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1.
Charred modern grass epidermis preserves the carbon isotopic composition of the parent plant photosynthetic pathway. Fifty-nine modern grasses and sedges were collected in lowland western Uganda. All charred epidermal samples from C 4 grasses or sedges preserve a carbon isotopic value within the range typical for C 4 plants (−17 to −10‰), and charred epidermal fragments from C 3 plants have carbon isotopic values between −30 and −26‰. The process of charring results in a slightly enriched carbon isotopic signature (−11.9‰ mean charred value as compared to −12.8‰ mean unaltered grass tissue value). δ 13C measurements of replicate samples from the same plant vary within 1–2‰, yet all values for the same plant stay within the expected values for the photosynthetic pathway of the plant. δ 13C measurements on >180-μm charred grass epidermal fragments extracted from surface sediment samples from three lakes on the lowland western Ugandan landscape confirm the predominant lowland C 4 grass input (δ 13C=−16 to −19‰). These results demonstrate the utility of using carbon isotopic analysis of charred grass epidermis to reconstruct C 3 vs. C 4 grassland assemblages on the landscape. Furthermore, such downcore δ 13C profiles can be used to highlight key zones of C 3 vs. C 4 grass change for which taxonomic analysis of fossil grass epidermis could provide more detailed information regarding grassland community composition. 相似文献
2.
The Upper Cretaceous Coon Creek Lagerstätte of Tennessee, USA, is known for its extremely well‐preserved mollusks and decapod crustaceans. However, the depositional environment of this unit, particularly its distance to the shoreline, has long been equivocal. To better constrain the coastal proximity of the Coon Creek Formation, we carried out a multiproxy geochemical analysis of fossil decapod (crab, mud shrimp) cuticle and associated sediment from the type section. Elemental analysis and Raman spectroscopy confirmed the presence of kerogenized carbon in the crabs and mud shrimp; carbon isotope (δ 13C) analysis of bulk decapod cuticle yielded similar mean δ 13C values for both taxa (?25.1‰ and ?26‰, respectively). Sedimentary biomarkers were composed of n‐alkanes from C 16 to C 36, with the short‐chain n‐alkanes dominating, as well as other biomarkers (pristane, phytane, hopanes). Raman spectra and biomarker thermal maturity indices suggest that the Coon Creek Formation sediments are immature, which supports retention of unaltered, biogenic isotopic signals in the fossil organic carbon remains. Using our isotopic results and published calcium carbonate δ 13C values, we modeled carbon isotope values of carbon sources in the Coon Creek Formation, including potential marine (phytoplankton) and terrestrial (plant) dietary sources. Coon Creek Formation decapod δ 13C values fall closer to those estimated for terrigenous plants than marine phytoplankton, indicating that these organisms were feeding primarily on terrigenous organic matter. From this model, we infer that the Coon Creek Formation experienced significant terrigenous organic matter input via a freshwater source and thus was deposited in a shallow, nearshore marine environment proximal to the shoreline. This study helps refine the paleoecology of nearshore settings in the Mississippi Embayment during the global climatic shift in the late Campanian–early Maastrichtian and demonstrates for the first time that organic δ 13C signatures in exceptionally preserved fossil marine arthropods are a viable proxy for use in paleoenvironmental reconstructions. 相似文献
3.
Question: The relationship between carbon‐13 in soil organic matter and C 3 and C 4 plant abundance is complicated because of differential productivity, litter fall and decomposition. As a result, applying a mass balance equation to δ 13C data from soils cannot be used to infer past C 3 and C 4 plant abundance; only the proportion of carbon derived from C 3 and C 4 plants can be estimated. In this paper, we compare δ 13C of surface soil samples with vegetation data, in order to establish whether the ratio of C 3:C 4 plants (rather than the proportion of carbon from C 3 and C 4 plants) can be inferred from soil δ 13C. Location: The Tsavo National Park, in southeastern Kenya. Methods: We compare vegetation data with δ 13C of organic matter in surface soil samples and derive regression equations relating the δ 13C of soil organic matter to C 3:C 4 plant abundance. We use these equations to interpret δ 13C data from soil profiles in terms of changes in inferred C 3:C 4 plant ratio. We compare our method of interpretation with that derived from a mass balance approach. Results: There was a statistically significant, linear relationship between the δ 13C of organic matter in surface soil samples and the natural logarithm of the ratio of C 3:C 4 plants in the 100m 2 surrounding the soil sample. Conclusions: We suggest that interpretation of δ 13C data from organic matter in soil profiles can be improved by comparing vegetation surveys with δ 13C of organic matter in surface soil samples. Our results suggest that past C 3 plant abundance might be under‐estimated if a mass balance approach is used. 相似文献
4.
