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1.
Logan J  Haas H  Deegan L  Gaines E 《Oecologia》2006,147(3):391-395
Nitrogen stable isotopes are frequently used in ecological studies to estimate trophic position and determine movement patterns. Knowledge of tissue-specific turnover and nitrogen discrimination for the study organisms is important for accurate interpretation of isotopic data. We measured δ15 N turnover in liver and muscle tissue in juvenile mummichogs, Fundulus heteroclitus, following a laboratory diet switch. Liver tissue turned over significantly faster than muscle tissue suggesting the potential for a multiple tissue stable isotope approach to study movement and trophic position over different time scales; metabolism contributed significantly to isotopic turnover for both liver and muscle. Nitrogen diet-tissue discrimination was estimated at between 0.0 and 1.2‰ for liver and –1.0 and 0.2‰ for muscle. This is the first experiment to demonstrate a significant variation in δ15 N turnover between liver and muscle tissues in a fish species.  相似文献   

2.
Estimates of diet derived from stable isotope analyses are sensitive to the accuracy of corrections made for diet-tissue fractionation. In particular, diet-tissue fractionation in reindeer Rangifer tarandus may be expected to differ significantly from the generic values often used in stable isotope dietary calculations, given the known values obtained from other ungulates and the complex digestive system and nutrient recycling characteristic of the species. We fed domestic reindeer a homogenous artificial diet of known isotopic value in order to directly determine diet-tissue isotopic fractionation of carbon and nitrogen, the main elements used in stable isotope dietary analyses. Diet-tissue fractionation values for blood plasma were +3.5 ± 0.1‰ (δ13C) and +4.2 ± 0.3‰ (δ15N). Diet-tissue fractionation values for whole blood were +3.7 ± 0.2‰ (δ13C) and +2.5 ± 0.3‰ (δ15N). Metabolic turnover rates were clearly sufficient for complete tissue replacement over the period of artificial feeding for blood plasma, but may not have been so for whole blood, especially for δ15N. Our values, except for whole blood δ15N, differ considerably from the generic values often used in dietary studies and interspecific comparisons of trophic niche. The results underline the importance of obtaining as specific as possible fractionation values for the species, tissue, and in some cases sex and physiological status of animals under examination, and the potential problems associated with assuming ‘generic’ fractionation values when comparing species, especially where digestive processes are dissimilar.  相似文献   

3.
Natural 15N abundance measurements of ecosystem nitrogen (N) pools and 15N pool dilution assays of gross N transformation rates were applied to investigate the potential of δ15N signatures of soil N pools to reflect the dynamics in the forest soil N cycle. Intact soil cores were collected from pure spruce (Picea abies (L.) Karst.) and mixed spruce-beech (Fagus sylvatica L.) stands on stagnic gleysol in Austria. Soil δ15N values of both forest sites increased with depth to 50 cm, but then decreased below this zone. δ15N values of microbial biomass (mixed stand: 4.7 ± 0.8‰, spruce stand: 5.9 ± 0.9‰) and of dissolved organic N (DON; mixed stand: 5.3 ± 1.7‰, spruce stand: 2.6 ± 3.3‰) were not significantly different; these pools were most enriched in 15N of all soil N pools. Denitrification represented the main N2O-producing process in the mixed forest stand as we detected a significant 15N enrichment of its substrate NO3 (3.6 ± 4.5‰) compared to NH4+ (−4.6 ± 2.6‰) and its product N2O (−11.8 ± 3.2‰). In a 15N-labelling experiment in the spruce stand, nitrification contributed more to N2O production than denitrification. Moreover, in natural abundance measurements the NH4+ pool was slightly 15N-enriched (−0.4 ± 2.0 ‰) compared to NO3 (−3.0 ± 0.6 ‰) and N2O (−2.1 ± 1.1 ‰) in the spruce stand, indicating nitrification and denitrification operated in parallel to produce N2O. The more positive δ15N values of N2O in the spruce stand than in the mixed stand point to extensive microbial N2O reduction in the spruce stand. Combining natural 15N abundance and 15N tracer experiments provided a more complete picture of soil N dynamics than possible with either measurement done separately.  相似文献   

