Differences In δ15N Values Research Articles

Inferring the ecology of willow warblers during their winter moult by sequential stable isotope analyses of remiges

We present a comparison of feather stable isotope (δ13C, δ15N) patterns representing the habitat and diet conditions for two subspecies of willow warblers Phylloscopus trochilus that breed in parapatry, but winter in different regions of sub‐Saharan Africa. Previous analyses have shown that on average winter moulted innermost primaries (P1) show subspecific differences in δ15N values, although individuals show substantial variation for both δ13C and δ15N within the subspecies. We examined whether corresponding variation in the timing of the winter moult, as reflected by consistent intra‐wing correlations for individual's δ13C and δ15N values, could explain some of the previously observed isotopic variation. Further, differential subspecific adaptations to winter precipitation patterns across Africa might result in a variable degree of site fidelity or itinerancy during moult. We found no consistent trend in isotopic values from innermost to outermost primaries, thus inter‐individual variation in the timing of moult does not explain the subspecific isotopic variation for P1. Patterns in wing feather δ13C and δ15N values indicated that 41% of the individuals from both subspecies shifted their diet or habitats during winter moult. Importantly, despite well‐documented itinerancy in willow warblers during the winter, 59% of the individuals had feather isotope values consistent with stable use of habitats or diets during winter moult. Repeatability analyses suggest that individuals of both subspecies initiate moult in similar habitats from year‐to‐year while feeding on isotopically similar diets.

Ecological influences on the difference in δ15N and δ13C values between fish tissues: implications for studies of temporal diet variation

AbstractTemporal diet changes can be detected by comparing δ13C and δ15N values between tissues with different isotopic turnover times. However, other factors contribute to disparities in δ13C and δ15N signatures between tissues and could confound the interpretation of stable isotope data. We examined the effects of ecological factors on differences in muscle and liver δ13C and δ15N signatures of Arctic charr [Salvelinus alpinus, (L.)] to determine whether their effects were large enough to influence studies of diet change. In addition, we examined whether differences in lipid content accounted for a significant portion of the differences in δ13C values between tissues. Results indicated that life history, diet, reproductive status and gender had significant effects on the differences in δ13C values between tissues, while gender and diet significantly affected differences in δ15N values. Differences in % lipid content between tissues also explained approximately 74% of the observed variation in the difference in δ13C values between tissues. The differences in δ13C values observed between muscle and liver tissues may be partially explained by physiological differences in lipid storage and use between tissues associated with ecological factor effects. These results demonstrate that ecological factors other than a change in diet can influence the differences in δ13C and δ15N signatures between muscle and liver tissues. However, the impact of ecological factors on studies of diet change depends on how the magnitude of the factor effects compared with a biologically meaningful change in diet.

Relative inputs of upwelled and atmospheric nitrogen to the eastern tropical North Atlantic food web: Spatial distribution of δ15N in mesozooplankton and relation to dissolved nutrient dynamics

The Eastern Tropical North Atlantic (ETNA) is characterised by a strong east to west gradient in the vertical upward flux of dissolved inorganic nitrogen to the photic zone. We measured the stable nitrogen isotope (δ15N) signatures of various zooplankton taxa covering twelve stations in the ETNA (04°–14°N, 016–030°W) in fall 2009, and observed significant differences in δ15N values among stations. These spatial differences in δ15N within zooplankton taxa exceeded those between trophic levels and revealed an increasing atmospheric input of nitrogen by N2 fixation and Aeolian dust in the open ocean as opposed to remineralised NO3− close to the NW African upwelling. In order to investigate the spatial distribution of upwelling-fuelled versus atmospheric-derived nitrogen more closely, we examined the δ15N signatures in size-fractionated zooplankton as well as in three widely distributed epipelagic copepod species on a second cruise in fall 2010 in the ETNA (02-17°35′N, 015–028°W). Copepods were sampled for δ15N and RNA/DNA as a proxy for nutritional condition on 25 stations. At the same stations, vertical profiles of chlorophyll-a and dissolved nutrients were obtained. High standing stocks of chl-a were associated with shallow mixed layer depth and thickening of the nutricline. As the nitracline was generally deeper and less thick than the phosphacline, it appears that non-diazotroph primary production was limited by N rather than P throughout the study area, which is in line with enrichment experiments during these cruises. Estimated by the δ15N in zooplankton, atmospheric sources of new N contributed less than 20% close to the African coast and in the Guinea Dome area and up to 60% at the offshore stations, depending on the depth of the nitracline. δ15N of the three different copepod species investigated strongly correlated with each other, in spite of their distinct feeding ecology, which resulted in different spatial patterns of nutritional condition as indicated by RNA/DNA.

