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In cirrhotic patients, compromised hepatocyte function combined with disturbed hepatic blood flow could affect hepato‐splanchnic substrate and metabolite fluxes and exacerbate fatigue during exercise. Eight cirrhotic patients performed incremental cycling trials (3 × 10 min; at light (28 [19–37] W; median with range), moderate (55 [41–69] W), and vigorous (76 [50–102] W) intensity). Heart rate increased from 68 (62–74) at rest to 95 (90–100), 114 (108–120), and 140 (134–146) beats/min (P < 0.05), respectively. The hepatic blood flow, as determined by constant infusion of indocyanine green with arterial and hepatic venous sampling, declined from 1.01 (0.75–1.27) to 0.69 (0.47–0.91) L/min (P < 0.05). Hepatic glucose output increased from 0.6 (0.5–0.7) to 1.5 (1.3–1.7) mmol/min, while arterial lactate increased from 0.8 (0.7–0.9) to 9.0 (8.1–9.9) mmol/L (P < 0.05) despite a rise in hepatic lactate uptake. Arterial ammonia increased in parallel to lactate from 47.3 (40.1–54.5) to 144.4 (120.5–168.3) μmol/L (P < 0.05), although hepatic ammonia uptake increased from 19.5 (12.4–26.6) to 69.5 (46.5–92.5) μmol/min (P < 0.05). Among the 14 amino acids measured, glutamate was released in the liver, while the uptake of free fatty acids decreased. During exercise at relatively low workloads, arterial lactate and ammonia levels were comparable to those seen in healthy subjects at higher workloads, while euglycemia was maintained due to sufficient hepatic glucose production. The accumulation of lactate and ammonia may contribute to exercise intolerance in patients with cirrhosis. [ABSTRACT FROM AUTHOR]

Abstract In cirrhotic patients, compromised hepatocyte function combined with disturbed hepatic blood flow could affect hepato‐splanchnic substrate and metabolite fluxes and exacerbate fatigue during exercise. Eight cirrhotic patients performed incremental cycling trials (3 × 10 min; at light (28 [19–37] W; median with range), moderate (55 [41–69] W), and vigorous (76 [50–102] W) intensity). Heart rate increased from 68 (62–74) at rest to 95 (90–100), 114 (108–120), and 140 (134–146) beats/min (P

Atmospheric ammonia (NH3) can affect nitrogen deposition, particle acidity, and gas-particle partitioning. Although the inorganic chemistry of NH3 in fine particulate (PM2.5) formation are well-constrained, the understanding of interactions between NH3 and secondary organic aerosol (SOA) are rather insufficient until recently. Laboratory studies indicate that NH3 molecule can react with SOA then forms nitrogen-containing organic compounds (NOCs), which can further react to form heterocyclic organic compounds. In this study, we use a modified version of the CMAQ model to simulate the potential importance of the SOA-ammonia uptake mechanism on air quality over China in summer and winter 2017, considering a range of assumed NH3 uptake coefficients (10−3–10−5). Our results show that uptake of NH3 by SOA leads to a decrease in gas-phase NH3 mixing ratio, by as much as 27.5% and 19.0% for the highest uptake coefficient scenario (10−3) in summer and winter, respectively. The largest reduction of ammonia occurs over the Sichuan Basin and the North China Plain. The reduction of gas-phase NH3 engenders a decrease of ammonium nitrate, by up to 30%, but has little impact on the ammonium sulfate concentration. Uptake of NH3 does not significantly affect SOA concentrations owing to overall moderate changes in aerosol acidity, and thus small effects on SOA formation from isoprene. Altogether, NH3 uptake led to a reduction in the average PM2.5 concentration up to 8.9% and 8.7% for the highest uptake coefficient (10−3) in summer and winter, respectively. These results highlight the need for better constraints on the NH3-SOA interactions.

Arginine is an important amino acid in plants, as it not only plays a structural role and serves as nitrogen storage but is also a precursor for various molecules, including polyamines and proline. Arginine is produced by argininosuccinate lyase (ASL) which catalyzes the cleavage of argininosuccinate to arginine and fumarate. ASL belongs to the fumarate lyase family and while many members of this family were well-characterized, little is known about plant ASLs. Here we present the first crystal structures of ASL from the model plant, Arabidopsis thaliana (AtASL). One of the structures represents the unliganded form of the AtASL homotetramer. The other structure, obtained from a crystal soaked in argininosuccinate, accommodates the substrate or the reaction products in one of four active sites of the AtASL tetramer. Each active site is located at the interface of three neighboring protomers. The AtASL structure with ligands allowed us to analyze the enzyme-substrate and the enzyme-product interactions in detail. Furthermore, based on our analyses, we describe residues of AtASL crucial for catalysis. The structure of AtASL gives the rationale for the open-to-close transition of the GSS mobile loop and indicates the importance of serine 333 from this loop for the enzymatic action of the enzyme. Finally, we supplemented the structural data with the identification of sequence motifs characteristic for ASLs., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Author(s). Published by Elsevier Inc. All rights reserved.)

