Soil carbon dioxide partial pressure and dissolved inorganic carbonate chemistry under elevated carbon dioxide and ozone

Global emissions of atmospheric CO₂ and tropospheric O₃ are rising and expected to impact large areas of the Earths forests. While CO₂ stimulates net primary production, O₃ reduces photosynthesis, altering plant C allocation and reducing ecosystem C storage. The effects of multiple air pollutants can alter belowground C allocation, leading to changes in the partial pressure of CO₂ (pCO₂) in the soil , chemistry of dissolved inorganic carbonate (DIC) and the rate of mineral weathering. As this system represents a linkage between the long- and short-term C cycles and sequestration of atmospheric CO₂, changes in atmospheric chemistry that affect net primary production may alter the fate of C in these ecosystems. To date, little is known about the combined effects of elevated CO₂ and O₃ on the inorganic C cycle in forest systems. Free air CO₂ and O₃ enrichment (FACE) technology was used at the Aspen FACE project in Rhinelander, Wisconsin to understand how elevated atmospheric CO₂ and O₃ interact to alter pCO₂ and DIC concentrations in the soil. Ambient and elevated CO₂ levels were 360±16 and 542±81 l l^–1, respectively; ambient and elevated O₃ levels were 33±14 and 49±24 nl l^–1, respectively. Measured concentrations of soil CO₂ and calculated concentrations of DIC increased over the growing season by 14 and 22%, respectively, under elevated atmospheric CO₂ and were unaffected by elevated tropospheric O₃. The increased concentration of DIC altered inorganic carbonate chemistry by increasing system total alkalinity by 210%, likely due to enhanced chemical weathering. The study also demonstrated the close coupling between the seasonal ¹³C of soil pCO₂ and DIC, as a mixing model showed that new atmospheric CO₂ accounted for approximately 90% of the C leaving the system as DIC. This study illustrates the potential of using stable isotopic techniques and FACE technology to examine long- and short-term ecosystem C sequestration.     

Soil respiration of Alaskan tundra at elevated atmospheric carbon dioxide concentrations

Summary CO₂ efflux from tussock tundra in Alaska that had been exposed to elevated CO₂ for 2.5 growing seasons was measured to assess the effect of long- and short-term CO₂ enrichment on soil respiration. Long-term treatments were: 348, 514, and 683 μll⁻¹ CO₂ and 680 μll⁻¹ CO₂+4°C above ambient. Measurements were made at 5 CO₂ concentrations between 87 and 680 μll⁻¹ CO₂. Neither long- or short-term CO₂ enrichment significantly affected soil CO₂ efflux. Tundra developed at elevated temperature and 680 μll⁻¹ CO₂ had slightly higher, but not statistically different, mean respiration rates compared to untreated tundra and to tundra under CO₂ control alone.     

Species-specific responses to atmospheric carbon dioxide and tropospheric ozone mediate changes in soil carbon

We repeatedly sampled the surface mineral soil (0–20 cm depth) in three northern temperate forest communities over an 11-year experimental fumigation to understand the effects of elevated carbon dioxide (CO₂) and/or elevated phyto-toxic ozone (O₃) on soil carbon (C). After 11 years, there was no significant main effect of CO₂ or O₃ on soil C. However, within the community containing only aspen ( Populus tremuloides Michx.), elevated CO₂ caused a significant decrease in soil C content. Together with the observations of increased litter inputs, this result strongly suggests accelerated decomposition under elevated CO_2. In addition, an initial reduction in the formation of new (fumigation-derived) soil C by O₃ under elevated CO₂ proved to be only a temporary effect, mirroring trends in fine root biomass. Our results contradict predictions of increased soil C under elevated CO₂ and decreased soil C under elevated O₃ and should be considered in models simulating the effects of Earth's altered atmosphere.     

