Natural Abundance Stable Isotope Research Articles

Stable C and N isotope ratios reveal soil food web structure and identify the nematode Eudorylaimus antarcticus as an omnivore–predator in Taylor Valley, Antarctica

Soil food webs of the McMurdo Dry Valleys, Antarctica are simple. These include primary trophic levels of mosses, algae, cyanobacteria, bacteria, archaea, and fungi, and their protozoan and metazoan consumers (including relatively few species of nematodes, tardigrades, rotifers, and microarthropods). These biota are patchily distributed across the landscape, with greatest faunal biodiversity associated with wet soil. Understanding trophic structure is critical to studies of biotic interactions and distribution; yet, McMurdo Dry Valley soil food web structure has been inferred from limited laboratory culturing and microscopic observations. To address this, we measured stable isotope natural abundance ratios of C (13C/12C) and N (15N/14N) for different metazoan taxa (using whole body biomass) to determine soil food web structure in Taylor Valley, Antarctica. Nitrogen isotopes were most useful in differentiating trophic levels because they fractionated predictably at higher trophic levels. Using 15N/14N, we found that three trophic levels were present in wet soil habitats. While cyanobacterial mats were the primary trophic level, the nematode Plectus murrayi, tardigrade Acutuncus antarcticus, and rotifers composed a secondary trophic level of grazers. Eudorylaimus antarcticus had a 15N/14N ratio that was 2–4‰ higher than that of grazers, indicating that this species is the sole member of a tertiary trophic level. Understanding the trophic positions of soil fauna is critical to predictions of current and future species interactions and their distributions for the McMurdo Dry Valleys, Antarctica.

The δ13C Value of Fingerstick Blood Is a Valid, Reliable, and Sensitive Biomarker of Sugar-Sweetened Beverage Intake in Children and Adolescents

Reliance on self-reported dietary intake methods is a commonly cited research limitation, and dietary misreporting is a particular problem in children and adolescents. Objective indicators of dietary intake, such as dietary biomarkers, are needed to overcome this research limitation. The added sugar (AS) biomarker δ13C, which measures the relative abundance of 13C to 12C, has demonstrated preliminary validity in adults. The purpose of this investigation was to determine the comparative validity, test-retest reliability, and sensitivity of the δ13C biomarker to detect AS and sugar-sweetened beverage (SSB) intake using fingerstick blood samples in children and adolescents. Children (aged 6-11 y, n = 126, 56% male, mean ± SD age: 9 ± 2 y) and adolescents (aged 12-18 y, n = 200, 44% male, mean ± SD age: 15 ± 2 y) completed 4 testing sessions within a 3-wk period. Participants' height, weight, demographic characteristics, and health history were determined at the first session; 24-h recalls were obtained at each visit and fingerstick blood samples were collected at visits 1 and 3. Samples were analyzed for δ13C value using natural abundance stable isotope mass spectrometry. δ13C value was compared with dietary outcomes in the full sample, and in child and adolescent subgroups. t Tests and correlational analyses were used to assess biomarker validity and reliability, whereas logistic regression and area under the receiver-operator characteristic curve (AUC) were used to evaluate sensitivity. Reported mean ± SD AS consumption was 82.2 ± 35.8 g/d and 329 ± 143 kcal/d, and SSB consumption was 222 ± 243 mL/d and 98 ± 103 kcal/d. Mean δ13C value was -19.65 ± 0.69‰, and was lower in children than in adolescents (-19.80 ± 0.67‰ compared with -19.56 ± 0.67‰, P = 0.002). δ13C values were similar across sessions (visit 1: -19.66 ± 0.68‰; visit 3: -19.64 ± 0.68‰; r = 0.99, P < 0.001) and were associated (P < 0.001) with intake of total AS (grams, kilocalories: r = 0.29) and SSB (milliliters, kilocalories: r = 0.35). The biomarker was able to better discriminate between high and low SSB consumers than high and low AS consumers, as demonstrated by the AUC (0.75 and 0.62, respectively). The δ13C biomarker is a promising, minimally invasive, objective biomarker of SSB intake in children and adolescents. Further evaluation using controlled feeding designs is warranted. Registered at clinicaltrials.gov as NCT02455388.