δ 13C values for freshwater aquatic plant matter varies from ?11 to ?50‰ and is not a clear indicator of photosynthetic pathway as in terrestrial plants. Several factors affect δ 13C of aquatic plant matter. These include: (1) The δ 13C signature of the source carbon has been observed to range from +1‰ for HCO 3? derived from limestone to ?30‰ for CO 2 derived from respiration. (2) Some plants assimilate HCO 3?, which is –7 to –11‰ less negative than CO 2. (3) C 3, C 4, and CAM photosynthetic pathways are present in aquatic plants. (4) Diffusional resistances are orders of magnitude greater in the aquatic environment than in the aerial environment. The greater viscosity of water acts to reduce mixing of the carbon pool in the boundary layer with that of the bulk solution. In effect, many aquatic plants draw from a finite carbon pool, and as in terrestrial plants growing in a closed system, biochemical discrimination is reduced. In standing water, this factor results in most aquatic plants having a δ 13C value similar to the source carbon. Using Farquhar's equation and other physiological data, it is possible to use δ 13C values to evaluate various parameters affecting photosynthesis, such as limitations imposed by CO 2 diffusion and carbon source. 相似文献
5.
An isotopic reconstruction of human dietary patterns and livestock management practices (herding, grazing, foddering, etc.) is presented here from the sites of Düzen Tepe and Sagalassos in southwestern Turkey. Carbon and nitrogen stable isotope ratios were determined from bone collagen extracted from humans ( n = 49) and animals ( n = 454) from five distinct time periods: Classical‐Hellenistic (400–200 BC), Early to Middle Imperial (25 BC–300 AD), Late Imperial (300–450 AD), Early Byzantine (450–600 AD), and Middle Byzantine (800–1200 AD). The humans had protein sources that were based on C 3 plants and terrestrial animals. During the Classical‐Hellenistic period, all of the domestic animals had δ 13C and δ 15N signatures that clustered together; evidence that the animals were herded in the same area or kept in enclosures and fed on similar foods. The diachronic analysis of the isotopic trends in the dogs, cattle, pigs, sheep, and goats highlighted subtle but distinct variations in these animals. The δ 13C values of the dogs and cattle increased (reflecting C 4 plant consumption) during the Imperial and Byzantine periods, but the pigs and the goats displayed little change and a constant C 3 plant‐based diet. The sheep had a variable δ 13C pattern reflecting periods of greater and lesser consumption of C 4 plants in the diet. In addition, the δ 15N values of the dogs, pigs, cattle, and sheep increase substantially from the Classical‐Hellenistic to the Imperial periods reflecting a possible increase in protein consumption, but the goats showed a decrease. Finally, these isotopic results are discussed in the context of zooarcheological, archeobotanical, and trace element evidence. Am J Phys Anthropol 149:157–171, 2012. © Wiley Periodicals, Inc. 相似文献
6.