Nitrogen and carbon isotope responses of Chinese cabbage and chrysanthemum to the application of liquid pig manure

The effects of the liquid pig manure (LM) used in organic farming on the natural abundance of ¹⁵N and ¹³C signatures in plant tissues have not been studied. We hypothesized that application of LM will (1) increase δ¹⁵N of plant tissues due to the high δ¹⁵N of N in LM as compared with soil N or inorganic fertilizer N, and (2) increase δ¹³C of plant tissues as a result of high salt concentration in LM that decreases stomatal conductance of plants. To test these hypotheses, variations in the δ¹⁵N and δ¹³C of Chinese cabbage (Brassica campestris L.) and chrysanthemum (Chrysanthemum morifolium Ramatuelle) with two different LMs (with δ¹⁵N of +15.6 and +18.2‰) applied at two rates (323 and 646 kg N ha^-1 for cabbage and 150 and 300 kg N ha^-1 for chrysanthemum), or urea (δ¹⁵N = -2.7‰) applied at the lower rate above for the respective species, in addition to the control (no N input) were investigated through a 60-day pot experiment. Application of LM significantly increased plant tissue δ¹⁵N (range +9.4 to +14.9‰) over the urea (+3.2 to +3.3‰) or control (+6.8 to 7.7‰) treatments regardless of plant species, strongly reflecting the δ¹⁵N of the N source. Plant tissue δ¹³C were not affected by the treatments for cabbage (range −30.8 to −30.2‰) or chrysanthemum (−27.3 to −26.8‰). However, cabbage dry matter production decreased while its δ¹³C increased with increasing rate of LM application or increasing soil salinity (P < 0.05), suggesting that salinity stress caused by high rate of LM application likely decreased stomatal conductance and limited growth of cabbage. Our study expanded the use of the δ¹⁵N technique in N source (organic vs. synthetic fertilizer) identification and suggested that plant tissue δ¹³C maybe a sensitive indicator of plant response to salinity stress caused by high LM application rates.     

Nitrogen dynamics and growth of seedlings of an N-fixing tree (Gliricidia sepium (Jacq.) Walp.) exposed to elevated atmospheric carbon dioxide

Summary Seeds of Gliricidia sepium (Jacq.) Walp., a tree native to seasonal tropical forests of Central America, were inoculated with N-fixing Rhizobium bacteria and grown in growth chambers for 71 days to investigate interactive effects of atmospheric CO₂ and plant N status on early seedling growth, nodulation, and N accretion. Seedlings were grown with CO₂ partial pressures of 350 and 650 bar (current ambient and a predicted partial pressure of the mid-21st century) and with plus N or minus N nutrient solutions to control soil N status. Of particular interest was seedling response to CO₂ when grown without available soil N, a condition in which seedlings initially experienced severe N deficiency because bacterial N-fixation was the sole source of N. Biomass of leaves, stems, and roots increased significantly with CO₂ enrichment (by 32%, 15% and 26%, respectively) provided seedlings were supplied with N fertilizer. Leaf biomass of N-deficient seedlings was increased 50% by CO₂ enrichment but there was little indication that photosynthate translocation from leaves to roots or that plant N (fixed by Rhizobium) was altered by elevated CO₂. In seedlings supplied with soil N, elevated CO₂ increased average nodule weight, total nodule weight per plant, and the amount of leaf nitrogen provided by N-fixation (as indicated by leaf ¹⁵N). While CO₂ enrichment reduced the N concentration of some plant tissues, whole plant N accretion increased. Results support the contention that increasing atmospheric CO₂ partial pressures will enhance productivity and N-fixing activity of N-fixing tree seedlings, but that the magnitude of early seedling response to CO₂ will depend greatly on plant and soil nutrient status.     

Intraguild interactions between spiders and ants and top-down control in a grassland food web

In most terrestrial ecosystems ants (Formicidae) as eusocial insects and spiders (Araneida) as solitary trappers and hunters are key predators. To study the role of predation by these generalist predators in a dry grassland, we manipulated densities of ants and spiders (natural and low density) in a two-factorial field experiment using fenced plots. The experiment revealed strong intraguild interactions between ants and spiders. Higher densities of ants negatively affected the abundance and biomass of web-building spiders. The density of Linyphiidae was threefold higher in plots without ant colonies. The abundance of Formica cunicularia workers was significantly higher in spider-removal plots. Also, population size of springtails (Collembola) was negatively affected by the presence of wandering spiders. Ants reduced the density of Lepidoptera larvae. In contrast, the abundance of coccids (Ortheziidae) was positively correlated with densities of ants. To gain a better understanding of the position of spiders, ants and other dominant invertebrate groups in the studied food web and important trophic links, we used a stable isotope analysis (¹⁵N and ¹³C). Adult wandering spiders were more enriched in ¹⁵N relative to ¹⁴N than juveniles, indicating a shift to predatory prey groups. Juvenile wandering and web-building spiders showed δ¹⁵N ratios just one trophic level above those of Collembola, and they had similar δ¹³C values, indicating that Collembola are an important prey group for ground living spiders. The effects of spiders demonstrated in the field experiment support this result. We conclude that the food resource of spiders in our study system is largely based on the detrital food web and that their effects on herbivores are weak. The effects of ants are not clear-cut and include predation as well as mutualism with herbivores. Within this diverse predator guild, intraguild interactions are important structuring forces.     