Dual isotope assessment of trophic dynamics of an intertidal infaunal community with seasonal shifts in food sources

Stable isotopes are widely used to better understand dietary contributions and trophic interactions in benthic ecosystems. However, the traditional δ13C–δ15N bi-plots can only depict food-web structure for a single time period or space location. Various statistical approaches have now been developed to identify food-web changes in temporal or spatial scale. In the present study, three community-level statistical methods, including circular statistics, Layman et al.’s metrics and Bayesian Ellipses, were applied to compare natural abundance stable carbon and nitrogen isotope data from primary producers, meiofauna (nematodes and copepods), and macrofauna (polychaetes) in an intertidal area between summer and winter. Bayesian mixing model was also used to determine the food source contributions of consumers. In summer, data suggested that benthic microalgae were a dominant food source for most consumers, except for nematode Spilophorella sp. which mainly relied on seagrass detritus. While in winter, there was a significant increase in reliance on seagrass-based carbon for the consumers. The trophic positions of polychaete Eunice sp. and nematode Ditlevsenella sp. also differed between summer and winter, suggesting that they could change their feeding habits and behaviour according to time and availability of food. Using community-based statistics to quantitatively analyze stable isotope data can thus elucidate changes in food-web structure. In addition, considerable variations in food source utilization and trophic level for the same genus or species may imply that the feeding ecology of benthic infauna should be interpreted with caution and cannot be relied on buccal morphology alone.

Symbiotic performance of grain and wild herbaceous legumes in the Okavango Delta and Tswapong region of Botswana

The low inherent soil fertility, especially nitrogen (N) constrains arable agriculture in Botswana. Nitrogen is usually added to soil through inorganic fertilizer application. In this study, biological nitrogen fixation by legumes is explored as an alternative source of N. The objectives of this study were to measure levels of N2 fixation by grain legumes such as cowpea, Bambara groundnut and groundnut in farmers’ fields as well as to estimated N2 fixation by indigenous herbaceous legumes growing in the Okavango Delta. Four flowering plants per species were sampled from the panhandle part of the Okavango Delta and Tswapong area. Nitrogen fixation was measured using the 15N stable isotope natural abundance technique. The δ15N values of indigenous herbaceous legumes indicated that they fixed N2 (−1.88 to +1.35 ‰) with the lowest value measured in Chamaecrista absus growing in Ngarange (Okavango Delta). The δ15N values of grain legumes growing on farmers’ fields ranging from −1.2 ‰ to +3.3 ‰ indicated that they were fixing N2. For grain legumes growing at most farms, %Ndfa were above 50% indicating that they largely depended on symbiotic fixation for their N nutrition. With optimal planting density, Bambara groundnuts on farmers’ fields could potentially fix over 90 kg N/ha in some parts of Tswapong area and about 60 kg N/ha in areas around the Okavango Delta. Results from this study have shown that herbaceous indigenous legumes and cultivated legumes play an important role in the cycling of N in the soil. It has also been shown that biological N2 on farmer’s field could potentially supply the much needed N for the legumes and the subsequent cereal crops if plant densities are optimized with the potential to increase food security and mitigate climate change.

Estimates of rhizosphere priming effects are affected by soil disturbance

Laboratory studies have shown that priming effects, caused by inputs of carbon into the rhizosphere, can change the rate of soil organic matter (SOM) decomposition and could have significant impacts on soil carbon cycling. However, there have been few studies in field conditions because of experimental constraints but field data are needed to improve models that forecast the effects of climate change on SOM decomposition rates and the impact of these changing rates on atmospheric CO2 concentration.In 2009 a fire at a Eucalyptus forest site in Australia killed all standing vegetation. Trenched plots were installed in 2010, approximately 12months after the fire, and were maintained plant-free for the subsequent year. In 2011, after forest re-growth outside the trenched plots, we compared SOM decomposition rates in the presence of plants (rooted plots) and in the absence of plants (trenched plots) using a natural abundance stable carbon isotope technique with minimal disturbance of the soil. We then compared our results to those obtained in another study conducted at the same time and the same plots using laboratory incubations of sieved soil samples.There was no difference in SOM decomposition rates between the trenched and the rooted plots estimated using our non-disruptive technique. In contrast, laboratory incubations of sieved soils highlighted a two-fold increase in SOM decomposition rates in the rooted plots compared with rates from the trenched plots. Our results suggest that rhizosphere priming may not actively influence soil carbon turnover in the undisturbed soil environment and question conclusions from laboratory incubation studies. We attribute the different findings from laboratory and field studies to the physical disturbance of the soil involved in laboratory incubations causing the release of previously protected substrates, making them available for decomposition.

