Higher Δ15 Research Articles

Leaf NPK stoichiometry, δ15 N, and apparent nutrient limitation of co-occurring carnivorous and noncarnivorous plants.

Previous meta-analyses suggested that carnivorous plants-despite access to N, P, and K from prey-have significantly lower leaf concentrations of these nutrients than noncarnivores. Those studies, however, largely compared carnivores in nutrient-poor habitats with noncarnivores in more nutrient-rich sites, so that the differences reported might reflect habitat differences as much as differences in nutrient-capture strategy. Here we examine three carnivorous and 12 noncarnivorous plants in the same nutrient-poor bog to compare their foliar nutrient concentrations, assess their patterns of nutrient limitation using leaf NPK stoichiometry, and estimate percentage N derived from prey by carnivores using a mixing model for stable N isotopes. We hypothesized that (1) carnivore leaf nutrient concentrations approach or exceed those of noncarnivores in the same nutrient-poor habitat; (2) species in different functional groups show different patterns of stoichiometry and apparent nutrient limitation; and (3) noncarnivores might show evidence of using other means of nutrient acquisition or conservation to reduce nutrient limitation. At Fallison Bog in northern Wisconsin, carnivorous plants (Drosera rotundifolia, Sarracenia purpurea, Utricularia macrorhiza) showed significantly lower leaf percentage C and N:P ratio, higher δ15 N, and no difference from noncarnivores in leaf N, P, K, and δ13 C. Sedges had significantly lower leaf percentage P, percentage C, and N:K ratio, and higher K:P ratio than nonsedges restricted to the Sphagnum mat, and may tap peat N via aerenchyma-facilitated peat oxidation (oxipeditrophy). Evergreen ericaceous shrubs exhibited significantly higher levels of percentage C and lower values of δ15 N than mat nonericads. Calla palustris-growing in the nutrient-rich moat at the bog's upland edge-had very high values of leaf N, K, δ15 N, and N:P ratio, suggesting that it may obtain nutrients from minerotrophic flows from the adjacent uplands and/or rapidly decaying peat. Stoichiometric analyses indicated that most species are N limited. A mixing model applied to δ15 N values for carnivores, noncarnivores, and insects produced an estimate of 50% of leaf N derived from prey for Utricularia, 42% for Sarracenia, and 41% for Drosera.

The occurrence and feeding of a critically endangered shark species, Oxynotus centrina in the Sea of Marmara.

The angular roughshark, Oxynotus centrina (Linnaeus, 1758), is a rarely encountered deep-sea demersal shark species. It is listed as "Critically Endangered" in the IUCN's regional assessment for the Mediterranean. Like other sharks and rays, the angular roughshark is subject to by-catch in demersal trawl and longline fisheries. This study contributes new knowledge on (a) its occurrence, size and sex information; and (b) its feeding ecology in the Sea of Marmara. Data were collected on total length (TL), total weight (TW) and sex from unpublished research survey reports performed intermittently from 1994 to 2020. According to the data, O. centrina was mainly recorded from the muddy sand bottom type in the southwest region of the Marmara Sea, which has higher biodiversity of macrobenthic species than other regions. Different methodological approaches were conducted to understand its feeding, such as DNA metabarcoding and isotope analysis to identify its prey spectra and assign a trophic level. Sampled specimens were full of liquid in their stomachs without any discernible visually identifiable prey items. The metabarcoding analyses were also unsuccessful in identifying any prey items. Trophic position calculation based on nitrogen isotope in muscles highlighted that O. centrina has the highest trophic position compared to the other sharks and rays in the Sea of Marmara. Although it was found mainly feeding on polychaetes and sipunculids species from previous studies, the δ15 N values made the authors cautious about the possibility of this species feeding on low trophic-level benthic invertebrates. Because the authors could not observe the presence of shark vitellus in the stomachs and could not succeed in DNA identification for prey, this study highlights high δ15 N values similar to top predators for O. centrina. The authors conclude that this species strategically feeds mainly on shark eggs to fulfil its nutritional requirements with minimum energy expenditure in line with its slow-moving behaviour and mouth morphology.