The sinks of dissolved inorganic nitrogen in surface water of wetland mesocosms

The transport of dissolved inorganic nitrogen in wetlands was investigated in the Yeyahu National Preserve of Beijing, China, from July to December 2011. Stable isotope of nitrogen (15N) as NH4+ was added to experimental mesocosms vegetated by seven common wetland plants. The 15N abundance in soil and various plant parts were measured at prescribed time intervals during the 49-day experimental period. Rapid 15N enrichments were found in soil and aquatic plants during the first sampling event at day 3 and typically increased linearly during the experimental period. Highest 15N enrichment was found in submerged plants, followed by emergent plants and soil. No differences in δ15N values were found among leaves and stems of all submerged plants (Potamogeton pusillus, Potamogeton perfoliatus, Najas marina, Ceratophyllum demersum), suggesting that these plant structures are equally effective in nitrogen assimilation. The roots of P. perfoliatus displayed the lowest enrichment compared to the stems and leaves. Such results indicate that submerged plant roots may have a relatively weak role in nitrogen assimilation. The two emergent plants (Phragmites australis and Typha orientalis) showed no differences in δ15N values among all plant parts, suggesting rapid translocation of nitrogen from roots to other portions. This experiment demonstrates that wetland soil and plant stems and leaves are major sinks for dissolved nitrogen from the surface water and provides valuable insight into nitrogen retention and transport pathways in various wetland plants.

Macroalgae δ15N values in well-mixed estuaries: Indicator of anthropogenic nitrogen input or macroalgae metabolism?

Although nitrogen stable isotope ratio (δ15N) in macroalgae is widely used as a bioindicator of anthropogenic nitrogen inputs to the coastal zone, recent studies suggest the possible role of macroalgae metabolism in δ15N variability. Simultaneous determinations of δ15N of dissolved inorganic nitrogen (DIN) along the land–sea continuum, inter-species variability of δ15N and its sensitivity to environmental factors are necessary to confirm the efficiency of macroalgae δ15N in monitoring nitrogen origin in mixed-use watersheds. In this study, δ15N of annual and perennial macroalgae (Ulva sp., Enteromorpha sp., Fucus vesiculosus and Fucus serratus) are compared to δ15N-DIN along the Charente Estuary, after characterizing δ15N of the three main DIN sources (i.e. cultivated area, pasture, sewage treatment plant outlet). During late winter and spring, when human activities produce high DIN inputs, DIN sources exhibit distinct δ15N signals in nitrate (NO3−) and ammonium (NH4+): cultivated area (+6.5 ± 0.6‰ and +9.0 ± 11.0‰), pasture (+9.2 ± 1.8‰ and +12.4‰) and sewage treatment plant discharge (+16.9 ± 8.7‰ and +25.4 ± 5.9‰). While sources show distinct δ15N-NO3− in this multiple source catchment, the overall mixture of NO3− sources – generally >95% DIN – leads to low variations of δ15N-NO3− at the mouth of the estuary (+7.7 to +8.4‰). Even if estuarine δ15N-NO3− values are not significantly different from pristine continental and oceanic site (+7.3‰ and +7.4‰), macroalgae δ15N values are generally higher at the mouth of the estuary. This highlights high anthropogenic DIN inputs in the estuary, and enhanced contribution of 15N-depleted NH4+ in oceanic waters. Although seasonal variations in δ15N-NO3− are low, the same temporal trends in macroalgae δ15N values at estuarine and oceanic sites, and inter-species differences in δ15N values, suggest that macroalgae δ15N values might be modified by the metabolic response of macroalgae to environmental parameters (e.g., temperature, light, DIN concentrations). Differences between annual and perennial macroalgae indicate both a higher integration time of perennial compared to annual macroalgae and the possible role of passive versus active uptake mechanisms. Further studies are required to characterize the sensitivity of macroalgae fractionation to variable environmental conditions and uptake mechanisms.

