δ15N Range Research Articles

Evaluation of nitrate contamination sources of unconfined groundwater in the North Han River basin of Korea using nitrogen isotope ratios

To evaluate the nitrate contamination sources of unconfined groundwater in the North Han River basin (127°45′E, 37°55′N), groundwater samples were collected monthly for three years (1997–1999) from 20 wells, and analyzed for the concentration (n=599) and δ15N (n=96) of NO3−N. Frequency distribution of NO3−N concentration showed that 43.9% of samples exceeded the national standard for drinking water (10 mg NO3−N L−1). During dry season between October and March, only 36.1% of a total of 296 samples had NO3−N concentration above 10 mg L−1. However, 51.5% of a total of 303 samples exceeded the standard level during wet season between April and September. Concentrations of nitrate increased with direction of groundwater flow. Wells located in vicinity of livestock feedlots showed consistently high nitrate concentration irrespective of precipitation pattern. The δ15N signatures of NO3− showed that in general, both15N-depleted source (e.g., chemical fertilizer) and15N-enriched source (e.g., compost and manure) impacted groundwater quality concurrently. The δ15N ranges of groundwater NO3− were +1.5–+12.9‰ in dry season and +3.2–+9.9‰ in wet season. These δ15N data suggested that the effects of compost and/or manure on nitrate concentration were apparent in dry season. A positive correlation (r2=0.32) between N concentration and δ15N of NO3− in dry season also suggested that nitrate concentration increased with increasing loading of nitrate from compost and/or manure wastewater. On the other hand, the relatively narrow range of δ15N and high nitrate concentration in wet season suggested that increased flush of nitrate derived from mixed sources such as fertilizer, compost, and manure during storm event resulted in high nitrate concentration. In this area, neither high δ15N values nor a negative correlation between δ15N and N concentration of NO3−, indicating denitrification was observed because of the coarse textured soil matrix. Therefore, reduction of nitrogen inputs through curtailment of fertilizer and compost application rates and appropriate treatment of livestock manure are the most effective and practical ways to prevent groundwater contamination.

Stable isotope composition of walleye: 15N accumulation with age and area-specific differences in δ13C

Stable nitrogen and carbon isotope ratios were measured for walleye (Stizostedion vitreum) collected across Lake Champlain, Vermont, to determine relationships between isotopic composition and diet, location of capture, length, weight, and age. Variation in δ13C values reflected area-specific differences in isotopic composition of organisms collected across the lake. A critical assumption in the application of isotope techniques is that a predictable relationship exists between the diet and isotopic composition of an organism. Our results indicate that isotopic fractionation factors may not be independent of age as has largely been assumed. By combining stable nitrogen and carbon isotope analysis with conventional stomach content analysis, we documented significant age effects in the δ15N composition of adult walleye that were not attributable to observed changes in diet. Age accounted for 81% of the variation in δ15N values of walleye (ages 2–27, N = 65, δ15N range = 15.3–19.2‰), providing evidence supporting 15N accumulation over the life span of walleye. Therefore, the risk of making faulty inferences of trophic position and food web interactions based on δ15N values may be increased when age is unknown. Our results indicate that metabolic effects associated with age require greater consideration in applications of stable isotope analysis.

Influence of inorganic nitrogen cycling on the δ15N of Lake Ontario biota

The seasonal variability in the δ15N of biota at the base of the pelagic food chain in Lake Ontario was studied during the period 1995-1997. Strong seasonal patterns were observed in the δ15N of dissolved inorganic nitrogen, different size fractions of particulate organic matter, and zooplankton split into size fractions of 110-210, 210-295, and >295 µm. Similar patterns were reflected in the δ15N of individual species: Diacyclops thomasi, Bosmina longirostris, Daphnia spp., and calanoid copepods. The seasonal fluctuation of primary producers was a function of the nitrogen enrichment of dissolved inorganic nitrogen during photosynthetic uptake and the δ15N of NH4+ and NO3- used by the algae. The large range in δ15N of primary producers suggests that recycling of nitrogen is an important determinant of δ15N at the base of the food web. The δ15N of individual species of zooplankton reflected the known feeding strategies and dietary preferences.