Aim In central Australia, the giant flightless bird Genyornis newtoni disappeared about 45–50 thousand years ago (ka). It has been reported that coincident with this extinction the carbon isotopic composition of preserved eggshells of the extant emu ( Dromaius novaehollandiae) shows an abrupt dietary shift from tropical grasses (C 4 photosynthesis) to temperate grasses and/or woody browse (C 3 photosynthesis). This abrupt shift has been interpreted as signalling ‘ecosystem collapse’ due to landscape burning by humans. We evaluate an alternative interpretation, that the shift in diet was not abrupt, but gradual, and caused by the weakening of the Australian monsoon. Location Lake Eyre, central Australia. Methods We re‐analysed a large, published dataset of emu diet δ 13C (inferred from δ 13C of preserved eggshells) spanning the last 140,000 years, using time‐series analysis. Using Akaike's information criterion, we compared two contrasting models: (1) there was an abrupt shift in δ 13C coincident with the extinction of Genyornis, assumed 47.5 ka; and (2) there was a gradual shift in δ 13C, correlated with reconstructed water level in Lake Eyre, a proxy for monsoon intensity. Results There was little evidence of an abrupt shift in emu diet δ 13C about 45–50 ka, but δ 13C appeared to steadily decrease between about 80 and 30 ka. Indeed, the model representing a correlation between δ 13C and lake level was more than seven times more likely than the model representing an abrupt shift at 47.5 ka. Main conclusions The emu eggshell isotopic record from Lake Eyre does not support the hypothesis that landscape burning by humans transformed a savanna?grassland mosaic into the modern desert scrub, contributing to the extinction of Genyornis. While our findings cast strong doubt on the foremost line of evidence that landscape burning by humans caused the megafaunal extinctions, and suggest that central Australia was becoming increasingly arid in the Late Pleistocene, the relative roles of hunting by humans and climate change in the megafaunal extinctions remain unresolved. 相似文献
7.
Abstract The growth, biomass δ 13C values, and ability to accumulate titratable acidity at night were compared in eight environmental treatments for Cremnophila linguifolia, Sedum greggii, and their F 1 hybrid. In the phytotron, differences in treatment daylength, day/night temperature and water availability were all found to have effects on total plant dry weight, nocturnal accumulation of titratable acidity and biomass δ 13C value of at least some of the genotypes. However, there were differences between the genotypes both in the magnitude and direction of response of the phenotypic properties to the treatment variables. The phytotron δ 13C values ranged from -12.9 to -19.2‰ for C. linguifolia, from -22.2 to -33.4‰ for S. greggii, and from -19.2 to -24.9‰ for the hybrid. After with-holding water for 76 h both C. linguifolia and the hybrid had midday Ψ leaf values of -0.23 MPa; however, S. greggii had a value of -1.05 MPa. In contrast to past observations of other species, the daily watered plants of C. linguifolia had less negative δ 13C values than did the plants watered only weekly. 相似文献
8.
Methods to trace source habitats and movement of parasitic natural enemies in agroecosystems are limited. This study demonstrates that stable carbon isotope analysis offers a valuable new method for revealing the movement of Microplitis mediator (Haliday) (Hymenoptera: Braconidae), a larval endoparasitoid of Helicoverpa armigera Hübner (Lepidoptera: Noctuidae), between C 3 and C 4 plants. Results indicate that M. mediator with δ 13C values of lower than −22‰ originate from a C 3 plant, whereas those with δ 13C values of higher than −19‰ develop on a C 4 plant. 相似文献
9.
Leaf respiration in the dark and its C isotopic composition (δ 13C R) contain information about internal metabolic processes and respiratory substrates. δ 13C R is known to be less negative compared to potential respiratory substrates, in particular shortly after darkening during light enhanced dark respiration (LEDR). This phenomenon might be driven by respiration of accumulated 13C‐enriched organic acids, however, studies simultaneously measuring δ 13C R during LEDR and potential respiratory substrates are rare. We determined δ 13C R and respiration rates (R) during LEDR, as well as δ 13C and concentrations of potential respiratory substrates using compound‐specific isotope analyses. The measurements were conducted throughout the diel cycle in several plant species under different environmental conditions. δ 13C R and R patterns during LEDR were strongly species‐specific and showed an initial peak, which was followed by a progressive decrease in both values. The species‐specific differences in δ 13C R and R during LEDR may be partially explained by the isotopic composition of organic acids ( e.g., oxalate, isocitrate, quinate, shikimate, malate), which were 13C‐enriched compared to other respiratory substrates ( e.g., sugars and amino acids). However, the diel variations in both δ 13C and concentrations of the organic acids were generally low. Thus, additional factors such as the heterogeneous isotope distribution in organic acids and the relative contribution of the organic acids to respiration are required to explain the strong 13C enrichment in leaf dark‐respired CO 2. 相似文献
10.
