Soil nitrification and foliar δ15N declined with stand age in trembling aspen and jack pine forests in northern Alberta,Canada

Aims

Seed germination and seedling emergence are vulnerable to water stress in arid environments. When precipitation is low and unpredictable during the early growing season, seeds near the sand surface often suffer from hydration/dehydration during germination. We investigated the responses of seedling emergence and survival of a sand dune grass with high sand stabilization value to amount and frequency of precipitation and depth of burial in sand.

Methods

Effects of amount and frequency of precipitation, burial and hydration/dehydration on seedling emergence of Leymus secalinus, were examined using standard procedures.

Results

Seedling emergence was affected by amount and frequency of monthly precipitation and depth of burial, and it decreased as precipitation frequency decreased with same amount of precipitation. Highest emergence percentage was obtained with 100 or 150 mm precipitation at 1–4 cm depth. Hydration/dehydration treatments decreased germination and increased dormancy percentage. Young seedlings with root lengths of 0–1 mm desiccated up to 30 days revived after rehydration.

Conclusions

Seedling emergence of L. secalinus is adapted to 150 mm monthly precipitation with frequency of 10–30 times per month, 1–4 cm burial depth and dehydration interval of 1–2 days. Alteration of amount and/or frequency of precipitation caused by climate change could markedly affect seedling emergence and population regeneration of this species.     

Spatial interpolation of N concentrations and δ15N values in the moss Hypnum cupressiforme collected in the forests of Slovenia

Mosses were collected from 103 locations and analyzed for N concentration and δ¹⁵N values. At each location, the samples were collected from two types of site: under the tree canopies and in the adjacent forest openings. A spatial correlation was detected, and ordinary kriging could be applied only to the N in the mosses that were collected in the openings. In contrast, a spatial correlation was not detected for N in the mosses that were collected under the canopies or for the δ¹⁵N of the mosses at either type of site. For those, the spatial interpolation was calculated as the sum of the regression predictions and the inverse distance weighted interpolation of residuals. The maps for the N at both site types (canopy/open) identified similar areas with increased N concentrations. The sole exception was the mosses collected under the canopy, for which some local emitters of NO_x were also identified.     

Ecosystem structure and soil-surface conditions drive the variability in the foliar δ13C and δ15N of Stipa tenacissima in semiarid Mediterranean steppes

We evaluated the effects of ecosystem composition and structure (species richness and diversity, cover and spatial attributes of vegetation), abiotic factors (climate and topographical features) and the condition of the bare-ground areas (evaluated using soil-surface indicators) on the performance of Stipa tenacissima [evaluated using foliar ¹³C, ¹⁵N, nitrogen concentration and the carbon-to-nitrogen (C:N) ratio] in 15 steppes of SE Spain. Foliar ¹³C values of S. tenacissima showed a low degree of variation in the studied steppes, with average values ranging from –24.1 to –22.9. Higher variation was found in the ¹⁵N values, which ranged from –5.5 to –2.4. The nitrogen concentration and the C:N ratio varied between 5.0 and 8.0 mg g^–1, and between 55.4 and 85.3, respectively. The ¹³C values became less negative with increasing spatial aggregation of perennial vegetation, while the C:N values increased with increasing perennial vegetation cover. The ¹⁵N values became more negative with increasing infiltration in the bare-ground areas, but the nitrogen concentration was not related to any of the environmental variables measured. Our results suggest that the relative importance of ecosystem structure and soil-surface conditions in the bare ground areas was higher than that of abiotic factors as determinants of the performance of S. tenacissima. The results also show that even subtle changes in these ecosystem features may lead to modifications in plant performance in semiarid S. tenacissima steppes, and thus to modifications in the associated ecosystem functions in the mid- to long-term.     