4.
We used the dual isotope method to study differences in nitrate export in two subwatersheds in Vermont, USA. Precipitation, soil water and streamwater samples were collected from two watersheds in Camels Hump State Forest, located within the Green Mountains of Vermont. These samples were analyzed for the δ15N and δ18O of NO3. The range of δ15N–NO3 values overlapped, with precipitation −4.5‰ to +2.0‰ (n = 14), soil solution −10.3‰ to +6.2‰ (n = 12) and streamwater +0.3‰ to +3.1‰ (n = 69). The δ18O of precipitation NO3 (mean 46.8 ± 11.5‰) was significantly different (P < 0.001) from that of the stream (mean 13.2 ± 4.3‰) and soil waters (mean 14.5 ± 4.2‰) even during snowmelt periods. Extracted soil solution and streamwater δ18O of NO3 were similar and within the established range of microbially produced NO3, demonstrating that NO3 was formed by microbial processes. The δ15N and δ18O of NO3 suggests that although the two tributaries have different seasonal NO3 concentrations, they have a similar NO3 source.  相似文献   

5.
Summary Plants from agricultural and natural upland ecosystem were investigated for15N content to evaluate the role of symbiotic N2-fixation in the nitrogen nutrition of soybean. Increased yields and lower δ15N values of nodulating soybeansvs, non-nodulating isolines gave semi-quantitative estimates of N2 fixation. A fairly large discrepancy was found between estimations by δ15N and by N yield at 0 kg N/ha of fertilizer. More precise estimates were made by following changes in plant δ15N when fertilizer δ15N was varied near15N natural abundance level. Clearcut linear relationships between δ15N values of whole plants and of fertilizer were obtained at 30 kg N/ha of fertilizer for three kinds of soils. In experimental field plots, nodulating soybeans obtained 13±1% of their nitrogen from fertilizer, 66±8% from N2 fixation and 21±10% from soil nitrogen in Andosol brown soil; 30%, 16% and 54% in Andosol black soil; 7%, 77% and 16% in Alluvial soil, respectively. These values for N2 fixation coincided with each corresponding estimation by N yield method. Other results include: 1)15N content in upland soils and plants was variable, and may reflect differences in the mode of mineralization of soil organics, and 2) nitrogen isotopic discrimination during fertilizer uptake (δ15N of plant minus fertilizer) ranged from −2.2 to +4.9‰ at 0–30 kg N/ha of fertilizer, depending on soil type and plant species. The proposed method can accurately and relatively simply establish the importance of symbiotic nitrogen fixation for soybeans growing in agricultural settings.  相似文献   

6.
The food webs of rocky infra-littoral ecosystems in the Mediterranean have been little studied. In this investigation stable isotopes and dietary data were compared in an attempt to describe features of the food webs concerned. δ13C and δ15N were determined for plants, invertebrates and fishes from the Bay of Calvi, Corsica. Dietary data were derived from the literature. δ13C of plants ranged from –8.59‰ to –33.74‰, of benthic invertebrates from –17.0‰ to –20.52‰, of planktonic invertebrates from –20.08‰ to –22.34‰ and of fishes from –16.27‰ to –19.59‰. δ15N was generally greater at higher trophic levels. δ15N of plants was 0.95–2.92‰, of benthic invertebrates 1.69–6.54‰, of planktonic invertebrates 3.51–6.82‰ and of fishes 4.63–9.77‰. 13C enrichment tended to be associated with benthic food chains and 13C depletion with planktonic chains. Stable-isotope data suggested more varied diets for many species than implied by gut-contents data. Omnivory and trophic plasticity were widespread, and many consumers fed lower down the food chain than previous studies had suggested. Both stable-isotope and gut-contents analysis resolved differences between fishes feeding on planktonic and benthic prey and indicated that the herbivorous fish Sarpa salpa fed on a diet substantially different from that of other fishes. Zooplankton were important in the diets of several consumers (both primary and secondary), as was plankton derived detritus. One species of fish previously identified as planktivorous was shown to feed largely on benthic organisms, whilst several species of benthic invertebrates may feed on plankton-derived detritus. Although herbivores seemed to obtain most of their C from macroalgae, δ15N data suggested that many of these animals supplemented their intake of N, although gut-contents analysis did not provide evidence for such uptake. The isotopic data have elucidated several features of the food web which we would not otherwise have detected. Received: 26 April 1999 / Accepted: 24 September 1999  相似文献   

