15N Levels Research Articles

Isotopic and molecular evidence for saprotrophic Marasmiaceae mycobionts in rhizomes of Gastrodia sesamoides

Gastrodia sesamoides, a common obligate mycoheterotrophic orchid species found in eastern Australia relies on a soil fungus to provide a source of carbon nutrition. The identity of this fungus is not known although in other studies of Gastrodia species a number of mycobionts have been suggested including Fomes and Mycena. In this study the fungal community of rhizomes of G. sesamoides has been identified via fungal ITS-DNA PCR amplification, cloning and sequencing. Although a number of fungi were identified by this approach the most common fungal ITS DNA within the orchid were saprotrophic members of the Marasmiaceae (Campanella and Marasmius spp.). Analysis of the natural carbon and nitrogen stable isotope abundances of stems of G. sesamoides showed an enrichment in 13C and low levels of 15N. These data suggest that G. sesamoides obtains its carbon parasitically from free-living saprotrophic fungi and not from an ectomycorrhizal fungal partner of a photosynthetic plant, as is common for other obligate mycoheterotrophic orchid species.

NITROGEN RETENTION IN URBAN LAWNS AND FORESTS

Lawns are a dominant cover type in urban ecosystems, and there is concern about their impacts on water quality. However, recent watershed-level studies suggest that these pervious areas might be net sinks, rather than sources, for nitrogen (N) in the urban environment. A 15N pulse-labeling experiment was performed on lawn and forest plots in the Baltimore (Maryland, U.S.A.) metropolitan area to test the hypothesis that lawns are a net sink for atmospheric-N deposition and to compare and contrast mechanisms of N retention in these vegetation types. A pulse of 15N-NO3-, simulating a precipitation event, was followed through mineral soils, roots, Oi-layer/thatch, aboveground biomass, microbial biomass, inorganic N, and evolved N2 gas over a one-year period. The 15N label was undetectable in gaseous samples, but enrichment of other pools was high. Gross rates of production and consumption of NO3- and NH4+ were measured to assess differences in internal N cycling under lawns and forests. Rates of N retention were similar during the first five days of the experiment, with lawns showing higher N retention than forests after 10, 70, and 365 days. Lawns had larger pools of available NO3- and NH4+; however, gross rates of mineralization and nitrification were also higher, leading to no net differences in NO3- and NH4+ turnover times between the two systems. Levels of 15N remained steady in forest mineral soils from day 70 to 365 (at 23% of applied 15N), but continued to accumulate in lawn mineral soils over this same time period, increasing from 20% to 33% of applied 15N. The dominant sink for N in lawn plots changed over time. Immobilization in mineral soils dominated immediately (one day) after tracer application (42% of recovered 15N); plant biomass dominated the short term (10 days; 51%); thatch and mineral-soil pools together dominated the medium term (70 days; 28% and 36%, respectively); and the mineral-soil pool alone dominated long-term retention (one year; 70% of recovered 15N). These findings illustrate the mechanisms whereby urban and suburban lawns under low to moderate management intensities are an important sink for atmospheric-N deposition.

Simultaneous analysis of microbial identity and function using NanoSIMS

Identifying the function of uncultured microbes in their environments today remains one of the main challenges for microbial ecologists. In this article, we describe a new method allowing simultaneous analysis of microbial identity and function. This method is based on the visualization of oligonucleotide probe-conferred hybridization signal in single microbial cells and isotopic measurement using high-resolution ion microprobe (NanoSIMS). In order to characterize the potential of the method, an oligonucleotide containing iodized cytidine was hybridized on fixed cells of Escherichia coli cultured on media containing different levels of 13C or 15N. Iodine signals could clearly be localized on targeted cells and the isotopic enrichment could be monitored at the single-cell level. The applicability of this new technique to the study of in situ ecophysiology of uncultured microorganisms within complex microbial communities is illustrated.