MOFs are considered as efficient NH 3 adsorbents for their high capacity but are accompanied by the collapse of MOFs. In this work, macromolecule-metal complexes (MMCs), which could provide metal sites like MOFs, were developed for reversible NH 3 uptake with high capacity with the assistance of the polymeric ligands. Based on the tunable structure of MMCs, the role of the polymeric ligands and metallic center was investigated. Thereinto, MMCs-3 with dual polymeric ligands presented higher NH 3 adsorption capacity and reversibility of adsorbents compared with MMCs containing a single polymeric ligand (MMCs-1 and MMCs-2). Combined with the NH 3 adsorption test, characterization of FT-IR, UV-vis, EPR spectroscopy, NH 3 -TPD measurement, and the DFT calculations, it was found that the neutral polymeric ligands PVIm contributed to improve the stability of MMCs-3 under a NH 3 atmosphere for the tough networks of PVIm-M(II), while the polymeric ligands with a carboxylate anion together with M(II) enhanced the NH 3 capacity for the feasible coordination of a carboxylate anion with M(II). The mechanism of NH 3 uptake by PVIm-Co-PVBA was proposed that the NH 3 was fixed through the coordination with Co(II) along with the departure of PVBA and the following hydrogen bonding interaction with PVBA, while the coordination between PVIm and Co(II) was not destroyed. Thus, MMCs-3 with dual polymeric ligands presented a higher NH 3 uptake capacity and stability. Optimally, PVIm-M-PVBA with the metal center of Co(II), Cu(II), and Ni(II) were obtained with a high capacity of 20.8-23.7 NH 3 mmol/g at 25 °C and 1 bar and a high selectivity of NH 3 over CO 2 (54.9-99.9) and N 2 (73.0-187.6) through the breakthrough measurement with a gas mixture of 0.2% NH 3 , 2% CO 2 , and 99.6% N 2 at 25 °C.

[Display omitted] • Metal chloride functionalized MOF-253(Al) was designed and prepared for NH 3 capture and separation. • NH 3 capture capacity of the optimal functionalized MOF is 3.33 times that of the pristine MOF. • Highly selective separation of NH 3 from NH 3 /N 2 /H 2 mixture was achieved. • The excellent performance is ascribed to NH 3 coordination toward Ni2+ and its hydrogen bonding with Cl− and carboxyl. Ammonia (NH 3) is produced from nitrogen (N 2) and hydrogen (H 2) by the Harber-Bosch process at high pressure and high temperature. Due to the reversibility and low conversion efficiency of the reaction, the product contains a large amount of raw gas, which seriously affects the production efficiency of ammonia. Therefore, it is essential to exploit high-performance adsorbents for separating NH 3 from NH 3 /N 2 /H 2 mixtures. Herein, we designed and synthesized a kind of metal–organic framework (MOF) composites by anchoring metal chloride (NiCl 2 , CoCl 2 , or SnCl 2) on the bipyridinium groups in the MOF-253(Al) pores. It is found that the optimal composite shows an NH 3 uptake capacity of 18.0 mmol/g at 25.0 °C and 1.0 bar, which is 3.33 times that of the pristine MOF-253(Al), and the composite exhibits a much faster adsorption kinetics of NH 3 than the pristine MOF-253(Al). Significantly, the excellent NH 3 uptake capacity at low pressure allows selective trapping of NH 3 from NH 3 /N 2 /H 2 at 25.0 °C with the selectivity coefficients of 5708 to N 2 and 2320 to H 2. Spectral measurements reveal that the synergistic action of NH 3 coordination to the metal in the MOF and the hydrogen bonding of NH 3 with both O atoms of the carboxyl groups and Cl− of the guest molecules account for such an excellent capture and selectivity. In general, the strategy developed here is simple and effective for improving the selectivity of NH 3 separation, which opens a new way for the design of high-performance solid adsorbents. [ABSTRACT FROM AUTHOR]