Site history affects soil and plant 15N natural abundances (δ15N) in forests of northern Vancouver Island, British Columbia

Abstract1. About 10 years after establishment, plantations of Western Redcedar (Thuja plicata Donn ex D. Don) on northern Vancouver Island, British Columbia become nutrient deficient and chlorotic, grow slowly, and are susceptible to invasion by the ericaceous shrub Salal (Gaultheria shallon Pursh.).2. To test the hypothesis that δ15N can be related to site histories (site disturbance, soil N dynamics and plant development), we measured soil and foliar δ15N in the summer of 1992 in 3‐year‐old (nutrient‐sufficient) and 10‐year‐old (nutrient‐deficient) plantations and in old‐growth stands. The foliar and soil δ15N values of the plantations and old‐growth forests were different and closely reflected site histories. Salal invasion and nutrient deficiency interacted to depress the growth of Redcedar in 10‐year‐old plantations.3. Site preparation destroyed the top soil organic layers (fresh and decaying litter) and forced Salal (ecto‐ and ericoid mycorrhizal) into the humus layer, where it was in direct competition with Redcedar, thereby disadvantaging arbuscular mycorrhizal/non‐mycorrhizal Redcedar in its nutrient acquisition during a period when N and P are severely limited.4. There was a large seasonal range of foliar δ15N (5·5 and 4·3‰ for 10‐year‐old Redcedar and Salal, respectively), and there was no relationship between foliar δ15N and measured rooting depth, demonstrating that rooting depths cannot be used to explain foliar δ15N variation among coexisting woody taxa.5. Foliar and soil δ15N declined with site age and with a presumed change from ‘open’ to ‘closed’ N cycling; the 15N‐depleting effects of mycorrhizal N transformations contributed to the observed δ15N decline.

Functional diversity in an Amazonian rainforest of French Guyana: a dual isotope approach (δ15N and δ13C).

Functional aspects of biodiversity were investigated in a lowland tropical rainforest in French Guyana (5°2'N, annual precipitation 2200 mm). We assessed leaf δ15N as a presumptive indicator of symbiotic N2 fixation, and leaf and wood cellulose δ13C as an indicator of leaf intrinsic water-use efficiency (CO2 assimilation rate/leaf conductance for water vapour) in dominant trees of 21 species selected for their representativeness in the forest cover, their ecological strategy (pioneers or late successional stage species, shade tolerance) or their potential ability for N2 fixation. Similar measurements were made in trees of native species growing in a nearby plantation after severe perturbation (clear cutting, mechanical soil disturbance). Bulk soil δ15N was spatially quite uniform in the forest (range 3-5‰), whereas average leaf δ15N ranged from -0.3‰ to 3.5‰ in the different species. Three species only, Diplotropis purpurea, Recordoxylon speciosum (Fabaceae), and Sclerolobium melinonii (Caesalpiniaceae), had root bacterial nodules, which was also associated with leaf N concentrations higher than 20 mg g-1. Although nodulated trees displayed significantly lower leaf δ15N values than non-nodulated trees, leaf δ15N did not prove a straightforward indicator of symbiotic fixation, since there was a clear overlap of δ15N values for nodulated and non-nodulated species at the lower end of the δ15N range. Perturbation did not markedly affect the difference δ15Nsoil - δ15Nleaf, and thus the isotopic data provide no evidence of an alteration in the different N acquisition patterns. Extremely large interspecific differences in sunlit leaf δ13C were observed in the forest (average values from -31.4 to -26.7‰), corresponding to intrinsic water-use efficiencies (ratio CO2 assimilation rate/leaf conductance for water vapour) varying over a threefold range. Wood cellulose δ13C was positively related to total leaf δ13C, the former values being 2-3‰ higher than the latter ones. Leaf δ13C was not related to leaf δ15N at either intraspecific or interspecific levels. δ13C of sunlit leaves was highest in shade hemitolerant emergent species and was lower in heliophilic, but also in shade-tolerant species. For a given species, leaf δ13C did not differ between the pristine forest and the disturbed plantation conditions. Our results are not in accord with the concept of existence of functional types of species characterized by common suites of traits underlying niche differentiation; rather, they support the hypothesis that each trait leads to a separate grouping of species.