Crop responses to 15N-labelled organic and inorganic nitrogen sources

A major challenge for low-external-input farming systems is to secure the N supply. Lack of synchrony between mineralization of organic N sources and plant N requirements is causing many growers to use different techniques to overcome this problem. One of these techniques is the application of soluble water extracts of different farm residues and plants. A field study was conducted to study the crop uptake of applied 15N-labeled alfalfa and clover extracts as compared to the N uptake from15N-glycine and three levels of 15N-(NH4)2SO4. The results show that total N accumulation in the field crops, squash and lettuce was primarily affected by the amount of added N (P ≤ 0.05) and not by the form in which the N was applied (P ≤ 0.05). The utilization efficiencies of N (pNdff) from plant extracts and glycine increased (P ≤ 0.05) gradually from 10, 20, to 30 days after application in contrast to (NH4)2SO4 which peaked in utilization efficiencies of 56% around 20 days after application. The pNdff reached 60%, 40% and 36% of the applied glycine, alfalfa and clover extracts, respectively in lettuce. Squash showed the same pattern during at 10, 20 and 30 days; however, the proportion of N derived from most of the treatments was higher in squash than in lettuce (P ≤ 0.05). These results indicate that plant extracts of clover and alfalfa can be used as efficient N fertilisers in low-external-input agroecosystems.

Fertilizer Requirements for Container-grown Buxus spp.

Abstract The objective of this study was to determine the nutritional requirements for container-grown boxwood and to determine if summer dormancy of boxwood can be removed via nutrition. Buxus sempervirens L. ‘Suffruticosa’, B. sempervirens L. ‘Vardar Valley’, and B. sinica var. insularis Nakai ‘Justin Brouwers’ were used for these studies. Various levels of Osmocote 15N–3.9P–9.8K (15N–9P2O5–12K2O) and a 10N–1.7P–4.9K (10–4–6) liquid fertilizer were applied to boxwood in a pine-bark substrate. Maximum shoot dry weight (OSDW) was achieved at 12 to 16 g (.42 to.56 oz) Osmocote per 3 liter (#1) container and 100 to125 ppm N from the liquid fertilizer. Leachate EC corresponding to OSDW ranged from 0.5 to 0.7 dS/m and 0.7 to 1.5 dS/m for Osmocote and the liquid fertilizer, respectively. Leaf tissue N levels corresponding to OSDW weight ranged from 3.1 to 4.3% for Osmocote and 5.0 to 5.5% for the liquid fertilizer. While the fertilizer requirements for boxwood OSDW were determined, additional growth flushes did not occur.

Distribution of 15N-Labeled Fertilizer Applied to Pecan: A Case Study

From March through June 1996, 15N-labeled fertilizer was applied to mature pecan trees [Carya illinoinensis (Wangehn.) K. Koch] in a commercial orchard to determine the fate of fertilizer-N in the tree and in the soil directly surrounding the tree. The concentrations of 15N and total N were determined within various tissue components and within the soil profile to a depth of 270 cm. By Nov. 1996, elevated levels of 15N were greatest at depths just above the water table (280 cm), suggesting a substantial loss of fertilizer-N to leaching. Recoveries of 15N from tissue and soil at the end of 1996 were 19.5% and 35.4%, respectively. Harvest removed 4.0% of the fertilizer-N applied, while 6.5% was recycled with leaf and shuck drop. In 1997, with no additional application of labeled fertilizer, the tissue components continued to exhibit 15N enrichment. By the end of the 1997 growing season, 15N levels decreased throughout the soil profile, with the most pronounced reduction at depths immediately above the water table. Estimated recoveries of 15N from pecan tissue (excluding root) and soil at the end of 1997 were 8.4% and 12.5%, respectively. In 1996 and 1997, 15N determinations indicated an accumulation of fertilizer-N in the tissues and a loss of fertilizer-N to the groundwater. Early spring growth, flowering, and embryo development used fertilizer-N applied the previous year, as well as that applied during the current year.

Synergism between Phyllobacterium sp. (N 2-fixer) and Bacillus licheniformis (P-solubilizer), both from a semiarid mangrove rhizosphere

Mangrove seedlings were treated with a mixture of two bacterial species, the slow-growing, N 2-fixing bacterium Phyllobacterium sp. and the fast-growing, phosphate-solubilizing bacterium Bacillus licheniformis, both isolated from the rhizosphere from black, white, and red mangroves of a semiarid zone. Nitrogen fixation and phosphate solubilization increased when the mixture was used compared to the effects observed when adding individual cultures, notwithstanding that there was no increase in bacterial multiplication under these conditions. Inoculation of black mangrove seedlings in artificial seawater showed the mixture performed somewhat better than inoculation of the individual bacterium; more leaves were developed and higher levels of 15N were incorporated into the leaves, although the total nitrogen level decreased. This study demonstrates that interactions between individual components of the rhizosphere of mangroves should be considered when evaluating these bacteria as plant growth promoters.