Variations in nitrogen isotopic values among various particle-sized fractions in modern soil in northwestern China

Ratios of stable nitrogen isotopes in organic matter derived from plants and preserved in soil are potential tracers for nitrogen cycles in natural ecosystems and valuable for evaluation of climate change. However, the relationship between nitrogen isotopic compositions in surface soil and in plant litter during the decomposition process from plant litter to soil organic matter is not well understood. By using nitrogen isotopic analysis of soil particle-sized fractions, nitrogen isotope discrimination between plant litter and surface soil organic matter in various modern ecosystems in northwestern China was conducted. The results of our study indicate that: (1) in general, the nitrogen isotopic compositions of particle-sized fractions from surface soil are different, and δ15N values increase from plant litter to fine soil organic matter; (2) the δ15N values in the soil particle-sized fractions become larger with increasing relative humidity and temperature, and the largest variation in the δ15N values is from −5.9‰ to −0.3‰; and (3) under a controlled climate, significant nitrogen isotope differences in δ15N values (Δδ15Nplant-soil) between plant litter and bulk soil organic matter were observed, with the values of 1.52 to 4.75 at various sites. Our results suggested that comparisons of Δδ15N values between bulk soil and the particle-sized fractions of soil could reveal the effect of humidity on transferring process of nitrogen from plant to soil in arid and semi-arid ecosystems.

Nitrogen isotopic signature of vegetables from the Slovenian market and its suitability as an indicator of organic production

The aim of this study was to obtain preliminary data on δ15N values in organically and conventionally grown vegetables available on the Slovenian market to determine whether there are any characteristic differences in δ15N values, which could be used as a marker to differentiate between organically and conventionally grown vegetables, namely vegetables grown with synthetic or organic nitrogen fertilisers. Therefore, 14 different varieties of organically and conventionally grown vegetables were analysed. The results obtained show clear differences in mean δ15N values between organically and conventionally grown vegetables (up to 6.3\\permil). However, due to overlapping results, it was not possible to differentiate between organically and conventionally grown counterparts in eight vegetable varieties, despite the differences in mean δ15N values. The results obtained indicate that the method has several limitations. Hence, δ15N could be a reliable indicator for organic production, but nevertheless, the conclusion has to be validated by background information.

Feeding ecology and phylogenetic structure of a complex neotropical termite assemblage, revealed by nitrogen stable isotope ratios

1. In the current ecological classification of termites, four feeding groups (I–IV) are recognised, corresponding to a gradient of decomposition from sound wood to highly mineralised organic matter in the soil. 2. Nitrogen stable isotopes (hereafter δ15N) were used to place termites from French Guiana rainforests along a wood-soil decomposition gradient, to test (i) whether feeding group assignation based on morphological characters was accurate and actually represented diet specialisation thresholds, and (ii) to what extent the dietary specialization of species is explained by phylogeny (phylogenetic autocorrelation). 3. δ15N values vary over a range of 13‰, suggesting that diet diversification contributes to the high species diversity in French Guiana. δ15N values span a similar interval in all Termitidae subfamilies. Ranges of different subfamilies broadly overlap, although each of them diversified preferentially on one side of the wood-soil decomposition gradient. Congeneric species share similar feeding habits, whereas distant species tend to feed on distinct substrates. 4. Feeding groups did not completely match stable isotope data: there was no discontinuity between Groups III and IV, and no correlation between anatomical criteria used to distinguish these groups and δ15N values. Nor was there any consistent difference in δ15N values between wood feeders of the families Rhinotermitidae (Group I) and Termitidae (Group II). We also suggest that species feeding outside the wood-soil gradient should be distinguished for their peculiar feeding requirements.