Species-dependent responses of juniper and spruce to increasing CO<Subscript>2</Subscript> concentration and to climate in semi-arid and arid areas of northwestern China

Qilian juniper (Sabina przewalskii Kom.) and Qinghai spruce (Picea crassifolia Kom.) represent different tree functional types, which can be found extensively in northwestern China. The former is drought-tolerant, whereas the latter is hygrophilous and shade-tolerant. We compared their intrinsic water-use efficiency (iWUE, inferred from carbon isotopic discrimination, δ¹³C, in their wood) as a function of atmospheric CO₂ concentration, [CO₂], and climate. δ¹³C of spruce was consistently about higher than that of juniper in semi-arid areas but was lower in arid areas. This difference was stable over time and demonstrated strong cross-correlations between species, although some subtle high-frequency (2 or 3 years) variations existed in both species, suggesting that regional climate may control carbon isotope discrimination. The ratio (the [CO₂] values in leaf intercellular and the atmosphere, respectively) of the juniper increased steadily over time, whereas that of the spruce showed a long-term downward trend. IWUE increased at all sites over the 150-year study period, mainly caused by increasing [CO₂]. The relationship between iWUE and [CO₂] reveals that the spruce was more sensitive than the juniper to increasing [CO₂], suggesting a species-specific adaptation to long-term environmental changes. Correlations between the high-frequency variations in stable carbon discrimination (Δ) and climate indicate similar intra-site responses to climate in both species, but different response strengths. Overall, complex interactions of temperature and moisture on stable carbon discrimination during current growth seasons of both species were environmental-determined. Regulation of gas exchange and reduced transpiration may influence water and energy budgets directly; therefore species-dependent responses of trees to elevated CO₂ should be considered in future research on global plant physiological ecology.     

Assimilate partitioning affects 13C fractionation of recently assimilated carbon in maize

Coupling ¹³C natural abundance and ¹⁴C pulse labelling enabled us to investigate the dependence of ¹³C fractionation on assimilate partitioning between shoots, roots, exudates, and CO₂ respired by maize roots. The amount of recently assimilated C in these four pools was controlled by three levels of nutrient supply: full nutrient supply (NS), 10 times diluted nutrient supply (DNS), and deionised water (DW). After pulse labelling of maize shoots in a ¹⁴CO₂ atmosphere, ¹⁴C was traced to determine the amounts of recently assimilated C in the four pools and the δ¹³C values of the four pools were measured. Increasing amounts of recently assimilated C in the roots (from 8% to 10% of recovered ¹⁴C in NS and DNS treatments) led to a 0.3‰ ¹³C enrichment from NS to DNS treatments. A further increase of C allocation in the roots (from 10% to 13% of recovered ¹⁴C in DNS and DW treatments) resulted in an additional enrichment of the roots from DNS to DW treatments by 0.3‰. These findings support the hypothesis that ¹³C enrichment in a pool increases with an increasing amount of C transferred into that pool. δ¹³C of CO₂ evolved by root respiration was similar to that of the roots in DNS and DW treatments. However, if the amount of recently assimilated C in root respiration was reduced (NS treatment), the respired CO₂ became 0.7‰ ¹³C depleted compared to roots. Increasing amounts of recently assimilated C in the CO₂ from NS via DNS to DW treatments resulted in a 1.6‰ δ¹³C increase of root respired CO₂ from NS to DW treatments. Thus, for both pools, i.e. roots and root respiration, increasing amounts of recently assimilated C in the pool led to a δ¹³C increase. In DW and DNS plants there was no ¹³C fractionation between roots and exudates. However, high nutrient supply decreased the amount of recently assimilated C in exudates compared to the other two treatments and led to a 5.3‰ ¹³C enrichment in exudates compared to roots. We conclude that ¹³C discrimination between plant pools and within processes such as exudation and root respiration is not constant but strongly depends on the amount of C in the respective pool and on partitioning of recently assimilated C between plant pools. Section Editor: H. Lambers     