The Methionine Chain Elongation Pathway in the Biosynthesis of Glucosinolates in Eruca sativa (Brassicaceae)

Glucosinolates are nitrogen- and sulfur-containing plant natural products that have become increasingly important in human affairs as flavor precursors, cancer-prevention agents, and crop protectants. While many glucosinolates are biosynthesized from common amino acids, the major glucosinolates in economically important species of the Brassicaceae, such as Brassica napus (oilseed rape), are thought to be formed from chain-elongated derivatives of methionine or phenylalanine. We investigated the chain elongation pathway for methionine that is involved in glucosinolate biosynthesis in Eruca sativa. Isotopically labeled methionine and acetate were administered to cut leaves and the major product, 4-methylthiobutylglucosinolate (isolated as its desulfated derivative), was analyzed by MS and NMR. Administration of [U-13C]methionine showed that the entire carbon skeleton of this amino acid, with the exception of the COOH carbon, is incorporated as a unit into 4MTB. Administration of [13C]- and [14C]acetate gave a labeling pattern consistent with the operation of a three-step chain elongation cycle which begins with the condensation of acetyl-CoA with a 2-oxo acid derived from methionine and ends with an oxidative decarboxylation forming a new 2-oxo acid with one additional methylene group. Administration of [15N]methionine provided evidence for the transfer of an amino group to the chain-elongated 2-oxo acid, forming an extended amino acid which serves as a substrate for the remaining steps of glucosinolate biosynthesis. The retention of a high level of 15N in the products suggests that the amino transfer reactions and the chain elongation cycle are localized in the same subcellular compartment.

Relationship between rate of crop growth at date of fertiliser N application and fate of fertiliser N applied to winter wheat

To investigate the relationship between the timing of fertiliser N applications and the N use efficiency of wheat, three field experiments with 15N were set up on winter wheat, on three different soils in France. Different crop N demands on the day of fertiliser application were obtained by varying either crop densities or date of fertiliser application. Labelled 15NH415NO3 was applied at tillering and during stem elongation. The 15N recovered from plant and soil at different dates after 15N addition and at maturity of wheat was measured. The fate of fertiliser N was rapidly determined, most of the fertiliser N accumulated in the wheat at maturity having been taken up within a few days of application. 15N recovery by the crop at final harvest (%) varied greatly (19–55% N applied) according to crop density, soil type and date of application. It was linearly related to the instantaneous crop growth rate calculated at the day of 15N application. The amount of fertiliser N immobilised in the soil was constant at 20 kg N ha−1, for all soil types and crop densities. Because residual mineral 15N in the soil at harvest was negligible and immobilisation was constant, the level of total 15N measured in the different N pools (soil+plant) reflected the% 15N uptake by the plant. There was consequently a negative linear relationship between the percentage of 15N not recovered for measurement, and crop growth rate (i.e. crop N demand) at date of fertiliser application. These results suggest that crop N demand at the time of N application determines the ability of the crop to compete for N with other processes, and may be a major factor determining the division of N between soil and crop.

Remobilization of Nitrogen from Storage Pools of Mature Apple Trees Depends on Nitrogen Status

Remobilization of reserve N and uptake of soil N in winter and spring were assessed in relation to the N status of trees. Ten-year-old `Newtown Pippin' apple trees on M.7A rootstock were fertilized to create moderately vigorous trees, trees with above-ground portions (tops) and roots relatively low in N (L/L), tops high in N and roots low in N (H/L), both tops and roots high in N (H/H), or tops low in N and roots high in N (L/H). Labeled (15N) fertilizers were used to tag the soil and frame and root N pools in the moderately vigorous trees prior to winter and spring remobilization. The level of 15N in the buds and new growth was monitored throughout winter and spring. Nitrogen stored in the aerial part of the tree was first to be remobilized to meet N requirements of the developing buds. Root and soil N reached the flower buds simultaneously. Trees of the L/H treatment transported labeled N upward to the bud as early as 9 Feb., even though average air temperature was close to 7°C, whereas L/L trees did not send any root-15N to the buds until 2.5 later. When trees received an abundance of N in the fall (H/H and L/H), their buds grew faster in the spring and they bloomed earlier compared with L/L and H/L trees. For root to shoot N translocation to start early (in winter), the bud needed to be low in N and the roots had to have adequate N reserves.