Influence of understory removal, thinning and P fertilization on N2fixation in a mature mesquite (Prosopis glandulosavar.glandulosa) stand

The natural abundance15N/14N method was used to estimate the influence of silvicultural and P fertilization treatments on N accretion, N2fixation and N partitioning among tissues in a mature mesquiteProsopis glandulosavar.glandulosastand in Texas. The silvicultural treatments consisted of understory removal, herbicide treatment of brushy resprouts, thinning trees to single stems and 100 kg ha−1P fertilization. The trees had a mean basal diameter of 17·8 cm with 8 to 35 cm range. The stand was slow growing with the increase in dry matter ranging from 0·465 Mg ha−1year−1to 0·701 Mg ha−1year−1for the 8 years after the treatments were applied. N accretion after 8 years ranged from 3·1 kg ha−1year−1to 4·4 kg ha−1year−1.Due to the range in δ15N of the leaves, twigs, branches and trunk, we used the weighted (by biomass) average δ15N per tree in calculations of the percent N derived from N2fixation (%Ndfa). There was considerable variability in δ15N of the reference plants, i.e. from 3·3 to 5·9. In contrast there was low variability in the background δ15N of nearby soils (7·0±1·0). As the total above-ground biomass δ15N of a grass grown outside the influence of mesquite (7·8±0·58) had the same δ15N as the soil (7·5±1·0), we used the grass outside the influence of mesquite and the weighted tree mean δ15N to calculate % of N derived from N2fixation.The decrease in intraspecific competition by thinning multistemed trees to single stemmed trees was the only treatment that significantly (p= 0·0001) increased growth. Interspecific competition, i.e. understory removal, did not increase growth. There were no significant differences in total N production or N fixation among treatment means. The most striking result was the highly positive correlation between tree δ15N and total N per tree and biomass per tree (R2= 0·90,F= 164·4, df. = 18, mean square error (MSE) = 0·155,p= 0·0001). This implies that the younger trees colonizing infertile soils relied more heavily on N2fixation than larger trees which accumulated 1200 kg ha−1more N under their canopies. The percentage N derived from N2fixation ranged from 63 to 73% in the various treatments. Despite the high percentage of N derived from N2fixation, the N2fixation of the stand was very low, i.e. 1·98 to 2·80 kg N ha−1year−1, due to the low growth of the stand. We believe that comparisons of the whole tree weighted δ15N to background soil δ15N provides a more reasonable approach to estimate % N2fixation than comparisons of leaves of fixers and reference plants.

Stable isotopes of carbon and nitrogen as indicators of diet and trophic structure of the fish community in a shallow hypereutrophic lake

Stable carbon (δ13C) and nitrogen (δ15N) isotopes were employed to elucidate energy flows and trophic interactions in Lake Apopka, a hypereutrophic lake in central Florida, U.S.A. Isotope compositions of lake biota ranged from −27·1 to −3·0‰ for δ13C, and from 3·7 to 13·9‰ for δ15N. The food web was based primarily on plankton production with diatoms, Microcystis and zooplankton dominating the diet of fish. Carbon isotope evidence showed that pico‐ and nano‐phytoplankton were not a direct carbon source for fish, but were important to zooplankton. δ15N mass balance estimates indicated that planktivorous fish obtained 48–85% of their diets from zooplankton. The ∼3‰ range of δ15N in gizzard shad reflected increasing dependence on zooplankton as fish grew whereas the positive relationship between total length and δ15N of largemouth bass reflected increasing predation on larger planktivorous fish with growth. The broad ranges of δ13C (−25·9 to −9·5‰) and δ15N (5·8 to 14·4‰) of blue tilapia were indicators of diet diversity. Two presumed omnivores (brown bullhead and white catfish) and piscivores (black crappie, largemouth bass and Florida gar) were found to depend on planktivorous fish. However, stable isotope data revealed no trophic links between blue tilapia, an abundant fish in the near‐shore area, and piscivores.

