15N Enrichment Research Articles

Using δ15N of Amino Acids and Nitrate to Investigate Particle Production and Transformation in the Ocean: A Case Study From the Eastern Tropical North Pacific Oxygen Deficient Zone

AbstractThe eastern tropical North Pacific oxygen deficient zone (ETNP‐ODZ) exhibits a distinct physical and biological environment compared to other oxygenated water columns, leading to a unique scenario of particulate organic matter (POM) production and vertical transport. To elucidate these biological pump processes, we present the first comparison of δ15N values of nitrate, phenylalanine (Phe), and glutamic acid (Glu) within two distinct size fractions of particles collected along a productivity gradient in the ETNP‐ODZ. Low δ15NPhe and δ15NGlu values in both particle pools at sites with prominent secondary chlorophyll maximum (SCM), compared to the ambient δ15N‐NO3−, suggest the presence of recycled N‐utilizing primary producers distinct from those at the primary chlorophyll maximum and their contribution to export. We observed reduced 15N enrichment of Phe in small particles and a narrower δ15NPhe disparity between the two particle size fractions compared to the results from oxic waters, likely due to slower heterotrophic microbial degradation of small particles. Unique δ15NPhe and δ15NGlu signatures of particles were found at the lower oxycline, potentially attributable to chemoautotrophic production and zooplankton mediation. These findings underscore the need for further investigations targeting particles generated at the SCM, their subsequent alteration by zooplankton, and the new production by chemoautotrophs. This will allow for a better evaluation of the efficiency of the biological pump in the globally expanding ODZs under contemporary climate change.

The Restoration Effect of Degraded Grassland Depends on the Response of Species Root Distribution to Resource Availability

Nitrogen (N) fertilization is an effective practice for restoring degraded grasslands, which might strongly depend on the rooting system and resource competition of individual plant species. The purpose of this study is to explore a method to distinguish the response of various plant root architectures to the resource availability in a mixed ecosystem in situ. Field experiments were conducted using isotope techniques in conjunction with a specialized experimental design at a semiarid grassland location featuring heavily grazed (HG) and moderately grazed (MG) grassland sites with different dominant species. The same amounts of water and 15N-labelled fertilizer were uniformly supplied by a tube fertigation system at soil depths of 0, 15 and 45 cm. At both the HG and MG sites, there was a significant increase in aboveground net primary production (ANPP), water use efficiency (WUE) and 15N use efficiency (15NUE) at the community level with increasing depths of fertigation. The ANPP and plant N uptake exhibited higher values at the HG site compared to those at the MG site, while 15NUE and 15N abundance were significantly lower at the HG site. The annual species Salsola collina Pall. exhibited the highest aboveground biomass (AGB) and 15N abundance compared to all other species. Furthermore, the 15N enrichment of S. collina increased with greater depths of 15N-labelled fertilization, indicating that S. collina might develop a more extensive root system in response to water and N addition in the degraded grassland. Our study highlights that using isotope methods could indirectly distinguish root distribution and resource acquisition. In the recovery of degraded grassland by N fertilizer, we should not only consider the aboveground biomass but also pay special attention to the resource competition of individual plant species due to the possible discrepancy in rooting systems.

A Novel System for Simultaneous Real-Time Determination of 15N-Enriched Ammonia, Hydroxylamine, Nitrite, and Nitrate.

Ammonium (NH4+), hydroxylamine (NH2OH), nitrite (NO2-), and nitrate (NO3-) account for the most important reactive nitrogen (N) species in the N cycle, playing a key role in N elimination and N retention, as well as the production of nitrogenous trace gases. However, it is still challenging to fulfill simultaneous real-time determination of all four N compounds enriched in 15N. This study successfully established a novel system by coupling an automatic simultaneous sample preparation unit to a membrane inlet mass spectrometer (4n-ASSP-MIMS) for rapid online 15N fraction analysis of all four key compounds in the N cycle. The limit of detection (LOD) was 20.22, 0.38, 0.17, and 0.25 μM for NH4+, NH2OH, NO2-, and NO3-, respectively, and relative error for 15N fraction determination was within 5% when 15N enrichment was higher than 5 atom %. This 4n-ASSP-MIMS system provides a first online analytical tool to capture the real-time isotopic signals during biogeochemical transformations of NH4+, NH2OH, NO2-, and NO3-, using the 15N tracing technique. Application of this system will dramatically advance our understanding of mechanisms involved in the N cycle.