Porous macromolecular-metal complexes (MMCs) constructed with stable macromolecular-Co(II) networks and flexible ligands are proposed to NH 3 uptake with high capacity of 11.41–22.01 mmol/g and reversibility. Thereinto, the macromolecular-Co(II) networks protect MMCs from collapse and the flexible ligands benefit to high NH 3 capacity and reversibility of MMCs. [Display omitted] • Porous macromolecular-metal complexes (MMCs) induced by Co(II) with polymers and flexible ligands were synthesized. • The synthesized Co(II)-MMCs used for NH 3 uptake with comparison of MOFs. • Co(II)-MMCs present high NH 3 uptake capacity and reversibility. • High NH 3 uptake capacity of Co(II)-MMCs is facilitated with the flexible ligands. • The stable macromolecular-Co(II) networks protect Co(II)-MMCs from collapse. MOFs are considered as potential adsorbents for their outstanding properties, but it's troublous for reversible ammonia adsorption or the collapse of MOFs' framework. Macromolecular-metal complexes (MMCs) are proposed to efficient and reversible NH 3 uptake at the first time with the consideration of the same coordinated unit to MOFs but with entropic advantage to assist the reversibility. In this work, the MMCs are developed with the construction of stable porous macromolecular-Co(II) networks and competitive flexible ligands to target the high capacity of 19.5 mmol/g under STP and reversibility of MMCs. The efficient NH 3 uptake of MMCs was proposed to the coordination of NH 3 with Co(II) along with the hydrogen bonding interaction with flexible ligand and the physical adsorption for the porosity of MMCs, meanwhile, the invariable macromolecular-Co(II) networks protect MMCs from collapse and comfine the flexible ligand. The comfined ligand would compete with NH 3 to coordinate with Co(II), which promotes the release of NH 3 and the reversibility of MMCs. The development of MMCs in this work provides a potential and competitive choice for NH 3 storage as well as other special applications for the tunable structure trough regulating the macromolecule, metallic center, and ligand. [ABSTRACT FROM AUTHOR]

Molecular analysis of cyanobacterial mat communities indicated that cyanobacteria, ammonia-oxidizing Archaea (AOA), and ammonia-oxidizing bacteria (AOB) coexist in those systems, competing for ammonium; this situation would imply competitive exclusion. We attempted to model how ammonia utilization niche partitioning occurs, and how ammonium levels can influence the interaction between those groups in a one-dimensional diffusionlimited system using Michaelis-Menten kinetics to describe ammonium consumption by each of those three groups. In our model, AOAs were able to dominate ammonium uptake by the community under most circumstances, except for unrealistically high (millimolar) levels of ammonium, where AOBs gained advantage. Cyanobacteria were unable to effectively compete for ammonium with either AOBs or AOAs throughout the mat at all ammonium concentrations and cell counts, suggesting that the presence of AOAs or AOBs forces cyanobacteria into nitrogen fixation mode. Such interaction can make cyanobacterial mats a net nitrogen source, as well as provide a carbon-independent energy transfer pathway from primary producers to the rest of the ecosystem. [ABSTRACT FROM AUTHOR]

Background: In plants, nitrate (NO3 -) nutrition gives rise to a natural N isotopic signature (δ15N), which correlates with the δ15N of the N source. However, little is known about the relationship between the δ15N of the N source and the 14N/15N fractionation in plants under ammonium (NH4 +) nutrition. When NH4 + is the major N source, the two forms, NH4 + and NH3, are present in the nutrient solution. There is a 1.025 thermodynamic isotope effect between NH3 (g) and NH4 + (aq) which drives to a different δ15N. Nine plant species with different NH4 +-sensitivities were cultured hydroponically with NO3 - or NH4 + as the sole N sources, and plant growth and δ15N were determined. Short-term NH4 +/NH3 uptake experiments at pH 6.0 and 9.0 (which favours NH3 form) were carried out in order to support and substantiate our hypothesis. N source fractionation throughout the whole plant was interpreted on the basis of the relative transport of NH4 + and NH3. Results: Several NO3 --fed plants were consistently enriched in 15N, whereas plants under NH4 + nutrition were depleted of 15N. It was shown that more sensitive plants to NH4 + toxicity were the most depleted in 15N. In parallel, N-deficient pea and spinach plants fed with 15NH4 + showed an increased level of NH3 uptake at alkaline pH that was related to the 15N depletion of the plant. Tolerant to NH4 + pea plants or sensitive spinach plants showed similar trend on 15N depletion while slight differences in the time kinetics were observed during the initial stages. The use of RbNO3 as control discarded that the differences observed arise from pH detrimental effects. Conclusions: This article proposes that the negative values of d15N in NH4 +-fed plants are originated from NH3 uptake by plants. Moreover, this depletion of the heavier N isotope is proportional to the NH4 +/NH3 toxicity in plants species. Therefore, we hypothesise that the low affinity transport system for NH4 + may have two components: one that transports N in the molecular form and is associated with fractionation and another that transports N in the ionic form and is not associated with fractionation. [ABSTRACT FROM AUTHOR]