Estuaries are complex systems where environmental fluctuations occur over distinct timescales due to local meteorological and large-scale climatic factors. Consequently, studies with low temporal resolution and taxonomic coverage may fail to detect isotopic variations in basal sources, providing biased interpretations of isotope mixing models. We investigated the seasonal and El Niño Southern Oscillation (ENSO)-driven interannual variations in δ13C, δ15N and C:N values among distinct basal sources and their implications for mixing models interpretation in a subtropical estuary. δ13C variations among sources differed in their magnitude and timescales, being large enough to confound source-specific values. Macroalgae and POM δ13C varied seasonally, whereas ENSO effects prevailed for C3 and C4 salt marsh plants, highlighting the contrasting influence of local versus remote environmental drivers on short- and long-lived primary producers, respectively. Peaks of δ15N were detected for all sources during short-term anthropogenic nutrient inputs. Isotope mixing model comparisons showed that overlooking isotopic variations in basal sources under distinct ENSO conditions can cause misinterpretation of local trophic interactions and nutrient cycling. The present study contributes to design appropriate sampling delineations in highly variable aquatic environments, emphasizing the importance of comprehensive, long-term monitoring of estuarine primary producers to encompass environmental drivers of stable isotopic variations. 相似文献
11.
Southern African forests are naturally fragmented yet hold a disproportionately high number of bird species. Carbon and nitrogen stable isotopes were measured in feathers from birds captured at Woodbush (n = 27 species), a large afromontane forest in the eastern escarpment of Limpopo province, South Africa. The δ 13C signatures of a range of forest plants were measured to categorise the food base. Most plants sampled, including two of five grass species, had δ 13C signatures typical of a C 3 photosynthetic pathway (?29.5 ± 1.9‰). Three grass species had a C 4 signature (?12.0 ± 0.6‰). Most bird species had δ 13C values representing a predominantly C 3‐based diet (?24.8‰ to ?20.7‰). δ 15N values were as expected, with higher levels of enrichment associated with a greater proportion of dietary animal matter. The cohesive isotopic niche defining most species (n = 22), where the ranges for δ 13C and δ 15N were 2.4‰ and 3.4‰, respectively, highlight the difficulties in understanding diets of birds in a predominantly C 3‐based ecosystem using carbon and nitrogen stable isotopes. However, variation in isotopic values between and within species provides insight into possible niche width and the use of resources by different birds within a forest environment. 相似文献
12.
Efforts to understand the cause of 12C versus 13C isotope fractionation in plants during photosynthesis and post‐photosynthetic metabolism are frustrated by the lack of data on the intramolecular 13C‐distribution in metabolites and its variation with environmental conditions. We have exploited isotopic carbon‐13 nuclear magnetic resonance ( 13C NMR) spectrometry to measure the positional isotope composition ( δ13C i, ‰) in ethanol samples from different origins: European wines, liquors and sugars from C 3, C 4 and crassulacean acid metabolism (CAM) plants. In C 3‐ethanol samples, the methylene group was always 13C‐enriched (~2‰) relative to the methyl group. In wines, this pattern was correlated with both air temperature and δ18O of wine water, indicating that water vapour deficit may be a critical defining factor. Furthermore, in C 4‐ethanol, the reverse relationship was observed (methylene‐C relatively 13C‐depleted), supporting the concept that photorespiration is the key metabolic process leading to the 13C distribution in C 3‐ethanol. By contrast, in CAM‐ethanol, the isotopic pattern was similar to but stronger than C 3‐ethanol, with a relative 13C‐enrichment in the methylene‐C of up to 13‰. Plausible causes of this 13C‐pattern are briefly discussed. As the intramolecular δ13C i‐values in ethanol reflect that in source glucose, our data point out the crucial impact on the ratio of metabolic pathways sustaining glucose synthesis. 相似文献
13.