7.
The effects of the liquid pig manure (LM) used in organic farming on the natural abundance of 15N and 13C signatures in plant tissues have not been studied. We hypothesized that application of LM will (1) increase δ15N of plant tissues due to the high δ15N of N in LM as compared with soil N or inorganic fertilizer N, and (2) increase δ13C of plant tissues as a result of high salt concentration in LM that decreases stomatal conductance of plants. To test these hypotheses, variations in the δ15N and δ13C of Chinese cabbage (Brassica campestris L.) and chrysanthemum (Chrysanthemum morifolium Ramatuelle) with two different LMs (with δ15N of +15.6 and +18.2‰) applied at two rates (323 and 646 kg N ha-1 for cabbage and 150 and 300 kg N ha-1 for chrysanthemum), or urea (δ15N = -2.7‰) applied at the lower rate above for the respective species, in addition to the control (no N input) were investigated through a 60-day pot experiment. Application of LM significantly increased plant tissue δ15N (range +9.4 to +14.9‰) over the urea (+3.2 to +3.3‰) or control (+6.8 to 7.7‰) treatments regardless of plant species, strongly reflecting the δ15N of the N source. Plant tissue δ13C were not affected by the treatments for cabbage (range −30.8 to −30.2‰) or chrysanthemum (−27.3 to −26.8‰). However, cabbage dry matter production decreased while its δ13C increased with increasing rate of LM application or increasing soil salinity (P < 0.05), suggesting that salinity stress caused by high rate of LM application likely decreased stomatal conductance and limited growth of cabbage. Our study expanded the use of the δ15N technique in N source (organic vs. synthetic fertilizer) identification and suggested that plant tissue δ13C maybe a sensitive indicator of plant response to salinity stress caused by high LM application rates.  相似文献   

8.
The use of stable isotopes to investigate animal diets, habitat use, and trophic level requires understanding the rate at which animals incorporate the 13C and 15N from their diets and the factors that determine the magnitude of the difference in isotopic composition between the animal’s diet and that of its tissues. We determined the contribution of growth and catabolic turnover to the rate of 13C and 15N incorporation into several tissues that can be sampled non-invasively (skin, scute, whole blood, red blood cells, and plasma solutes) in two age classes of a rapidly growing ectotherm (loggerhead turtles, Caretta caretta). We found significant differences in C and N incorporation rates and isotopic discrimination factors (Δ13C = δ13Ctissues − δ13Cdiet and Δ15N = δ15Ntissues − δ15Ndiet) among tissues and between age classes. Growth explained from 26 to 100% of the total rate of incorporation in hatchling turtles and from 15 to 52% of the total rate of incorporation in juvenile turtles. Because growth contributed significantly to the rate of isotopic incorporation, variation in rates among tissues was lower than reported in previous studies. The contribution of growth can homogenize the rate of isotopic incorporation and limit the application of stable isotopes to identify dietary changes at contrasting time scales and to determine the timing of diet shifts. The isotopic discrimination factor of nitrogen ranged from −0.64 to 1.77‰ in the turtles’ tissues. These values are lower than the commonly assumed average 3.4‰ discrimination factors reported for whole body and muscle isotopic analyses. The increasing reliance on non-invasive and non-destructive sampling in animal isotopic ecology requires that we recognize and understand why different tissues differ in isotopic discrimination factors.  相似文献   