Dietary shifts and diversities of individual life histories reveal cultural dynamics and interplay of millets and rice in the Chengdu Plain, China during the Late Neolithic (2500–2000 cal. BC)

We undertook a more comprehensive analyses than our previous study (Yi et al., International Journal of Osteoarchaeology, 2018, 28, 636-644) at the Gaoshan site (~2500-2000 cal. BC) to expand our understanding of the different roles of rice and millets to human subsistence strategies, diachronic shift of human diets, diversity of human life histories and cultural influence (dental ablation) to human population. Carbon and nitrogen isotopic analyses of human (ribs and long bones, n = 68) and animal (n = 15) bones as well as carbonized seeds from rice and millets (n = 8) were undertaken. Human bones were directly AMS-14 C dated. In addition, sequential sampling of 16 individuals with varied age and sex and cultural contexts for isotopic analysis was also conducted. The calibrated dating results (n = 27) indicate that there existed two periods of occupation at the site, ~2500-2200 BC and ~2200-1900 BC. Moderate high δ15 N values were found in the crops. Isotopic data from bone collagen suggest that the humans mainly relied on C3 -based animal protein and were supplemented by C4 -based foods. This isotopic pattern is also seen in pigs. Significant difference of human δ13 C values (Mann-Whitney U test, p < 0.05) between the two periods was observed. Compared to the bones, the isotopic data of human dentine serial sections show a much wider variability and higher δ13 C values. The crops (millets and rice) were probably manured. Human individuals subsisted on rice/millet agriculture and pigs in general. In combination with the radiocarbon dates, the increase of C3 (rice) consumption by the humans was found from the early (2500-2200 cal. BC) to late (2200-2000 cal. BC) periods, indicating the intensification of rice agriculture through time. However, four categories of human life histories during childhood are identified given different isotopic profiles of dentine sections, demonstrating that C3 (rice) and C4 (millets)-based foods played different roles in human lives. Even though, there were similar weaning practices among the human populations. It is surprising that human individuals with dental ablation, cultural characteristic in the Middle and Lower Yangtze River Valley, consumed more millets during childhood in the early period than those without dental ablation in the late period. Our study here provides novel insights into cultural dynamics and the interplay between rice and millets in rice-millet agricultural system during the Late Neolithic in Southwest China.

Quantifying capital versus income breeding: New promise with stable isotope measurements of individual amino acids.

Capital breeders accumulate nutrients prior to egg development, then use these stores to support offspring development. In contrast, income breeders rely on local nutrients consumed contemporaneously with offspring development. Understanding such nutrient allocations is critical to assessing life-history strategies and habitat use. Despite the contrast between these strategies, it remains challenging to trace nutrients from endogenous stores or exogenous food intake into offspring. Here, we tested a new solution to this problem. Using tissue samples collected opportunistically from wild emperor penguins Aptenodytes forsteri, which exemplify capital breeding, we hypothesized that the stable carbon (δ13 C) and nitrogen (δ15 N) isotope values of individual amino acids (AAs) in endogenous stores (e.g. muscle) and in egg yolk and albumen reflect the nutrient sourcing that distinguishes capital versus income breeding. Unlike other methods, this approach does not require untested assumptions or diet sampling. We found that over half of essential AAs had δ13 C values that did not differ between muscle and yolk or albumen, suggesting that most of these AAs were directly routed from muscle into eggs. In contrast, almost all non-essential AAs differed in δ13 C values between muscle and yolk or between muscle and albumen, suggesting de novo synthesis. Over half of AAs that have labile nitrogen atoms (i.e. 'trophic' AA) had higher δ15 N values in yolk and albumen than in muscle, suggesting that they were transaminated during their routing into egg tissue. This effect was smaller for AAs with less labile nitrogen atoms (i.e. 'source' AA). Our results indicate that the δ15 N offset between trophic-source AAs (Δ15 Ntrophic-source ) may provide an index of the extent of capital breeding. The value of emperor penguin Δ15 NPro-Phe was higher in yolk and albumen than in muscle, reflecting the mobilization of endogenous stores; in comparison, the value of Δ15 NPro-Phe was similar across muscle and egg tissue in previously published data for income-breeding herring gulls Larus argentatus smithsonianus. Our results provide a quantitative basis for using AA δ13 C and δ15 N, and isotopic offsets among AAs (e.g. Δ15 NPro-Phe ), to explore the allocation of endogenous versus exogenous nutrients across the capital versus income spectrum of avian reproduction.

