Natural Abundance Stable Isotope Research Articles

Benthic food-web succession in a developing salt marsh

Ecological succession has long been a focal point for research, and knowledge of underlying mechanisms is required if scientists and managers are to successfully promote recov- ery of ecosystem function following disturbance. We addressed the influence of bottom-up pro- cesses on successional assemblage shifts in salt marshes, ecosystems with strong physical gradi- ents, and how these shifts were reflected in the trophic characteristics of benthic fauna. We tracked the temporal development of infaunal community structure and food-web interactions in a young, created salt marsh and an adjacent natural marsh in Mission Bay, California, USA (1996−2003). Macro faunal community succession in created Spartina foliosa habitats occurred rapidly, with infaunal densities reaching 70% of those in the natural marsh after 1 yr. Community composition shifted from initial dominance of insect larvae (surface-feeding microalgivores) to increased dominance of oligo chaetes (subsurface-feeding detritivores) within the first 7 yr. Iso- topic labeling of microalgae, N2-fixing cyanobacteria, S. foliosa and bacteria revealed direct links (or absence thereof) between these basal food sources and specific consumer groups. In combina- tion with the compositional changes in the macroinvertebrate fauna, the trophic patterns indi- cated an increase in food-web complexity over time, reflecting resource-driven marsh succession. Natural abundance stable isotope ratios of salt marsh consumers (infaunal and epifaunal macroin- vertebrates, and fish) initially reflected distinctions in trophic structure between the created and natural marsh, but these diminished during successional development. Our findings suggest that changing resource availability is one of the important drivers of succession in benthic communi- ties of restored wetlands in Southern California.

Soil Carbon Sequestration in Sorghum Cropping Systems

Management practices can influence both the quantity of soil organic carbon (SOC) and its distribution into different fractions or pools. We investigated SOC sequestration potentials of cropping systems in near-surface (0–5 cm) samples through soil size and density fractionation coupled with acid hydrolysis and natural abundance of stable isotopes (δ13C) in a 20-year field study in the southern Great Plains in 2002. Treatments included two tillage regimens, conventional and no tillage (NT), in combination with two cropping systems: continuous grain sorghum [Sorghum bicolor (L.) Moench.] (CS) and a sorghum-wheat (Triticum aestivum L.)-soybean [Glycine max (L.) Merr.] (SWSoy) rotation, and with or without N fertilization. Tillage and cropping sequence significantly affected SOC distribution and the natural abundance of δ13C in different fractions. Samples from CS exhibited δ13C values ranging from −15‰ to −20‰, suggesting most SOC was derived from this C4 crop species. The δ13C values for soils from SWSoy varied from −20‰ to −22‰, reflecting a mixed input from C3-derived and C4-derived residue input. For whole soil and all aggregate-size fractions, SOC concentrations were significantly higher for NT than conventional tillage. However, the effects of cropping system and N fertilization on SOC interacted with tillage. Greater SOC for enhanced cropping (SWSoy) or N fertilization was observed only under NT. The fraction of <53 μm represented a greater proportion of soil than other aggregate-size fractions. Our long-term study indicated that SOC and its various fractions, including more resistant, can be increased by NT with enhanced cropping and N fertilization.

Contributions of the [NiFe]- and [FeFe]-hydrogenase to H2production inShewanella oneidensisMR-1 as revealed by isotope ratio analysis of evolved H2

Shewanella oneidensis MR-1 encodes both a [NiFe]- and an [FeFe]-hydrogenase. While the output of these proteins has been characterized in mutant strains expressing only one of the enzymes, the contribution of each to H2 synthesis in the wild-type organism is not clear. Here, we use stable isotope analysis of H2 in the culture headspace, along with transcription data and measurements of the concentrations of gases in the headspace, to characterize H2 production in the wild-type strain. After most of the O2 in the headspace had been consumed, H2 was produced and then consumed by the bidirectional [NiFe]-hydrogenase. Once the cultures were completely anaerobic, a new burst of H2 synthesis catalyzed by both enzymes took place. Our data are consistent with the hypothesis that at this point in the culture cycle, a pool of electrons is shunted toward both hydrogenases in the wild-type organisms, but that in the absence of one of the hydrogenases, the flux is redirected to the available enzyme. To our knowledge, this is the first use of natural-abundance stable isotope analysis of a metabolic product to elucidate substrate flux through two alternative enzymes in the same cellular system.

