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1.
The δ 15N values of adult holometabolous insects exceed those of larvae, but otherwise little information on terrestrial invertebrates has been obtained in food‐web analyses using stable isotope ratios (δ 15N, δ 13C). Changes in δ 13C during metamorphosis and differences between males and females have not been examined. We collected the larvae and cocoons of Euthrix potatoria (L.) (Lepidoptera: Lasiocampidae) in the field and used them to assess the species’ isotopic fractionation. Each emerged moth was divided into five body parts. We conducted stable N and C isotope analyses for each body part, as well as for cocoons and exuviae, and also compared stable isotope ratios between sexes. We confirmed δ 15N enrichment through metamorphosis and estimated that δ 15N enrichment is accomplished by the relative concentration of 15N due to the excretion of copious meconium, which contains abundant 14N. We also observed changes in δ 13C values through metamorphosis. Both isotope values tended to change more in males than in females. The proportion of the whole‐adult weight represented by meconium was higher in males than in females, suggesting that high meconium secretion in males contributes to the sexual difference in δ 15N. These phenomena may be common in Holometabola, which require a pupal stage. For more accurate food‐web assessments, it is important to consider stable isotope changes during different life cycles, as well as sexual differences. 相似文献
2.
Trophic fractionation of carbon and nitrogen isotopes (Δδ 13C, Δδ 15N) was examined using previously complied databases for food chains in Lake Biwa, Lake Baikal, and Mongolian grassland. The following two features were clarified: (1) For each ecosystem, the ratios of trophic fractionation of carbon and nitrogen isotopes (Δδ 15N/Δδ 13C) throughout food chain could be obtained as the slope of linear regression line on the δ 15N–δ 13C plot. (2) Further, analysis of covariance (ANCOVA) revealed the slopes on δ 15N–δ 13C were not significantly different among these various ecosystems and allowed us to have the regression by setting δ 15N as the response variable: δ 15N = 1.61 δ 13C + [ecosystem specific constant] with standard errors of [±0.41] and [±9.7] for the slope and the intercept, respectively. It was suggested that the slope of the regression (or the ratio Δδ 15N/Δδ 13C) could be applicable to more complicated food webs in case nitrogen and carbon isotope ratios of primary producers can be assumed constant in space and time within the ecosystems. The results from simple linear regression analyses coincided well with the ANCOVA results for these ecosystems, although there was some discrepancy between the results of the two statistical analyses. Possible factors that govern the linear relationship between δ 15N and δ 13C along a food chain are discussed together with a new scope for the stable isotope food chain analyses. 相似文献
3.
To analyze the stable isotope ratios of small‐bodied invertebrates, the entire animal is typically sacrificed and processed, which is problematic for threatened or endangered species. Appendages which are regenerated could be used to infer whole‐body isotope ratios, but differences in turnover rates and isotopic signatures among tissues may confound such an approach. We tested the hypothesis that the δ 13C and δ 15N of whole‐body tissue for freshwater amphipods could be predicted from the δ 13C and δ 15N of walking legs, with the goal of estimating body δ 13C and δ 15N of Gammarus acherondytes, a United States federally endangered species. To test this, we analyzed the δ 13C and δ 15N of walking legs and bodies of five species of amphipods from geographically distant areas (Idaho, Illinois, and Washington) in the United States. The general relationships of whole‐body isotope ratios of C and N as a function of leg isotope ratios were linear and had slopes of one. In the range of the data, leg δ 13C was slightly lower than body δ 13C, indicating some tissue‐specific fractionation, while δ 15N was similar for legs and bodies. Our data suggest that legs can be used to predict body isotope ratios in freshwater amphipods. This approach provides an additional tool to help researchers understand the biology of small, endangered invertebrates without sacrificing individuals. This is especially useful in cave ecosystems where populations are naturally sparse. 相似文献
4.