Seasonal variation in natural abundance of δ13C and 15N in Salicornia brachiata Roxb. populations from a coastal area of India

ABSTRACTHigh and fluctuating salinity is characteristic for coastal salt marshes, which strongly affect the physiology of halophytes consequently resulting in changes in stable isotope distribution. The natural abundance of stable isotopes (δ13C and δ15N) of the halophyte plant Salicornia brachiata and physico-chemical characteristics of soils were analysed in order to investigate the relationship of stable isotope distribution in different populations in a growing period in the coastal area of Gujarat, India. Aboveground and belowground biomass of S. brachiata was collected from six different populations at five times (September 2014, November 2014, January 2015, March 2015 and May 2015). The δ13C values in aboveground (−30.8 to −23.6 ‰, average: −26.6 ± 0.4 ‰) and belowground biomass (−30.0 to −23.1 ‰, average: −26.3 ± 0.4 ‰) were similar. The δ13C values were positively correlated with soil salinity and Na concentration, and negatively correlated with soil mineral nitrogen. The δ15N values of aboveground (6.7–16.1 ‰, average: 9.6 ± 0.4 ‰) were comparatively higher than belowground biomass (5.4–13.2 ‰, average: 7.8 ± 0.3 ‰). The δ15N values were negatively correlated with soil available P. We conclude that the variation in δ13C values of S. brachiata was possibly caused by soil salinity (associated Na content) and N limitation which demonstrates the potential of δ13C as an indicator of stress in plants.

土壌-植物系における炭素,窒素,酸素,イオウ動態解析のための安定同位体自然存在比の利用 : 1994年以降の研究の展開

土壌-植物系における炭素,窒素,酸素,イオウ動態解析のための安定同位体自然存在比の利用 : 1994年以降の研究の展開

Process Patent Protection via Analysis of Stable Isotope Ratios

Process patent protection via the analysis of natural-abundance stable isotopes has been demonstrated as an approach to extend the effective life of bio/pharmaceutical patents. The high specificity of isotope ratio analysis compared to other approaches (for example, concentrations of organic impurities or trace metals) allows the isotopic analysis to differentiate processes that were not previously resolvable by less precise analytical methods. Here we summarize the rationale for, and some selected case studies of, this emerging field. We review: (i) the systematics of stable isotope chemistry, (ii) approaches to instrumental analysis of stable isotopes, (iii) the biogeochemical origin of stable isotopic fingerprints, (iv) equilibrium versus kinetic isotope effects on those fingerprints, (v) categories of application of process patent protection, and (vi) case histories of application. The three reviewed cases include one of nutraceutical false advertising, one of a small molecule antibiotic drug product ...

Isotope Tracing of Long-Term Cadmium Fluxes in an Agricultural Soil.

Globally widespread phosphate fertilizer applications have resulted in long-term increases in the concentration of cadmium (Cd) in soils. The accumulation of this biotoxic, and bioaccumulative metal presents problems for the management of soil-plant-animal systems, because the magnitude and direction of removal fluxes (e.g., crop uptake, leaching) have been difficult to estimate. Here, Cd isotopic compositions (δ114/110Cd) of archived fertilizer and soil samples from a 66 year-long agricultural field trial in Winchmore, New Zealand, were used to constrain the Cd soil mass balance between 1959 and 2015 AD, informing future soil Cd accumulation trajectories. The isotopic partitioning of soil Cd sources in this system was aided by a change in phosphate source rocks in 1998 AD, and a corresponding shift in fertilizer isotope composition. The dominant influence of mixing between isotopically distinct Cd end-members was confirmed by a Bayesian modeling approach. Furthermore, isotope mass balance modeling revealed that Cd removal processes most likely increased in magnitude substantially between 2000 and 2015 AD, implying an increase in Cd bioaccumulation and/or leaching over that interval. Natural-abundance stable isotopes are introduced here as a powerful tool for tracing the fate of Cd in agricultural soils, and potentially the wider environment.