Elevated levels of δ15N in riverine Painted Turtles (Chrysemys picta): trophic enrichment or anthropogenic input?

The natural abundance of stable isotopes of elements in animal tissue is influenced by both biotic and abiotic factors. Biotically, animals feeding at higher trophic levels are enriched in the ratio of 15N:14N (δ15N) relative to their food resources owing to the preferential excretion of 14N. Abiotically, increases in δ15N may also reflect different sources of biologically available nitrogen, including nitrogen resulting from denitrification of inorganic fertilizer. We studied variation in δ15N among freshwater turtle populations to assess spatial variation in δ15N and to determine whether this variation can be attributed to differences in nitrogen source or trophic enrichment. We examined nitrogen and carbon stable isotope ratios in duckweed (genus Lemna L.) and in Painted Turtles (Chrysemys picta (Schneider, 1783)) in aquatic ecosystems expected to be differentially affected by agricultural activity and denitrification of inorganic fertilizer. Across sites, C. picta δ15N was strongly correlated with Lemna δ15N and was elevated in sites influenced by agricultural activity. Furthermore, trophic position of turtles was not associated with δ15N but was consistent with expected values for primary consumers in freshwater systems, indicating that differences in tissue δ15N could be attributed to differences in initial sources of nitrogen in each ecosystem. Our results suggest that care must be taken when attributing differences in isotopic values of animal populations to trophic factors.

New insights on the trophic ecology of bathyal communities from the methane seep area off Concepción, Chile (~36° S)

AbstractStudies of the trophic structure in methane‐seep habitats provide insight into the ecological function of deep‐sea ecosystems. Methane seep biota on the Chilean margin likely represent a novel biogeographic province; however, little is known about the ecology of the seep fauna and particularly their trophic support. The present study, using natural abundance stable isotopes, reveals a complex trophic structure among heterotrophic consumers, with four trophic levels supported by a diversity of food sources at a methane seep area off Concepción, Chile (~36° S). Although methanotrophy, thiotrophy and phototrophy are all identified as carbon fixation mechanisms fueling the food web within this area, most of the analysed species (87.5%) incorporate carbon derived from photosynthesis and a smaller number (12%) use carbon derived from chemosynthesis. Methane‐derived carbon (MDC) incorporation was documented in 22 taxa, including sipunculids, gastropods, polychaetes and echinoderms. In addition, wide trophic niches were detected in suspension‐feeding and deposit‐feeding taxa, possibly associated with the use of organic matter in different stages of degradation (e.g. from fresh to refractory). Estimates of Bayesian standard ellipses area (SEAB) reveal different isotopic niche breadth in the predator fishes, the Patagonian toothfish Dissostichus eleginoides and the combtooth dogfish Centroscyllium nigrum, suggesting generalist versus specialist feeding behaviors, respectively. Top predators in the ecosystem were the Patagonian toothfish D. eleginoides and the dusky cat shark, Bythaelurus canescens. The blue hake Antimora rostrata also provides a trophic link between the benthic and pelagic systems, with a diet based primarily on pelagic‐derived carrion. These findings can inform accurate ecosystem models, which are critical for effective management and conservation of methane seep and adjacent deep‐sea habitats in the Southeastern Pacific.

Mercury isotopes in a forested ecosystem: Implications for air‐surface exchange dynamics and the global mercury cycle

