共查询到20条相似文献,搜索用时 0 毫秒
1.
Disentangling effects of growth and nutritional status on seabird stable isotope ratios 总被引:1,自引:0,他引:1
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
2.
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
3.
Recent studies have shown that anguillid eel populations in habitats spanning the marine–freshwater ecotone can display extreme
plasticity in the range of catadromy expressed by individual fishes. Carbon and nitrogen stable isotope analysis was used
to differentiate between European eels (Anguilla anguilla) collected along a short (2 km) salinity gradient ranging from <1‰ to ~30‰ in Lough Ahalia, a tidal Atlantic lake system.
Significant differences were recorded in mean δ13C, δ15N and C:N values from eels collected from fresh, brackish and marine-dominated basins. A discriminant analysis using these
predictor variables correctly classified ca. 85% of eels to salinity zone, allowing eels to be classified as freshwater (FW),
brackish (BW) or marine (MW) residents. The results of the discriminant analysis also suggested that a significant proportion
of eels moved between habitats (especially between FW and BW). Comparisons of several key population parameters showed significant
variation between eels resident in different salinity zones. Mean condition and estimated age was significantly lower in MW
eels, whilst observed length at age (a correlate of growth) was significantly higher in MW eels, intermediate in BW and lowest
in FW eels. This study has demonstrated that the ecology of eels found along a short salinity gradient can be extremely plastic
and that stable isotope analysis has considerable utility in demonstrating intra-population variation in diadromous fishes. 相似文献
4.
Carbon and nitrogen stable isotopes are frequently used to study energy sources and food web structure in ecosystems, and
more recently, to study the effects of anthropogenic stress on aquatic ecosystems. We investigated the effect of nutrient
enrichment on δ13C and δ15N in fine (FPOM), coarse (CPOM) particulate organic matter, periphyton, invertebrates and fish in nine boreal streams in south-central
Sweden. In addition, we analysed the diet of benthic consumers using stable isotope data. Increases in δ15N of periphyton (R
2 = 0.88), CPOM (0.78), invertebrates (0.92) and fish (0.89) were related to nutrient enrichment. In contrast, δ13C signatures did not change along the nutrient gradient. Our results show that δ15N has potential as a sensitive indicator of nutrient enrichment in boreal streams. Carbon and nitrogen isotopes failed to
elucidate putative diets of selected aquatic consumers. Indeed, comparison of low- and high-impact sites showed that δ13C of many consumers were found outside the ranges of basal resource δ13C. Moreover, ranges of basal resource δ13C and δ15N overlapped at both low and high sites, making discrimination between the importance of allochthonous and autochthonous production
difficult. Our findings show that a fractionation rate of 3.4‰ is not always be appropriate to assess trophic interactions,
suggesting that more studies are needed on fractionation rates along gradients of impairment.
Handling editor: M. Power 相似文献
5.
Halophila ovalis on the intertidal flat of SW Thailand maintains its presence under heavy grazing by dugongs; nevertheless, the site provides
lower availability of nutrients in the water column to the seagrasses due to emersion during ebb tide. To examine the seasonality
of the nutrient status of fast-growing H. ovalis in the intertidal flat, and to assess whether emersion at low tide affects the availability of nutrients and inorganic carbon,
monthly changes in carbon (C), nitrogen (N), and phosphorus (P) concentrations of H. ovalis were determined. Carbon and nitrogen stable isotope ratios were also determined to examine the carbon and nitrogen sources.
CNP concentrations of the leaves and petioles of H. ovalis at the intertidal flat were considerably lower than those of the same species previously reported from a subtidal environment
throughout the observation period of 8 months. The δ13C of the above-ground parts of H. ovalis was lower than in previous reports, suggesting that CO2 was sufficient for its growth. The CNP concentrations of H. ovalis on the intertidal flat increased after a decrease in grazing pressure by dugongs. It is suggested that H. ovalis sustained rapid growth on the intertidal flat by lowering its nutrient concentrations under heavy grazing by dugongs to maintain
its presence.
Received: March 2, 2001 / Accepted: June 6, 2001 相似文献
6.