Mycorrhizal Hyphal Turnover as a Dominant Process for Carbon Input into Soil Organic Matter

The atmospheric concentration of CO₂ is predicted to reach double current levels by 2075. Detritus from aboveground and belowground plant parts constitutes the primary source of C for soil organic matter (SOM), and accumulation of SOM in forests may provide a significant mechanism to mitigate increasing atmospheric CO₂ concentrations. In a poplar (three species) plantation exposed to ambient (380 ppm) and elevated (580 ppm) atmospheric CO₂ concentrations using a Free Air Carbon Dioxide Enrichment (FACE) system, the relative importance of leaf litter decomposition, fine root and fungal turnover for C incorporation into SOM was investigated. A technique using cores of soil in which a C₄ crop has been grown (δ¹³C −18.1‰) inserted into the plantation and detritus from C₃ trees (δ¹³C −27 to −30‰) was used to distinguish between old (native soil) and new (tree derived) soil C. In-growth cores using a fine mesh (39 μm) to prevent in-growth of roots, but allow in-growth of fungal hyphae were used to assess contribution of fine roots and the mycorrhizal external mycelium to soil C during a period of three growing seasons (1999–2001). Across all species and treatments, the mycorrhizal external mycelium was the dominant pathway (62%) through which carbon entered the SOM pool, exceeding the input via leaf litter and fine root turnover. The input via the mycorrhizal external mycelium was not influenced by elevated CO₂, but elevated atmospheric CO₂ enhanced soil C inputs via fine root turnover. The turnover of the mycorrhizal external mycelium may be a fundamental mechanism for the transfer of root-derived C to SOM.     

The effect of cold-induced increased metabolic rate on the rate of <Superscript>13</Superscript>C and <Superscript>15</Superscript>N incorporation in house sparrows (<Emphasis Type="Italic">Passer domesticus</Emphasis>)

Animals with high metabolic rates are believed to have high rates of carbon and nitrogen isotopic incorporation. We hypothesized that (1) chronic exposure to cold, and hence an increase in metabolic rate, would increase the rate of isotopic incorporation of both ¹³C and ¹⁵N into red blood cells; and (2) that the rate of isotopic incorporation into red blood cells would be allometrically related to body mass. Two groups of sparrows were chronically exposed to either 5 or 22°C and switched from a ¹³C-depleted C₃-plant diet to a more ¹³C-enriched C₄-plant one. We used respirometry to estimate the resting metabolic rate of birds exposed chronically to our two experimental temperatures. The allometric relationship between the rate of ¹³C incorporation into blood and body mass was determined from published data. The of birds at 5°C was 1.9 times higher than that of birds at 22°C. Chronic exposure to a low temperature did not have an effect on the rate of isotopic incorporation of ¹⁵N save for a very small effect on the incorporation of ¹³C. The isotopic incorporation rate of ¹³C was 1.5 times faster than that of ¹⁵N. The fractional rate of ¹³C incorporation into avian blood was allometrically related to body mass with an exponent similar to −1/4. We conclude that the relationship between metabolic rate and the rate of isotopic incorporation into an animal’s tissues is indirect. It is probably mediated by protein turnover and thus more complex than previous studies have assumed.     

δ13C and δ15N changes after dietary shift in veliger larvae of the slipper limpet Crepidula fornicata: an experimental evidence

δ¹³C and δ¹⁵N measurements are still poorly conducted in benthic invertebrate larvae. To assess the δ¹³C and δ¹⁵N changes occurring after a dietary shift, experiments were conducted on veliger larvae of Crepidula fornicata fed with two cultured microalgae (Isochrysis galbana and Pavlova lutheri) of known isotopic composition, ¹³C-enriched and ¹⁵N-depleted compared to the initial values of the larvae. Rapid changes in larval δ¹³C and δ¹⁵N were observed after the dietary shift, with an increase in δ¹³C and a decrease in δ¹⁵N. After 19 days of feeding, isotopic equilibrium was still not reached, a period which is close to the duration of the pelagic life of the larvae. This implies that the isotopic composition measured in field-collected larvae might only partly reflect actual larval feeding but also the parental isotopic signature, especially during the early developmental stages. Isotopic measurements in marine invertebrate larvae should thus be interpreted cautiously. In planktonic food web investigations, the study of field-collected larvae of different size/developmental stage may reduce potential misinterpretations.     