Nitrogen (N) isotope patterns are useful for understanding carbon and nitrogen dynamics in mycorrhizal systems but questions remain about how different N forms, fungal symbionts, and N availabilities influence δ 15N signatures. Here, we studied how biomass allocation and δ 15N patterns in Pinus sylvestris L. cultures were affected by nitrogen supply rate (3% per day or 4% per day relative to the nitrogen already present), nitrogen form (ammonium versus nitrate), and mycorrhizal colonization by fungi with a greater ( Laccaria laccata) or lesser ( Suillus bovinus) ability to assimilate nitrate. Mycorrhizal (fungal) biomass was greater with ammonium than with nitrate nutrition for Suillus cultures but similar for Laccaria cultures. Total biomass was less with nitrate nutrition than with ammonium nutrition for nonmycorrhizal cultures and was less in mycorrhizal cultures than in nonmycorrhizal cultures. The sequestration of available N by mycorrhizal fungi limited plant N supply. This limitation and the higher energetic cost of nitrate reduction than ammonium assimilation appeared to control plant biomass accumulation. Colonization decreased foliar δ 15N by 0.5 to 2.2‰ (nitrate) or 1.7 to 3.5‰ (ammonium) and increased root tip δ 15N by 0 to 1‰ (nitrate) or 0.6 to 2.3‰ (ammonium). Root tip δ 15N and fungal biomass on root tips were positively correlated in ammonium treatments ( r 2?=?0.52) but not in nitrate treatments ( r 2?=?0.00). Fungal biomass on root tips was enriched in 15N an estimated 6–8‰ relative to plant biomass in ammonium treatments. At high nitrate availability, Suillus colonization did not reduce plant δ 15N. We conclude that: (1) transfer of 15N-depleted N from mycorrhizal fungi to plants produces low plant δ 15N signatures and high root tip and fungal δ 15N signatures; (2) limited nitrate reduction in fungi restricted transfer of 15N-depleted N to plants when nitrate is supplied and may account for many field observations of high plant δ 15N under such conditions; (3) plants could transfer assimilated nitrogen to fungi at high nitrate supply but such transfer was without 15N fractionation. These factors probably control plant δ 15N patterns across N availability gradients and were here incorporated into analytical equations for interpreting nitrogen isotope patterns in mycorrhizal fungi and plants. 相似文献
5.
A stable isotope study was carried out to investigate the feeding ecology of the common goby Pomatoschistus microps larvae (Krøyer, 1838), and to assess differences in the response of planktonic food web to nutrient enrichment, in two ecosystems from the Southern European coast with different levels of historical pollution (estuaries of the Minho and Lima Rivers). At each estuary and time (July 2012, November 2012, February 2013, and June 2013), the fish larvae of two size classes (class 0: 0–10 mm; class 1: 10–15 mm), particulate organic matter (POM), and pelagic zooplankton were collected. The stable isotope mixing model SIAR revealed that, despite temporal differences in the relative proportion of prey items ingested, in both estuaries P. microps larvae feed on both planktonic-hyperbenthic food sources, predating mainly on copepods (from 34% to 60%), Mysidacea (from 16% to 28%), and brachyuran zoea (from 14% to 29%). Fish larvae size classes did not differ significantly for δ 15N, and exhibited a very narrow range of the δ 13C signature. Enriched δ 15N values of all biota in the Lima estuary throughout the study period, with a marked nitrogen enrichment in colder months, are indicative of higher anthropogenic inputs of nitrogen (e.g. sewage and industrial discharges, agriculture) into this system. The δ 15N values of fish larvae and other planktonic groups can be a sensitive bioindicator, because they are highly correlated with the nitrogen content of water (ammonium), indicating that this element has transferred through the planktonic food web. Enriched carbon isotope ratios were observed in warmer months, in both estuaries, and the heavier δ 13C values in Lima are best explained by differences in the degree of marine influence. This research emphasises the utility of stable isotopes in trophic interactions studies, highlighting the relevance of the stable nitrogen isotope of zooplanktonic communities as a reliable bioindicator to detect patterns of anthropogenic nitrogen contamination in estuarine ecosystems. 相似文献
6.