Use of stable nitrogen isotope signatures of anthropogenic organic matter in the coastal environment: The case study of the Kosirina Bay (Murter Island, Croatia)

In this study stable nitrogen isotope ratios of particulate matter POM, zooplankton and selected biota such as Mytilus galloprovincialis were used to assess the impact of anthropogenically derived organic matter from the untreated domestic sewage, municipal and industrial effluents on the coastal ecosystem of the Kosirina Bay (Murter Island). The differences in δ15N values observed in POM and organisms collected in Kosirina Bay as compared to POM and biota sampled at unaffected sites from the southern part of the Kornati Island and highly impacted Pirovac Bay revealed only a very minor effects of anthropogenic inputs of nutrients and organic matter which is most probably derived from a sewage outfall south of the Tužbina Island.

Climate and ecosystem 15N natural abundance along a transect of Inner Mongolian grasslands: Contrasting regional patterns and global patterns

Nitrogen isotopes provide integrated information about nitrogen cycling in terrestrial ecosystems. This study explores the regional patterns of ecosystem 15N abundance along a 1200 km transect in Inner Mongolian grasslands and their relationships with climate. Results indicate that climatic variables control approximately 50% of the variation in ecosystem 15N abundance along the transect. Ecosystem 15N abundance decreases as both mean annual precipitation (MAP) and mean annual temperature (MAT) increase. Regional patterns obtained from our study differ from reported global patterns. Ecosystem 15N abundance is negatively correlated with MAT along the eastern part of the transect, while a positive correlation between MAT and ecosystem 15N is apparent at the global scale. MAP exerts stronger controls on ecosystem 15N abundance along the western part of the transect than what is shown in a global regression model. Ecosystem 15N abundance in the western part of the transect is substantially higher (ca. 3‰) than the values projected by a global model. The Δ15Nsoil‐plant (‰) (difference in δ15N values between plant and soil) values in Inner Mongolian grasslands are not significantly correlated with either MAP or MAT; but Δ15Nsoil‐plant values are positively correlated with MAT and negatively correlated with MAP at the global scale. These conflicting trends strongly indicate that climatic controls on ecosystem 15N abundance are scale‐dependent. Regional N deposition gradients, grazing‐induced ammonia volatilization, and variation in plant‐soil types are among the possible causes of these inconsistencies.

Assessing the symbiotic dependency of grain and tree legumes on N2 fixation for their N nutrition in five agro-ecological zones of Botswana

To assess the symbiotic dependency of grain and shrub/tree legumes within five agro-ecological zones of Botswana, fully expanded leaves of the test species were sampled from about 26 study sites within Ngwaketse, Gaborone, Central, Ghanzi and Kalahari agro-ecological zones. Isotopic analysis revealed significant differences in δ15N values of the grain legumes [cowpea (Vigna unguiculata L. Walp), Bambara groundnut (Vigna subterranea L. Verde.), and groundnut (Arachis hypogaea L.)] from the 26 farming areas in both 2005 and 2006. Estimates of %Ndfa of leaves also showed significant differences between farming areas, with cowpea deriving more than 50% of its N nutrition from symbiotic fixation. In terms of distribution, many more symbiotic shrub/tree species were found in the wetter Ngwaketse agro-zone compared to the fewer numbers in the drier Kalahari region. Acacias were the more dominant species at all sites. Leaf δ15N values of shrub/tree species also varied strongly across Botswana, with 11 out of 18 of these legumes deriving about 50%, or more, of their N from symbiotic N2 fixation.Acacia caffra, in particular, obtained as much as 93.6% of its N nutrition from symbiotic fixation in the wetter Ngwaketse agro-zone. This study has shown that grain legumes sampled from farmer’s fields in Botswana obtained considerable amounts of their N from symbiotic fixation. We have also shown that shrub and tree legumes probably play an important role in the N economy of the savanna ecosystems in Botswana. However, the decline in the number of functional N2-fixing shrub/tree legumes along an aridity gradient suggests that soil moisture is a major constraint to N2 fixation in the tree legumes of Botswana.