Growth and accumulation of 15N in rice inoculated with the parent and a nitrogenase-derepressed mutant strain of Anabaena variabilis

Rice plants were grown in a temperature-controlled greenhouse in 1.2 L glass fleakers. The rooting media were inoculated with either the parent strain or a nitrogenase-derepressed mutant strain (which excreted NH4 4 + produced by nitrogenase) of the cyanobacterium Anabaena variabilis and exposed to 15N 2. Dry matter and total N accumulated in the roots and shoots of plants inoculated with the mutant strain were significantly greater than from plants inoculated with the parent strain. Significantly higher levels of 15N 2 accumulated in the roots and shoots of plants inoculated with the mutant strain, which indicated that more fixed N was readily available for root uptake and assimilation. Roots and shoots of uninoculated plants exposed to 15N 2 had a small, consistent but nonsignificant increase in levels of 15N compared with treatments that were exposed only to the natural atmospheric abundance of the isotope. These results show that the NH 4 +-excreting mutant strain of A. variabilis has the potential to increase N input for plant growth in rice production systems.

Stable C and N isotope values of selected components of a tropical australian sugarcane ecosystem

The δ13C and δ15N values of sugarcane plant tissues, decomposing harvest residues, soil and the casts and body tissues of the earthwormPontoscolex corethrurus were determined. Little variation in δ13C values was found between plant parts. The δ13C values of the decomposing harvest residues declined and became more variable after 148 days of exposure in the field. In the decomposing residues, δ13C values of the neutral detergent fibre fraction were similar to those of the whole tissues while those of the proximate lignin were more negative. The δ15N values of the residues also declined over time after a short initial delay.P. corethrurus populations are more intimately associated with the roots of sugarcane than with the bulk soil. Tissue δ13C values suggest that the earthworm diet is similar to or more enriched in13C than sugarcane tissues and is substantially more enriched than the soil C. Earthworm tissues have similar levels of15N enrichment to both the soil and plant tissues. These data are consistent with the hypothesis that this earthworm derives much of its assimilated C relatively directly from organic matter associated with the roots and decomposing harvest residues.

Nitrogen isotopic analyses by isotope-ratio-monitoring gas chromatography/mass spectrometry

Amino acids containing natural-abundance levels of 15N were derivatized and analyzed isotopically using a technique in which individual compounds are separated by gas chromatography, combusted on-line, and the product stream sent directly to an isotope-ratio mass spectrometer. For samples of N 2 gas, standard deviations of ratio measurement were better than 0.1% 0 (Units for δ are parts per thousand or per million (% 0).) for samples larger than 400 pmol and better than 0.5% 0 for samples larger than 25 pmol (0.1% 0 15N is equivalent to 0.00004 atom % 15N). Results duplicated those of conventional, batchwise analyses to within 0.05% 0. For combustion of organic compounds yielding CO 2/N 2 ratios between 14 and 28, in particular for N-acetyl n-propyl derivatives of amino acids, δ values were within 0.25% 0 of results obtained using conventional techniques and standard deviations were better than 0.35% 0. Pooled data for measurements of all amino acids produced an accuracy and precision of 0.04 and 0.23% 0, respectively, when 2 nmol of each amino acid was injected on column and 20% of the stream of combustion products was delivered to the mass spectrometer.

Protein metabolism in vegans

To elucidate the mechanisms of adaptation to a low-energy and low-protein vegan diet, we carried out dietary surveys and nitrogen balance studies five times during one year on two women and a man who ate raw brown rice, raw green vegetables, three kinds of raw roots, fruit and salt daily. Individual subjects modified this vegan diet slightly. The mean daily energy intake of the subjects was 18, 14, and 32 kcal/kg, of body weight. The loss of body weight was about 10% of the initial level. The daily nitrogen balance was -32, -33, and -11 mg N/kg of body weight. In spite of the negative nitrogen balance, the results of routine clinical tests, initially normal, did not change with the vegan diet. Ten months after the start of the vegan diet, the subjects were given 15N urea orally. The incorporation of 15N into serum proteins suggested that these subjects could utilize urea nitrogen for body protein synthesis. The level of 15N in serum proteins was close to the level in other normal adult men on a low-protein diet with adequate energy for 2 weeks.