Stable isotopes can be used to elucidate ecological relationships in community and trophic studies. Findings are calibrated
against baselines, e.g. from a producer or primary consumer, assumed to act as a reference to the isotopic context created
by spatio-temporal attributes such as geography, climate, nutrient, and energy sources. The ability of an organism to accurately
represent a community base depends on how, and over what time-scale, it assimilates ambient materials. Freshwater mussels
have served as references for trophic studies of freshwater communities and as indicators of change in nutrient pollution
load or source. Their suitability as reference animals has not yet been fully explored, however. We conducted a series of
studies examining the suitability of freshwater mussels as isotopic baselines, using their ability to reflect variation in
ambient nutrient loads as a case scenario. (1) We analyzed bivalve foot tissue δ15N and δ13C from 22 stream reaches in the Piedmont region of North Carolina, USA to show that compositions varied substantially among
locations. Site mean bivalve δ13C values correlated with site ambient particulate organic matter (POM) δ13C values, and site mean bivalve δ15N values correlated with site ambient water dissolved δ15N-NO3 values. (2) Similarity of results among sample types demonstrated that the minimally invasive hemolymph sample is a suitable
substitute for foot tissue in δ15N analyses, and that small sample sizes generate means representative of a larger population. Both findings can help minimize
the impact of sampling on imperiled freshwater mussel populations. (3) In a bivalve transplantation study we showed that hemolymph
δ15N compositions responded to a shift in ambient dissolved δ15N-NO3, although slowly. The tissue turnover time for bivalve hemolymph was 113 days. We conclude that bivalves serve best as biomonitors
of chronic, rather than acute, fluctuations in stream nutrient loads, and provide initial evidence of their suitability as
time-integrated isotopic baselines for community studies. 相似文献
7.
Rodolfo A. Golluscio Amy T. Austin Guillermo C. García Martínez Marina Gonzalez-Polo Osvaldo E. Sala Robert B. Jackson 《Ecosystems》2009,12(4):686-697
We explored the net effects of grazing on soil C and N pools in a Patagonian shrub–grass steppe (temperate South America).
Net effects result from the combination of direct impacts of grazing on biogeochemical characteristics of microsites with
indirect effects on relative cover of vegetated and unvegetated microsites. Within five independent areas, we sampled surface
soils in sites subjected to three grazing intensities: (1) ungrazed sites inside grazing exclosures, (2) moderately grazed
sites adjacent to them, and (3) intensely grazed sites within the same paddock. Grazing significantly reduced soil C and N
pools, although this pattern was clearest in intensely grazed sites. This net effect was due to the combination of a direct
reduction of soil N content in bare soil patches, and indirect effects mediated by the increase of the cover of bare soil
microsites, with lower C and N content than either grass or shrub microsites. This increase in bare soil cover was accompanied
by a reduction in cover of preferred grass species and standing dead material. Finally, stable isotope signatures varied significantly
among grazed and ungrazed sites, with δ15N and δ13C significantly depleted in intensely grazed sites, suggesting reduced mineralization with increased grazing intensity. In
the Patagonian steppe, grazing appears to exert a negative effect on soil C and N cycles; sound management practices must
incorporate the importance of species shifts within life form, and the critical role of standing dead material in maintaining
soil C and N stocks and biogeochemical processes.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
Author Contributions RAG designed study, performed research, analyzed data, wrote the paper; ATA designed study, wrote the paper; CGGM designed
study, performed research, analyzed data; MGP performed research; OES designed study; RBJ designed study, contributed new
methods. 相似文献
8.
Marc Estiarte Josep Peñuelas Constancia López-Martínez Ramon Pérez-Obiol 《Vegetation History and Archaeobotany》2008,17(6):667-674
The palaeoenvironment of a former coastal lagoon in the south eastern Iberian Peninsula (San Rafael, Almeria, Spain) were
inferred from one core analyzed for particulate organic matter content (POM) together with its C/N, δ13C, δ15N to depict the biogeochemical record from the Late Glacial to the Holocene. The results, complemented by previously reported
pollen assemblages, indicate the appearance of a freshwater lagoon at 7300 b.p. (uncalibrated 14C age), its salinization at 6200 b.p. and its disappearance at 4400 b.p. The period of existence of the lagoon coincided with a period of wetter conditions as inferred from terrestrial vegetation.