Comparison of natural abundance and enriched 15N methods to quantify nitrogen fertilizer recovery in maize under field conditions

Comparison of natural abundance and enriched 15N methods to quantify nitrogen fertilizer recovery in maize under field conditions

Source, transport, and fate of nitrate in shallow groundwater in the eastern Niger Delta

The eastern Niger Delta region in Nigeria is a hotspot for reactive nitrogen pollution due to extensive animal husbandry, pit latrine usage, and agricultural practices. Despite the high level of human activity, the sources and processes affecting nitrogen in groundwater remain understudied. Groundwater nitrate (NO3−) concentrations are highly variable, with some areas recording values well above the safe drinking water threshold of 50 mg/L. This is particularly true near municipal sewage systems. Elevated nitrite (NO2−) and ammonium (NH4+) concentrations were also detected in the study area. Sewage analysis revealed NO3− concentrations ranging from 1 to 145 mg/L, NO2− from 0.2 to 2 mg/L, and notably high NH4+ concentrations. A comparison of major ions indicated that 71%, 90%, 87%, and 92% of groundwater samples surpassed reference site levels for calcium (Ca2+), sodium (Na+), potassium (K+), and chloride (Cl−), respectively, pointing to sewage as a likely source of contamination. The NO3−/Cl− ratios at several sites suggested that most groundwater NO3− originates from human waste. Stable isotope analysis of NO3− showed a general enrichment in 15N and, in some cases, a depletion in 18O, indicating that the NO3− originates from sewage-derived NH4+ nitrification. Although denitrification, a process that reduces NO3−, is present, the high dissolved oxygen (DO) and NO3− levels in the groundwater suggest that denitrification is insufficient to fully mitigate NO3− pollution. Consequently, there is a risk of NO3− leaching from shallow aquifers into the Niger Delta’s surface waters and ultimately into the coastal ocean.

Measuring 15N and 13C Enrichment Levels in Sparsely Labeled Proteins Using High-Resolution and Tandem Mass Spectrometry.

Isotope labeling of both 15N and 13C in selected amino acids in a protein, known as sparse labeling, is an alternative to uniform labeling and is particularly useful for proteins that must be expressed using mammalian cells, including glycoproteins. High levels of enrichment in the selected amino acids enable multidimensional heteronuclear NMR measurements of glycoprotein three-dimensional structure. Mass spectrometry provides a means to quantify the degree of enrichment. Mass spectrometric measurements of tryptic peptides of a selectively labeled glycoprotein expressed in HEK293 cells revealed complicated isotope patterns which consisted of many overlapping isotope patterns from intermediately labeled peptides, which complicates the determination of the label incorporation. Two challenges are uncovered by these measurements. Metabolic scrambling of amino groups can reduce the 15N content of enriched amino acids or increase the 15N in nontarget amino acids. Also, undefined, unlabeled medium components may dilute the enrichment level of labeled amino acids. The impact of this unexpected metabolic scrambling was overcome by simulating isotope patterns for all isotope-labeled peptide states and generating linear combinations to fit to the data. This method has been used to determine the percent incorporation of 15N and 13C labels and has identified several metabolic scrambling effects that were previously undetected in NMR experiments. Ultrahigh mass resolution is also utilized to obtain isotopic fine structure, from which enrichment levels of 15N and 13C can be assigned unequivocally. Finally, tandem mass spectrometry can be used to confirm the location of heavy isotope labels in the peptides.

A Novel Approach for Real-Time Determination of 15N-Enriched Hydroxylamine.

Hydroxylamine (NH2OH) is a critical precursor of nitrous oxide (N2O) and key intermediate in the nitrogen cycle. However, the conversion of NH2OH is very fast, and the lack of real-time 15N analytical methods for NH2OH hinders the on-time capture of its biochemical signals in the N cycle. To bridge this gap, we developed a novel approach for real-time determination of 15N-enriched NH2OH. In this approach, an automated sample inlet unit was coupled to a membrane-inlet mass spectrometer, and NH2OH was converted to N2O by sodium hypochlorite for analysis. The interference of carbon dioxide was successfully removed by an ascarite trap, and the N2O signal showed good linearity over the targeted NH2OH concentrations. The limit of detection and limit of quantification of this approach were 0.38 and 1.28 μM, respectively, and 15N enrichment can be accurately detected when the 15N enrichment is higher than 5 atom %. This approach provides a first online analytical tool to capture real-time NH2OH transforming signals using the 15N tracing technique, which will advance mechanism studies of the N cycle.