Effect of light regimes on the utilisation of an exogenous carbon source by freshwater biofilm bacterial communities

This study reports the novel use of nucleic acid stable isotope probing (NA-SIP) to identify metabolically active ([¹³C]-acetate assimilating) bacteria in freshwater biofilms. Currently, a little is known of the factors affecting the structure and activity of these complex microbial biofilm communities, although it is likely that they are influenced by riparian vegetation through attenuation of light and alteration of allochthonous inputs of carbon. NA-SIP was used to investigate the effect of varying light regimes on [¹³C]-acetate assimilating bacteria within laboratory biofilm microcosms. Differences in clone libraries of 16S rRNA and rRNA genes from ¹³C-labelled and unlabelled nucleic acids indicated differential uptake of acetate and the rapid transfer of ¹³C to organisms at a higher trophic level. Biofilm communities incubated in the dark changed least over time and retained a significant fraction of phototrophic organisms. Incubation under elevated light caused the greatest change in bacterial community structure. Contrary to expectation, a complete loss of chlorophyll containing organisms occurred within this treatment, challenging current thinking that elevated light promotes communities dominated by photoautotrophs in nutrient enriched environments.     

Competition and drought limit the response of water-use efficiency to rising atmospheric carbon dioxide in the Mediterranean fir <Emphasis Type="Italic">Abies pinsapo</Emphasis>

The gas-exchange and radial growth responses of conifer forests to climatic warming and increasing atmospheric CO₂ have been widely studied. However, the modulating effects of variables related to stand structure (e.g., tree-to-tree competition) on those responses are poorly explored. The basal-area increment (BAI) and C isotope discrimination (C stable isotope ratio; δ¹³C) in the Mediterranean fir Abies pinsapo were investigated to elucidate the influences of stand competition, atmospheric CO₂ concentrations and climate on intrinsic water-use efficiency (WUEi). We assessed the variation in δ¹³C of tree-rings from dominant or co-dominant trees subjected to different degrees of competition. A high- (H) and a low-elevation (L) population with contrasting climatic constraints were studied in southern Spain. Both populations showed an increase in long-term WUEi. However, this increase occurred more slowly at the L site, where a decline of BAI was also observed. Local warming and severe droughts have occurred in the study area over the past 30 years, which have reduced water availability more at lower elevations. Contrastingly, trees from the H site were able to maintain high BAI values at a lower cost in terms of water consumption. In each population, trees subjected to a higher degree of competition by neighboring trees showed lower BAI and WUEi than those subjected to less competition, although the slopes of the temporal trends in WUEi were independent of the competitive micro-environment experienced by the trees. The results are consistent with an increasing drought-induced limitation of BAI and a decreasing rate of WUEi improvement in low-elevation A. pinsapo forests. This relict species might not be able to mitigate the negative effects of a decrease in water availability through a reduction in stomatal conductance, thus leading to a growth decline in the more xeric sites. An intense and poorly asymmetric competitive environment at the stand level may also act as an important constraint on the adaptive capacity of these drought-sensitive forests to climatic warming. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Controlling for anthropogenically induced atmospheric variation in stable carbon isotope studies

Increased use of stable isotope analysis to examine food-web dynamics, migration, transfer of nutrients, and behavior will likely result in expansion of stable isotope studies investigating human-induced global changes. Recent elevation of atmospheric CO₂ concentration, related primarily to fossil fuel combustion, has reduced atmospheric CO₂ δ¹³C (¹³C/¹²C), and this change in isotopic baseline has, in turn, reduced plant and animal tissue δ¹³C of terrestrial and aquatic organisms. Such depletion in CO₂ δ¹³C and its effects on tissue δ¹³C may introduce bias into δ¹³C investigations, and if this variation is not controlled, may confound interpretation of results obtained from tissue samples collected over a temporal span. To control for this source of variation, we used a high-precision record of atmospheric CO₂ δ¹³C from ice cores and direct atmospheric measurements to model modern change in CO₂ δ¹³C. From this model, we estimated a correction factor that controls for atmospheric change; this correction reduces bias associated with changes in atmospheric isotopic baseline and facilitates comparison of tissue δ¹³C collected over multiple years. To exemplify the importance of accounting for atmospheric CO₂ δ¹³C depletion, we applied the correction to a dataset of collagen δ¹³C obtained from mountain lion (Puma concolor) bone samples collected in California between 1893 and 1995. Before correction, in three of four ecoregions collagen δ¹³C decreased significantly concurrent with depletion of atmospheric CO₂ δ¹³C (n ≥ 32, P ≤ 0.01). Application of the correction to collagen δ¹³C data removed trends from regions demonstrating significant declines, and measurement error associated with the correction did not add substantial variation to adjusted estimates. Controlling for long-term atmospheric variation and correcting tissue samples for changes in isotopic baseline facilitate analysis of samples that span a large temporal range.     