Stable isotopes in tropical tree rings: theory, methods and applications

Summary The notion that many tropical tree species form annual growth rings has triggered research on their growth and its environmental drivers over long periods of time. Even more recently, a large number of studies have also analysed the natural abundance of stable isotopes in tropical tree rings. The rapid developments in this young field call for a review. Here, we focus on stable isotopes of carbon (13C), oxygen (18O) and nitrogen (15N). We start by explaining how environmental and physiological effects define the isotopic composition of wood in tropical trees. Abundance of 13C is mainly driven by water, light and nutrient availability. Here 18O values are chiefly determined by those of rainwater and additionally by rooting depth and factors determining leaf water evaporation. The 15N levels are determined by the 15N signature of nitrogen uptake, which in turn depends in complex ways on various processes in the nitrogen cycle. We then discuss methodological aspects of isotopes studies in tropical tree rings. An important requirement is that rings are reliably dated. Furthermore, a key methodological concern is that temporal changes in isotopic values can be confounded by tree‐size driven changes, which can be avoided by sampling from a fixed diameter range or accounted for statistically. Next, 50 studies are reviewed that measured stable isotopes of C, O, and/or N in tree rings of a total of 85 tropical tree species. Temporal variation in both δ13C and δ18O was correlated with precipitation and El Niño Southern Oscillation variability. Seasonality in δ13C and δ18O was successfully used for delimiting visually non‐distinct annual rings. Tropical tree responses to increasing atmospheric [CO2] were effectively quantified, using δ13C as a measure of intrinsic water use efficiency. And finally, anthropogenic changes in the nitrogen cycle in tropical forests have been inferred from δ15N. We conclude with methodological and ecophysiological recommendations for isotope studies in tropical tree rings. Future perspectives include the analysis of intramolecular isotopic distributions of isotopes in glucose that can advance our understanding of environmental effects on tropical tree physiology. Finally, we recommend that tropical tree ring isotope data are deposited in open access databases. A lay summary is available for this article.

Stable Isotopes Reveal Rapid Cycling of Soil Nitrogen after Manure Application.

Understanding the fate of applied nitrogen (N) in agricultural soils is important for agronomic, environmental, and human health reasons, but it is methodologically difficult to study at the field scale. Natural abundance stable isotope measurements (δN) were used in this field study with micrometeorological measurements of nitrous oxide (NO) emissions to identify the biogeochemical processes responsible for rapid N transformations immediately after application of liquid dairy manure. Fifteen samplings occurred between 16 Mar. 2012 and 5 Apr. 2013, with a focus on spring manure application (before and after) and a winter snowmelt period. Concentrations and δN values of ammonium (NH), nitrate (NO), NO, and total N were measured throughout the year. Approximately 56 (±7)% of the NH-N applied in the spring could not be accounted for 3 d after manure application and was presumably lost by ammonia volatilization before it was tilled into the soil and/or removed from the inorganic N pool by microbial assimilation. Almost all of the remaining manure-NH (95 ± 1.1%) was converted within 3 wk to NO and NO by nitrification and nitrifier-denitrification, respectively. The in situ N isotope effect for nitrification (ε) was calculated to be -32.0 (±5.3)‰. Overall, field-scale measurements of δN at natural abundance provided valuable information that was used to distinguish sources of NH (manure vs. soil organic N) and to follow the production and consumption of NO and the pathways of NO production in soil.

Freshwater Contributions and Nitrogen Sources in a South Texas Estuarine Ecosystem: a Time-Integrated Perspective from Stable Isotopic Ratios in the Eastern Oyster (Crassostrea virginica)

Freshwater inflows to Texas estuaries vary widely due to regional climate fluctuations and are being substantially altered by human activities. The natural abundance stable isotope ratios of carbon and nitrogen in oyster adductor muscle were used to acquire a time-integrated view of freshwater and nitrogen contributions to the Mission-Aransas National Estuarine Research Reserve in South Texas. The study objective was to determine the influence of inputs from the San Antonio and Guadalupe rivers, which deliver approximately 1.6 km3 of water to the mid-Texas coastal region annually. A transect of sampling stations extending from the head of San Antonio Bay (northeast of the Reserve boundary) to the Aransas Pass ship channel (roughly 70 km to the southwest) was visited multiple times between 2009 and 2011. Carbon isotopic values increased from approximately −25 to −17‰ while δ15N values decreased from approximately +16 to +10‰ between the bay and ship channel. This range of carbon isotope values translates into time-integrated freshwater fractions as high as 0.8 (1 = 100% fresh) at the most inland sampling station to freshwater fractions around zero approaching the Gulf of Mexico. Contributions from the San Antonio and Guadalupe rivers to waters of the reserve vary between wet and dry years, but overall, the data suggest that these rivers are persistent and substantial sources of fresh water and nitrogen to the reserve. This study emphasizes the importance of connectivity and lateral exchanges among bays/lagoons when considering potential sources of fresh water and nitrogen that control ecosystem structure and function.