In the epiphytic tillandsioids, Guzmania monostachia, Werauhia sanguinolenta, and Guzmania lingulata (Bromeliaceae), juvenile plants exhibit an atmospheric habit, whereas in adult plants the leaf bases overlap and form water-holding tanks. CO 2 gas-exchange measurements of the whole, intact plants and δ 13C values of mature leaves demonstrated that C 3 photosynthesis was the principal pathway of CO 2 assimilation in juveniles and adults of all three species. Nonetheless, irrespective of plant size, all three species were able to display features of facultative CAM when exposed to drought stress. The capacity for CAM was the greatest in G. monostachia, allowing drought-stressed juvenile and adult plants to exhibit net CO 2 uptake at night. CAM expression was markedly lower in W. sanguinolenta, and minimal in G. lingulata. In both species, low-level CAM merely sufficed to reduce nocturnal respiratory net loss of CO 2. δ 13C values were generally less negative in juveniles than in adult plants, probably indicating increased diffusional limitation of CO 2 uptake in juveniles. 相似文献
14.
Photosynthetic activity in carbonate‐rich benthic microbial mats located in saline, alkaline lakes on the Cariboo Plateau, B.C. resulted in pCO 2 below equilibrium and δ 13C DIC values up to +6.0‰ above predicted carbon dioxide (CO 2) equilibrium values, representing a biosignature of photosynthesis. Mat‐associated δ 13C carb values ranged from ~4 to 8‰ within any individual lake, with observations of both enrichments (up to 3.8‰) and depletions (up to 11.6‰) relative to the concurrent dissolved inorganic carbon (DIC). Seasonal and annual variations in δ 13C values reflected the balance between photosynthetic 13C‐enrichment and heterotrophic inputs of 13C‐depleted DIC. Mat microelectrode profiles identified oxic zones where δ 13C carb was within 0.2‰ of surface DIC overlying anoxic zones associated with sulphate reduction where δ 13C carb was depleted by up to 5‰ relative to surface DIC reflecting inputs of 13C‐depleted DIC. δ 13C values of sulphate reducing bacteria biomarker phospholipid fatty acids (PLFA) were depleted relative to the bulk organic matter by ~4‰, consistent with heterotrophic synthesis, while the majority of PLFA had larger offsets consistent with autotrophy. Mean δ 13C org values ranged from ?18.7 ± 0.1 to ?25.3 ± 1.0‰ with mean Δ 13C inorg‐org values ranging from 21.1 to 24.2‰, consistent with non‐CO 2‐limited photosynthesis, suggesting that Precambrian δ 13C org values of ~?26‰ do not necessitate higher atmospheric CO 2 concentrations. Rather, it is likely that the high DIC and carbonate content of these systems provide a non‐limiting carbon source allowing for expression of large photosynthetic offsets, in contrast to the smaller offsets observed in saline, organic‐rich and hot spring microbial mats. 相似文献
15.
Abstract: While atmospheric species of bromeliads have narrow leaves, densely covered with water‐absorbing trichomes throughout their life cycles, many tank bromeliads with broad leaves, forming phytotelmata, go through an atmospheric juvenile phase. The effect of the different habits and the phase change in tank‐forming bromeliads on water and nutrient relations was investigated by analysing the relationship between plant size, C/N ratios and the natural abundance of 13C and 15N in five epiphytic bromeliad species or morphospecies of a humid montane forest in Xalapa, Mexico. The atmospheric species Tillandsia juncea and T. butzii exhibited full crassulacean acid metabolism, with δ 13C values (mean ‐ 15.3 ‰ and ‐ 14.7 ‰, respectively) independent of size. In Tillandsia species with C 3 photosynthesis, δ 13C decreased with increasing plant size, indicating stronger drought stress in juveniles. The increase of the C/N ratio with size suggests that, at least in heteroblastic bromeliads, the availability of water is more limiting during early growth, and that limitations of nitrogen supply become more important later on, when water stored in the tank helps to bridge dry periods, reducing water shortage. δ 15N values of the two atmospheric species were very negative (‐ 12.6 ‰ and ‐ 12.2 ‰, respectively) and did not change with plant size. Tank‐forming bromeliads had less negative δ 15N values (c ‐ 6 ‰), and, in species with atmospheric juveniles and tank‐forming adults, δ 15N values increased significantly with plant size. These differences do not appear to be an effect of the isotopic composition of N sources, but rather reflect N availability and limitation and stress‐induced changes in 15N discrimination. 相似文献
16.