Forests mediate the biogeochemical cycling of mercury (Hg) between the atmosphere and terrestrial ecosystems; however, there remain many gaps in our understanding of these processes. Our objectives in this study were to characterize Hg isotopic composition within forests, and use natural abundance stable Hg isotopes to track sources and reveal mechanisms underlying the cycling of Hg. We quantified the stable Hg isotopic composition of foliage, forest floor, mineral soil, precipitation, and total gaseous mercury (THg(g)) in the atmosphere and in evasion from soil, in 10‐year‐old aspen forests at the Rhinelander FACE experiment in northeastern Wisconsin, USA. The effect of increased atmospheric CO2 and O3 concentrations on Hg isotopic composition was small relative to differences among forest ecosystem components. Precipitation samples had δ202Hg values of −0.74 to 0.06‰ and ∆199Hg values of 0.16 to 0.82‰. Atmospheric THg(g) had δ202Hg values of 0.48 to 0.93‰ and ∆199Hg values of −0.21 to −0.15‰. Uptake of THg(g) by foliage resulted in a large (−2.89‰) shift in δ202Hg values; foliage displayed δ202Hg values of −2.53 to −1.89‰ and ∆199Hg values of −0.37 to −0.23‰. Forest floor samples had δ202Hg values of −1.88 to −1.22‰ and ∆199Hg values of −0.22 to −0.14‰. Mercury isotopes distinguished geogenic sources of Hg and atmospheric derived sources of Hg in soil, and showed that precipitation Hg only accounted for ~16% of atmospheric Hg inputs. The isotopic composition of Hg evasion from the forest floor was similar to atmospheric THg(g); however, there were systematic differences in δ202Hg values and MIF of even isotopes (∆200Hg and ∆204Hg). Mercury evasion from the forest floor may have arisen from air‐surface exchange of atmospheric THg(g), but was not the emission of legacy Hg from soils, nor re‐emission of wet‐deposition. This implies that there was net atmospheric THg(g) deposition to the forest soils. Furthermore, MDF of Hg isotopes during foliar uptake and air‐surface exchange of atmospheric THg(g) resulted in the release of Hg with very positive δ202Hg values to the atmosphere, which is key information for modeling the isotopic balance of the global mercury cycle, and may indicate a shorter residence time than previously recognized for the atmospheric mercury pool.

The magnitude of spatial and temporal variation in δ15N and δ13C differs between taxonomic groups: Implications for food web studies

Understanding variability in stable isotope abundance is essential for effective hypothesis testing and evaluating food sources, trophic levels and food web structure. The magnitude and sources of variability are likely to differ among taxonomic and functional groups. We aimed to quantify variability of δ13C and δ15N for 16 species representing seven distinct taxonomic groups of benthic invertebrates and autotrophs in a marine ecosystem. We quantified the magnitude of variability among individuals or shoots separated by metres, among eight sites separated by kilometres, and between two survey occasions separated by months. δ13C varied by as much as 7‰ for primary producers, 4‰ for consumers, while δ15N varied by as much as 9‰ and 2‰ respectively. Variation in δ15N of seagrass was largely accounted for by differences among sites, while variation in δ13C was mainly attributable to shoots collected a few metres apart. Compared to seagrasses, variation in macroalgae was mainly explained by differences between the two survey occasions for δ15N and among individuals collected a few metres apart for δ13C. Variation was generally lower for consumers and typically explained by differences among individuals for δ15N but displayed inconsistent patterns for δ13C. Dual isotope Bayesian mixing models showed that the potential contributions of food sources for herbivorous consumers varied among sites and between survey occasions, and also that there was high variability or uncertainty in the contributions of sources within sites. The relative consistency in the main sources of variation among broad taxonomic groups in autotrophs suggests that aspects of physiology that are phylogenetically conserved might be important influences on variation in natural abundances of stable isotopes. In comparison, the sources of variability were less consistent within and among broad consumer groups, suggesting complex interactions between consumers and their food sources.

Trophic ecology of a freshwater sponge (Spongilla lacustris) revealed by stable isotope analysis

The vital roles that sponges play in marine habitats are well-known. However, sponges inhabiting freshwaters have been largely ignored despite having widespread distributions and often high local abundances. We used natural abundance stable isotope signatures of carbon and nitrogen (δ 13C and δ 15N) to infer the primary food source of the cosmopolitan freshwater sponge Spongilla lacustris. Our results suggest that S. lacustris feed largely on pelagic resources and may therefore link pelagic and benthic food webs. A facultative association between S. lacustris and endosymbiotic green algae caused S. lacustris to have significantly depleted carbon and nitrogen signatures that may reflect carbon and nitrogen exchange between sponges and their symbiotic algae. Isotopic data from specialist sponge consumers demonstrated that sponges hosting zoochlorellae were the major component of the diet of the spongillafly Climacia areolaris and the sponge-eating caddisfly Ceraclea resurgens suggesting that the symbiosis between freshwater sponges and algae is important to sponge predator trophic ecology. Our results help define the role of sponges in freshwater ecosystems and shed new light on the evolution and ecological consequences of a complex tri-trophic symbiosis involving freshwater sponges, zoochlorellae, and spongivorous insects.