Nitrogen content of Lemna gibba medium was measured in order to examine changes in concentrations of three nitrogen forms, ammonia, nitrate and nitrite, induced by the presence of cadmium over 10 days. Cadmium concentration of 100 μg L-1 caused a complete inhibition (100 %) of growth and a 74.4 % decrease of ammonia production. However, control experiments using Lemna gibba plants sterilized by sodium hypochiorite and diluted alcohol solutions before culturing suggested that NH4+ present in experimental medium originated from microorganisms activity. Slight increase of nitrite was observed in response to cadmium. Significant increase of NO3 concentrations (159 ± 32 to 385 ± 23 mg L-1) was observed in the medium of plants exposed 10 days to 100 μg L-1 Cd2+. Decrease in cadmium content of the culture medium after 10 days of incubation indicated a strong relationship between cadmium uptake, toxicity, and inhibition of nitrate absorption. Changes in nitrate concentration are proposed as a marker of cadmium contamination in water. [ABSTRACT FROM AUTHOR]

Metal-organic frameworks (MOFs) are potential adsorbents for ammonia capture. However, exploration of high-performing MOFs for ammonia adsorption from humid air is still challenging due to the competitive adsorption between ammonia and water molecules. In this work, high-throughput computational screening of 2932 CoRE MOFs for ammonia capture from humid air was carried out by grand canonical Monte Carlo (GCMC) simulations. It was found that the affinities or Henry's constant of MOFs towards ammonia and water molecules play a more important role in determining the ammonia capture performance than their structural properties. Hydrophobic MOFs exhibited higher ammonia selectivity, while hydrophilic MOFs possessed higher ammonia uptake regardless of the strong adsorption competition from water molecules. The coefficient describing the impacts of water adsorption on ammonia uptake (IC H2O_NH3) revealed that although water adsorption promoted the ammonia uptake in the MOFs with IC H2O_NH3 < 0, their ammonia uptake is still lower than that with IC H2O_NH3 > 0 due to their ultra-low affinity towards ammonia. Moreover, analysis of top performers from screening demonstrated that the competitive adsorption between ammonia and water can be identified by their similar density and potential energy distributions of adsorbed ammonia and water molecules. [Display omitted] • High-throughput computational screening of MOFs for ammonia capture from humid air was reported. • Affinities of MOFs towards ammonia and water plays important roles in ammonia capture. • High-performing MOFs exhibited high ammonia uptake regardless of the adsorption competition between ammonia and water molecules. • Similar potential energy distributions suggested the competitive adsorption between ammonia and water. [ABSTRACT FROM AUTHOR]

[Display omitted] • Porous poly ionic liquids functionalized with c obaltous thiocyanate (PIL-TAs) were synthesized. • PIL-TAs present superhigh NH 3 uptake capacity and reversibility. • Superhigh NH 3 uptake capacity of PIL-TAs is facilitated with the cooperative interactions of NH 3. • The competitive thiocyanate with NH 3 benefit to the feasible NH 3 release and reversible PIL-TAs. In this work, porous PIL-TAs were reported for superhigh capacity and reversible NH 3 uptake, where PIL-TAs are cobaltous thiocyanate (Co(II)(SCN) 4 2-, TA) functionalized poly ionic liquids (PILs). Due to the structural support of TA as well as the interactions between TA and imidazolium cation, the PIL-TAs possess porous structure and optimized properties. Superhigh NH 3 uptake capacity of 13.2–20.1 mmol g−1 at STP of PIL-TAs was achieved, which is higher than most of the reported sorbents. In detail, the cooperative interactions including the NH 3 coordinating with Co(II) instead of SCN- and hydrogen bonding of C2H in imidazolium (C2H...NH 3) benefit to high capacity of NH 3 uptake. Reversely, the NH 3 release would be directed by the competition of the free SCN- with NH 3 to reform PIL-TAs. The competitive interaction with SCN- promotes NH 3 release by temperature and pressure swinging, which contributes the adsorbents recyclable. The cooperative and competitive interactions make these PIL-TAs possess high gas capacity and feasible desorption, which might be a potential strategy for designing novel gas sorbents. [ABSTRACT FROM AUTHOR]