The lagoon’s salinization was not related to a decrease in precipitation but to a stronger maritime influence since there
were no parallel changes in terrestrial vegetation. Salinization caused an increase in δ13C, associated with a higher relative presence of C4 plants, and an increase in δ15N, due to a decrease in plant N demand. The late period of the lagoon, from about 5100 to 4400 b.p., shows a progressive drying and salinization not detected in isotopes but reflected in a decrease in POM, and in the pollen
records. Increases in δ15N were related to increases in salinity within the lagoon, and are indicative of a more open N cycle, because the absence
of changes in terrestrial vegetation rules out changes in the catchment area as the cause for changes in δ15N. 相似文献
9.
We determined the flight capabilities and feeding habits of adults of nine silphine beetle species and illustrated their relationship.
We examined the silphine beetles for the presence or absence of flight muscles and estimated their feeding habits by comparing
the carbon and nitrogen stable isotope ratios for them with those of necrophagous nicrophorine species and carnivorous carabine
species. Three species (Silpha longicornis, S. perforata and Phosphuga atrata) completely lacked individuals with flight muscles, and one species (Eusilpha japonica) showed flight muscle dimorphism. Stable isotope analysis suggested that these species were carnivores, mainly feeding on
soil invertebrates. Most flight species showed higher isotopic ratios than the flightless species. Some of them have isotopic
ratios close to those of the nicrophorine species, suggesting that these species mainly feed on vertebrate carcasses. Flightless
silphine species would have limited ability to search for patchy and unpredictable carcass resources. Further studies are
necessary to understand the adaptive evolution of flight capability and the feeding habits in this group. 相似文献
10.
Linda M. Campbell Sylvester B. Wandera Robert J. Thacker D. George Dixon Robert E. Hecky 《Environmental Biology of Fishes》2005,74(3-4):247-260
Synopsis Nile perch, Lates niloticus, and Nile tilapia, Oreochromis niloticus, were originally transplanted from Lake Albert in western Uganda to the African Great Lakes, Lake Victoria and Lake Kyoga,
where they are partially implicated in reduction of the fish species diversity. Lake Albert is facing multiple environmental
changes, including declining fish species diversity, hyper-eutrophication, hypoxia, and reduced fish catches. To examine the
role of Nile perch and Nile tilapia in the food web in their native Lake Albert, we estimated their diets using stable nitrogen
and carbon isotopes. In Lake Albert, the tilapiine congeners (closely related species), Tilapia zillii, Oreochromis leucostictus, and Sarethorodon galilaeus, and the centropomid Nile perch congener, Lates macrophthalmus, have narrower diet breath in the presence of the native O. niloticus and L. niloticus. A computerized parameter search of dietary items for five commercially important fish species (Hydrocynus forskahlii, Bagrus bayad, L. niloticus, Alestes baremose and Brycinus nurse) was completed using a static isotopic mixing model. The outcome of the simulation for most fish species compared favorably
to previously published stomach contents data for the Lake Albert fishes dating back to 1928, demonstrating agreement between
stable isotope values and analyses of stomach contents. While there were some indications of changes in the diets of L. niloticus and A. baremose diets over the past 20 years in parallel with other changes in the lake, for the most part, food web structure in this lake
remained stable since 1928. The Lake Albert fish assemblage provides insight into the invasion success of L. niloticus and O. niloticus. 相似文献
11.
Resolving temporal variation in vertebrate diets using naturally occurring stable isotopes 总被引:12,自引:0,他引:12
Assessments of temporal variation in diets are important for our understanding of the ecology of many vertebrates. Ratios
of naturally occurring stable isotopes in animal tissues are a combination of the source elements and tissue specific fractionation
processes, and can thus reveal dietary information. We review three different approaches that have been used to resolve temporal
diet variation through analysis of stable isotopes. The most straightforward approach is to compare samples from the same
type of tissue that has been sampled over time. This approach is suited to address either long or short-term dietary variation,
depending on sample regime and which tissue that is sampled. Second, one can compare tissues with different metabolic rates.