Assessment of nutritional subsidies to freshwater mussels using a multiple natural abundance isotope approach

Summary Freshwater mussels are a particularly imperilled group of aquatic organisms. To date, there is limited understanding of their dietary and nutritional subsidies, including those from both autochthonous (aquatic) and allochthonous (terrestrial) sources. The aim of this study was to evaluate the relative contributions of potential nutritional resources to mussel biomass using a Bayesian mixing model (MixSIAR) for (i) bulk suspended versus benthic organic matter (OM) pools, (ii) general autochthonous versus allochthonous OM sources and (iii) specific individual OM sources. Nutritional subsidies of five species of freshwater mussels were assessed using natural abundance stable isotopes (δ13C, δ15N, δ2H) and natural radiocarbon (Δ14C). The ranges for each of the four isotopes were similar across all species and sampling dates, and mussel δ13C and δ15N values were similar to those measured in previous studies. Despite relatively low variability in this study, species and temporal differences in mussel δ15N, δ2H and Δ14C values were generally significant (P &lt; 0.05). Based on δ13C values alone, bulk suspended OM subsidised mussel biomass to a greater extent than bulk benthic OM (c. 60 and 40% respectively). Modelling of multiple isotopes indicated that while the collective allochthonous nutritional sources accounted for a significant part (c. 33%) of mussel biomass, autochthonous materials collectively subsidised mussel tissue at about twice this level (c. 67). Significant amounts of 14C‐depleted aged carbon were incorporated into freshwater mussel tissue (mean equivalent 14C age = 1325 ± 68 years BP). Potential nutritional sources were also mostly aged, including benthic algae (1226 ± 556 years BP), zooplankton (1682 years BP), terrestrial soil (10 588 ± 4330 years BP) and suspended particulate OM (1443 ± 57 years BP). No single potential individual nutritional OM source explained the multiple isotopic values of freshwater mussel tissue, indicating that these organisms have a multi‐source and potentially opportunistic diet. Benthic algae and phytoplankton were quantitatively the most important individual OM subsidies to freshwater mussel nutrition at our study site (27 and 19% respectively). Observed seasonal shifts in freshwater mussel isotopic values were minimal, suggesting (i) that mussels may not be seasonally limited by individual nutritional resources and/or (ii) the replacement times of C, N and H in freshwater mussel tissues make detection of temporal nutritional subsidy shifts – if they occur – difficult to discern.

Use of Multi-Carbon Sources by Zooplankton in an Oligotrophic Lake in the Tibetan Plateau

We applied natural abundance stable isotope δ13C and radiocarbon ∆14C analyses to investigate trophic linkages between zooplankton and their potential food sources (phytoplankton, submersed plants, and allochthonous organic carbon) in Lake Nam Co, one of the largest oligosaline and oligotrophic lakes in the Tibetan Plateau, in south-west China. The δ13C and ∆14C levels of the calanoid copepod Arctodiaptomus altissimus pectinatus indicate that it uses different carbon sources. Thus, based on a two-isotope mixing model, our results suggested that recently synthesized but 14C-depleted primary producers (phytoplankton and submersed plants) were the most important sources of carbon, together contributing 92.2% of the zooplankton biomass. Allochthonous organic carbon and dissolved organic carbon constituted 4.7% and 3.1% of the carbon in the diet of zooplankton, respectively. Our findings from Lake Nam Co suggest that the carbon in the food webs of lakes located in a glaciated environment originates from various sources of different ages.