Nitrogen availability and N‐cycling dynamics across ecosystems play a critical role in plant functioning and species distribution. Measurements of 15N natural abundance provides a way to assess ecosystem N dynamics, and the range of nitrogen stable isotope values (δ 15N) for plants in an ecosystem can indicate divergent strategies for N uptake. We tested the hypotheses that the N‐rich seasonally dry forest would have higher soil and leaf δ 15N and a smaller range of leaf δ 15N values compared to the N‐poor cerradão (savanna woodland). We measured N concentration and δ 15N in two soil depths and leaves of 27 woody species in cerradão and 26 in seasonally dry forest. As expected, total soil N concentration decreased while soil δ 15N value increased with soil depth. Regardless of soil depth, seasonally dry forest soils had higher δ 15N and total N concentration compared to cerradão soils. Foliar δ 15N values varied from ?6.4‰ to 5.9‰ in cerradão and from ?2.3‰ to 8.4‰ in seasonally dry forest plants. Phylogenetically independent contrasts analysis and comparisons of δ 15N mean values of the most abundant species and species co‐occurring in both sites confirmed the hypothesis of higher δ 15N for seasonally dry forest in comparison to cerradão. These results corroborate the expectation of higher soil and leaf δ 15N values in sites with higher soil N availability. However, except for the most abundant species, no across‐site leaf–soil (δ 15N leaf –δ 15N soil) differences (Δδ 15N) were found suggesting that differences in leaf δ 15N between cerradão and seasonally dry forest are driven by differences in soil δ 15N. Variation of leaf δ 15N was large in both sites and only slightly higher in cerradão, suggesting high diversity of N use strategies for both cerradão and seasonally dry forest communities. 相似文献
7.
Carbon (δ 13C) and nitrogen (δ 15N) stable isotope variations in baleen plates of sei whales ( Balaenoptera borealis) stranded after a mass mortality event in Chilean Patagonia were investigated to assess potential dietary and migratory patterns. Carbon and nitrogen isotope ratios of seven baleens from six individuals were analyzed. The δ 13C values ranged from ? 19.1 to ? 15.9‰ and the δ 15N values from 8.7 to 15.4‰. Variations of up to 2.9‰ for δ 13C and 5.3‰ for δ 15N were observed within one baleen. Carbon and nitrogen isotope records of each baleen were significantly correlated and showed recurring oscillations confirmed by wavelet analyses. Oscillations slightly differed in periodicity indicating variable baleen growth rates between 10.0 and 16.5 cm/year. Food sources of the whales are discussed in terms of available isotope data for potential prey taxa and potential migratory behavior on the basis of latitudinal isotope gradients of particulate organic matter. Cyclicity could be explained by regular migrations of the sei whales from subtropical calving areas to high‐latitude foraging grounds. δ 15N records of baleens differed between individuals eventually pointing to diverse feeding and migratory preferences among sei whale individuals. 相似文献
8.
Ferriferous savannas, also known as cangas in Brazil, are nutrient-impoverished ecosystems adapted to seasonal droughts. These ecosystems support distinctive vegetation physiognomies and high plant diversity, although little is known about how nutrient and water availability shape these ecosystems. Our study was carried out in the cangas from Carajás, eastern Amazonia, Brazil. To investigate the N cycling and drought adaptations of different canga physiognomies and compare the findings with those from other ecosystems, we analyzed nutrient concentrations and isotope ratios (δ 13C and δ 15N) of plants, litter, and soils from 36 plots distributed in three physiognomies: typical scrubland (SB), Vellozia scrubland (VL), and woodland (WD). Foliar δ 15N values in cangas were higher than those in savannas but lower than those in tropical forests, indicating more conservative N cycles in Amazonian cangas than in forests. The lower δ 15N in savanna formations may be due to a higher importance of mycorrhizal species in savanna vegetation than in canga vegetation. Elevated δ 13C values indicate higher water shortage in canga ecosystems than in forests. Foliar and litter nutrient concentrations vary among canga physiognomies, indicating differences in nutrient dynamics. Lower nutrient availability, higher C:N ratios, and lower δ 15N values characterize VL, whereas WD is delineated by lower δ 13C values and higher soil P. These results suggest lower water restriction and lower P limitation in WD, whereas VL shows more conserved N cycles due to lower nutrient availability. Differences in nutrient and water dynamics among physiognomies indicate different ecological processes; thus, the conservation of all physiognomies is required to ensure the maintenance of functional diversity in this unique ecosystem. 相似文献
9.