Difference in δ15N Signatures among Plant Parts of PerennialSpecies Subjected to Drought Stress with Special Referenceto the Contribution of Symbiotic N2-fixation to Plant N

The 15N natural abundance method has been widely used for evaluation of symbiotic N2-fixation. The method inevitably requires a reference plant that reflects soil derived δ15N similar to that in a N2-fixing target plant, for estimating the contribution of fixed N2. However, it is often difficult to select a suitable reference plant. Recently, an alternative method was proposed using the difference in δ15N values between shoots and nodulated roots, which did not require a reference plant per se. Whether this method is applicable to a wide range of N2-fixing plants having different growth habits and symbiosis types remains to be verified. To test the applicability of this method for perennial plants, we examined the difference in δ15N values between shoot and nodulated root (∆δ15Ns-nr), and that between shoot and root (∆δ15Ns-r) in 6-month-old plants grown in pots with different soil moisture regimes. The relationships between ∆δ15Ns-nr and the percentage of N derived from atmospheric N2 (%Ndfa) calculated from the conventional 15N natural abundance method, and between ∆δ15Ns-r and %Ndfa were analyzed in N2-fixing legume Lespedeza cuneata and N2-fixing non-legume Elaeagnus pungens and Myrica rubra. A close correlation was found between ∆δ15Ns-nr and %Ndfa as well as between ∆δ15Ns-r and %Ndfa in Lespedeza cuneata, while no correlation was found in N2-fixing non-legume species. The results indicated that ∆δ15N signatures could be useful for estimating %Ndfa for N2-fixing perennial legume (Lespedeza cuneata) in the first growth season but might not be applicable for N2-fixing actinorhizal plants.

Evidence for secondary planktonic transport of post‐larvae of seagrass‐associated King George whiting

Stomach contents and stable isotope analyses were used to determine if secondary planktonic dispersal of King George whiting Sillaginodes punctata post‐larvae from shallow inshore habitats in a large embayment in south‐eastern Australia was initiated by wave disturbance. Benthic harpacticoid copepods, which live in and amongst seagrass shoots, were found in the stomachs of S. punctata caught offshore in the plankton only during rough weather. Stable isotope analyses showed that the base of nutritional support, estimated from values of δ13C, of S. punctata collected in the plankton changed significantly during rough (waves > 0·25 m) compared to calm (waves < 0·25 m) weather conditions. Values of δ13C collected from S. punctata in the plankton during rough weather were more consistent with S. punctata values found in the seagrass. Sillaginodes punctata collected in the plankton and seagrass during rough and calm weather failed to show differences in δ15N values. Dietary and isotope analyses support a model whereby newly arrived S. punctata larvae can be resuspended from seagrass beds and dispersed offshore by wave action during onshore winds. Secondary planktonic dispersal in S. punctata would provide a mechanism by which seagrass beds further inside Port Phillip Bay are colonized.

Influence of mycorrhizal associations on foliar δ 15 N values of legume and non-legume shrubs and trees in the fynbos of South Africa: Implications for estimating N 2 fixation using the 15 N natural abundance method

In this study, we examined the use of the 15N natural abundance method to quantify the percentage N derived from fixation of atmospheric N2 in honeybush (Cyclopia spp.) shrubs and trees in the fynbos, South Africa. Non-fixing shrubs and trees of similar phenology to the Cyclopia species were chosen as reference plants. These reference plants were selected to cover a range of mycorrhizal associations (ericoid mycorrhizal, arbuscular mycorrhizal and non-mycorrhizal). Isotopic analysis revealed a wide range of foliar δ15N values for the reference plants, including many very negative values. The marked differences in δ15N values were defined by the mycorrhizal status of the reference plant species, with the ericoid and arbuscular mycorrhizal plants showing lower foliar δ15N values relative to their non-mycorrhizal counterparts. In contrast, the δ15N values of the N2-fixing Cyclopia species were uniformly clustered around zero, from −0.11‰ to −1.43‰. These findings are consistent with the observation that mycorrhizal fungi discriminate against the heavier 15N isotope during transfer of N from the fungus to the host plant, leaving the latter depleted in 15N (i.e. with a more negative δ15N value). However, a major assumption of the 15N natural abundance method for estimating N2 fixation is that both legume and reference plant should have the same level of fractionation associated with N uptake. But, because mycorrhizal associations may strongly affect the level of fractionation during N uptake and transfer, the test legume should belong to the same mycorrhizal group as the chosen reference plant species. As shown in this study, if the mycorrhizal status of the legume and the reference plant differs, or cannot be assessed, then the 15N natural abundance technique cannot be used to quantitatively estimate N2 fixation.