Since the elements in a given tissue have been assimilating during time spans specific to its metabolic rate, tissues with
different metabolic rates will reflect dietary records over different periods. Third, comparisons of sections from tissues
with progressive growth, such as hair, feathers, claws and teeth, will reveal temporal variation since these tissues will
retain isotopic values in a chronological order. These latter two approaches are mainly suited to address questions regarding
intermediate and short-term dietary variation. Knowledge of tissue specific metabolic rates, which determine the molecular
turnover for a specific tissue, is of central importance for all these comparisons. Estimates of isotopic fractionation between
source and measured target are important if specific hypotheses regarding the source elements are addressed. Estimates of
isotopic fractionation, or at least of differences in fractionation between tissues, are necessary if different tissues are
compared. We urge for more laboratory experiments aimed at improving our understanding of differential assimilation of dietary
components, isotopic fractionation and metabolic routing. We further encourage more studies on reptiles and amphibians, and
generally more studies utilizing multiple tissues with different turnover rates. 相似文献
12.
We conducted stable oxygen and carbon isotope analyses for otoliths of Atlantic salmon (Salmo salar), in an attempt to develop a reference database on isotopic variability among private and federal hatcheries in Maine which
currently support the salmon aquaculture industry and recovery of endangered populations. During the first phase of our study,
we collected 40–50 sagittal otoliths of juvenile Atlantic salmon from each of the five hatcheries and analyzed for stable
oxygen and carbon isotope ratios (18O/16O or δ18O, and 13C/12C or δ13C). Combination of δ18O and δ13C signatures in otoliths showed that the five hatcheries can be clearly separated and chemically distinguished. By identifying
stable isotopic variations of otoliths from different hatchery settings, we were able to establish some isotopic criteria
or standards to assign a likelihood that an individual Atlantic salmon came from a specific hatchery within the reference
database. If successful, a diagnostic tool that can provide definitive information on identification of the hatchery origin
could serve as a novel marking technique, and the chemical method may provide a more effective alternative to DNA analysis
for mixed stocks. Overall our isotopic data from otoliths support the hypothesis that there are detectable differences between
the five hatcheries, and multiple statistical analyses indicated that we can correctly distinguish individual Atlantic salmon
into a hatchery with high confidence. 相似文献
13.
Characterization of photosynthetic pathway of plant species growing in the eastern Tibetan plateau using stable carbon isotope composition 总被引:1,自引:0,他引:1
The photosynthetic pathway of plant species collected at Menyuan, Henan, and Maduo sites, east of Tibetan Plateau, China,
during the growing season were studied using stable carbon isotopes in leaves. The 232 samples leaves analyzed belonged to
161 species, 30 families, and 94 genera. The δ13C values (from −24.6 to − 29.2 ‰) indicated that all the considered species had a photosynthetic C3 pathway. The absence of plant species with C4 photosynthetic pathway might be due to the extremely low air temperature characterizing the Tibetan Plateau. The average
δ13C value was significantly (p<0.05) different between annuals and perennials at the three considered study sites. Hence the longer-lived species had greater
water-use efficiency (WUE) than shorter-lived species, that is, longer-lived species are better adapted to the extreme environmental
conditions of the Tibetan Plateau. 相似文献
14.
In most terrestrial ecosystems ants (Formicidae) as eusocial insects and spiders (Araneida) as solitary trappers and hunters
are key predators. To study the role of predation by these generalist predators in a dry grassland, we manipulated densities
of ants and spiders (natural and low density) in a two-factorial field experiment using fenced plots. The experiment revealed
strong intraguild interactions between ants and spiders. Higher densities of ants negatively affected the abundance and biomass
of web-building spiders. The density of Linyphiidae was threefold higher in plots without ant colonies. The abundance of Formica cunicularia workers was significantly higher in spider-removal plots. Also, population size of springtails (Collembola) was negatively
affected by the presence of wandering spiders. Ants reduced the density of Lepidoptera larvae. In contrast, the abundance
of coccids (Ortheziidae) was positively correlated with densities of ants. To gain a better understanding of the position
of spiders, ants and other dominant invertebrate groups in the studied food web and important trophic links, we used a stable
isotope analysis (15N and 13C). Adult wandering spiders were more enriched in 15N relative to 14N than juveniles, indicating a shift to predatory prey groups. Juvenile wandering and web-building spiders showed δ15N ratios just one trophic level above those of Collembola, and they had similar δ13C values, indicating that Collembola are an important prey group for ground living spiders. The effects of spiders demonstrated
in the field experiment support this result. We conclude that the food resource of spiders in our study system is largely
based on the detrital food web and that their effects on herbivores are weak. The effects of ants are not clear-cut and include
predation as well as mutualism with herbivores. Within this diverse predator guild, intraguild interactions are important
structuring forces. 相似文献
15.
