1%, with an average value of 47.5 ± 14.6%, to NH4+ and 32.3%–71.2%, with an average of 53.4 ± 16.1%, to ambient NH3 during December 2018 in Beijing. Results from CMAQ-ISAM suggest non-agricultural sources contributed on average 66.2 ± 1.9% to ambient NH4+ and 66.4 ± 1.9% to ambient NH3 during December 2018. Results from this study suggest that agricultural and non-agricultural sources nearly equally contributed to NH3 and NH4+ in urban Beijing during December 2018 with an uncertainty range of 13%–19% between isotope-based methods and CTM method.

Formation Mechanisms and Source Apportionments of Nitrate Aerosols in a Megacity of Eastern China Based On Multiple Isotope Observations

AbstractInorganic nitrate (NO3−) is a crucial component of fine particulate matter (PM2.5) in haze events in China. Understanding the formation mechanisms of nitrate and the sources of NOx was critical to control the air pollution. In this study, measurements of multiple isotope compositions of nitrate (δ18O‐NO3−, δ17O‐NO3−, and δ15N‐NO3−) in PM2.5 were conducted in Hangzhou from 9 October 2015 to 24 August 2016. Our results showed that oxygen anomaly of nitrate (Δ17O‐NO3−: 20.0‰–37.9‰) and nitrogen isotope of nitrate (δ15N‐NO3−: −2.9‰ to 18.1‰) values were higher in winter and lower in summer. Based on Δ17O‐NO3− observation and a Bayesian model, NO3 radical chemistry was found to dominate the nitrate formation in winter, while photochemical reaction (NO2 + OH) was the main pathway in summer. After considering the nitrogen isotopic fractionation in the NOx(g)‐NO3−(p) conversion, the average contributions of coal combustion, vehicle exhausts, biomass burning, and soil emission were 50% ± 9%, 19% ± 12%, 26% ± 15%, and 5% ± 4%, respectively, to nitrate aerosols during the whole sampling period. Coal combustion was the most important nitrate source in Hangzhou, especially in winter (∼56%). The contribution of soil emission increased significantly in summer due to active soil microbial processes under high temperature environment.

Abiotic reduction of nitrate to ammonium by iron (oxy)(hydr)oxides and its stable isotope (δ15N, δ18O) dynamics

In this study, the kinetics and stable isotopic fractionation of nitrate (NO3−) reduction in Fe(II)/Fe(III) homogeneous/heterogeneous systems were investigated at circumneutral pH (i.e., pH 6.5 to 7.5) and ambient temperature (24 °C). Of the suite of iron (oxy)(hydr)oxides examined, NO3− reduction by Fe(II) was only significant in the presence of layered double hydroxide green rust (GR) minerals. Comparison of type 1 and type 2 GRs demonstrated that type 1 (GR(Cl−)) had the fastest NO3− reducing ability, with an Fe(II)-normalized pseudo first-order kinetic rate constant of kobs (NO3−) = 16.2 × 101 M−1 d−1. Anion concentrations (SO42− or Cl−) and the ratio of [Fe(II)] to [Fe(III)] also influenced reduction kinetics. Ammonium (NH4+) was the predominant reaction end-product (>50%) with the remaining reduced N species concluded to be comprised of N2(g) as other intermediates (nitrite (NO2−), nitric oxide (NO) and nitrous oxide (N2O) were not detected during NO3− reduction. Apparent nitrogen (δ15N-NO3−) isotopic fractionation (15ε) for GR(Cl−) and GR(SO42−) were were calculated to be 12.9 (CI: 8.2, 16)‰ and 37.9 (CI: 34, 41)‰, respectively. Similarly, oxygen (δ18O-NO3−) isotopic fractionation (18ε) differed between GR(Cl−) and GR(SO42−): 4.53 (CI: 2.5, 5.8)‰ and 14.4 (CI: 12, 16)‰. However, the ratio of 18ε:15ε for both minerals could be fitted to a linear regression of slope 0.369 (CI: 0.361, 0.377). These findings reaffirm the importance of GR minerals to NO3− reduction, especially in iron-rich systems hosting dynamic redox oscillations, including hyporheic zones, estuarine sediments and groundwater aquifers. These results also demonstrate that N and O stable isotope kinetic fractionation analyses as well as the ratio of 18ε:15ε can provide a means to distinguish between this abiotic anoxic reaction from anaerobic bacterial NO3− reduction processes.

What controls aerosol δ15N-NO3−? NOx emission sources vs. nitrogen isotope fractionation

Atmospheric δ15N-NO3− has been used to reveal NOx (NO + NO2) sources as NO3− is the ultimate sink of NOx. However, it remains questionable whether the nitrogen isotope fractionation among NOy (NO, NO2, NO3, N2O5, HNO3 and NO3−) engender the misjudgment of NOx emission sources by affecting δ15N-NOy. To explore this issue, we integrated the dataset of aerosol δ15N-NO3− values and ratios of fNO2 (fNO2 = NO2/(NO2 + NO)), calculated the nitrogen isotope fractionation factors (Δs) among NOy, compared the total energy consumption in Beijing-Tianjin-Hebei region (BTH) from 2013 to 2018. Results showed that, although the total energy consumption structure changed from 2013 to 2018 in BTH, there were fewer interannual variances of aerosol δ15N-NO3− values. Nitrogen isotope fractionation factors between NO and NO2 (Δ0), NO2 and NO3 (Δ2), NO2 and N2O5 (Δ3), NO2 and ClONO2 (Δ4) also displayed less interannual variations from 2013 to 2018 in BTH. But both aerosol δ15N-NO3− and Δs displayed significant seasonal patterns, and there was significant relationship between monthly aerosol δ15N-NO3− and Δs, which suggested that Δs have important influence on shaping aerosol δ15N-NO3− and further discriminating NOx emission sources. This study implies that we should refine the Δs when employing atmospheric δ15N-NO3− to quantify NOx source allocation.