Influence of former agricultural land use on net nitrate production in forest soils

SummaryExcept where nitrate is added to the soil artificially, nitrate is leached from forest soils only if it is produced. Although the factors influencing nitrification have been widely studied, nitrification activity still cannot be simply predicted from ecosystem characteristics. In France, about half of the present forest area was agricultural in 1850. Previous work suggested that former cultivation could be a major factor influencing nitrogen availability in forest soils.Using laboratory incubations, we compared the net production of ammonium and nitrate in soils from formerly manured lands planted with conifers 70–100 years ago with that in soils of surrounding ancient coniferous forests. Net nitrate production, available P content, and natural abundance of nitrogen 15, δ15N, were greater in soils from formerly manured plots than other land, whereas the C:N ratio of the soil was less. The difference in net nitrate production between previously manured sites and adjacent ancient forests was related to differences in δ15N values in the soil but not evidently to other soil properties. Because soil δ15N increases with the intensity of organic manuring, these results suggest that nitrification in forest soils depends on previous manurial practices under agriculture. In this context, the soil δ15N might be used as an indicator of both previous agricultural land use and potential nitrification. Because a significant proportion of West European forests grow on previously cultivated soils, past land use should be taken into account when evaluating the risks of nitrate leaching from forests.

Stable isotopic composition of carbon and nitrogen and nitrogen content in vascular epiphytes along an altitudinal transect*

ABSTRACTThe foliar content of nitrogen and the relative abundances of 13C and 15N were analysed in vascular epiphytes collected from six sites along an altitudinal gradient from tropical dry forests to humid montane forests in eastern Mexico. The proportion of epiphyte species showing crassulacean acid metabolism (CAM) (atmospheric bromeliads, thick‐leaved orchids, Cactaceae, and Crassulaceae) decreased with increasing elevation and precipitation from 58 to 6%. Atmospheric bromeliads, almost all of which had δ13C values indicating CAM, were more depleted in 15N (x=−10·9‰± 2·11) than the C3 bromeliads which form water‐storing tanks (−6·05‰± 2·26). As there was no difference in δ15N values between C3 and CAM orchids, the difference in bromeliads was not related to photosynthetic pathways but to different nitrogen sources. While epiphytes with strong 15N depletion appear to obtain their nitrogen mainly from direct atmospheric deposition, others have access to nitrogen in intercepted water and from organic matter decomposing on branches and in their phytotelmata. Bromeliads and succulent orchids had a lower foliar nitrogen content than thin‐leaved orchids, ferns and Piperaceae. Ground‐rooted hemi‐epiphytes exhibited the highest nitrogen contents and δ15N values.

Field comparison of δ15N values and growth of alder provenances and species

Abstract Growth and δ15N values in the leaves of three alder species (Alnus glutinosa, A. cordata, and A. rubra) originating from various locations were examined. Natural 15N abundance was used to screen the N2 fixing capacity of these plants. We compared 4-y-old trees growing on two different soil types at two densities. Our results showed that A. rubra did not grow at a French site (Miribel) and the performance of the other alders was clearly controlled by both the origin and soil properties. In a sandy soil, significant differences in the δ15N values were observed in the different provenances of A. glutinosa without any effect on their growth. Genetic variability among alder genotypes was detectable only in a soil with low fertility at a high tree density. Differences in growth and δ15N values disappeared when the tree density was low or when the soil was fertile. Our results indicate that the difference in adaptation of species and provenances to local climatic and soil conditions must be taken into a...