Correlations between foliar δ15N and nitrogen concentrations may indicate plant-mycorrhizal interactions

Nitrogen isotope measurements may provide insights into changing interactions among plants, mycorrhizal fungi, and soil processes across environmental gradients. Here, we report changes in δ¹⁵N signatures due to shifts in species composition and nitrogen (N) dynamics. These changes were assessed by measuring fine root biomass, net N mineralization, and N concentrations and δ¹⁵N of foliage, fine roots, soil, and mineral N across six sites representing different post-deglaciation ages at Glacier Bay, Alaska. Foliar δ¹⁵N varied widely, between 0 and –2‰ for nitrogen-fixing species, between 0 and –7‰ for deciduous non-fixing species, and between 0 and –11‰ for coniferous species. Relatively constant δ¹⁵N values for ammonium and generally low levels of soil nitrate suggested that differences in ammonium or nitrate use were not important influences on plant δ¹⁵N differences among species at individual sites. In fact, the largest variation among plant δ¹⁵N values were observed at the youngest and oldest sites, where soil nitrate concentrations were low. Low mineral N concentrations and low N mineralization at these sites indicated low N availability. The most plausible mechanism to explain low δ¹⁵N values in plant foliage was a large isotopic fractionation during transfer of nitrogen from mycorrhizal fungi to plants. Except for N-fixing plants, the foliar δ¹⁵N signatures of individual species were generally lower at sites of low N availability, suggesting either an increased fraction of N obtained from mycorrhizal uptake (f), or a reduced proportion of mycorrhizal N transferred to vegetation (T _r). Foliar and fine root nitrogen concentrations were also lower at these sites. Foliar N concentrations were significantly correlated with δ¹⁵N in foliage of Populus, Salix, Picea, and Tsuga heterophylla, and also in fine roots. The correlation between δ¹⁵N and N concentration may reflect strong underlying relationships among N availability, the relative allocation of carbon to mycorrhizal fungi, and shifts in either f or T _r. Received: 14 December 1998 / Accepted: 16 August 1999     

White spruce foliar δ13C and δ15N indicate changed soil N availability by understory removal and N fertilization in a 13-year-old boreal plantation

Background and Aims

Phosphorus (P) mineralisation from crop residues is usually predicted from total P or carbon: phosphorus (C: P) ratios. However, these measures have limited accuracy as they do not take into account the presence of different P species that may be mineralised at different rates. In this study P speciation was determined using solution ³¹P nuclear magnetic resonance (NMR) spectroscopy to understand the potential fate of residue P in soils.

Methods

Mature above-ground biomass of eight different crops sampled from the field was portioned into stem, chaff and seed.

Results

The main forms of P detected in stem and chaff were orthophosphate (25–75 %), phospholipids (10–40 %) and RNA (5–30 %). Phytate was the dominant P species in seeds, and constituted up to 45 % of total P in chaff but was only detected in minor amounts (<1 %) in stem residue. The majority (65–95 %) of P in stems was water-extractable, and most of this was detected as orthophosphate. However, this includes organic P that may have been hydrolysed during the water extraction.

Conclusions

This study indicates that the majority of residue P in aboveground plant residues has the potential to be delivered to soil in a form readily available to plants and soil microorganisms.

     

Patterns of δ^13C and δ^15N in wolverine Gulo gulo tissues from the Brooks Range,Alaska

Knowledge of carnivore diets is essential to understand how carnivore populations respond demographically to variations in prey abundance. Analysis of stable isotopes is a useful complement to traditional methods of analyzing carnivore diets. We used data on δ^13C and δ^15N in wolverine tissues to investigate patterns of seasonal and annual diet variation in a wolverine Gulo gulo population in the western Brooks Range, Alaska, USA. The stable isotope ratios in wolverine tissues generally reflected that of terrestrial carnivores, corroborating previous diet studies on wolverines. We also found variation in δ^13C and δ^15N both between muscle samples collected over several years and between tissues with different assimilation rates, even after correcting for isotopic fractionation. This suggests both annual and seasonal diet variation. Our results indicate that data on δ^13C and δ^15N holds promise for qualitative assessments of wolverine diet changes over time. Such temporal variation may be important indicators of ecological responses to environmental perturbations, and we suggest that more refined studies of stable isotopes may be an important tool when studying temporal change in diets of wolverines and similar carnivores [ Current Zoology 55 （3）： 188- 192, 2009].     