UNDERSTANDING AND OVERCOMING BASELINE ISOTOPIC VARIABILITY IN RUNNING WATERS

Natural abundances of stable isotopes in lotic food webs yield valuable information about sources of organic matter for consumers and trophic structure. However, interpretation of isotopic information can be challenging in the face of variability in organisms at the base of food webs. Unionid and dreissenid mussels, commonly used as baseline organisms in lakes, are uncommon in many river settings and can have variable diets, thus making them unsuitable as a universal baseline for many river food web studies and often forcing reliance on more common benthic insects for this purpose. Turnover rates of body carbon and nitrogen in insects are relatively rapid (1 to 50 days half-life). These rapid turnover rates in primary consumers can result in considerable temporal variability in δ13C that rivals that of algae (>10‰ range within a site). This suggests that using primary consumers as a surrogate baseline for algae may not circumvent the problem of temporal variability and the resultant mismatch of sources with longer-lived, slow-growing secondary and tertiary consumers. There are several strategies for reducing the influence of these confounding factors when bivalves with a known diet are not present. These include sampling over large spatial scales and correlating δ13C of consumers with the source of interest (e.g. benthic algae), sampling baseline organisms multiple times in the weeks preceding sampling of larger consumers (particularly in response to large changes in discharge) and using algal-detrital separation methods and multiple tracers as much as possible. Incorporating some of these recommendations and further exploring variability at the base of the food web will potentially provide greater insights into consumer–resource coupling in running waters and more robust conclusions about food web structure and energy flow in these dynamic systems. Copyright © 2012 John Wiley & Sons, Ltd.

Stable isotope composition of microfauna supports the occurrence of biologically fixed nitrogen from cyanobacteria in desert soil food webs

Biological soil crusts of the arid Southwest US are habitat for a greater diversity of microfauna than soils with absent or physically disturbed crusts. The cyanobacteria of desert soil crusts fix nitrogen biologically, but it is not known whether these cyanobacteria are consumed by microfauna and are a significant component of the soil food web. We suspect that stylet-bearing nematodes and tardigrades are the most likely microfauna to graze on cyanobacteria, so we used natural abundance stable isotopes to test whether cyanobacteria are indeed a significant component of their diet in the field. We found that stable isotope composition of microinvertebrate biomass supports the hypothesis that cyanobacteria are a potential prey item in the food chain leading up to tardigrades and dorylaim-type nematodes in desert soils, but does not support the same hypothesis for Tylenchidae in these communities. Continued development of stable isotope methods may help to refine our understanding of feeding behaviors in belowground desert soil food webs.

Enzymatic activities and stable isotope patterns of ectomycorrhizal fungi in relation to phylogeny and exploration types in an afrotropical rain forest

Ectomycorrhizal (ECM) fungi obtain both mineral and simple organic nutrients from soil and transport these to plant roots. Natural abundance of stable isotopes (¹⁵N and ¹³C) in fruit bodies and potential enzymatic activities of ECM root tips provide insights into mineral nutrition of these mutualistic partners. By combining rDNA sequence analysis with enzymatic and stable isotope assays of root tips, we hypothesized that phylogenetic affinities of ECM fungi are more important than ECM exploration type, soil horizon and host plant in explaining the differences in mineral nutrition of trees in an African lowland rainforest. Ectomycorrhizal fungal species belonging to extraradical mycelium-rich morphotypes generally displayed the strongest potential activities of degradation enzymes, except for laccase. The signature of ¹⁵N was determined by the ECM fungal lineage, but not by the exploration type. Potential enzymatic activities of root tips were unrelated to ¹⁵N signature of ECM root tip. The lack of correlation suggests that these methods address different aspects in plant nutrient uptake. Stable isotope analysis of root tips could provide an additional indirect assessment of fungal and plant nutrition that enables enhancement of taxonomic coverage and control for soil depth and internal nitrogen cycling in fungal tissues.