9.
Eshetu  Zewdu  Högberg  Peter 《Plant and Soil》2000,222(1-2):109-117
We used the natural abundance of 15N in soils in forests, pastures and cultivated lands in the Menagesha and Wendo-Genet areas of Ethiopia to make inferences about the N cycles in these ecosystems. Since we have described the history of these sites based on variations in 13C natural abundance, patterns of δ15N and δ13C values were compared to determine if shifts of 15N correlate with shifts of vegetation. At Menagesha, a > 500-yr-old planted forest, we found δ15N values from −8.8 to +3.5‰ in litter, from −3.5 to +4.5‰ in 0–10 cm soil layer, and from −1.5 to +6.8‰ at >20 cm soil depth. The low δ15N in litter and surface mineral soils suggests that a closed N cycle has operated for a long time. At this site, the low δ13C of the surface horizon and the high δ13C of the lower soil horizons is clear evidence of a long phase of C4 grass dominance or cultivation of C4 crops before the establishment of the forest >500 years ago. In contrast, at Wendo-Genet, high δ13C of soils reveals that most of the land has been uncovered by forests until recently. Soil δ15N was high throughout (3.4–9.8‰), and there were no major differences between forested, cultivated and pasture soils in δ15N values of surface mineral soils. The high δ15N values suggest that open N cycles operate in the Wendo-Genet area. From the points of view of soil fertility management, it is interesting that tall forest ecosystems with relatively closed N cycling could be established on the fairly steep slopes at Menagesha after a long period of grass vegetation cover or cultivation. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   

10.
Abstract Freshwater ecosystems derive organic carbon from both allochthonous and autochthonous sources. We studied the relative contributions of different carbon sources to zooplankton in a small, polyhumic, steeply stratified lake, using six replicate surface-to-sediment enclosures established during summer and autumn 2004. We added 13C-enriched bicarbonate to the epilimnion of half the enclosures for three weeks during each season and monitored carbon stable isotope ratios of DIC, DOC, POC and Daphnia, along with physical, chemical and biological variables. During summer, 13C-enriched DIC (δ13C up to 44 ± 7.2‰) was soon taken up by phytoplankton (δ13C up to −5.1 ± 13.6‰) and was transmitted to Daphnia13C up to −1.7 ± 7.2‰), demonstrating consumption of phytoplankton. In contrast, during autumn, 13C-enriched DIC (δ13C up to 56.3 ± 9.8‰) was not transmitted to Daphnia, whose δ13C became progressively lower (δ13C down to −45.6 ± 3.3‰) concomitant with decreasing methane concentration. Outputs from a model suggested phytoplankton contributed 64–84% of Daphnia diet during summer, whereas a calculated pelagic carbon mass balance indicated only 30–40% could have come from phytoplankton. Although autumn primary production was negligible, zooplankton biomass persisted at the summer level. The model suggested methanotrophic bacteria contributed 64–87% of Daphnia diet during autumn, although the calculated carbon mass balance indicated a contribution of 37–112%. Thus methanotrophic bacteria could supply virtually all the carbon requirement of Daphnia during autumn in this lake. The strongly 13C-depleted Daphnia values, together with the outputs from the models and the calculated carbon mass balance showed that methanotrophic bacteria can be a greater carbon source for Daphnia in lakes than previously suspected.  相似文献   

11.
Carbon isotopic composition of soils subjected to C3–C4 vegetation change can be used to estimate C turnover in bulk soil and in soil organic matter (SOM) pools with fast and intermediate turnover rates. We hypothesized that the biological availability of SOM pools is inversely proportional to their thermal stability, so that thermogravimetry can be used to separate SOM pools with contrasting turnover rates. Soil samples from a field plot cultivated for 10.5 years with the perennial C4 plant Miscanthus×gigantheus were analyzed by thermogravimetry coupled with differential scanning calorimetry (DSC). Three SOM fractions were distinguished according to the differential weight losses and exothermic or endothermic reactions measured by DSC. The δ13C and δ15N values of these three fractions obtained by gradual soil heating were measured by IRMS. The weight losses up to 190 °C mainly reflected water evaporation because no significant C and N losses were detected and δ13C and δ15N values of the residual SOM remained unchanged. The δ13C values (−16.4‰) of SOM fraction decomposed between 190 and 390 °C (containing 79% of total soil C) were slightly closer to that of the Miscanthus plant tissues (δ13C = −11.8‰) compared to the δ13C values (−16.8‰) of SOM fraction decomposed above 390 °C containing the residual 21% of SOM. Thus, the C turnover in the thermally labile fraction was faster than that in thermally stable fractions, but the differences were not very strong. Therefore, in this first study combining TG-DSC with isotopic analysis, we conclude that the thermal stability of SOM was not very strongly related to biological availability of SOM fractions. In contrast to δ13C, the δ15N values strongly differed between SOM fractions, suggesting that N turnover in the soil was different from C turnover. More detailed fractionation of SOM by thermal analysis with subsequent isotopic analysis may improve the resolution for δ13C.  相似文献   