The Burmanniaceae contain several lineages of achlorophyllous mycoheterotrophic plants that associate with arbuscular mycorrhizal fungi (AMF). Here we investigate the isotopic profile of a green and potentially mycoheterotrophic plant in situ, Burmannia coelestis, and associated reference plants. We generated δ 13C and δ 15N stable isotope profiles for five populations of B. coelestis. Burmannia coelestis was significantly enriched in 13C relative to surrounding C 3 reference plants and significantly depleted in 13C relative to C 4 reference plants. No significant differences were detected in 15N enrichment between B. coelestis and reference plants. The isotopic profiles measured are suggestive of partial mycoheterotrophy in B. coelestis. Within the genus Burmannia transitions to full mycoheterotrophy have occurred numerous times, suggesting that some green Burmannia species are likely to be partially mycoheterotrophic but in many conditions this mode of nutrition may only be detectable using natural abundance stable isotopic methods, such as when associated with C 4 mycorrhizal plants. 相似文献
17.
Biogenic calcretes associated with a regional Cretaceous to Paleogene subaerial unconformity and an intraformational composite (polygenic) surface in Upper Cretaceous intra-platform peritidal successions in central Dalmatia and eastern Istria, Croatia (Adriatic-Dinaridic Carbonate Platform), were analyzed for their δ 13C and δ 18O signatures in order to provide insight into the conditions of subaerial exposure and calcrete development. The distinctly negative δ 13C signatures of biogenic calcretes marking the regional subaerial unconformity differ considerably from the δ 13C values of the host marine limestones. This indicates carbon isotope exchange of primary marine CaCO 3 with CO 2 released by root and rhizomicrobial respiration and subsequent precipitation of pedogenic calcrete. The range of δ 13C (from ?13.1 to ?8.2 ‰ Vienna PeeDee Belemnite standard, VPDB) and δ 18O (from ?10.1 to ?6.1 ‰ VPDB) values of calcretes are similar to those reported from calcretes elsewhere, and the δ 13C values of biogenic calcretes with typical Microcodium aggregates (?13.1 to ?12.3 ‰ VPDB) at the ?ibenik locality are very close to, or at the lower limit of, values for soil carbonates formed in isotopic equilibrium with soil CO 2. These values are expected for authigenic pedogenic carbonates formed under the influence of C 3 plant communities, without influence from heavier carbon from pre-existing carbonate and lack of input of atmospheric CO 2. Such low δ 13C values support the interpretation of Microcodium aggregates as being precipitated under a direct biological control within the soil, although the relationship between formation mechanisms and stable isotope signatures of Microcodium needs further investigation. The δ 13C values (?4.4 to ?3.6 ‰ VPDB) of rhizogenic calcretes formed inside firmground Thalassinoides burrows of the composite surface at the ?ibenik locality are more negative than the δ 13C values of the host marine limestones, which confirms that the composite surface went through a phase of meteoric pedo(dia)genesis. However, the overall δ 13C values of calcretes are less negative than expected, which might reflect contamination from associated primary marine carbonate. This study represents the first detailed stable isotope investigation of calcretes from carbonate successions of the External Dinarides, and the results may be applied to discontinuities present in other shallow-water carbonate rock successions. 相似文献
18.