Vertical modeling of the nitrogen cycle in the eastern tropical South Pacific oxygen deficient zone using high‐resolution concentration and isotope measurements

AbstractMarine oxygen deficient zones (ODZs) have long been identified as sites of fixed nitrogen (N) loss. However, the mechanisms and rates of N loss have been debated, and traditional methods for measuring these rates are labor‐intensive and may miss hot spots in spatially and temporally variable environments. Here we estimate rates of heterotrophic nitrate reduction, heterotrophic nitrite reduction (denitrification), nitrite oxidation, and anaerobic ammonium oxidation (anammox) at a coastal site in the eastern tropical South Pacific (ETSP) ODZ based on high‐resolution concentration and natural abundance stable isotope measurements of nitrate (NO3−) and nitrite (NO2−). These measurements were used to estimate process rates using a two‐step inverse modeling approach. The modeled rates were sensitive to assumed isotope effects for NO3− reduction and NO2− oxidation. Nevertheless, we addressed two questions surrounding the fates of NO2− in the ODZ: (1) Is NO2− being primarily reduced to N2 or oxidized to NO3− in the ODZ? and (2) what are the contributions of anammox and denitrification to NO2− removal? Depth‐integrated rates from the model suggest that 72–88% of the NO2− produced in the ODZ was oxidized back to NO3−, while 12–28% of NO2− was reduced to N2. Furthermore, our model suggested that 36–74% of NO2− loss was due to anammox, with the remainder due to denitrification. These model results generally agreed with previously measured rates, though with a large range of uncertainty, and they provide a long‐term integrated view that compliments incubation experiments to obtain a broader picture of N cycling in ODZs.

A novel tool for stable nitrogen isotope analysis in aqueous samples.

Bulk stable isotope analysis (BSIA) of dissolved matter (e.g. dissolved organic carbon, total nitrogen bound (TNb ), etc.) is of particular importance since this pool is a prime conduit in the cycling of N and C. Studying the two elemental pools is of importance, as transformation and transport processes of N and C are inextricably linked in all biologically mediated systems. No system able to analyze natural abundance stable carbon and nitrogen isotope composition in aqueous samples (without offline sample preparation) and simultaneously has been reported so far. Extension of the high-temperature combustion (HTC) system, to be capable of measuring TNb stable nitrogen isotope composition, is described in this study. To extend the TOC analyzer to be capable of measuring TNb , modifications from the HTC high-performance liquid chromatography/isotope ratio mass spectrometry (HPLC/IRMS) interface were implemented and expanded. A reduction reactor for conversion of NOx into N2 was implemented into the new developed system. The extension addresses mainly the development of the focusing unit for nitrogen and a degassing device for online separation of TNb from molecular nitrogen (N2 ) prior to injection. The proof of principle of the system was demonstrated with different compound solutions. In this initial testing, the δ15 NAIR-N2 values of the tested compounds were determined with precision and trueness of typically ≤0.5‰. Good results (u≤0.5‰) could be achieved down to a TNb concentration of 40 mgN/L and acceptable results (u≤1.0‰) down to 5 mgN/L. In addition, the development resulted in the first system reported to be suitable for simultaneous and direct δ13 C and δ15 N BSIA of aqueous samples. The development resulted in the first system shown to be suitable for both δ13 C and δ15 N direct BSIA in aqueous samples. This system could open up new possibilities in SIA-based research fields. Copyright © 2016 John Wiley & Sons, Ltd.

The tiny-leaved orchid Cephalanthera subaphylla obtains most of its carbon via mycoheterotrophy.

The evolution of mycoheterotrophy has been accompanied by extreme reductions in plant leaf size and photosynthetic capacity. Partially mycoheterotrophic plants, which obtain carbon from both photosynthesis and their mycorrhizal fungi, include species with leaves of normal size and others that are tiny-leaved. Thus, plant species may lose their leaves in a gradual process of size reduction rather than through a single step mutation. Little is known about how the degree of mycoheterotrophy changes during reductions in leaf size. We compared the degree of mycoheterotrophy among five Japanese Cephalanthera species, four with leaves of normal size (Cephalanthera falcata, Cephalanthera erecta, Cephalanthera longibracteata and Cephalanthera longifolia), one with tiny leaves (Cephalanthera subaphylla), and one albino form of C. falcata (as reference specimens for fully mycoheterotrophic plants). The levels of mycoheterotrophy were determined by stable isotope natural abundance analysis. All Cephalanthera species were relatively enriched in 13C and 15N in comparison with surrounding autotrophic plants. Cephalanthera subaphylla was strongly enriched in 13C and 15N to levels similar to the albinos. Species with leaves of normal size were significantly less enriched in 13C than C. subaphylla and the albinos. Thus, C. subaphylla was strongly mycoheterotrophic, obtaining most of its carbon from mycorrhizal fungi even though it has tiny leaves; species with leaves of normal size were partially mycoheterotrophic. Hence, during the evolutionary pathway to full mycoheterotrophy, some plant species appear to have gained strong mycoheterotrophic abilities before completely losing foliage leaves.