12.
CO2 applied for Free-Air CO2 Enrichment (FACE) experiments is strongly depleted in 13C and thus provides an opportunity to study C turnover in soil organic matter (SOM) based on its δ 13C value. Simultaneous use of 15N labeled fertilizers allows N turnover to be studied. Various SOM fractionation approaches (fractionation by density, particle size, chemical extractability etc.) have been applied to estimate C and N turnover rates in SOM pools. The thermal stability of SOM coupled with C and N isotopic analyses has never been studied in experiments with FACE. We tested the hypothesis that the mean residence time (MRT) of SOM pools is inversely proportional to its thermal stability. Soil samples from FACE plots under ambient (380 ppm) and elevated CO2 (540 ppm; for 3 years) treatments were analyzed by thermogravimetry coupled with differential scanning calorimetry (TG-DSC). Based on differential weight losses (TG) and energy release or consumption (DSC), five SOM pools were distinguished. Soil samples were heated up to the respective temperature and the remaining soil was analyzed for δ 13C and δ 15N by IRMS. Energy consumption and mass losses in the temperature range 20–200°C were mainly connected with water volatilization. The maximum weight losses occurred from 200–310°C. This pool contained the largest amount of carbon: 61% of the total soil organic carbon in soil under ambient treatment and 63% in soil under elevated CO2, respectively. δ 13C values of SOM pools under elevated CO2 treatment showed an increase from −34.3‰ of the pool decomposed between 20–200°C to −18.1‰ above 480°C. The incorporation of new C and N into SOM pools was not inversely proportional to its thermal stability. SOM pools that decomposed between 20–200 and 200–310°C contained 2 and 3% of the new C, with a MRT of 149 and 92 years, respectively. The pool decomposed between 310–400°C contained the largest proportion of new C (22%), with a MRT of 12 years. The amount of fertilizer-derived N after 2 years of application in ambient and elevated CO2 treatments was not significantly different in SOM pools decomposed up to 480°C having MRT of about 60 years. In contrast, the pool decomposed above 480°C contained only 0.5% of new N, with a MRT of more than 400 years in soils under both treatments. Thus, the separation of SOM based on its thermal stability was not sufficient to reveal pools with contrasting turnover rates of C and N. Responsible Editor: Bernard Nicolardot.  相似文献   

13.
15N abundances of current needles of Norway spruce collected during 23 yrs of a forest fertilization experiment were studied in order to follow ecosystem gains and losses of N. Unlabelled ammonium nitrate at four rates (N0–N3), phosphorus at three rates (P0–P2), and potassium plus other elements including micronutrients at two rates (K0–K1), had been applied to plots in a complete factorial design. Nitrogen had been applied annually at average rates of 0, 34, 68 and 102 kg N ha-1 yr-1. Tree growth had responded positively to additions of N, but the response was remarkably more positive to the N2P2K1 treatment. In N1 treatments, δ15N (‰) declined over time. This was consistent with an earlier study, and should reflect a change in 15N abundance towards that of fertilizer N (minus discrimination during uptake), which in turn means accretion of most of the N added. As in the earlier study, in which N3 plots lost most of the N added, the present N3 plots showed an increasing δ15N (‰). This pattern was not significantly affected by additions of P and K plus other elements, although a weak negative effect of P on N accretion was indicated, i.e. there was a tendency δ15N (‰) to be higher when P was added. This, and another recent result based on an N budget, shows that so-called revitalization fertilization may well increase growth of trees, but also promotes losses of N from the ecosystem. As in the previous study, a decline in δ15N (‰) on control plots provided evidence of contamination. Given a removal of 100 kg N ha-1 at stem harvest and a leaching of 2 kg N ha-1 yr-1, our data on 15N suggest that a load of 9 kg N ha-1 yr-1 would saturate the ecosystem after 100 years. This load is only about twice the annual deposition at the site.  相似文献   