Scaling up taxon-specific microbial traits to predict community-level microbial activity in agricultural systems

Soil microorganisms perform many important ecosystem functions including nitrogen (N) cycling which dictates plant productivity in agricultural ecosystems. Despite the importance of these communities, connecting microbial composition with ecosystem function has been a long standing challenge. Taxon-specific substrate assimilation traits, measured with quantitative stable isotope probing (qSIP), may provide a means to scale from microbial community composition to community-level process rates. To test the potential for scaling up taxon-specific N assimilation to predict community-level rates of carbon mineralization, N mineralization, and N immobilization we measured soil properties, microbial activity, and N assimilation using 15N qSIP in soils from six distinct farm systems. N assimilation, measured as DNA 15N enrichment, varied among taxa and within taxa across farms. Taxon specific N assimilation was aggregated to calculate a community-weighted mean, which when combined with measures of microbial biomass was used to estimate new microbial biomass N production. This estimate of new microbial biomass production reflects the growth of active microbes over the incubation period and related to microbial activity. The new microbial biomass N produced was predictive of soil C and N mineralization rates, explaining 37–47% of the observed variation across the six farming systems. This approach highlights the ability of trait-based methods to relate microbial community structure data to microbially mediated functional process rates. Such advances may enhance our ability to understand and manage microbially mediated processes, such as N cycling, in both natural and agricultural ecosystems.

Carbon and nitrogen sources in tropical coastal lagoon food webs under variable hydrological conditions

Whilst the impact of continental and marine nutrient sources on the ecological functioning of coastal food webs is well investigated in temperate regions, tropical ecosystems remain less well understood, in particular coastal lagoons. In this study, the sources of carbon and nitrogen in a coastal lagoon system in the southeast of Madagascar are traced using stable isotopes of carbon (δ13C) and nitrogen (δ15N). Three interconnected coastal lagoons with different degree of river influence and eutrophication are investigated. Their food webs are assessed with respect to spatial and seasonal (wet and dry seasons) variations in lagoon water conditions, as well as with respect to the sources of nutrients and organic matters. Results show that river input is the main source of NO3−, and that NO3− supply is significantly higher during the wet season. In contrast, NH4+ is produced internally in the lagoon, and concentrations are higher during the dry season. A lower δ13C value observed in particulate organic matter (POM; proxy of phytoplankton) indicates terrestrial-riverine carbon inputs, which generally have low δ13C values. During the dry season, exceptionally high δ15N values of lagoon POM suggest the uptake of newly produced lagoon water NH4+ and/or the 15N enrichment of lagoon POM pool due to mineralization, resulting in 15N enrichment in consumers. Moreover, δ13C and δ15N values of consumers (fishes and invertebrates) reflect predominantly those of lagoon POM and sediment organic matter (SOM), suggesting that consumers primarily depend on lagoon POM and SOM as sources of carbon and nitrogen. The δ15N values in consumers further indicate that some species feed on more than one trophic level, suggesting flexible foraging strategy of consumers as a function of food source availability. This study demonstrates the roles of both river inflow and sediment in supplying carbon and nitrogen to a coastal lagoon food web, documenting the ecological implications of seasonal variations in lagoon hydrological conditions.

Stable Isotope Signatures Illuminate Diverse Nutritional Strategies in Rhizoctonias and Their Orchid Partners.