Relationships between leaf δ15 N and leaf metallic nutrients.

Nitrogen (N) isotopic ratios (δ15 N values) of plants are primarily determined by the δ15 N values of their N sources. Metallic nutrients affect plant N uptake. However, there is little knowledge of the relationships between leaf δ15 N values and leaf metallic nutrients. The δ15 N values are often lower in soil nitrate (NO3 - ) than in ammonium (NH4 + ) due to large isotopic fractionation during nitrification. Plants acquire more NO3 - than NH4 + when accumulating high potassium (K), calcium (Ca) and magnesium (Mg) to maintain charge balance. In addition, plants that absorb more NO3 - than NH4 + increase the soil pH and decrease the availability of iron (Fe), manganese (Mn) and zinc (Zn). We therefore hypothesized that leaf δ15 N values correlate negatively with K, Ca and Mg contents, while positively with Fe, Mn and Zn contents. Leaves of non-N-fixing plants were sampled across an approx. 6000 km transect in China and their δ15 N values and metallic nutrient content were determined using elemental analyzer/isotope ratio mass spectrometry. Inconsistent with the hypothesis, leaf δ15 N values correlated positively with leaf K, Ca and Mg, indicating higher δ15 N values of soil NO3 - than NH4 + . Higher δ15 N values of soil NO3 - revealed stronger denitrification than nitrification in the study regions because isotopic fractionation occurs during both processes. Leaf δ15 N values correlated negatively with Fe, relating to decreases in soil Fe availability, which might be attributed to oxidation of Fe2+ to Fe3+ supplying electrons for denitrification, while greater uptake of NO3 - than NH4 + of plants increases soil pH. Leaf δ15 N values correlated positively with Zn and did not correlate with Mn. These observed relationships between leaf δ15 N values and metallic nutrients, except Mn, were independent of vegetation or soil types. This study has enriched our knowledge of associations between metallic nutrients and N cycling in plant-soil systems, especially for the roles of Fe in soil N transformations and K, Ca and Mg in plant N uptake.

Seasonality and interindividual variation in mandrill feeding ecology revealed by stable isotope analyses of hair and blood.

Mandrills are large-bodiedterrestrial forest primates living in particularly large social groups of several hundred individuals. Following these groups in the wild to assess differences in diet over time as well as among individuals is demanding. We here use isotope analyses in blood and hair obtained during repeated captures of 43 identified free-ranging mandrills (Mandrillus sphinx) from Southern Gabon, to test how dietary variation relates to the season as well as an individual's age and sex. We measured the stable carbon (δ13 C‰) and nitrogen (δ15 N‰) isotope ratios in 46 blood and 214 hair section samples as well as from a small selection of mandrill foods (n = 24). We found some seasonal isotopic effects, with lower δ13 C values but higher δ15 N values observed during the highly competitive long dry season compared to the fruit-rich long rainy season. Variation in δ13 C was further predicted by individual age, with higher δ13 C values generally found in younger individuals suggesting that they may consume more high canopy fruit than older individuals, or that older individuals consume more low canopy foliage. The best predictor for δ15 N values was the interaction between age and sex, with mature and reproductively active males revealing the highest δ15 N values, despite the observation that males consume substantially less animal food items than females. We interpret high δ15 N values in these mature male mandrill blood and hair sections to be the result of nutritional stress associated with intense male-male competition, particularly during mating season. This is the first study showing isotopic evidence for nutritional stress in a free-ranging primate species and may spark further investigations into male mandrill diet and energy balance.

Isotopic niche partitioning in two sympatric howler monkey species.