Relationships between nitrogen fixation and growth in Robinia pseudoacacia seedlings: A functional growth‐analysis approach using 15N

The effects of NO3− fertilization on N2 fixation and growth of black locust (Robinia pseudoacacia L.) seedlings were examined by growth analyses. Seedlings were grown under low (0.25 mM) and high (2.00 mM) NO3− provided with each watering. In one experiment, seedlings were given NO3− with enriched levels of 15N (11.03%) to identify N accumulation from soil and atmospheric sources. Curves describing accretion of total dry weight and N were fit as functions of accumulated photosynthetically active radiation (PAR). Relative growth rates (RGR) and nitrogen productivities (NP) were derived from these curves and examined as functions of seedling size and/or internal N concentration. Early in the experiments seedlings receiving the higher amount of NO3− grew faster, acquired more N, had greater internal nitrogen concentrations (N1) and had higher NP than seedlings receiving the lower amounts. Seedlings receiving higher N also acquired most of their N from NO3−, whereas those receiving lower N acquired most from N2 fixation. Later in the experiments, no differences in growth rae, N1 or NP were observed. Furthermore, midway through the experiments seedlings receiving the higher amounts of NO3− began to acquire the majority of N from N2 fixation. At the end of the two experiments, seedlings given higher NO3− were 16 and 78% larger in dry weight. Differences in size were due to growth differences that occurred only during the first half of the experiments. During this period, the faster growth of seedlings given higher NO3− was attributed to greater access to utilizable N and lower energy costs associated with the acquisition and/or utilization of NO3−, relative to N2. Unlike the situation in non‐N2‐fixing plants, differences in available soil N had no influence on biomass allocation to roots, stems or leaves in black locust. When soil N became limiting, N2 fixation capacity increased, rather than root mass, to acquire more N.

Effect of NO3 on N2 fixation and nitrogenous solutes of xylem in two nodulated West African geocarpic legumes, Kersting's bean (Macrotyloma geocarpum L.) and Bambara groundnut (Vigna subterranea L.)

Nodulation, nitrogen (N2) fixation and xylem sap composition were examined in sand cultured plants of Bambara groundnut (Vigna subterranea L.) and Kersting's bean (Macrotyloma geocarpum L.) inoculated with Bradyrhizobium strain CB756 and supplied via the roots for a 4 week period from the third week onwards with different levels of (15N)-nitrate (0–15 mM). The separate contributions of nitrate and N2 to plant nitrogen were measured by isotope dilution. Increasing levels of nitrate inhibited nodule growth (measured as dry matter or nodule N) of both species parallel with decreased dependence on symbiotically-fixed N. Specific nodule activity (N2 fixed g nodule dry−1 d−1 of nodules) was reduced progressively with time in V. subterranea at higher (5 or 15 mM) levels of NO3, but this was not so for M. geocarpum. Root xylem bleeding sap of both species showed ureides (allantoin and allantoic acid) as predominant (>90%) solutes of nitrogen when plants were relying solely on atmospheric N. Levels of ureide and glutamine decreased and those of asparagine and nitrate in xylem increased with increasing level of applied nitrate. Relative levels of xylem ureide-N were positively correlated (R2=0.842 for M. geocarpum and 0.556 for V. subterranea), and the ratio of asparagine to glutamine in xylem exudate negatively correlated (R2=0.955 for M. geocarpum and 0.736 for V. subterranea) with plant reliance on nitrogen fixation. The data indicate that xylem sap analyses might be useful for indirect field assays of nitrogen fixation by the species and that Kersting's bean might offer some potential as a symbiosis in which N2 fixation is relatively tolerant of soil N.

Response of grape cultivars to nitrogen and phosphorus grown with water harvesting

Abstract Two fertilizer studies were conducted on mature vineyards established with a water harvesting system on a White House sandy loam (fine, mixed, thermic, Ustollic Haplargid) soil at the University of Arizona Oracle Agricultural Center. In one study, two grape (Vitis vinifera L.) cultivars, ‘Cabernet Sauvignon’ and ‘Sauvignon blanc’, were treated with different levels of 15N and P fertilizer and tested for tissue NO3‐N and total‐P content. In second study, eleven grape varieties were treated with three levels of N. Tissue samples were analysed for total P and NO3‐N content, and the ratio of petiole‐P to leaf blade‐P was determined. When sufficient quantities of both nutrients were provided, N and P interacted positively resulting in increased grape yields. The petiole‐P to leaf blade‐P ratio correctly monitored the P status of the vines.