Understanding the nutritional ecology of orchids, particularly those in symbiosis with rhizoctonias, presents a complex challenge. This complexity arises partly from the absence of macroscopic fruit bodies in rhizoctonias, which impedes the acquisition of their stable isotope data. In this study, we investigated the fungal associations and isotopic signatures in the pelotons of Stigmatodactylus sikokianus (associated with non-ectomycorrhizal [non-ECM] rhizoctonias) and Chamaegastrodia shikokiana (associated with ECM rhizoctonias). Our research reveals elevated levels of 13C enrichment in S. sikokianus plants and their pelotons, similar to those found in fully mycoheterotrophic orchids and their mycobionts. Interestingly, C. shikokiana plants and their pelotons exhibited even higher levels of 13C and 15N enrichment than many other fully mycoheterotrophic species. Our findings imply that both ECM and saprotrophic mycobionts, including certain rhizoctonias, can fulfill the carbon needs of highly mycoheterotrophic orchids. This finding also indicates that 13C enrichment can be an indicator of mycoheterotrophy in at least some rhizoctonia-associated orchids, despite the typically low 13C enrichment in non-ECM rhizoctonias. Our demonstration of partial mycoheterotrophy in S. sikokianus suggests a broader prevalence of this nutritional strategy among orchids, given that almost all orchids are associated with non-ECM rhizoctonias.

Evidence for Surprising Heavy Nitrogen Isotopic Enrichment in Comet 46P/Wirtanen’s Hydrogen Cyanide

46P/Wirtanen is a Jupiter-family comet, probably originating from the solar system’s Kuiper Belt, that now resides on a 5.4 yr elliptical orbit. During its 2018 apparition, comet 46P passed unusually close to the Earth (within 0.08 au), presenting an outstanding opportunity for close-up observations of its inner coma. Here we present observations of HCN, H13CN, and HC15N emission from 46P using the Atacama Compact Array. The data were analyzed using the SUBLIME non-LTE radiative transfer code to derive 12C/13C and 14N/15N ratios. The HCN/H13CN ratio is found to be consistent with a lack of significant 13C fractionation, whereas the HCN/HC15N ratio of 68 ± 27 (using our most conservative 1σ uncertainties), indicates a strong enhancement in 15N compared with the solar and terrestrial values. The observed 14N/15N ratio is also significantly lower than the values of ∼140 found in previous comets, implying a strong 15N enrichment in 46P’s HCN. This indicates that the nitrogen in Jupiter-family comets could reach larger isotopic enrichments than previously thought, with implications for the diversity of 14N/15N ratios imprinted into icy bodies at the birth of the solar system.

Ammonia synthesis via an engineered nitrogenase assembly pathway in Escherichia coli.

Heterologous expression of nitrogenase has been actively pursued because of the far-reaching impact of this enzyme on agriculture, energy and environment. Yet, isolation of an active two-component, metallocentre-containing nitrogenase from a non-diazotrophic host has yet to be accomplished. Here, we report the heterologous synthesis of an active Mo-nitrogenase by combining genes from Azotobacter vinelandii and Methanosarcina acetivorans in Escherichia coli. Metal, activity and EPR analyses demonstrate the integrity of the metallocentres in the purified nitrogenase enzyme; whereas growth, nanoSIMS and NMR experiments illustrate diazotrophic growth and 15N enrichment by the E. coli expression strain, as well as accumulation of extracellular ammonia upon deletion of the ammonia transporter that permits incorporation of thus-generated N into the cellular mass of a non-diazotrophic E. coli strain. As such, this study provides a crucial prototype system that could be optimized/modified to enable future transgenic expression and biotechnological adaptations of nitrogenase.

Reconstructing diet after the fall of the Wari Empire in the central Andean Highlands: A study of human dental calculus

Stable isotopes from bones and teeth have been used to reconstruct human lifeways in bioarchaeological research for over 30 years. Recently, there have been efforts to use less invasive methods of analysis that meet ethical standards and do not compromise the integrity of human skeletal remains. To that end, this study examines dental calculus from human teeth as a proxy for paleodietary reconstruction. This current work builds on a handful of studies that have tested the use of dental calculus for reconstructing diet, which have shown mixed results. This study contributes to those ongoing efforts to improve methods and capacities in paleodiet research. The sample is comprised of dental calculus from individuals from the Wari (600–1000 CE) and post-Wari periods (1000–1400 CE). Individuals from both time periods with stable carbon and nitrogen isotope data from the calculus are directly compared to the stable isotope results from human bone collagen, dental carbonates, and bone carbonates. Results from dental calculus show δ13C that range from -20.5 ‰ to −12.3 ‰, consistent with the use of C4 plants (maize) as a major component of the diet. The δ13C from calculus are isotopically lighter and more varied than those obtained from collagen, and this is likely explained by the distinct composition of calculus compared to that of bone collagen and apatite. The δ15N from calculus range from +8.9 ‰ to +18.8 ‰, which is heavier than expected for highland maize-based diets in the Peruvian Andes. This may be explained by 15N enrichment from aridity or crop fertilization (e.g., camelid dung), especially during the post Wari period, a time of social upheaval and severe drought. The differences in stable isotope values may also be partially explained by the diet-sourced isotopes in bone collagen (e.g., averaging the last 5–10 years of life) versus that of dental calculus (e.g., averaging the last few years of life). Documenting these differences in stable isotope ratios from distinct components may aid in richer understandings of past diets and provide additional ways to compare diet through time and space. Indeed, as more researchers begin analyzing the stable isotope ratios from dental calculus, we can minimize destructive techniques and make direct comparisons between studies that use dental calculus. We suggest that this is an important expansion of our analytical toolkit.