Reliability of δ13C and δ15N in faeces for reconstructing savanna herbivore diet

We tested the reliability of herbivore faecal δ¹³C and δ¹⁵N values for reconstructing diet through review of an extensive database derived from a 3-year study of ungulates in South Africa's Kruger National Park. Faeces are a useful material for stable isotope studies of diet because they record dietary turnover at very short time scales, and because sampling is non-invasive. However, the validity of faecal isotope proxies may be questioned because they represent only undigested food remains. Results from Kruger Park confirm that free-ranging browsers have faecal δ¹³C consistent with C₃ feeding, grazer faeces are C₄, and mixed-feeder faeces intermediate. Although the respective ranges do not overlap, there is significant variation in faecal δ¹³C of browsers and grazers (～2.0–4.0‰) across space and through time. We demonstrate that most (～70%) of this variation can be ascribed to corresponding patterns of variation in the δ¹³C of C₃ and C₄ plants, respectively, re-enforcing the fidelity of faecal isotope proxies for diet but highlighting a need for mixing models that control for variations in plant δ¹³C in order to achieve accurate diet reconstructions. Predictions for the effects of climate (rainfall) and ecophysiology on ¹⁵N-abundance variations in mammals do not persist in faeces. Rather, faecal δ¹⁵N tracks changes in plant δ¹⁵N, with further fractionation occurring primarily due to variations in dietary protein (reflected by %N). Controlling for these effects, we show that a dual-isotope multiple source mixing model (Isosource) can extend diet reconstructions for African savanna herbivores beyond simplified C₃/C₄ distinctions, although further understanding of variations in mammal δ¹⁵N are needed for greater confidence in this approach.     

Influence of mycorrhizal associations on foliar δ15N values of legume and non-legume shrubs and trees in the fynbos of South Africa: Implications for estimating N2 fixation using the 15N 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 ¹⁵N values for the reference plants, including many very negative values. The marked differences in ¹⁵N values were defined by the mycorrhizal status of the reference plant species, with the ericoid and arbuscular mycorrhizal plants showing lower foliar ¹⁵N values relative to their non-mycorrhizal counterparts. In contrast, the ¹⁵N 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 ¹⁵N 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.     

Jack pine foliar δ15N indicates shifts in plant nitrogen acquisition after severe wildfire and through forest stand development

Background and aims

Natural abundance of the stable nitrogen (N) isotope ¹⁵N can elucidate shifts in plant N acquisition and ecosystem N cycling following disturbance events. This study examined the potential relationship between foliar δ¹⁵N and depth of plant N acquisition (surface organic vs. mineral soil) and nitrification as conifer stands develop following stand-replacing wildfire.

Methods

We measured foliar δ¹⁵N along an 18-site chronosequence of jack pine (Pinus banksiana) stands, 1 to 72 years in age post-wildfire. Foliar δ¹⁵N was compared to total δ¹⁵N of the organic (Oe + Oa) and mineral (0–15 cm) soil horizons, and organic horizon N mineralization and nitrification as functions of total mineralization.

Results

Foliar δ¹⁵N declined with stand age, yet wildfire effects were heterogeneous. Jack pine seedlings on burned, mineral soil patches in the youngest stand were significantly more enriched than those on unburned, organic patches (P?=?0.007). High foliar values in the youngest stands relative to mineral-horizon δ¹⁵N indicate that nitrification also likely contributed to seedling enrichment.

Conclusions

Our results suggest jack pine seedlings on burned patches obtain N from the mineral soil with potentially high nitrification rates, whereas seedlings on unburned patches and increasingly N-limited, mature jack pine acquire relatively more N from organic horizons.     

Patterns of δ~(13) C and δ~(15) N in wolverine Gulo gulo tissues from the Brooks Range,Alaska

Knowledge of carnivore diets is essential to understand how carnivore populations respond demographically to variations in prey abundance.Analysis of stable isotopes is a useful complement to traditional methods of analyzing carnivore diets.We used data on δ 13 C and δ 15 N in wolverine tissues to investigate patterns of seasonal and annual diet variation in a wolverine Gulo gulo population in the western Brooks Range,Alaska,USA.The stable isotope ratios in wolverine tissues generally reflected that of terr...     

Shade tolerance of temperate Asian bamboos: a harbinger of their naturalization in Pacific Northwest coniferous forests?