Elevated CO2 and O3 modify N turnover rates, but not N2O emissions in a soybean agroecosystem

In order to predict and mitigate future climate change, it is essential to understand plant-mediated effects of elevated CO2 (eCO2) and O3 (eO3) on N-cycling, including N2O emissions. This is of particular interest for agroecosystems, since N-cycling and N2O emissions are responsive to adaptive management. We investigated the interaction of soil moisture content with eCO2 and eO3 on potential N2O emissions from SoyFACE during a 28-day laboratory incubation experiment. We also assessed field N2O fluxes during 2 soybean-growing seasons. In addition, we sought to link previously observed changes in soybean growth and production to belowground processes over a longer time scale by analyzing changes in natural abundance stable isotope ratios of soil N (δ15N). This method relies on the concept that soil δ15N can only change when inputs or outputs with an isotope signature different from that of soil N are altered. We found no major effects of eCO2 and eO3 on laboratory and field measured N2O emissions. Natural abundance isotope analyses suggested, however, a decrease in belowground allocation of biologically fixed N in combination with decreased total gaseous N loss by eCO2, resulting in a tighter N cycle in the longer-term. In contrast, the isotope data suggested an increase in belowground allocation of biologically fixed N under eO3, leading to increased gaseous N loss, most likely in the form of N2. Given that effects of eCO2 and eO3 on N pools and instantaneous transformation rates in surface soil layers of this agroecosystem have been minimal, our results illustrate the importance of evaluating longer-term changes in N turnover rates. We conclude that eCO2 decelerates whereas eO3 accelerates N-cycling in the longer-term, but feedback through changed N2O emissions is not occurring in this soybean system.

Reliance on prey‐derived nitrogen by the carnivorous plant Drosera rotundifolia decreases with increasing nitrogen deposition

• Carnivory in plants is presumed to be an adaptation to a low-nutrient environment. Nitrogen (N) from carnivory is expected to become a less important component of the N budget as root N availability increases. • Here, we investigated the uptake of N via roots versus prey of the carnivorous plant Drosera rotundifolia growing in ombrotrophic bogs along a latitudinal N deposition gradient through Sweden, using a natural abundance stable isotope mass balance technique. • Drosera rotundifolia plants receiving the lowest level of N deposition obtained a greater proportion of N from prey (57%) than did plants on bogs with higher N deposition (22% at intermediate and 33% at the highest deposition). When adjusted for differences in plant mass, this pattern was also present when considering total prey N uptake (66, 26 and 26 μg prey N per plant at the low, intermediate and high N deposition sites, respectively). The pattern of mass-adjusted root N uptake was opposite to this (47, 75 and 86 μg N per plant). • Drosera rotundifolia plants in this study switched from reliance on prey N to reliance on root-derived N as a result of increasing N availability from atmospheric N deposition.

Response of Sphagnum papillosum and Drosera rotundifolia to Reduced and Oxidized Wet Nitrogen Deposition

We transplanted Sphagnum ‘turfs’ containing abundant Drosera rotundifolia into an existing nitrogen deposition experiment at Whim Moss near Edinburgh. These mesocosms received simulated N deposition as either NH 4 + or NO 3 - , to give total N deposition rates of approximately 8, 16 or 32, or 64 kg N ha-1 year-1. Simulated N deposition was added in a realistic way (i.e., with rainfall throughout the year). The δ15N of this added N was elevated relative to background N. We measured the tissue chemistry and δ15N of Sphagnum papillosum and D. rotundifolia over two years after transplant. Our aim was to determine uptake of the deposited N and the impact on S. papillosum tissue chemistry and D. rotundifolia tissue chemistry and ecology. We found clear, significant impacts of N deposition on S. papillosum, with increased capitula N content and reduced C:N ratio. Increased δ15N indicated uptake of deposited N. The response of D. rotundifolia was less clear with impacts only at the highest rate of N deposition. There was no evidence of differential uptake of reduced or oxidized wet N deposition by either S. papillosum or D. rotundifolia. Using the natural abundance stable isotope method we estimated the minimum contribution of prey N to the total N in D. rotundifolia to be 35%. The results suggest that differences in the uptake of reduced or oxidized wet N deposition might not be ecologically significant when wet N deposition is added realistically. They also support the suggestion that a model of N dynamics in Sphagnum-dominated ecosystems that includes the role of Sphagnum as a small-scale ecosystem engineer, is required to predict vascular plant responses to N deposition accurately.