Automated Assignment of 15N And 13C Enrichment Levels in Doubly-Labeled Proteins.

Uniform enrichment of 15N and 13C in proteins is commonly employed for 2D heteronuclear NMR measurements of the three-dimensional protein structure. Achieving a high degree of enrichment of both elements is important for obtaining high quality data. Uniform labeling of proteins and glycoproteins expressed in higher organisms (yeast or mammalian cell lines) is more challenging than expression in Escherichia coli, a prokaryote that grows on simple, chemically defined media but does not provide appropriate eukaryotic modifications. It is difficult to achieve complete incorporation of both heavy isotopes, and quality control measures are important for quantitating the level of their enrichment. Mass spectrometry measurements of the isotopic distribution of the intact protein or its proteolytic fragments provide the means to assess the enrichment level. A mass accuracy of 1 ppm or better is shown to be required to distinguish the correct combination of 13C and 15N enrichment due to subtle shifts in peak centroids with differences in the underlying, but unresolved, isotopic fine structure. A simple computer program was developed to optimize the fitting of experimental isotope patterns to statistically derived distributions. This method can determine the isotopic abundance from isotope patterns and isotopologue masses in conventional MS data for peptides, intact proteins, and glycans. For this purpose, MATLAB's isotope simulator, isotopicdist, has been modified to permit the variation of 15N and 13C enrichment levels and to perform a two-dimensional grid search of enrichment levels of both isotopes. We also incorporated an alternate isotope simulator, js-emass, into MATLAB for use in the same fitting program. Herein we benchmark this technique on natural abundance ubiquitin and uniformly [15N,13C]-labeled ubiquitin at both the intact and peptide level, outline considerations for data quality and mass accuracy, and report several improvements we have made to the previously reported analysis of the [15N,13C]-enriched human IgG Fc domain, a glycoprotein that has been expressed in Saccharomyces cerevisiae.

Thermokarst landscape exhibits large nitrous oxide emissions in Alaska’s coastal polygonal tundra

Global atmospheric concentrations of nitrous oxide have been increasing over previous decades with emerging research suggesting the Arctic as a notable contributor. Thermokarst processes, increasing temperature, and changes in drainage can cause degradation of polygonal tundra landscape features resulting in elevated, well-drained, unvegetated soil surfaces that exhibit large nitrous oxide emissions. Here, we outline the magnitude and some of the dominant factors controlling variability in emissions for these thermokarst landscape features in the North Slope of Alaska. We measured strong nitrous oxide emissions during the growing season from unvegetated high centered polygons (median (mean) = 104.7 (187.7) µg N2O-N m−2 h−1), substantially higher than mean rates associated with Arctic tundra wetlands and of similar magnitude to unvegetated hotspots in peat plateaus and palsa mires. In the absence of vegetation, isotopic enrichment of 15N in these thermokarst features indicates a greater influence of microbial processes, (denitrification and nitrification) from barren soil. Findings reveal that the thermokarst features discussed here (~1.5% of the study area) are likely a notable source of nitrous oxide emissions, as inferred from chamber-based estimates. Growing season emissions, estimated at 16 (28) mg N2O-N ha−1 h−1, may be large enough to affect landscape-level greenhouse gas budgets.