Stable isotope ratios of carbon (δ 13C) and nitrogen (δ 15N) were used to investigate feeding patterns of larval and early juvenile pelagic fishes in slope waters of the Gulf of Mexico. Contribution of organic matter supplied to fishes and trophic position within this pelagic food web was estimated in 2007 and 2008 by comparing dietary signatures of the two main producers in this ecosystem: phytoplankton [based on particulate organic matter (POM)] and Sargassum spp. Stable isotope ratios of POM and pelagic Sargassum spp. were significantly different from one another with δ 13C values of POM depleted by 3–6‰ and δ 15N values enriched by 2 relative to Sargassum spp. Stable isotope ratios were significantly different among the five pelagic fishes examined: blue marlin Makaira nigricans, dolphinfish Coryphaena hippurus, pompano dolphinfish Coryphaena equiselis, sailfish Istiophorus platypterus and swordfish Xiphias gladius. Mean δ 13C values ranged almost 2 among fishes and were most depleted in I. platypterus. In addition, mean δ 15N values ranged 4–5 with highest mean values found for both C. hippurus and C. equiselis and the lowest mean value for M. nigricans during both years. Increasing δ 13C or δ 15N with standard length suggested that shifts in trophic position and diet occurred during early life for several species examined. Results of a two‐source mixing model suggest approximately an equal contribution of organic matter by both sources (POM = 55%; pelagic Sargassum spp. = 45%) to the early life stages of pelagic fishes examined. Contribution of organic matter, however, varied among species, and sensitivity analyses indicated that organic source estimates changed from 2 to 13% for a δ 13C fractionation change of ±0·25‰ or a δ 15N fractionation change of ± 1·0‰ relative to original fractionation values. 相似文献
10.
Vegetation greenness has increased across much of the global land surface over recent decades. This trend is projected to continue—particularly in northern latitudes—but future greening may be constrained by nutrient availability needed for plant carbon (C) assimilation in response to CO 2 enrichment (eCO 2). eCO 2 impacts foliar chemistry and function, yet the relative strengths of these effects versus climate in driving patterns of vegetative greening remain uncertain. Here we combine satellite measurements of greening with a 135 year record of plant C and nitrogen (N) concentrations and stable isotope ratios (δ 13C and δ 15N) in the Northern Great Plains (NGP) of North America to examine N constraints on greening. We document significant greening over the past two decades with the highest proportional increases in net greening occurring in the dries and warmest areas. In contrast to the climate dependency of greening, we find spatially uniform increases in leaf‐level intercellular CO 2 and intrinsic water use efficiency that track rising atmospheric CO 2. Despite large spatial variation in greening, we find sustained and climate‐independent declines in foliar N over the last century. Parallel declines in foliar δ 15N and increases in C:N ratios point to diminished N availability as the likely cause. The simultaneous increase in greening and decline in foliar N across our study area points to increased N use efficiency (NUE) over the last two decades. However, our results suggest that plant NUE responses are likely insufficient to sustain observed greening trends in NGP grasslands in the future. 相似文献
12.