Determining Degradation and Synthesis Rates of Arabidopsis Proteins Using the Kinetics of Progressive 15N Labeling of Two-dimensional Gel-separated Protein Spots

The growth and development of plant tissues is associated with an ordered succession of cellular processes that are reflected in the appearance and disappearance of proteins. The control of the kinetics of protein turnover is central to how plants can rapidly and specifically alter protein abundance and thus molecular function in response to environmental or developmental cues. However, the processes of turnover are largely hidden during periods of apparent steady-state protein abundance, and even when proteins accumulate it is unclear whether enhanced synthesis or decreased degradation is responsible. We have used a (15)N labeling strategy with inorganic nitrogen sources coupled to a two-dimensional fluorescence difference gel electrophoresis and mass spectrometry analysis of two-dimensional IEF/SDS-PAGE gel spots to define the rate of protein synthesis (K(S)) and degradation (K(D)) of Arabidopsis cell culture proteins. Through analysis of MALDI-TOF/TOF mass spectra from 120 protein spots, we were able to quantify K(S) and K(D) for 84 proteins across six functional groups and observe over 65-fold variation in protein degradation rates. K(S) and K(D) correlate with functional roles of the proteins in the cell and the time in the cell culture cycle. This approach is based on progressive (15)N labeling that is innocuous for the plant cells and, because it can be used to target analysis of proteins through the use of specific gel spots, it has broad applicability.

Dominance of biologically produced nitrate in upland waters of Great Britain indicated by stable isotopes

Atmospheric deposition of nitrogen (N) compounds is the major source of anthropogenic N to most upland ecosystems, where leaching of nitrate (NO 3 − ) into surface waters contributes to eutrophication and acidification as well as indicating an excess of N in the terrestrial catchment ecosystems. Natural abundance stable isotopes ratios, 15N/14N and 18O/16O (the “dual isotope” technique) have previously been used in biogeochemical studies of alpine and forested ecosystems to demonstrate that most of the NO 3 − in upland surface waters has been microbially produced. Here we present an application of the technique to four moorland catchments in the British uplands including a comparison of lakes and their stream inflows at two sites. The NO 3 − concentrations of bulk deposition and surface waters at three sites are very similar. While noting the constraints imposed by uncertainty in the precise δ18O value for microbial NO 3 − , however, we estimate that 79–98% of the annual mean NO 3 − has been microbially produced. Direct leaching of atmospheric NO 3 − is a minor component of catchment NO 3 − export, although greater than in many similar studies in forested watersheds. A greater proportion of atmospheric NO 3 − is seen in the two lake sites relative to their inflow streams, demonstrating the importance of direct NO 3 − deposition to lake surfaces in catchments where terrestrial ecosystems intercept a large proportion of deposited N. The dominance of microbial sources of NO 3 − in upland waters suggests that reduced and oxidised N deposition may have similar implications in terms of contributing to NO 3 − leaching.

Natural abundance stable isotopes and dual isotope tracer additions help to resolve resources supporting a saltmarsh food web

We used a combination of natural abundance stable isotopes (NASI) and multiple isotope additions to examine the importance of local and tidally imported basal resources to infauna in a tidal, saltmarsh creek within the Plum Island Estuary, Massachusetts, U.S.A. NASI analysis was conducted on primary producers and infauna prior to isotope additions. While algal resources as a group had distinct NASI values compared to Spartina spp. resources, various algal resources [sediment-associated algae (local) and phytoplankton (tidally imported)] had similar NASI leading to low dietary resolution with mixing models. To separate primary producer isotope values, we conducted a large-scale, 42 day 15N-addition to the whole saltmarsh creek, which enriched tidally imported (i.e., phytoplankton) and local algae resources. We also conducted small-scale additions of enriched 13C to 1-m 2 plots within the creek receiving the 15N addition to separately label local resources. Enriched 15N and 13C were taken up by filamentous algae and associated epiphytic diatoms and benthic microalgae creating distinct isotope values in local algal resources. In addition, changes in 15N-tracer uptake by phytoplankton during the first week of the 15N-addition provided a unique enrichment pattern that was followed through the food web. Both NASIs and isotope additions indicated that Spartina spp. was not an important basal food resource to infauna, lending credence to the notion that saltmarsh food webs don't often rely on macrophyte detritus. Isotope additions indicated that various types of algae, both local and tidally imported, were important to the diets of infauna, including polychaetes, tanaids and harpacticoid copepods. Isotope additions increased diet resolution and revealed less accurate basal resource contributions determined with NASI alone. While many food web studies rely on NASI, we feel that the combination of NASI and isotope additions is needed in systems with similar primary producer values to more accurately determine resource contributions, to increase resolution of dietary contributions and to determine the dietary importance of local basal resources.