A pilot study on the isotopic characterization of feeding habits of Diptera in a tropical rain forest

Flies (Diptera) are among the most diverse groups of insects and are known to utilize various food resources, including plants, detritus, microbial tissues, and fresh and dead animal tissues. However, their feeding habits in the field remain poorly understood. We conducted a pilot study to apply stable nitrogen (N) and carbon (C) isotope techniques to examine the feeding habits of flies in a tropical rain forest in Sarawak, Malaysia. The fly samples comprised 13 families and >18 species. The results showed significant differences in nitrogen and carbon isotope ratios (δ15N and δ13C) among families and species within a family. The observed pattern is largely consistent with their known feeding habits; flies that use carcasses and carrion as diets (e.g., Sarcophagidae and Calliphoridae) have significantly higher δ15N values than those likely utilizing plant-based diets (e.g., Cecidomyiidae). There were significant differences in δ15N values among the six species of Calliphoridae, which is consistent with insect succession on carcasses. The differences in δ15N may be explained by the use of carrion at different stages of decomposition, because microbial decomposition can lead to the 15N enrichment. Tachinid flies had relatively low δ13C values, reflecting the use of lepidopterans as a host. This pilot study shows that the δ15N and δ13C values of flies provide insights into the diversity of feeding habits of fly communities, which could also serve as an indicator of resource availability in an entire ecosystem.

Mode of carbon gain and fungal associations of Neuwiedia malipoensis within the evolutionarily early-diverging orchid subfamily Apostasioideae.

The earliest-diverging orchid lineage, Apostasioideae, consists only of two genera: Apostasia and Neuwiedia. Previous reports of Apostasia nipponica indicated a symbiotic association with an ectomycorrhiza-forming Ceratobasidiaceae clade and partial utilization of fungal carbon during the adult stage. However, the trophic strategy of Neuwiedia throughout its development remains unidentified. To further improve our understanding of mycoheterotrophy in the Apostasioideae, this study focused on Neuwiedia malipoensis examining both the mycorrhizal association and the physiological ecology of this orchid species across various development stages. We identified the major mycorrhizal fungi of N. malipoensis protocorm, leafy seedling and adult stages using molecular barcoding. To reveal nutritional resources utilized by N. malipoensis, we compared stable isotope natural abundances (δ13C, δ15N, δ2H, δ18O) of different developmental stages with those of autotrophic reference plants. Protocorms exhibited an association with saprotrophic Ceratobasidiaceae rather than ectomycorrhiza-forming Ceratobasidiaceae and the 13C signature was characteristic of their fully mycoheterotrophic nutrition. Seedlings and adults were predominantly associated with saprotrophic fungi belonging to the Tulasnellaceae. While 13C and 2H stable isotope data revealed partial mycoheterotrophy of seedlings, it is unclear to what extent the fungal carbon supply is reduced in adult N. malipoensis. However, the 15N enrichment of mature N. malipoensis suggests partially mycoheterotrophic nutrition. Our data indicated a transition in mycorrhizal partners during ontogenetic development with decreasing dependency of N. malipoensis on fungal nitrogen and carbon. The divergence in mycorrhizal partners between N. malipoensis and A. nipponica indicates different resource acquisition strategies and allows various habitat options in the earliest-diverging orchid lineage, Apostasioideae. While A. nipponica relies on the heterotrophic carbon gain from its ectomycorrhizal fungal partner and thus on forest habitats, N. malipoensis rather relies on own photosynthetic carbon gain as an adult, allowing it to establish in habitats as widely distributed as those where Rhizoctonia fungi occur.

The Use of High-Density UN Fuel in Heat-Pipe Microreactors

Heat-pipe microreactors (HPMRs) are very small-scale nuclear reactors that employ heat pipes (HPs) for heat removal. HPMRs can be easily integrated with other forms of renewable energies, can be used for emergency responses to disaster relief zones, can be deployed in remote locations not connected to the grid, and can be removed from sites and replaced by new ones. HPMRs can also be used for space missions as HPs do not rely on gravity for heat transfer. Conventional fuel materials, such as uranium oxide (UO2) and uranium oxycarbide (UCO), are currently considered in most existing HPMR designs, but ceramic uranium nitride (UN) fuel that has high uranium density, high thermal conductivity, and high melting point may become a better fuel candidate. Through neutronics calculations, this paper assesses the impact of using UN fuel in HPMRs with two different neutron spectra (fast and thermal) and two different fuel forms [traditional solid fuel pellets and TRi-structural-ISOtropic (TRISO) fuel compacts]. It was concluded that retrofitting HPMRs with UN fuel has the potential to reduce the initial 235U enrichment requirement by ~3 wt% (to keep the same cycle length) or increase the cycle length (by keeping the same initial 235U enrichment), which enables more compact and transportable HPMR core designs. However, using UN fuel decreases the control element worth [by up to 20% for the Special Purpose Reactor (SPR) and 5% for HP-MR] and is up to 80% more costly. Increasing 15N enrichment can further decrease the initial 235U enrichment requirement and increase the control element worth but is more costly. Compared to fast-spectrum HPMRs fueled with solid pellet fuels, retrofitting UN fuel is more suitable for thermal-spectrum HPMRs fueled with TRISO fuel compacts, where the neutron spectrum hardening caused by using UN is less significant.