Ecological similarity between species can lead to interspecific trophic competition. However, when ecologically similar species coexist, they may differ in foraging strategies and habitat use, which can lead to niche partitioning. As the body tissues of consumers contain a stable isotope signature that reflects the isotopic composition of their diet, stable isotope analysis is a useful tool to study feeding behavior. We measured the isotopic niche width, which is a proxy for trophic niche width, of mantled (Alouatta palliata) and black (A. pigra) howler monkeys. Specifically, studied populations in allopatry and sympatry to assess whether these species showed niche partitioning. Between 2008 and 2012, we collected hair samples from 200 subjects (113 black and 87 mantled howler monkeys) and used continuous flow isotope ratio mass spectrometry to estimate δ13 C and δ15 N. We described the isotopic niche width of each species in allopatry and sympatry with the Bayesian estimation of the standard ellipse areas. In allopatry, isotopic niche width and isotopic variation were similar in both species. In sympatry, black howler monkeys had a significantly broader isotopic niche, which was mainly determined by high δ15 N values, and included the majority of mantled howler monkeys' isotopic niche. The isotopic niche of mantled howler monkeys did not differ between sympatry and allopatry. The coexistence of these ecologically similar species may be linked to trophic niche adjustments by one species, although the particular features of such adjustments (e.g., dietary, spatial, or sensory partitioning) remain to be addressed.

The trophic niche of sculpins Cottus spp. in forage fish assemblages of boreal lakes.

We compared the trophic niches of freshwater sculpins Cottus spp. with those of other co-habiting forage fishes in two groups of boreal lakes with distinct habitats and fish communities. Near North Lakes (45° 00' to 47° 30' N) were deeper, stratified and contained lake trout Salvelinus namaycush as the apex piscivore, whereas Far North Lakes (51° 10' to 52° 20' N) were shallower, did not stratify and contained pike Esox lucius and walleye Sander vitreus as the apex piscivores. Trophic niches of sculpins and other forage fishes were compared based on niche metrics calculated from muscle stable carbon (δ13 C) and nitrogen (δ15 N) isotope ratios. In Near North Lakes, sculpins were found almost exclusively in deep, offshore waters and their niche positions reflected a greater reliance on pelagic production (lower δ13 C) and a higher trophic elevation (higher δ15 N) compared with most other forage fishes. Furthermore, sculpins in Near North Lakes tended to have larger trophic niches (occupied greater area in δ13 C- δ15 N space), particularly in the food chain (δ15 N) dimension, than other cohabiting forage fishes. In contrast, sculpins in Far North Lakes were commonly found in the nearshore and had trophic niche positions and sizes that were similar to those of the other cohabiting forage fishes. This study illustrates the flexibility in the realised trophic niches of sculpins in relation to habitat availability and fish community composition in boreal lakes.

Diet and adult age-at-death among mobile foragers: A synthesis of bioarcheological methods.

The research explores whether the combined study of cortical bone histology, bone morphology, and dietary stable isotopes can expand insights into past human health and adaptations, particularly dietary sufficiency and life span. Midthoracic rib cortices from 54 South African Late Holocene adult skeletons (28 M, 24 F, two sex undetermined) are assessed by transmitted-light microscopy for cross-sectional area measurements, osteon area (On.Ar), osteon population density, and presence/absence of secondary osteon variants. Values for δ13 Cbone collagen , δ15 Nbone collagen , 14 C dates, Southwestern and Southern Cape geographic regions, body size measures, estimated ages-at-death from both morphological and histological methods are integrated into analyses, which include Spearman correlations, χ2 tests and Kruskal-Wallis ANOVAs. There is reduced On.Ar variability with higher δ15 N (r = -.41, p = .005); rib %cortical area and δ15 N are negatively correlated in the Southern Cape group (r = -.60, p = .03). Osteon variants are more common in older adults; histological ages at death are significantly older than those determined from gross morphology. We found bone tissue relationships with measures of diet composition, but indicators of dietary adequacy remain elusive. Relationships of tissue quality and isotopes suggest that some Southern Cape adults lived long lives. Osteon variants are associated with age-at-death; some association with diet remains possible. Gross morphological methods appear to underestimate adult ages-at-death, at least among small-bodied adults.

Interpretation of bulk nitrogen and carbon isotopes in archaeological foodcrusts on potsherds.