Bamboos native to temperate East Asian coniferous forests arrive with increasing frequency in the United States as horticultural imports, and some are becoming naturalized locally. Given the strong floristic and physiognomic similarities between East Asian and western North American coniferous forests, we asked whether these introduced bamboos could tolerate the varied light regimes within coniferous forests in their new range. Seven temperate Asian bamboos and one North American bamboo (Arundinaria gigantea) were grown within shade structures in an experimental garden; these structures reduced ambient light to three light levels (50, 70, 90 % shade) that occur routinely within coniferous forests in the Pacific Northwest. Species’ responses under these light levels were measured by their light response curves to photosynthesis, resource allocation to light or carbon harvesting centers inferred by CO₂ response curves, and shifts amongst forms of leaf Chlorophyll. Bashania fargesii has lower chlorophyll content and photosynthetic rates under high shade (90 %) relative to other Asian species and to B. fargesii grown in 50 and 70 % shade. Bashania fargesii, Sasa kurilensis and A. gigantea also displayed lower photosynthetic rates under 90 % shade compared to plants grown in 50 and 70 % shade and lower electron transport capacity under 70 and 90 % shade compared to plants grown in 50 % shade. In contrast, Pleioblastus chino, Pleioblastus distichus, Pseudosasa japonica, Sasa palmata and Sasaella ramosa display strong tolerance of low light. Our results indicate these five Asian bamboos (and others yet to be introduced) could skirt a major environmental barrier to new species establishment in these North American forests. Measuring a species’ light response curve offers a reliable, rapid means to assess an immigrant species’ potential to tolerate forests’ varied light regimes.     

Temporal records of δ13C and δ15N in North Pacific pinnipeds: inferences regarding environmental change and diet

Sea lion and seal populations in Alaskan waters underwent various degrees of decline during the latter half of the twentieth century and the cause(s) for the declines remain uncertain. The stable carbon (¹³C/¹²C) and nitrogen (¹⁵N/¹⁴N) isotope ratios in bone collagen from wild Steller sea lions (Eumetopias jubatus), northern fur seals (Callorhinus ursinus) and harbor seals (Phoca vitulina) from the Bering Sea and Gulf of Alaska were measured for the period 1951-1997 to test the hypothesis that a change in trophic level may have occurred during this interval and contributed to the population declines. A significant change in '¹⁵N in pinniped tissues over time would imply a marked change in trophic level. No significant change in bone collagen '¹⁵N was found for any of the three species during the past 47 years in either the Bering Sea or the Gulf of Alaska. However, the ¹⁵N in the Steller sea lion collagen was significantly higher than both northern fur seals and harbor seals. A significant decline in '¹³C (almost 2 ‰ over the 47 years) was evident in Steller sea lions, while a declining trend, though not significant, was evident in harbor seals and northern fur seals. Changes in foraging location, in combination with a trophic shift, may offer one possible explanation. Nevertheless, a decrease in '¹³C over time with no accompanying change in '¹⁵N suggests an environmental change affecting the base of the foodweb rather than a trophic level change due to prey switching. A decline in the seasonal primary production in the region, possibly resulting from decreased phytoplankton growth rates, would exhibit itself as a decline in '¹³C. Declining production could be an indication of a reduced carrying capacity in the North Pacific Ocean. Sufficient quantities of optimal prey species may have fallen below threshold sustaining densities for these pinnipeds, particularly for yearlings and subadults who have not yet developed adequate foraging skills.     

Research article: small-scale spatial variation of δ13C and δ15N isotopes in Antarctic carbon sources and consumers

Regional food web studies that fail to account for small-scale isotopic variability can lead to a mismatch between an organism’s inferred and true trophic position. Misinterpretation of trophic status may result, substantially limiting spatial and temporal comparability of food web studies. We sampled several carbon sources and consumers in a nested design to assess the variability of food web members across small spatial scales (100 s of m to several km) in regions around the Windmill Islands and Vestfold Hills in East Antarctica. For carbon sources, δ¹³C in sea ice POM was particularly variable between locations (km apart) and between sites (100 s of m apart) with replicate samples varying by up to 16‰. Macroalgae δ¹³C was less variable (replicate samples ranging up to 6.9‰ for the red alga Iridaea cordata), yet still differed between locations. Sediment POM and pelagic POM were the least variable, displaying minimal differences between locations or sites for δ¹³C and δ¹⁵N. Three out of eight consumers were significantly different between locations for δ¹³C, and five out of eight for δ¹⁵N, with the fish Trematomus bernacchii the most variable for both δ¹³C and δ¹⁵N. At smaller scales, the amphipod Paramorea walkeri showed significant variation between sites in δ¹³C but not in δ¹⁵N. We attribute small-scale variability to the dynamic physical environment for carbon sources in coastal systems and a close coupling of diet to habitat for consumers. We highlight the need to account for small-scale spatial variation in sampling designs for regional food web studies.     