14.
Sears J  Hatch SA  O'Brien DM 《Oecologia》2009,159(1):41-48
A growing number of studies suggest that an individual’s physiology affects its carbon and nitrogen stable isotope signatures, obscuring a signal often assumed to be only a reflection of diet and foraging location. We examined effects of growth and moderate food restriction on red blood cell (RBC) and feather δ15N and δ13C in rhinoceros auklet chicks (Cerorhinca monocerata), a piscivorous seabird. Chicks were reared in captivity and fed either control (75 g/day; n = 7) or ~40% restricted (40 g/day; n = 6) amounts of high quality forage fish. We quantified effects of growth on isotopic fractionation by comparing δ15N and δ13C in control chicks to those of captive, non-growing subadult auklets (n = 11) fed the same diet. To estimate natural levels of isotopic variation, we also collected blood from a random sample of free-living rhinoceros auklet adults and chicks in the Gulf of Alaska (n = 15 for each), as well as adult feather samples (n = 13). In the captive experiment, moderate food restriction caused significant depletion in δ15N of both RBCs and feathers in treatment chicks compared to control chicks. Growth also induced depletion in RBC δ15N, with chicks exhibiting lower δ15N when they were growing the fastest. As growth slowed, δ15N increased, resulting in an overall pattern of enrichment over the course of the nestling period. Combined effects of growth and restriction depleted δ15N in chick RBCs by 0.92‰. We propose that increased nitrogen-use efficiency is responsible for 15N depletion in both growing and food-restricted chicks. δ15N values in RBCs of free-ranging auklets fell within a range of only 1.03‰, while feather δ15N varied widely. Together, our captive and field results suggest that both growth and moderate food restriction can affect stable isotope ratios in an ecologically meaningful way in RBCs although not feathers due to greater natural variability in this tissue.  相似文献   

15.
When using stable isotopes as dietary tracers it is essential to consider effects of nutritional state on isotopic fractionation. While starvation is known to induce enrichment of 15N in body tissues, effects of moderate food restriction on isotope signatures have rarely been tested. We conducted two experiments to investigate effects of a 50–55% reduction in food intake on δ15N and δ13C values in blood cells and whole blood of tufted puffin chicks, a species that exhibits a variety of adaptive responses to nutritional deficits. We found that blood from puffin chicks fed ad libitum became enriched in 15N and 13C compared to food-restricted chicks. Our results show that 15N enrichment is not always associated with food deprivation and argue effects of growth on diet–tissue fractionation of nitrogen stable isotopes (Δ15N) need to be considered in stable isotope studies. The decrease in δ13C of whole blood and blood cells in restricted birds is likely due to incorporation of carbon from 13C-depleted lipids into proteins. Effects of nutritional restriction on δ15N and δ13C values were relatively small in both experiments (δ15N: 0.77 and 0.41‰, δ13C: 0.20 and 0.25‰) compared to effects of ecological processes, indicating physiological effects do not preclude the use of carbon and nitrogen stable isotopes in studies of seabird ecology. Nevertheless, our results demonstrate that physiological processes affect nitrogen and carbon stable isotopes in growing birds and we caution isotope ecologists to consider these effects to avoid drawing spurious conclusions.  相似文献   