Foodcrust, the charred deposit adhering to the surface of containers, is a possible source of information on the function of ancient vessels and the subsistence of prehistoric humans. While the carbon isotope ratios in those materials are useful in detecting the usage of C4 plants, the reliability of nitrogen isotopic signatures has not been fully investigated. The validity of bulk nitrogen isotope ratios has previously been investigated in coastal or riverine environments, where multiple resources from terrestrial and aquatic ecosystems were available, but not in terrestrial settings which provide a simpler mixing of terrestrial animals and plants. Hence, we conducted an exhaustive study on charred deposits on potsherds at two inland archaeological sites belonging to prehistoric Jomon hunter-gathers in central Japan, focusing on δ15 N values and atomic N/C ratios determined using an isotope ratio mass spectrometer and an elemental analyzer, respectively. For both sites, the δ15 N values showed significant correlations with the N/C ratios among samples from the inner surface, suggesting that these have recorded animal contribution. Furthermore, previous studies of Neolithic pottery from North Europe and Far East Russia bearing strong marine signatures had shown reasonably higher δ15 N values and N/C ratios in comparison with our data from terrestrial settings. On the other hand, some charred materials probably originating from plant starch showed lower values with both parameters. Samples from the outer surface produced less meaningful isotopic and elemental ratios altered by a thermal effect and/or contamination from soot. When the samples of foodcrusts were selected carefully from the inner surface, bulk nitrogen isotopes and N/C ratios reflect the composition of what was cooked or processed in containers. This will provide useful information for understanding the human adaptation from the Late Pleistocene to the Holocene in conjunction with residual lipid analyses.

Soil organic carbon stability in forests: Distinct effects of tree species identity and traits.

Rising atmospheric CO2 concentrations have increased interest in the potential for forest ecosystems and soils to act as carbon (C) sinks. While soil organic C contents often vary with tree species identity, little is known about if, and how, tree species influence the stability of C in soil. Using a 40year old common garden experiment with replicated plots of eleven temperate tree species, we investigated relationships between soil organic matter (SOM) stability in mineral soils and 17 ecological factors (including tree tissue chemistry, magnitude of organic matter inputs to the soil and their turnover, microbial community descriptors, and soil physicochemical properties). We measured five SOM stability indices, including heterotrophic respiration, C in aggregate occluded particulate organic matter (POM) and mineral associated SOM, and bulk SOM δ15 N and ∆14 C. The stability of SOM varied substantially among tree species, and this variability was independent of the amount of organic C in soils. Thus, when considering forest soils as C sinks, the stability of C stocks must be considered in addition to their size. Further, our results suggest tree species regulate soil C stability via the composition of their tissues, especially roots. Stability of SOM appeared to be greater (as indicated by higher δ15 N and reduced respiration) beneath species with higher concentrations of nitrogen and lower amounts of acid insoluble compounds in their roots, while SOM stability appeared to be lower (as indicated by higher respiration and lower proportions of C in aggregate occluded POM) beneath species with higher tissue calcium contents. The proportion of C in mineral associated SOM and bulk soil ∆14 C, though, were negligibly dependent on tree species traits, likely reflecting an insensitivity of some SOM pools to decadal scale shifts in ecological factors. Strategies aiming to increase soil C stocks may thus focus on particulate C pools, which can more easily be manipulated and are most sensitive to climate change.

Assimilation of nitrogen and carbon isotopes from fish diets to otoliths as measured by nanoscale secondary ion mass spectrometry.

Nitrogen and carbon stable isotope ratios (δ15 N and δ13 C values) of carbonate-bound organic materials in otoliths can provide information to address the biological and ecological functions of fish. Correct interpretation of otolith δ15 N and δ13 C profiles requires knowledge of the metabolic routes of nitrogen and carbon isotopes. However, the isotopic assimilation of δ15 N and δ13 C compositions from diets to otoliths has rarely been investigated. This study traced the daily nitrogen and carbon isotopic assimilation between diets and otoliths using nanoscale secondary ion mass spectrometry (NanoSIMS). Isotopically labeled algae (Tetraselmis chui) were fed to tilapia (Oreochromis niloticus) for 14-17days. NanoSIMS and conventional isotope ratio mass spectrometry were used to measure δ15 N and δ13 C variations in the otoliths and fish muscle, respectively. Otolith δ15 N values abruptly surged from natural abundance levels by 1000-2300‰ after the fish ate 15 N-spiked algae with δ15 N values of approximately 2200‰. However, the δ15 N values of fish muscle increased to only approximately 500‰ at the end of the feeding experiment. Much higher δ15 N values (3700-14 000‰) and moderate δ13 C values (60-200‰) were detected in the otoliths after the tilapia ate 15 N- and 13 C-spiked algae with a δ15 N value of 36667‰ and a δ13 C value of 272‰. Mapping analysis showed sub-micrometer-scale distribution of 15 N embedded in the otolith growth increments with a low-to-high δ15 N signal after the tilapia shifted diets from non-spiked to 15 N-labeled algae. These results suggest that otolith nitrogen and carbon isotopes from food were directly assimilated on the same day. Food is the major and in some cases only source of otolith nitrogen isotopes but makes only a partial contribution to otolith carbon isotopes. Therefore, the δ15 N values recorded in the sclerochronological layers of the otoliths can be used to determine the trophic levels, food sources and diet changes of fish.