Natural15N abundances in plant parts of groundnut (Arachis hypogaea)

In recent reports (Kohl et al. 1980; Rennie et al. 1982; Shearer et al. 1980; Yoneyama et al. 1986a, b) the 15N dilution method for the determination of natural levels of 15N was used to estimate the proportion of symbiotically fixed nitrogen and combined nitrogen during nitrogen accumulation in leguminous plants. However, a heterogeneous distribution of l5N in plant parts was sometimes observed, which made it difficult to apply this method for the selection of appropriate parts for the estimation.

STABILIZATION OF RESIDUAL C AND N IN SOIL

Three Manitoba soils varying in clay and organic matter content were mixed with each of four plant residue amendments: (1) a control where no plant or fertilizer materials were added; (2) 14C- and 15N-labelled wheat straw; (3) 14C- and 15N-labelled wheat straw plus 15N-labelled KNO3; and (4) 14C- and 15N-labelled prebloom alfalfa residue. The soils were incubated at 20 °C and 75% field capacity for 90 d. Soil samples were collected at 0, 7, 30, and 90 d of incubation. Two humic acid fractions were obtained from the amended soils. Fraction A was obtained by Na4P2O7 extraction and Fraction B was recovered from the remaining residue by sonication. Significant (P < 0.05) differences were observed in the incorporation of 14C and 15N into both fractions. The effect of clay content on C and N incorporation was most marked in Fraction B: C incorporation in all treatments tended to be higher in the Red River Clay soil than in the Newdale (bottom of the knoll) soil; N incorporation in the straw and alfalfa treatments tended to be higher in the Red River Clay soil but, in the straw-plus-fertilizer treatment, N incorporation tended to be higher in the Newdale (bottom of the knoll) soil. The effect of the form of C and N in the residue was most apparent in the incorporation of N into Fraction B: in all soils the level of 15N in the alfalfa treatment was significantly (P < 0.05) higher than that in the straw plus fertilizer treatment. Key words: Plant residue management, incorporation, incubation, C and N turnover, KNO3 fertilizer

New and Conventional Evaporative Systems in Concentrating Nitrogen Samples Prior to Isotope‐ratio Analysis

AbstractStudies were conducted to quantify and compare the efficiencies of various evaporative systems used in evaporating 15N samples prior to mass spectrometric analysis. Two new forced‐air systems were designed and compared with a conventional forced‐air system and with an open‐air drybath technique for effectiveness in preventing atmospheric contamination of evaporating samples. One of the new system designs was compared with the conventional forced‐air evaporative system and with the open‐air drybath technique for levels of 15N cross‐contamination among evaporating samples. The forced‐air evaporative systems significantly reduced the time needed to evaporate samples as compared to the open‐air drybath technique; samples were evaporated to dryness in 2.5 h with the forced‐air systems as compared to 8 to 10 h on the open‐air drybath. The effectiveness of a given forced‐air system to prevent atmospheric contamination of evaporating samples was significantly affected by the flow rate of the airstream flowing over the samples; the optimum flow rate was different for each of three forced‐air systems tested. When operated at optimum air flow rate, the forced‐air evaporative systems reduced atmospheric contaminants to <0.2µg N per sample, even when a contaminant of ∼77 µg NH3‐N per sample vial was added upstream of the scrubbing systems. The average atmospheric contaminant N found in samples evaporated on the open‐air drybath was 0.3 µ N, indicating very low concentrations of atmospheric NH3 during this study. Both these contaminant levels are sufficiently low to be of little or no significance in most tracer N studies. However, in previous studies we have experienced significant contamination of 15N samples evaporated on an open‐air drybath because the level of contaminant N in the laboratory atmosphere varied and could not be adequately controlled, even when NH3‐containing chemicals, cleansers, and paints were not stored or used in or around the working area. Average cross‐contaminant levels of 0.28, 0.20, and 1.01 µ of N were measured between samples evaporated on the open‐air dry‐bath, the newly‐designed forced‐air system, and the conventional forced‐air system, respectively. The cross‐contamination level is significantly higher on the conventional forced‐air system than on the other two systems, and could significantly alter the atom % 15N of high‐enriched, low [N] evaporating samples. The data from this study demonstrate that effectiveness in preventing atmospheric contamination and potential for cross‐contamination vary from system to system; thus, a given evaporative system should be carefully tested before being used in routine 15N sample processing.