Effect of host shoot clipping on carbon and nitrogen sources for arbuscular mycorrhizal fungi 总被引:2,自引:0,他引:2
The effect of clipping of the host-plant shoot on the sources of carbon and nitrogen for the arbuscular mycorrhizal (AM) fungus Gigaspora margarita was determined by measuring 13C in spores and hyphae in cocultures of C3 and C4 plants and by differential 15N labeling. C3 and C4 plants, which have different δ13C values, were grown in the same container separated by a series of hyphal compartments. The C3 and C4 plants were applied with 14N- and 15N-urea, respectively. After clipping of the C3 shoots, spore δ
13C gradually approached that of the C4 roots. Hyphal δ
13C paralleled that of spores. Spore % 15N was similar to that of mineral N in the C4 plant compartment. Thus C in G. margarita coming from the clipped plants decreased with time. This demonstrates that C in AM fungi comes from living plants, whilst
the N in spores comes mostly from the soil.
Accepted: 28 November 2000 相似文献
16.
Atmospheric CO2 concentration has increased by 25% over the preindustrial level. A parallel increase in C concentration and decreases in N
concentration and δ13C of plants grown throughout this century have been observed in plant specimens stored in herbaria. We tested our previous
results in a study of 12 more species collected in the western Mediterranean throughout this century (1920–1930, 1945–1955,
and 1985–1990) and tree rings of Quercus pubescens from the same area. These changes were accompanied by apparent increases in condensed tannin concentration. A decreasing trend
in δ15N both in herbarium material and tree rings was also found, indicating that ecosystems might cope with higher plant N demand
by decreasing N losses and increasing N fixation and mineralization. These results may contribute to a better understanding
of the effects of global change on carbon and nitrogen cycling.
Received: 12 November 1995 / Accepted: 17 May 1996 相似文献
17.
Nitrogen content and δ15N (nitrogen stable isotope ratio) were measured in different parts of the sporophyte of Undaria pinnatifida (Harvey) Suringar from two bays in the northeastern Japan. There were clear differences between the thalli collected in winter
and in summer: high nitrogen content and low average δ15N were observed for the thalli collected in winter, while the opposite pattern was found for the thalli from summer. In addition,
the pattern of internal δ15N distribution in the thallus changed with season. It is possible that the cause for these seasonal differences in the algae
was the seasonal change in environmental conditions, because in winter water is normally rich in nitrogen, while in summer
it is poor. U. pinnatifida sporophyte may be useful as an indicator of nitrogen sources in coastal waters, but consideration must be given to the effect
of isotope fractionation on δ15N of the plant, especially in winter. Potential may exist for the use of different parts of the thallus to indicate nitrogen
sources at different periods, but more investigation is necessary to accomplish this. 相似文献
18.
Animals with high metabolic rates are believed to have high rates of carbon and nitrogen isotopic incorporation. We hypothesized
that (1) chronic exposure to cold, and hence an increase in metabolic rate, would increase the rate of isotopic incorporation
of both 13C and 15N into red blood cells; and (2) that the rate of isotopic incorporation into red blood cells would be allometrically related
to body mass. Two groups of sparrows were chronically exposed to either 5 or 22°C and switched from a 13C-depleted C3-plant diet to a more 13C-enriched C4-plant one. We used respirometry to estimate the resting metabolic rate of birds exposed chronically to our two experimental temperatures. The allometric relationship between the rate of 13C incorporation into blood and body mass was determined from published data. The of birds at 5°C was 1.9 times higher than that of birds at 22°C. Chronic exposure to a low temperature did not have an effect
on the rate of isotopic incorporation of 15N save for a very small effect on the incorporation of 13C. The isotopic incorporation rate of 13C was 1.5 times faster than that of 15N. The fractional rate of 13C incorporation into avian blood was allometrically related to body mass with an exponent similar to −1/4. We conclude that
the relationship between metabolic rate and the rate of isotopic incorporation into an animal’s tissues is indirect. It is
probably mediated by protein turnover and thus more complex than previous studies have assumed. 相似文献
19.