The foliar stable N isotope ratio (δ 15N) can provide integrated information on ecosystem N cycling. Here we present the δ 15N of plant and soil in four remote typical tropical rainforests (one primary and three secondary) of southern China. We aimed to examine if (1) foliar δ 15N in the study forests is negative, as observed in other tropical and subtropical sites in eastern Asia; (2) variation in δ 15N among different species is smaller compared to that in many N-limited temperate and boreal ecosystems; and (3) the primary forest is more N rich than the younger secondary forests and therefore is more 15N enriched. Our results show that foliar δ 15N ranged from ?5.1 to 1.3 ‰ for 39 collected plant species with different growth strategies and mycorrhizal types, and that for 35 species it was negative. Soil NO 3 ? had low δ 15N (?11.4 to ?3.2 ‰) and plant NO 3 ? uptake could not explain the negative foliar δ 15N values (NH 4 + was dominant in the soil inorganic-N fraction). We suggest that negative values might be caused by isotope fractionation during soil NH 4 + uptake and mycorrhizal N transfer, and by direct uptake of atmospheric NH 3/NH 4 +. The variation in foliar δ 15N among species (by about 6 ‰) was smaller than in many N-limited ecosystems, which is typically about or over 10 ‰. The primary forest had a larger N capital in plants than the secondary forests. Foliar δ 15N and the enrichment factor (foliar δ 15N minus soil δ 15N) were higher in the primary forest than in the secondary forests, albeit differences were small, while there was no consistent pattern in soil δ 15N between primary and secondary forests. 相似文献
13.
Stable carbon (C) and nitrogen (N) isotope ratios of sedimentary organic matter (OM) can reflect the biogeochemical history of aquatic ecosystems. However, diagenetic processes in sediments may alter isotope records of OM via microbial activity and preferential degradation of isotopically distinct organic components. This study investigated the isotope alteration caused by preferential degradation in surface sediments sampled from a eutrophic reservoir in Germany. Sediments were treated sequentially with hot water extraction, hydrochloric acid hydrolysis, hydrogen peroxide oxidation and di-sodium peroxodisulfate oxidation to chemically simulate preferential degradation pathways of sedimentary OM. Residue and extracts from each extraction step were analyzed using elemental analyzer-isotope ratio mass spectrometry and solid-state 13C nuclear magnetic resonance spectroscopy. Our results show that stable C and N isotope ratios reacted differently to changes in the biochemical composition of sedimentary OM. Preferential degradation of proteins and carbohydrates resulted in a 1.2‰ depletion of 13C, while the isotope composition of 15N remained nearly the same. Sedimentary δ15N values were notably altered when lignins and lipids were oxidized from residual sediments. Throughout the sequential fractionation procedure, δ13C was linearly correlated with the C:N of residual sediments. This finding demonstrates that changes in biochemical composition caused by preferential degradation altered δ13C values of sedimentary OM, while this trend was not observed for δ15N values. Our study identifies the influence of preferential degradation on stable C isotope ratios and provide additional insight into the isotope alteration caused by post-depositional processes. 相似文献
14.
To determine the dominant processes controlling nitrogen (N) dynamics in soils and increase insights into soil N cycling from nitrogen isotope (δ 15N) data, patterns of 15N enrichment in soil profiles were compiled from studies on tropical, temperate, and boreal systems. The maximum 15N enrichment between litter and deeper soil layers varied strongly with mycorrhizal fungal association, averaging 9.6 ± 0.4‰ in ectomycorrhizal systems and 4.6 ± 0.5‰ in arbuscular mycorrhizal systems. The 15N enrichment varied little with mean annual temperature, precipitation, or nitrification rates. One main factor controlling 15N in soil profiles, fractionation against 15N during N transfer by mycorrhizal fungi to host plants, leads to 15N-depleted plant litter at the soil surface and 15N-enriched nitrogen of fungal origin at depth. The preferential preservation of 15N-enriched compounds during decomposition and stabilization is a second important factor. A third mechanism, N loss during nitrification and denitrification, may account for large 15N enrichments with depth in less N-limited forests and may account for soil profiles where maximum δ 15N is at intermediate depths. Mixing among soil horizons should also decrease differences among soil horizons. We suggest that dynamic models of isotope distributions within soil profiles that can incorporate multiple processes could provide additional information about the history of nitrogen movements and transformations at a site. 相似文献
15.