Soil organic carbon dynamics in cleared temperate forest spodosols converted to maize cropping

In southwest France, sandy spodosols have developed from Quaternary sandy eolian deposits. On these soils, numerous forest lands have been converted to continuous intensive maize cropping. A chronosequence study is realized by comparing organic C pools and ¹³C natural abundance of one forested and 6 agricultural sites, whose ages of cultivation range from 4 to 32 yr. ¹³C ratio is found to increase with time of cultivation. After 3 decades of intensive maize cropping, about half of the initial organic C content in the forest topsoil layer has disappeared. The fraction of C derived from maize crop increases during the first decades of cultivation, but its level is significantly lower than those observed in other soils, which indicates a high mineralization rate of organic C. In this context, soil characteristics associated to intensive agricultural practices lead to a rapid and large loss of C, whereas inputs from maize seem to have only a very small long-term contribution.     

Elevated carbon dioxide and ozone alter productivity and ecosystem carbon content in northern temperate forests

Three young northern temperate forest communities in the north‐central United States were exposed to factorial combinations of elevated carbon dioxide (CO₂) and tropospheric ozone (O₃) for 11 years. Here, we report results from an extensive sampling of plant biomass and soil conducted at the conclusion of the experiment that enabled us to estimate ecosystem carbon (C) content and cumulative net primary productivity (NPP). Elevated CO₂ enhanced ecosystem C content by 11%, whereas elevated O₃ decreased ecosystem C content by 9%. There was little variation in treatment effects on C content across communities and no meaningful interactions between CO₂ and O₃. Treatment effects on ecosystem C content resulted primarily from changes in the near‐surface mineral soil and tree C, particularly differences in woody tissues. Excluding the mineral soil, cumulative NPP was a strong predictor of ecosystem C content (r² = 0.96). Elevated CO₂ enhanced cumulative NPP by 39%, a consequence of a 28% increase in canopy nitrogen (N) content (g N m^?2) and a 28% increase in N productivity (NPP/canopy N). In contrast, elevated O₃ lowered NPP by 10% because of a 21% decrease in canopy N, but did not impact N productivity. Consequently, as the marginal impact of canopy N on NPP (?NPP/?N) decreased through time with further canopy development, the O₃ effect on NPP dissipated. Within the mineral soil, there was less C in the top 0.1 m of soil under elevated O₃ and less soil C from 0.1 to 0.2 m in depth under elevated CO₂. Overall, these results suggest that elevated CO₂ may create a sustained increase in NPP, whereas the long‐term effect of elevated O₃ on NPP will be smaller than expected. However, changes in soil C are not well‐understood and limit our ability to predict changes in ecosystem C content.     

Resolving temporal variation in vertebrate diets using naturally occurring stable isotopes

Assessments of temporal variation in diets are important for our understanding of the ecology of many vertebrates. Ratios of naturally occurring stable isotopes in animal tissues are a combination of the source elements and tissue specific fractionation processes, and can thus reveal dietary information. We review three different approaches that have been used to resolve temporal diet variation through analysis of stable isotopes. The most straightforward approach is to compare samples from the same type of tissue that has been sampled over time. This approach is suited to address either long or short-term dietary variation, depending on sample regime and which tissue that is sampled. Second, one can compare tissues with different metabolic rates. Since the elements in a given tissue have been assimilating during time spans specific to its metabolic rate, tissues with different metabolic rates will reflect dietary records over different periods. Third, comparisons of sections from tissues with progressive growth, such as hair, feathers, claws and teeth, will reveal temporal variation since these tissues will retain isotopic values in a chronological order. These latter two approaches are mainly suited to address questions regarding intermediate and short-term dietary variation. Knowledge of tissue specific metabolic rates, which determine the molecular turnover for a specific tissue, is of central importance for all these comparisons. Estimates of isotopic fractionation between source and measured target are important if specific hypotheses regarding the source elements are addressed. Estimates of isotopic fractionation, or at least of differences in fractionation between tissues, are necessary if different tissues are compared. We urge for more laboratory experiments aimed at improving our understanding of differential assimilation of dietary components, isotopic fractionation and metabolic routing. We further encourage more studies on reptiles and amphibians, and generally more studies utilizing multiple tissues with different turnover rates.     