Distinct isotopic signatures reveal effect of ecoregion on small mammals of Ghana.

Species reside in dynamic environments, simultaneously experiencing variations in climatic conditions, habitat availability and quality, interspecific interactions, and anthropogenic pressures. We investigated variation in foraging ecology of the small mammal community between land-use classifications (i.e., protected national parks and unprotected lands abutting them) in Mole National Park (MNP) and Digya National Park (DNP), representing distinct ecoregions of Ghana. In 5,064 trap nights, we sampled 153 individuals of 23 species within the 2 national parks and adjacent lands outside protected boundaries to describe variation in community composition. We also used δ13 C and δ15 N isotopic ratios from fresh feces to determine main effects and interactions between land use and ecoregion on trophic structure in species and communities of small mammals. Small mammals exhibited distinct community assemblages between ecoregions (i.e., national parks): Gerbilliscus guineae, Hybomys trivirgatus, Malacomys edwardsi, Lemniscomys bellieri, L. zebra, and Taterillus gracilis were only captured in the dry savanna ecoregion of MNP. Additionally, isotopic signatures for nitrogen were significantly lower in MNP (2.83 ± 0.17‰) compared to DNP (4.97 ± 0.33‰), indicating that small mammals occupied different trophic levels between ecoregions. The most common species, Praomys daltoni exhibited variation in isotopic signatures between ecoregions and land use, with higher δ15 N found within MNP boundaries. We found no distinction in δ13 C at the community or species level within or across protected areas. Ultimately, understanding shifts in the ecology of species can inform predictions about community structure and ecosystem function under future environmental and anthropogenic scenarios.

Maize (Zea mays) consumption in the southern andes (30°-31° S. Lat): Stable isotope evidence (2000 BCE-1540 CE).

The timing and dietary role of maize agriculture is central to archaeological discussions in the Andean region. In the semi-arid region of northern Chile (SARNC), archaeological models propose that maize was adopted during the Early Ceramic period in tandem with pottery and sedentism. Through stable isotope (SI) analyses, of bone collagen and apatite, this study assesses the timing of maize introduction, diachronic changes (2,000 BCE to 1,540 CE.), and synchronic dietary variability in the prehistoric SARNC. Fifty-two prehistoric individuals from SARNC were analyzed for δ13 Cap , δ13 Ccol, and δ15 N. Descriptive statistics were used to characterize the results by period and location (inland and coast). Between-periods (ANOVA or Kruskal-Wallis tests), and synchronic comparisons (inland vs. coast; Student's t-tests), were conducted. A SIAR model was run to further evaluate dietary changes. Dietary interpretations are based on food web data. Coastal groups show significant changes in the diet during the Middle (900-1,000CE; enrichment in δ13 C), and Late Intermediate periods (100-1450CE; when the Δ13 Cap-col is above 5.2‰). In the inland, significant changes in SI occurred in the Late Intermediate period (δ13 C enrichment). In the Late period, the inland diet became enriched for δ15 N. Synchronic comparisons showed coastal individuals to have higher δ15 N. The popularization of maize in the SARNC was not associated with the appearance of pottery and/or sedentism, and its role as a dietary staple was a late phenomenon (c.a. 1,000CE). The results obtained in this study show that the adoption and consumption of maize varied dramatically in the Southern Andes.