Stable isotope ratios (δ 13C and δ 15N) have been used extensively to trace nutrients from Pacific salmon, but salmon transfer more than carbon and nitrogen to stream ecosystems, such as phosphorus, minerals, proteins, and lipids. To examine the importance of these nutrients, metrics other than isotopes need to be considered, particularly when so few studies have made direct links between these nutrients and how they affect riparian organisms. Our study specifically examined δ 13C and δ 15N of riparian organisms from salmon and non‐salmon streams in Idaho, USA, at different distances from the streams, and examined whether the quality of riparian plants and the body condition of invertebrates varied with access to these nutrients. Overall, quality and condition metrics did not mirror stable isotope patterns. Most notably, all riparian organisms exhibited elevated δ 15N in salmon streams, but also with proximity to both stream types suggesting that both salmon and landscape factors may affect δ 15N. The amount of nitrogen incorporated from Pacific salmon was low for all organisms (<20%) and did not correlate with measures of quality or condition, probably due to elevated δ 15N at salmon streams reflecting historical salmon runs instead of current contributions. Salmon runs in these Idaho streams have been declining, and associated riparian ecosystems have probably seen about a 90% reduction in salmon‐derived nitrogen since the 1950s. In addition, our results support those of other studies that have cautioned that inferences from natural abundance isotope data, particularly in conjunction with mixing models for salmon‐derived nutrient percentage estimates, may be confounded by biogeochemical transformations of nitrogen, physiological processes, and even historical legacies of nitrogen sources. Critically, studies should move beyond simply describing isotopic patterns to focusing on the consequences of salmon‐derived nutrients by quantifying the condition and fitness of organisms putatively using those resources. 相似文献
16.
The natural abundance of nitrogen (N) stable isotopes (δ 15N) has the potential to enhance our understanding of the ecosystem N cycle at large spatial scales. However, vegetation and soil δ 15N patterns along climatic and edaphic gradients have not yet been fully understood, particularly for high-altitude ecosystems. Here we determined vegetation and soil δ 15N in alpine grasslands on the Tibetan Plateau by conducting four consecutive regional surveys during 2001–2004, and then examined their relationships with both climatic and edaphic variables. Our results showed that both vegetation and soil N in Tibetan alpine grasslands were more 15N-enriched than global averages. Vegetation δ 15N did not exhibit any significant trend along the temperature gradient, but decreased significantly with an increase in precipitation amount. In contrast, soil δ 15N did not vary with either mean annual temperature or precipitation. Our results also indicated that soil δ 15N exhibited a slight increase with clay content, but decreased with soil carbon:nitrogen ratio. A general linear model analysis revealed that variations in vegetation δ 15N were dominantly determined by climatic variables, whereas soil δ 15N was related to edaphic variables. These results provide clues for potential climatic and edaphic regulations on ecosystem N cycle in these high-altitude regions. 相似文献
17.
Carbon and nitrogen stable isotope ratios (δ 13C and δ 15N) have been used for more than two decades in analyses of food web structure. The utility of isotope ratio measurements is based on the observation that consumer δ 13C values are similar (<1‰ difference) to those of their diet, while consumer δ 15N values are about 3‰ higher than those of their diet. The technique has been applied most often to aquatic and aboveground terrestrial food webs. However, few isotope studies have examined terrestrial food web structure that includes both above- and belowground (detrital) components. Here, we review factors that may influence isotopic signatures of terrestrial consumers in above- and belowground systems. In particular, we emphasize variations in δ 13C and δ 15N in belowground systems, e.g., enrichment of 13C and 15N in soil organic matter (likely related to soil microbial metabolism). These enrichments should be associated with the high 13C (~3‰) enrichment in belowground consumers relative to litter and soil organic matter and with the large variation in δ 15N (~6‰) of the consumers. Because such enrichment and variation are much greater than the trophic enrichment generally used to estimate consumer trophic positions, and because many general predators are considered dependent on energy and material flows from belowground, the isotopic variation in belowground systems should be taken into account in δ 13C and δ 15N analyses of terrestrial food webs. Meanwhile, by measuring the δ 13C of key predators, the linkage between above- and belowground systems could be estimated based on observed differences in δ 13C of primary producers, detritivores and predators. Furthermore, radiocarbon ( 14C) measurements will allow the direct estimation of the dependence of predators on the belowground systems. 相似文献
18.