Stable isotopes of carbon and nitrogen as indicators of diet and trophic structure of the fish community in a shallow hypereutrophic lake

Stable carbon (δ13C) and nitrogen (δ15N) isotopes were employed to elucidate energy flows and trophic interactions in Lake Apopka, a hypereutrophic lake in central Florida, U.S.A. Isotope compositions of lake biota ranged from −27·1 to −3·0‰ for δ13C, and from 3·7 to 13·9‰ for δ15N. The food web was based primarily on plankton production with diatoms, Microcystis and zooplankton dominating the diet of fish. Carbon isotope evidence showed that pico- and nano-phytoplankton were not a direct carbon source for fish, but were important to zooplankton. δ15N mass balance estimates indicated that planktivorous fish obtained 48–85% of their diets from zooplankton. The ∼3‰ range of δ15N in gizzard shad reflected increasing dependence on zooplankton as fish grew whereas the positive relationship between total length and δ15N of largemouth bass reflected increasing predation on larger planktivorous fish with growth. The broad ranges of δ13C (−25·9 to −9·5‰) and δ15N (5·8 to 14·4‰) of blue tilapia were indicators of diet diversity. Two presumed omnivores (brown bullhead and white catfish) and piscivores (black crappie, largemouth bass and Florida gar) were found to depend on planktivorous fish. However, stable isotope data revealed no trophic links between blue tilapia, an abundant fish in the near-shore area, and piscivores.

The chemistry of marine petroleum seeps

Natural oil seepage contributes approximately 10% (0.6 × 106 metric tons per year) of the annual input of petroleum hydrocarbons to the marine environment and half of that arises from the continental shelves of the circum-Pacific. Chemical properties of submarine petroleum seeps are reviewed, focusing on those of the Southern California Borderland, in relation to crude oil produced in the same geographic area, the compositional changes due to weathering in the marine environment, and the hydrocarbon and trace element compositions of the surrounding seawater and adjacent sediments. One purpose has been to determine whether the hydrocarbon compositions of seawater or surface sediments reflect the composition of nearby seepage, and can be used to identify previously unknown petroleum sources. The analytical procedures included identification of saturated hydrocarbons by gas chromatography and gas chromatography/mass spectrometry; stable isotopic ratios of sulfur, nitrogen and carbon on gas and oil, and the δ13C for various liquid-solid chromato-graphic fractions. The range in δ34S covered by the California oils is −4 to +15.9%., and the δ15N range is from +5.6 to +11%.. Carbon isotopic ratios for total oils exhibit a relatively narrow range (−22.1 to −27.3%.); however, chemical fractions show a relatively greater variation (−21.5 to −31.5%.), with the hexane fraction lighter than the total oil, and the n-alkanes representing the lightest of all fractions measured. The content of nitrogen and sulfur, their isotopic ratios, and the δ13C of chromatographic fractions, serve to distinguish oils and seeps derived from different formations. Comparison of gas chromatographic/mass spectrometric data from a seep and from a crude oil produced nearby show cycloalkane compositional differences attributable to origins, rather than to differences in weathering history. Data presented demonstrate that analyses of the water column for enrichment in high-molecular-weight hydrocarbons or trace metals adjacent to seep areas may produce equivocal results. Sediment analysis appears to give a more reliable indication of petroleum contribution. Anthropogenic hydrocarbons and trace metals, derived from shipping lanes, harbor activities, or sewage disposal systems, can exceed natural concentrations and thereby alter the distribution pattern. However, marine sediments receiving petroleum contributions can be differentiated from those in which only biogenic hydrocarbons are present.