Abstract: The red seaweed Asparagopsis armata (Harvey; Rhodophytae, Bonnemaisoniaceae) produces biologically active secondary metabolites that are valuable natural ingredients for cosmetics and medicine and its cultivation may therefore be a profitable venture. The tetrasporophyte of this species (“Falkenbergia rufolanosa”) was successfully tank-cultivated as a continuous biofilter for the effluent of a commercial fish farm in southern Portugal. Optimal stocking density for highest biomass yield and a low level of other algal species in winter and late spring was 5×g centrifuged fresh weight l−1. The effect of total ammonia nitrogen supply (TAN flux) on biofiltration and biomass yield was investigated in winter and spring. Results revealed that A. armata is currently the seaweed-biofilter with the highest TAN removal of up to 90 μmol l−1 h−1 at a TAN flux of about 500 μmol l−1 h−1. In the tanks used, this is equivalent to a removal of up to 14.5 g TAN m−2 day−1. At a lower TAN flux of about 40 μmol l−1 h−1, TAN removal by A. armata is more than double to what is reported at this flux for another successful seaweed biofilter, the genus Ulva. Monthly variation of A. armata biomass yield peaked in May and was lowest in January. At TAN fluxes between 300 and 400 μmol l−1 h−1, an average water temperature of 21.7 °C and a total daily photon flux density of 47 Mol m−2, seaweed yield was over 100 g DW m−2 day−1 with a recorded maximum of 119 g. During spring, autumn and early summer, the biomass of A. armata within the experimental tanks doubled every week. A model for the up scaling of this finfish integrated aquaculture of A. armata varies the investment in biofilter surface area and estimates the return in biofiltration and biomass yield. Highest TAN removal efficiencies will only be possible at low TAN fluxes and a very large biofilter area, resulting in a low production of biomass per unit area. To remove 50% of TAN from the effluent (1 mt Sparus aurata; 21 °C), 28 m2 of biofilter, designed to support a water turnover rate of 0.8 Vol h−1 would be necessary. This system produces 6.1 kg FW (1.5 kg DW) of A. armata per day and has the potential to turn biofiltration into an economically sustained, beneficial side effect. [Copyright &y& Elsevier]

Abstract: An NAD+-dependent glutamate dehydrogenase (GDH; EC 1.4.1.24) was cloned from the ruminal ciliate protozoan, Entodinium caudatum. The gene had high sequence similarity to GDH genes from the Bacteroides (class) – a class of bacteria which is highly represented in the rumen. When expressed in Escherichia coli the enzyme had a high affinity for ammonia and α-ketoglutarate (apparent K m of 2.33 and 0.71mM, respectively) and a low affinity for glutamate (apparent K m of 98mM). GDH activity and GDH mRNA concentration were increased by incubating washed E. caudatum cells with ammonia and antibiotics. These results suggest that the GDH is an anabolic enzyme catalysing the assimilation of ammonia by E. caudatum in the rumen and that the gene was probably acquired by lateral gene transfer from a ruminal bacterium. [Copyright &y& Elsevier]

Partitioning gaseous water-soluble organic compounds (WSOC) to the aerosol phase is a major formation pathway of atmospheric secondary organic aerosols (SOA). However, the fundamental mechanism of the WSOC-partitioning process remains elusive. By simultaneous measurements of both gas-phase WSOC (WSOCg) and aerosol-phase WSOC (WSOCp) and formic and acetic acids at a rural site in the Yangtze River Delta (YRD) region of China during winter 2019, we showed that WSOCg during the campaign dominantly partitioned to the organic phase in the dry period (relative humidity (RH) < 80%) but to aerosol liquid water (ALW) in the humid period (RH > 80%), suggesting two distinct SOA formation processes in the region. In the dry period, temperature was the driving factor for the uptake of WSOCg. In contrast, in the humid period, the factors controlling WSOCg absorption were ALW content and pH, both of which were significantly elevated by NH 3 through the formation of NH 4 NO 3 and neutralization with organic acids. Additionally, we found that the relative abundances of WSOCp and NH 4 NO 3 showed a strong linear correlation throughout China with a spatial distribution consistent with that of NH 3 , further indicating a key role of NH 3 in WSOCp formation at a national scale. Since WSOCp constitutes the major part of SOA, such a promoting effect of NH 3 on SOA production by elevating ALW formation and WSOCg partitioning suggests that emission control of NH 3 is necessary for mitigating haze pollution, especially SOA, in China.