Long-term changes of the δ15N natural abundance of plants and soil in a temperate grassland

Tracing back the N use efficiency of long-term fertilizer trials is important for future management recommendations. Here we tested the changes in natural N-isotope composition as an indicator for N- management within a long-term fertilization lysimeter experiment in a low mountain range pasture ecosystem at Rengen (Eifel Mountains), Germany. Cattle slurry (δ¹⁵N?=?8.9?±?0.5‰) and mineral fertilizers (calcium ammonium nitrate; δ¹⁵N?=??1.0?±?0.2‰) were applied at a rate between 0 and 480 kg N ha^?1?yr^?1 throughout 20 years from 1985 onwards. In 2006, samples were taken from different grass species, coarse and fine particulate soil organic matter, bulk soil and leachates. Total soil N content hardly changed during fertilization experiment. As also N leaching has been small within the stagnant water regime, most N was lost through the gaseous phase beside plant uptake and cutting. Unlike N uptake by plants, the process of N volatilization resulted in strong discrimination against the ¹⁵N isotope. As a consequence, the δ¹⁵N values of top soil samples increased from 1.8?±?0.4‰ to 6.0?±?0.4‰ and that of the plants from ?1.2?±?1.3‰ to 4.8?±?1.2‰ with increasing N fertilizer rate. Samples receiving organic fertilizer were most enriched in δ¹⁵N. The results suggest that parts of the fertilizer N signal was preserved in soils and even discovered in soil organic matter pools with slow N turnover. However, a ¹⁵N/¹⁴N isotope fractionation of up to 1.5‰ added to the δ¹⁵N values recovered in soils and plants, rendering the increase in δ¹⁵N value a powerful indicator to long-term inefficient N usage and past N management in the terrestrial environment.     

Shoot δ15N correlates with genotype and salt stress in barley

Given a uniform N source, the ¹⁵N of barley shoots provided a genotypic range within treatments and a separation between control and salt-stress treatments as great as did ¹³C^*. Plant ¹⁵N has been represented in the literature as a bioassay of external source ¹⁵N and used to infer soil N sources, thus precluding consideration of the plant as a major cause in determining its own 8¹⁵N. We believe this to be the first report of plant ¹⁵N as a genetic trait. No mechanistic model is needed for use of ¹⁵N as a trait in controlled studies; however, a qualitative model is suggested for further testing.Symbol ¹⁵N (or ¹³C) the difference between: (1) the ratio of heavy to light isotopes of the element in a sample and (2) that of its reference standard     

Differences in δ15N and δ13C between embryonic and maternal tissues of the ovoviviparous bluntnose sixgill shark Hexanchus griseus

Stable nitrogen (δ¹⁵N) and carbon (δ¹³C) isotope ratios from muscle, liver and yolk were analysed from the mother and embryos of an ovoviviparous shark, Hexanchus griseus. Embryonic liver and muscle had similar δ¹⁵N and δ¹³C ratios or were depleted in heavy isotopes, compared to the same maternal somatic and reproductive yolk tissues, but no relationship existed between δ¹⁵N or δ¹³C and embryo length, as expected, because a switch to placental nourishment is lacking in this species. This study expands the understanding of maternal nourishment and embryonic stable isotope differences in ovoviviparous sharks.     

Stratification of δ13C values of leaves in Amazonian rain forests

Summary The contribution of soil respiration to the photosynthesis of the shade flora in the Amazon forest was evaluated by measuring the ¹³C values of leaves collected at different levels in two forest communities. Canopy leaves have an average ¹³C of-30.5 in the podsol forest and-28.7 in the laterite forest. Leaves from plants in the lower forest strata have a significantly lower value of-35.2 in the podsol forest and-34.3 in the laterite forest.Mailing address of the first author: Before May 31, 1980: Department of Biological Sciences, Stanford University, Stanford, California 94305 USA. After May 31: Centro de Ecologia, IVIC Aptdo. 1827. Caracas, Venezuela     

Variability in δ13C and δ15N trophic discrimination factors for teleost fishes: a meta-analysis of temperature and dietary effects

Reviews in Fish Biology and Fisheries - Stable isotope analysis (SIA) is widely used to assess animal diet and movements, requiring accurate estimates of trophic discrimination factors (TDFs)....