Ecologists use stable isotopes to infer diets and trophic levels of animals in food webs, yet some assumptions underlying these inferences have not been thoroughly tested. We used laboratory‐reared colonies of Solenopsis invicta Buren (Hymenoptera: Formicidae: Solenopsidini) to test the effects of metamorphosis, diet, and lipid storage on carbon and nitrogen stable isotope ratios. Effects of metamorphosis were examined in ant colonies maintained on a control diet of domestic crickets and sucrose solution. Effects of a diet shift were evaluated by adding a tuna supplement to select colonies. Effects of lipid content on stable isotopes were tested by treating worker ants with polar and non‐polar solvents. δ 13C and δ 15N values of larvae, pupae, and workers were measured by mass spectrometry on whole‐animal preparations. We found a significant effect of colony age on δ 13C, but not δ 15N; larvae, pupae, and workers collected at 75 days were slightly depleted in 13C relative to collections at 15 days (Δδ 13C = ?0.27‰). Metamorphosis had a significant effect on δ 15N, but not δ 13C; tissues of each successive developmental stage were increasingly enriched in 15N (pupae, +0.5‰; workers, +1.4‰). Availability of tuna resulted in further shifts of about +0.6‰ in isotope ratios for all developmental stages. Removing fat with organic solvents had no effect on δ 13C, but treatment with a non‐polar solvent resulted in enriched δ 15N values of +0.37‰. Identifying regular patterns of isotopic enrichment as described here should improve the utility of stable isotopes in diet studies of insects. Our study suggests that researchers using 15N enrichment to assess trophic levels of an organism at different sites need to take care not to standardize with immature insect herbivores or predators at one site and mature ones at another. Similar problems may also exist when standardizing with holometabolous insects at one site and spiders or hemimetabolous insects at another site. 相似文献
19.
Northern forests are exposed to relatively high ammonia inputs due to high atmospheric deposition and the common practise of forest fertilization. It is not known how increased soil NH 4
+concentrations affect acquisition of symbiosis-mediated N from organic sources. We examined the effect of inorganic N and P availability on N acquisition from alanine by 43 weeks old birch ( Betula pendula) seedlings in symbiosis with the ectomycorrhizal fungus Paxillus involutus. The seedlings were exposed for 9 weeks to nutrient additions equivalent to 43 kg N and 6.4 kg P ha -1 (low N and P availability), 250 kg N and 38 kg P ha -1(high N and P availability) or to 250 kg N and 6.4 kg P ha -1 (high N and low P availability). Carbon and nitrogen allocation between the symbionts was assessed by exposing the foliage to 14CO 2 and the mycelium to 15N-alanine or 15NH 4
+ simultaneously and measuring the distribution of the isotopic tracers after a three-day chase period. High inorganic N combined with low P availability did not have marked effect on symbiosis-mediated N uptake from alanine, whilst high N and P availability reduced alanine-derived 15N translocation by the fungus to the plant. Shoot 15N concentration and concentration of 14C in the extramatrical mycelium correlated significantly across treatments pointing to controlled reciprocity of transactions between the partners. 相似文献
20.
This study presents the latitudinal variation (from 60° 30′ N to 68° 2′ N latitude) of natural abundances of 15N in the foliage, humus and soils of boreal forests of Finland. Our results clearly showed that N concentration of the foliage
did not change significantly with latitudes but their 15N values were significantly higher in higher latitude sites relative to that of the mid and lower latitude sites, indicating
the different forms of N uptake at the higher latitudes compared to the lower latitudes. We assume that the higher foliage
δ 15N values of the higher latitudes trees might be due to either more openness of N cycle (greater proportional N losses) in
these latitudes compared to the sites of southern latitudes (lower N losses) or the differences in their mycorrhizal associations.
Regression analysis showed that the temperature was the main factor influencing the 15N natural abundance of humus and soil of all forest ecosystems, both before and after clear-cut, whereas rainfall was the
main controlling factor to the foliage 15N. Possible reasons behind the increasing δ 15N natural abundances of plants and soils with increasing latitudes are discussed in this paper. The clear-cut did not show
any specific trend on the 15N fractionation in humus and soil, i.e. both 15N-enrichment and -depletion occurred after clear-cut. 相似文献
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