The analysis of stable isotope composition (delta13C, delta15N, delta18O) of phloem-transported organic matter is a useful tool for assessing short-term carbon and water balance of trees. A major constraint of the general application of this method to trees at natural field sites is that the collection of phloem sap with the "phloem bleeding" technique is restricted to particular species and plant parts. To overcome this restriction, we compared the contents (amino compounds and sugars) and isotope signatures (delta13C, delta15N, delta18O) of phloem sap directly obtained from incisions in the bark (bleeding technique) with phloem exudates where bark pieces were incubated in aqueous solutions (phloem exudation technique with and without chelating agents [EDTA, polyphosphate] in the initial sampling solution, which prevent blocking of sieve tubes). A comparable spectrum of amino compounds and sugars was detected using the different techniques. O, C, or N compounds in the initial sampling solution originating from the chelating agents always decreased precision of determination of the respective isotopic signatures, as indicated by higher standard deviation, and/or led to a significant difference of mean delta as compared to the phloem bleeding technique. Hence, depending on the element from which the ratio of heavy to light isotope is determined, compounds lacking C, N, and/or O should be used as chelating agents in the exudation solution. In applying the different techniques, delta13C of organic compounds transported in the phloem of the twig (exudation technique with polyphosphate as chelating agent) were compared with those in the phloem of the main stem (phloem bleeding technique) in order to assess possible differences in carbon isotope composition of phloem carbohydrates along the tree axis. In July, organic compounds in the stem phloem were significantly enriched in 13C by > 1.3 per thousand as compared to the twig phloem, whereas this effect was not observed in September. Correlation analysis between delta13C and stomatal conductance (Gs) revealed the gradient from the twigs to the stem observed in July may be attributed to temporal differences rather than to spatial differences in carbon isotope composition of sugars. As various authors have produced conflicting results regarding the enrichment/depletion of 13C in organic compounds in the leaf-to-stem transition, the different techniques presented in this paper can be used to provide further insight into fractionation processes associated with transport of C compounds from leaves to branches and down the main stem. 相似文献
20.
The aspen free-air CO2 and O3 enrichment (FACTS II–FACE) study in Rhinelander, Wisconsin, USA, is designed to understand the mechanisms by which young northern deciduous forest ecosystems respond to elevated atmospheric carbon dioxide (CO2) and elevated tropospheric ozone (O3) in a replicated, factorial, field experiment. Soil respiration is the second largest flux of carbon (C) in these ecosystems, and the objective of this study was to understand how soil respiration responded to the experimental treatments as these fast-growing stands of pure aspen and birch + aspen approached maximum leaf area. Rates of soil respiration were typically lowest in the elevated O3 treatment. Elevated CO2 significantly stimulated soil respiration (8–26%) compared to the control treatment in both community types over all three growing seasons. In years 6–7 of the experiment, the greatest rates of soil respiration occurred in the interaction treatment (CO2 + O3), and rates of soil respiration were 15–25% greater in this treatment than in the elevated CO2 treatment, depending on year and community type. Two of the treatments, elevated CO2 and elevated CO2 + O3, were fumigated with 13C-depleted CO2, and in these two treatments we used standard isotope mixing models to understand the proportions of new and old C in soil respiration. During the peak of the growing season, C fixed since the initiation of the experiment in 1998 (new C) accounted for 60–80% of total soil respiration. The isotope measurements independently confirmed that more new C was respired from the interaction treatment compared to the elevated CO2 treatment. A period of low soil moisture late in the 2003 growing season resulted in soil respiration with an isotopic signature 4–6‰ enriched in 13C compared to sample dates when the percentage soil moisture was higher. In 2004, an extended period of low soil moisture during August and early September, punctuated by a significant rainfall event, resulted in soil respiration that was temporarily 4–6‰ more depleted in 13C. Up to 50% of the Earth’s forests will see elevated concentrations of both CO2 and O3 in the coming decades and these interacting atmospheric trace gases stimulated soil respiration in this study. 相似文献