16.
Feathers are used commonly for stable isotope analysis to assess the foraging ecology and migration patterns of birds. However, these studies often require knowledge of species-specific feather isotopic discrimination factors (the differences in isotopic ratios between a species’ diet and feathers), which can be influenced by a species’ physiological state during molt. In this study, we determined the isotopic discrimination factors (Δ13Cdiet−feather and Δ15Ndiet−feather) between adult gentoo penguin (Pygoscelis papua) diet and feathers in a controlled study. In addition, we tested whether molt duration or the magnitude of voluntary dietary reduction during molt influenced isotopic discrimination, as previous studies have found that nutritional stress can exaggerate 15N enrichment and in some cases lead to 13C depletion in feathers. Contrary to this hypothesis, we found no effect of molt duration or dietary reduction on discrimination factors, suggesting that isotopic discrimination is not linearly related to these measures of fasting intensity in penguins. Furthermore, we found that the range of Δ15Ndiet−feather found in several species of penguins, which fast while they molt, was similar to discrimination factors in fish-eating birds, which do not fast during molt. It is likely that species-specific metabolic adaptations that limit nutritional stress while fasting and variation in their relative reliance on endogenous vs. dietary pools during feather growth may confound the use of Δ15Ndiet−feather as a general measure of nutritional stress when comparing among species.  相似文献   

17.
 Quantifying pathways of energy transfer between plants, pests, and beneficial insects is a necessary step toward maintaining pest stable agroecosystems in the absence of chemical subsidies. A diet switching experiment utilizing a predatory ladybird beetle, Hippodamia variegata (Goeze), evaluated the use of naturally occurring stable C and N isotopes as an economically feasible and safe method for quantifying pathways of energy flow within agroecosystems. Stable isotope values of the ladybird beetle Coleomegilla maculata lengi (Timberlake) collected from an agroecosystem were used to estimate the relative amount of C and N derived from agricultural plants and incorporated by ladybird beetles based on mass balance equations. At the beginning of the diet-switching experiment δ13C and δ15N values of H. variegata (–12.0‰ and 6.3‰, respectively) differed by –0.2‰ and 2.9‰ from the aphids that were provided exclusively as their diet. These data are consistent with previous estimates of trophic level isotope effects. After switching the diet of H. variegata to an alternative food, isotope values of H. variegata gradually shifted toward expected values for individuals fed this diet (–22.9‰ and 8.8‰ for δ13C and δ15N values, respectively). Isotope values of another ladybird beetle, C. maculata, collected from the field indicated that in May, alfalfa and maize (pollen) obtained in the previous year contributed 32% and 68% of the C or N to the diets of these individuals and in August, 52%, 6%, and 42% of the C or N assimilated by these insects was derived from alfalfa, wheat, and maize, respectively. These data are consistent with expectations based on the relative abundance of C. maculata in various crops during the season. The field and laboratory data are a clear indication that isotope values are sensitive to dietary changes on a relatively short time scale (days) and provide a strong basis for the use stable C and N isotope to trace energy flow patterns of these beneficial organisms within agroecosystems. Received: 17 November 1995 / Accepted: 20 June 1996  相似文献   

18.
Faecal stable isotope compositions reflect wildlife diets, if digestive processes along the gastrointestinal tract (GIT) do not alter diet–faeces isotopic relationships in an unpredictable way. We investigated 13C and 15N compositions of digesta along the ruminant GIT, using Saanen dairy goats kept on pure grass hay or browse for >20 days. Isotopic changes occurred in the ventral rumen, and in the small intestine, where digesta had significantly higher δ13C and δ15N (associated with lower C or higher N content, respectively) values relative to other GIT sites. However, effects on isotope fractionation were small (∼1.0‰ for δ13C and ∼ 2.0‰ for δ15N), and were reversed in the hindgut such that faecal isotope compositions did not differ from the foregut. No other substantial isotopic changes occurred across GIT sites, despite the morphophysiological complexity of the ruminant GIT. We found similarly small differences across GIT components of rheem gazelles (Gazella leptoceros) fed a mixture of C3 lucerne and C4 grass, although in this case faeces were 15N-depleted relative to other GIT components. Along with differences in δ15N between goats fed browse or grass, this result implies a systematic difference in diet–faeces δ15N relationships, contingent on the botanical composition of ruminant diets. Thus, while our results support faecal δ13C as a reliable proxy for wildlife diets, further work on factors influencing faecal 15N abundance is needed. Finally, we note high levels of isotopic variability between individuals fed the same diets, even accounting for the relatively short duration of the experiments, suggesting an important influence of stochasticity on isotope fractionation.  相似文献   