Plants with the C 3, C 4, and crassulacean acid metabolism (CAM) photosynthetic pathways show characteristically different discriminations against 13C during photosynthesis. For each photosynthetic type, no more than slight variations are observed within or among species. CAM plants show large variations in isotope fractionation with temperature, but other plants do not. Different plant organs, subcellular fractions and metabolises can show widely varying isotopic compositions. The isotopic composition of respired carbon is often different from that of plant carbon, but it is not currently possible to describe this effect in detail. The principal components which will affect the overall isotope discrimination during photosynthesis are diffusion of CO 2, interconversion of CO 2 and HCO ?3, incorporation of CO 2 by phosphoenolpyruvate carboxylase or ribulose bisphosphate carboxylase, and respiration. Theisotope fractionations associated with these processes are summarized. Mathematical models are presented which permit prediction of the overall isotope discrimination in terms of these components. These models also permit a correlation of isotope fractionations with internal CO 2 concentrations. Analysis of existing data in terms of these models reveals that CO 2 incorporation in C 3 plants is limited principally by ribulose bisphosphate carboxylase, but CO 2 diffusion also contributes. In C 4 plants, carbon fixation is principally limited by the rate of CO 2 diffusion into the leaf. There is probably a small fractionation in C 4 plants due to ribulose bisphosphate carboxylase. 相似文献
19.
Using bulk tissue and fatty acid 13C analysis we investigated major trophic pathways from soil microorganisms to microbial consumers to predators in conventional versus organic farming systems planted for the first time with maize. Organic farming led to an increase in microbial biomass in particular that of fungi as indicated by phospholipid fatty acids (PLFAs). Microbial PLFAs reflected the conversion from C 3 to C 4 plants by a shift in δ 13C of 2‰, whereas the isotopic signal in fatty acids (FAs) of Collembola was much more pronounced. In the euedaphic Protaphorura fimata the δ 13C values in maize fields exceeded that in C 3 (soybean) fields by up to 10‰, indicating a close relationship between diet and vegetation cover. In the epedaphic Orchesella villosaδ 13C values shifted by 4‰, suggesting a wider food spectrum including carbon of former C 3 crop residues. Differences in δ 13C of corresponding FAs in consumers and resources were assessed to assign food web links. P. fimata was suggested as root and fungal feeder in soybean fields, fungal feeder in conventional and leaf consumer in organically managed maize fields. O. villosa likely fed on root and bacteria under soybean, and bacteria and fungi under maize. Comparison of δ 13C values in FAs of the cursorial spider Pardosaagrestis and O. villosa implied the latter as important prey species in soybean fields. In contrast, the web‐building spider Mangora acalypha showed no predator–prey relationship with Collembola. The determination of δ 13C values in trophic biomarker FAs allowed detailed insight into the structure of the decomposer food web and identified diet‐shifts in both consumers at the base of the food web and in top predators in organic versus conventional agricultural systems. The results indicate changes in major trophic links and therefore carbon flux through the food web by conversion of conventional into organic farming systems. 相似文献
20.
Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae) is a serious worldwide pest of stored cereal grains that also has the ability to breed in non‐agricultural host plant material. Stable isotope signatures (concentrations of isotopes) were used as internal tissue markers to determine dietary differences among adult R. dominica and to make inferences about source habitats of field‐trapped insects. Adult R. dominica collected near granaries or from non‐agricultural forested sites near Stillwater, OK, USA, and insects reared on selected hosts under laboratory conditions were studied to determine the carbon and nitrogen isotope signatures. Laboratory‐reared R. dominica showed δ 13C (stable isotope ratio of carbon) values similar to the host on which they developed with an enrichment of about 1 in the insect body. Insects reared on seeds of wheat and oak, which have C 3 photosynthetic pathways, showed much depleted δ 13C values (–23.7 and –26.2, respectively) in comparison to insects reared on seeds of corn, a C 4 photosynthetic plant (–11.3). A majority of the field‐collected R. dominica showed δ 13C values similar to expectations for a C 3 host. However, a few field‐collected insects had δ 13C signatures similar to the C 4 plant‐reared insects in the laboratory experiment. Stored grain of C 4 crops were lacking at many of the sample field sites. These results suggest that R. dominica occurs on either C 3‐ or C 4‐based hosts in the field, and point to utilization of non‐grain C 4 plants as hosts. Our studies indicated that 13C isotope is a reliable marker to infer types of hosts used in the feeding history of R. dominica. 相似文献
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