Nitrate Source Identification and Nitrification-denitrification at the Sediment-water Interface

Identifying nitrate sources and its transformation mechanisms are important for nitrate pollution control in surface water. The columnar core sediment samples in West Lake were taken in different seasons. The transformation of nitrogen at the sediment-water interface was studied using nitrogen and oxygen isotopes, stable isotope analysis in R (SIAR) and acetylene inhibition method in the West Lake, Hangzhou. The results showed that the concentration gradient of both NO3-and NH4+ existed at the sediment-water interface. NO3- concentrations decreased from bottom water to pore water and NO3- was accumulated in sediments. NH4+ concentrations increased from bottom water to pore water and NH4+ was released from sediments. Nitrate sources in bottom water where nitrification exited were sewage (manure), soil nitrogen, chemical fertilizer and precipitation. Sewage (manure) was the major nitrogen contributor (60.8%) in summer. Particularly high δ15 N values in pore water indicated that there was strong denitrification at the sediment-water interface in West Lake. The average nitrification rate and denitrification rate at the sediment-water interface were 2.85 mmol·(m2·d)-1 and 23.51 μmol·(m2·d)-1, respectively. The sediment-water interface played a role in nitrogen removal process in aquatic environment. Seasonal and spatial variations of nitrification rates and denitrification rates were found in this study. Temperature and dissolved oxygen were the main influential factors for the transformation of nitrogen at the sediment-water interface in West Lake.

Silicon and nitrogen cycling in the upwelling area off Peru: A dual isotope approach

We present a comparison of the dissolved stable isotope composition of silicate (δ30Si(OH)4) and nitrate (δ15 inline image) to investigate the biogeochemical processes controlling nutrient cycling in the upwelling area off Peru, where one of the globally largest Oxygen Minimum Zones (OMZs) is located. Besides strong upwelling of nutrient rich waters mainly favoring diatom growth, an anticyclonic eddy influenced the study area. We observe a tight coupling between the silicon (Si) and nitrogen (N) cycles in the study area. Waters on the shelf showed high Si(OH)4 concentrations accompanied by diminished inline image concentration as a consequence of intense remineralization, high Si fluxes from the shelf sediments, and N-loss processes such as anammox/denitrification within the OMZ. Correspondingly, the surface waters show low δ30Si(OH)4 values (+2‰) due to low Si utilization but relatively high δ15 inline image (+13‰) values due to upwelling of waters influenced by N-loss processes. In contrast, as a consequence of the deepening of the thermocline in the eddy center, a pronounced Si(OH)4 depletion led to the highest δ30Si(OH)4 values (+3.7‰) accompanied by high δ15 inline image values (+16‰). In the eddy center, high inline image: Si(OH)4 ratios favored the growth of non-siliceous organisms (Synechococcus). Our data show that upwelling processes and the presence of eddies play important roles controlling the nutrient cycles and therefore also exert a major influence on the phytoplankton communities in the Peruvian Upwelling. Our findings also show that the combined approach of δ30Si(OH)4 and δ15 inline image can improve our understanding of paleo records as it can help to disentangle utilization and N-loss processes.

Natural 15 N abundance in fruit bodies of ectomycorrhizal fungi from boreal forests.

The 15 N natural abundance and N concentrations of fruit bodies from 70 species (23 genera) of ectomycorrhizal fungi found in boreal forests are presented. Large intraspecific and intrageneric differences were found, e.g. 8.3‰15 N in the species Dermocybe crocea and 12.6‰ in the genus Cortinarius. In addition, significant differences in both δ15 N and %N were found between different parts of fruit bodies, with cap material giving consistently higher values. Proteins and amino acids were enriched by 9.7±0.4‰ (mean ± 1 SE) relative to chitin, irrespective of the part of the fruit body examined. Chitin had δ15 N values similar to that of plant hosts. The higher δ15 N and %N values of the caps than of the stipes probably reflect a higher portion of proteins and amino acids in the caps. The δ15 N of mycorrhizal fungi can be a function of the N species used (organic N, NH4 + , NO3 - ), the depth of soil at which the mycelium occurs, and metabolic fractionations. The metabolic fractionations, e.g. potential transaminations during the flux of N from the soil through the fungus to the plant, make it difficult, at present, to make inferences about sources of N based on δ15 N values alone. No effect of sample drying temperature on δ15 values of fungal material was detected.