Nonlinear models of 15N partitioning kinetics in late-lactation dairy cows from individually labeled feed ingredients

Few studies have examined the N kinetics of individual feeds with stable isotope tracing. We hypothesized that N partitioning to milk and excreta pools as well as the rates of the processes that drive this partitioning would differ for alfalfa silage, corn silage, corn grain, and soybean meal. Feed ingredients were endogenously labeled with 15N and included in 4 diets to create treatments with the same dietary composition and different labeled feed. Diets were fed to 12 late-lactation dairy cows for 4 d (96 h) and feces, urine, and milk collection proceeded during the 4 d of 15N enrichment and for 3 d (80 h) after cessation of label feeding. Nonlinear models of 15N enrichment and decay were fit to milk (MN), urine (UN), and fecal N (FN) in R with the nlme package, and feed-specific parameter estimates were compared. The estimated proportions of feed N that were excreted in feces supported our understanding that N from soybean meal and corn grain is more digestible than N from alfalfa and corn silage. Estimates for the N partitioning between milk and urine from the 2 concentrate feeds (soybean meal and corn grain) indicated that UN/MN ratios were less than or equal to 1:1, indicating either more or equal nitrogen partitioning to milk compared with urine. It is important to maintain factual accuracy in representing the results rather than implying a desired outcome unsupported by the data. In contrast, UN/MN ratios for forage feeds (corn and alfalfa silage) were >1:1, indicating more N partitioning to urine than milk. The modeled proportion of total FN that originated from feed N was 82.2%, which is in line with previous research using a similar 15N measurement timeframe. However, the proportion of urinary and MN originating from feed N was much lower (60.5% for urine, 57.9% for milk), suggesting that approximately 40% of urinary and MN directly originate from body N sources related to protein turnover.

Stable isotopes (15N) facilitate non-invasive labelling of large quantities of macroinvertebrates across different species and feeding types.

While macroinvertebrate dispersal operates at the individual level, predictions of their dispersal capabilities often rely on indirect proxies rather than direct measurements. To gain insight into the dispersal of individual specimens, it is crucial to mark (label) and capture individuals. Isotopic enrichment with 15N is a non-invasive method with the potential of labelling large quantities of macroinvertebrates. While the analysis of 15N is widely utilised in food web studies, knowledge on the specific utility of isotopic enrichment with 15N for mass labelling of macroinvertebrate individuals across different taxa and feeding types is limited. Previous studies have focused on single species and feeding types, leaving gaps in our understanding of the broader applicability of this method. Therefore, this study aimed to test and compare isotopic mass enrichment across several macroinvertebrate taxa and feeding types. We released 15NH4Cl at five stream reaches in North-Rhine Westphalia, Germany, and successfully enriched 12 distinct macroinvertebrate taxa (Crustacea and Insecta). Significant enrichment was achieved in active and passive filter feeders, grazers, shredders and predators, and predominantly showed positive correlations with the enrichment of the taxa's main food sources phytobenthos and particulate organic matter. Enrichment levels rose rapidly and peaked at distances between 50 m and 300 m downstream of the isotopic inlet; significant enrichment occurred up to 2000 m downstream of the isotopic inlet in all feeding types. Macroinvertebrate density estimates on the stream bottom averaged to a total of approximately 3.4 million labelled individuals of the 12 investigated taxa, thus showing the high potential of isotopic (15N) enrichment as a non-invasive method applicable for mass labelling across different macroinvertebrate feeding types. Hence, isotopic enrichment can greatly assist the analysis of macroinvertebrate dispersal through mark-and-recapture experiments, as it allows to measure the movement at the level of individual specimens.