Influence of an intervention targeting a reduction in sugary beverage intake on the δ13C sugar intake biomarker in a predominantly obese, health-disparate sample.

Controversy exists surrounding the health effects of added sugar (AS) and sugar-sweetened beverage (SSB) intakes, primarily due to a reliance on self-reported dietary intake. The purpose of the current investigation was to determine if a 6-month intervention targeting reduced SSB intake would impact δ13C AS intake biomarker values. A randomized controlled intervention trial. At baseline and at 6 months, participants underwent assessments of anthropometrics and dietary intake. Fasting fingerstick blood samples were obtained and analysed for δ13C value using natural abundance stable isotope MS. Statistical analysis included descriptive statistics, correlational analyses and multilevel mixed-effects linear regression analysis using an intention-to-treat approach. Rural Southwest Virginia, USA. Adults aged ≥18 years who consumed ≥200 kcal SSB/d (≥837 kJ/d) were randomly assigned to either the intervention (n 155) or a matched-contact group (n 146). Participants (mean age 42·1 (sd 13·4) years) were primarily female and overweight (21·5 %) or obese (57·0 %). A significant group by time difference in δ13C value was detected (P<0·001), with mean (sd) δ13C value decreasing in the intervention group (pre: -18·92 (0·65) ‰, post: -18·97 (0·65) ‰) and no change in the comparison group (pre: -18·94 (0·72) ‰, post: -18·92 (0·73) ‰). Significant group differences in weight and BMI change were also detected. Changes in biomarker δ13C values were consistent with changes in self-reported AS and SSB intakes. The δ13C sugar intake biomarker assessed using fingerstick blood samples shows promise as an objective indicator of AS and SSB intakes which could be feasibly included in community-based research trials.

Application of Stable Isotopes in Ecosystem Research

The ecological applications of natural abundance of stable isotopes benefited the understanding of the ecosystem processes. Natural isotopic effects ensuing from difference in physico-chemical properties, fractionation and discrimination of elements such as carbon, hydrogen, oxygen, nitrogen, sulphur, strontium and calcium, have been used to reinforce understanding the various aspects of ecosystem analysis and to revisit some old, baffling processes with new perspectives. Stable isotopes have provided insight into not only the relationship between biodiversity and ecosystem structure and functioning, but also strengthened our knowledge about the possible mechanisms of ecosystem functioning in various ecosystems and biomes. This review elucidates the role of natural abundance of stable isotopes in benefiting ecosystem analysis.

Niche‐dependent trophic position distributions among primary, secondary and tertiary consumers

Many consumers display flexible feeding strategies that vary among individuals or populations, through life‐history, or spatiotemporally. Despite the recognized influence of flexible feeding on the structure and dynamics of food webs, the consequences of these feeding strategies on the actual shape and characteristics of trophic position distributions have received less attention. We proposed and tested several a priori hypotheses to predict the likely effect of niche‐dependent (e.g. herbivore, secondary consumer) foraging on the shape and statistical properties of consumer trophic position distributions using natural abundance stable isotope data from a diverse dataset of consumers. We found evidence that the structural characteristics of consumer trophic position distributions varied as a function of trophic niche. Herbivores and tertiary consumers tended to be ‘packed’ closely near their mean trophic position, with few individuals realizing trophic positions markedly higher or lower than the mean. Conversely, secondary consumers often displayed broad trophic position distributions with many individuals dispersed away from the center of the distribution. We examined the effect of applying constant versus dynamic isotope trophic fractionation models and found that both models yielded similar although not identical results. Our findings suggest that trophic level omnivory supports a larger fraction of consumer diet at intermediate trophic positions than at either the lowest or the highest positions in aquatic food webs. These results suggest that vertical trophic niche declines among higher order consumers despite general evidence that the range of potential foraging options (i.e. horizontal trophic niche) tends to increase at higher trophic positions. Although further work is needed to test the generality of these patterns in other ecosystems, proactively examining trophic position distributions and reporting appropriate measures of central tendency (e.g. arithmetic versus geometric means) will increase the accuracy of individual trophic studies as well as the applicability of results for meta‐analytical food web models.