The influence of light on nitrogen cycling and the primary nitrite maximum in a seasonally stratified sea

In the seasonally stratified Gulf of Aqaba Red Sea, both NO 2 - release by phytoplankton and NH 4 + oxidation by nitrifying microbes contributed to the formation of a primary nitrite maximum (PNM) over different seasons and depths in the water column. In the winter and during the days immediately following spring stratification, NO 2 - formation was strongly correlated ( R 2 = 0.99) with decreasing irradiance and chlorophyll, suggesting that incomplete NO 3 - reduction by light limited phytoplankton was a major source of NO 2 - . However, as stratification progressed, NO 2 - continued to be generated below the euphotic depth by microbial NH 4 + oxidation, likely due to differential photoinhibition of NH 4 + and NO 2 - oxidizing populations. Natural abundance stable nitrogen isotope analyses revealed a decoupling of the δ 15N and δ 18O in the combined NO 3 - and NO 2 - pool, suggesting that assimilation and nitrification were co-occurring in surface waters. As stratification progressed, the δ 15N of particulate N below the euphotic depth increased from −5‰ to up to +20‰. N uptake rates were also influenced by light; based on 15N tracer experiments, assimilation of NO 3 - , NO 2 - , and urea was more rapid in the light (434 ± 24, 94 ± 17, and 1194 ± 48 nmol N L −1 day −1 respectively) than in the dark (58 ± 14, 29 ± 14, and 476 ± 31 nmol N L −1 day −1 respectively). Dark NH 4 + assimilation was 314 ± 31 nmol N L −1 day −1, while light NH 4 + assimilation was much faster, resulting in complete consumption of the 15N spike in less than 7 h from spike addition. The overall rate of coupled urea mineralization and NH 4 + oxidation (14.1 ± 7.6 nmol N L −1 day −1) was similar to that of NH 4 + oxidation alone (16.4 ± 8.1 nmol N L −1 day −1), suggesting that mineralization of labile dissolved organic N compounds like urea was not a rate limiting step for nitrification. Our results suggest that assimilation and nitrification compete for NH 4 + and that N transformation rates throughout the water column are influenced by light over diel and seasonal cycles, allowing phytoplankton and nitrifying microbes to contribute jointly to PNM formation. We identify important factors that influence the N cycle throughout the year, including light intensity, substrate availability, and microbial community structure. These processes could be relevant to other regions worldwide where seasonal variability in mixing depth and stratification influence the contributions of phytoplankton and non-photosynthetic microbes to the N cycle.

Tracing sources of soil nitrate using the dual isotopic composition of nitrate in 2 M KCl-extracts

Soils comprise a critical interface between the atmosphere, lithosphere, hydrosphere and biosphere, and play a major role in the cycling of nitrogen (N), an element crucial to plant growth. Isotope techniques constitute a powerful tool to study the origin and fate of N compounds (e.g. NO 3 −) within the environment including soils. The objective of our study was to test the usefulness of the isotope composition of soil NO 3 − extracted with 2 M KCl (soil NO 3) as a tool to investigate the origin and fate of NO 3 − in the environment. Specifically issues related to repeat extractions, crop type, length of fertilization, and soil depth were addressed. Soils from four contrasting agricultural management regimes were sampled. Within the relatively confined study area (4 ha), the isotopic compositions of soil NO 3 differed markedly due to management treatments (up to 6 and 17‰ for δ 15N and δ 18O, respectively), but were repeatable among replicate plots (±1‰). Differences in both δ 15N and δ 18O values were observed between legume and non-legume treatments, as well as fertilized versus non-fertilized treatments, which were larger than the variability observed between replicate plots. Differences in the isotopic composition of extractable soil nitrate were not limited to the surface layer, but also occurred within deeper soil layers. This study indicates that the analysis of the natural abundance stable isotope composition of soil NO 3 may provide a promising additional tool for tracing the origin and fate of NO 3 − in the soil zone.

Natural abundance of stable isotopes reveals the diversity of carnivorous plant diets

Ever since carnivorous plants were first described and studied by early naturalists (e.g., Darwin 1875), there has been continued interest in the relationship between the uptake of nutrients from invertebrate prey and the growth and performance of these plants. Although it is now thought that multiple nutrients appear to limit the growth of most carnivorous plants (Ellison 2006), of particular interest has been nitrogen (N), as many studies have shown that carnivorous plants can obtain extra N from the digestion of their prey.