19.
The semi-diurnal tidal regime (≥2 m) in the Paria Gulf on the Atlantic coast of Venezuela, and the flat landscape of the region, allow the penetration for tens of km of marine waters into the rivers draining the northeastern coastal plain of the country. The levels of salinity, tidal flooding, and sedimentation decrease perpendicularly from the river channel toward the back swamps. The vegetation varies sequentially from fringe mangroves along the river margins, to back swamps containing forests dominated by Pterocarpus officinalis, herbaceous communities of Lagenocarpus guianensis, and palm swamps with Mauritia flexuosa, Chrysobalanus icaco, and Tabebuia spp. This environmental structure was used to test the hypotheses that: (a) mangrove distribution is strongly associated with salinity of interstitial water, and (b) they occupy areas where tidal influence and sediment dynamics determine a relatively open N cycle. Analyses of soil, water, and plants along a 1.5 km transect located near the confluence of the Guanoco and San Juan Rivers (Sucre and Monagas States, Venezuela) revealed that: (a) conductivity decreased from 11 to 0.2 mmhos cm−1 from the river fringe to the internal swamp, whereas Na in the same stretch decreased from 100 to 2 μM; (b) average leaf tissue concentrations of Na, P, and N decreased significantly along the transect; (c) P. officinalis showed a large Na-exclusion capacity indicated by positive K/Na ratios from 8 to 200, and Crinum erubescens counteracted Na by accumulating K above 1,000 mmol kg−1; (d) leaves varied widely in δ 13C (−25.5 to −32‰) and δ 15N (4 to −10.5‰) values. Samples were aggregated according to soil carbon content corresponding to those of the mangrove forest belt (5–28 mol C kg−1; 0–650 from river fringe) and those of the back swamps (40–44 mol C kg−1; 700–1,500 m from river fringe). The concentrations of Na, P, and N (in mmol kg−1) and δ 15N values (in ‰) were significantly higher in the mangrove forest compared to the back swamp (Na 213 vs. 88; P 41 vs. 16; N 1,535 vs. 727; δ 15N 1.5 vs. −3.7), indicating that the fringe forest was not nutrient limited. These results support the hypotheses that mangroves are restricted to the more-saline sections of the transect, and that the fringe forest has a more open N cycle, favoring 15N accumulation within the system.  相似文献   

20.
To determine the feasibility of using stable isotopes to track diet shifts in wild gag, Mycteroperca microlepis, populations over seasonal timescales, we conducted a repeated measures diet-shift experiment on four adult gag held in the laboratory. Fish were initially fed a diet of Atlantic mackerel, Scomber scombrus, (mean δ13C = −21.3‰ ± 0.2, n = 20) for a period of 56 days and then shifted to a diet of pinfish, Lagodon rhomboids, (mean δ13C = −16.6‰ ± 0.6, n = 20) for the 256 day experiment. We developed a non-lethal surgical procedure to obtain biopsies of the muscle, liver, and gonad tissue monthly from the same four fish. We then determined the δ13C value of each tissue by isotope ratio mass spectrometry. For the gonad tissue we used the relationship between C/N and lipid content to correct for the influence of lipids on δ13C value. We observed a significant shift in the δ13C values of all of the tissues sampled in the study. Carbon turnover rates varied among the three tissues, but the shift in diet from mackerel to pinfish was clearly traceable through analysis of δ13C values. The turnover rates for muscle tissue were 0.005‰ day−1, and for gonad tissue was 0.009‰ day−1. Although it is generally thought that tissue turnover rates in ectotherms are driven primarily by growth, we found that metabolic rate can be a major factor driving tissue turnover in adult gag.  相似文献   

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