The internal pool of ammonia in strains of unicellular and filamentous cyanobacteria was found to be 6-12 nmol·mg protein. In nitrate grown Anacystis nidulans R-2 the pool size averaged 12 nmol·mg protein, which corresponds to 2.3 mM, and was little affected by N-source or medium pH during growth. Cells from NH-limited continuous culture contained comparable pools, and cell yield was independent of medium pH (7.2-8.5). The internal pool was not bound to macromolecules. The pool fell transiently to about one-third within 2 h on shifting cells to N-free medium, but was slowly regenerated over 24 h. Added ammonia was removed from solution by illuminated cell suspensions at a linear rate, adequate to supply biosynthetic needs, to residual concentrations less than 5 μM. An apparent K of less than 1 μM can be inferred. Uptake rates were independent of N-source during growth, and of assay pH over the range 6.2-8.7. Bicarbonate was needed for uptake, but the rate of uptake was not influenced by the simultaneous presence of NaNO (10 mM) or CHNHCl (0.15 mM). Uptake was energydependent, and was eliminated in dark, anaerobic conditions or by the addition of protonophores. Uptake was also strongly inhibited by dicyclohexylearbodiimide, an ATPase inhibitor, by - SH reagents and methionine sulfoximine, suggesting that interference with energy supply or with ammonia metabolism prevented further entry into the cells. [ABSTRACT FROM AUTHOR]

The uptake of ammonia and O by washed cells of Nitrosomonas has been followed simultaneously and continuously using electrode techniques. The stoichiometry of NHoxidation, O uptake and NOproduction was 1 : 1.5 : 1.0 and for NHOH oxidation a ratio of 1 for O : NO. A variety of inhibitors of electron transport and metals as well as uncouplers restricted ammonia uptake more markedly than O utilization. There is good evidence for the involvement of copper in the NHuptake process. A quinacrine fluorescence technique has been used to study the proton extrusion by washed cells on adding NHCl and NHOH respectively as substrates. The uptake of NHwas followed by the extrusion of H and this process was depressed by those inhibitors which were also effective in the electrode experiments. A requirement for copper is also established for the translocation of protons into the medium, resulting from the uptake of NHby cells. [ABSTRACT FROM AUTHOR]

Abstract Background In plants, nitrate (NO3-) nutrition gives rise to a natural N isotopic signature (δ15N), which correlates with the δ15N of the N source. However, little is known about the relationship between the δ15N of the N source and the 14N/15N fractionation in plants under ammonium (NH4+) nutrition. When NH4+ is the major N source, the two forms, NH4+ and NH3, are present in the nutrient solution. There is a 1.025 thermodynamic isotope effect between NH3 (g) and NH4+ (aq) which drives to a different δ15N. Nine plant species with different NH4+-sensitivities were cultured hydroponically with NO3- or NH4+ as the sole N sources, and plant growth and δ15N were determined. Short-term NH4+/NH3 uptake experiments at pH 6.0 and 9.0 (which favours NH3 form) were carried out in order to support and substantiate our hypothesis. N source fractionation throughout the whole plant was interpreted on the basis of the relative transport of NH4+ and NH3. Results Several NO3--fed plants were consistently enriched in 15N, whereas plants under NH4+ nutrition were depleted of 15N. It was shown that more sensitive plants to NH4+ toxicity were the most depleted in 15N. In parallel, N-deficient pea and spinach plants fed with 15NH4+ showed an increased level of NH3 uptake at alkaline pH that was related to the 15N depletion of the plant. Tolerant to NH4+ pea plants or sensitive spinach plants showed similar trend on 15N depletion while slight differences in the time kinetics were observed during the initial stages. The use of RbNO3 as control discarded that the differences observed arise from pH detrimental effects. Conclusions This article proposes that the negative values of δ15N in NH4+-fed plants are originated from NH3 uptake by plants. Moreover, this depletion of the heavier N isotope is proportional to the NH4+/NH3 toxicity in plants species. Therefore, we hypothesise that the low affinity transport system for NH4+ may have two components: one that transports N in the molecular form and is associated with fractionation and another that transports N in the ionic form and is not associated with fractionation.