Isotopic signatures and nutrient relations of plants inhabiting brackish wetlands in the northeastern coastal plain of Venezuela

The semi-diurnal tidal regime (≥2 m) in the Paria Gulf on the Atlantic coast of Venezuela, and the flat landscape of the region, allow the penetration for tens of km of marine waters into the rivers draining the northeastern coastal plain of the country. The levels of salinity, tidal flooding, and sedimentation decrease perpendicularly from the river channel toward the back swamps. The vegetation varies sequentially from fringe mangroves along the river margins, to back swamps containing forests dominated by Pterocarpus officinalis, herbaceous communities of Lagenocarpus guianensis, and palm swamps with Mauritia flexuosa, Chrysobalanus icaco, and Tabebuia spp. This environmental structure was used to test the hypotheses that: (a) mangrove distribution is strongly associated with salinity of interstitial water, and (b) they occupy areas where tidal influence and sediment dynamics determine a relatively open N cycle. Analyses of soil, water, and plants along a 1.5 km transect located near the confluence of the Guanoco and San Juan Rivers (Sucre and Monagas States, Venezuela) revealed that: (a) conductivity decreased from 11 to 0.2 mmhos cm⁻¹ from the river fringe to the internal swamp, whereas Na in the same stretch decreased from 100 to 2 μM; (b) average leaf tissue concentrations of Na, P, and N decreased significantly along the transect; (c) P. officinalis showed a large Na-exclusion capacity indicated by positive K/Na ratios from 8 to 200, and Crinum erubescens counteracted Na by accumulating K above 1,000 mmol kg⁻¹; (d) leaves varied widely in δ ¹³C (−25.5 to −32‰) and δ ¹⁵N (4 to −10.5‰) values. Samples were aggregated according to soil carbon content corresponding to those of the mangrove forest belt (5–28 mol C kg⁻¹; 0–650 from river fringe) and those of the back swamps (40–44 mol C kg⁻¹; 700–1,500 m from river fringe). The concentrations of Na, P, and N (in mmol kg⁻¹) and δ ¹⁵N values (in ‰) were significantly higher in the mangrove forest compared to the back swamp (Na 213 vs. 88; P 41 vs. 16; N 1,535 vs. 727; δ ¹⁵N 1.5 vs. −3.7), indicating that the fringe forest was not nutrient limited. These results support the hypotheses that mangroves are restricted to the more-saline sections of the transect, and that the fringe forest has a more open N cycle, favoring ¹⁵N accumulation within the system.     

Stable Isotopic and Trace Elemental Compositions of Otoliths and the Stock Structure of Pacific Cod, Gadus macrocephalus

Synopsis We examined the stock composition and life history of Pacific cod, Gadus macrocephalus, in the northeast Pacific Ocean from sagittal otoliths collected from three marine fishing areas in Washington State, U.S.A. We analyzed both stable isotope ratios (δ¹⁸O and δ¹³C) and trace elemental concentrations (Sr, Mg, Na, Fe, Mn) for these otoliths. The combination of δ¹⁸O and δ¹³C, and correlation of δ¹⁸O vs. 1000Sr/Ca and 1000Mg/Ca showed clear separations between North Puget Sound and coastal cod, suggesting there might be two different spawning stocks in the region. The North Puget Sound cod might represent an ‘Estuary-type’ from the Strait of Georgia, whereas coastal cod might represent an ‘Ocean-type’ from the Pacific west coast. Isotopic variations from five representative otoliths also showed a two-stage life history for Pacific cod, and a critical transition period for cod in migration to the ocean or in age of sexual maturity. These chemical interpretations and conclusions appear in agreement with biological observations of Pacific cod, and are consistent with results of previous studies for other marine fish species in nearby areas.     

Grazing and Ecosystem Carbon Storage in the North American Great Plains

Isotopic signatures of ¹³C were used to quantify the relative contributions of C₃ and C₄ plants to whole-ecosystem C storage (soil+plant) in grazed and ungrazed sites at three distinct locations (short-, mid- and tallgrass communities) along an east–west environmental gradient in the North American Great Plains. Functional group composition of plant communities, the source and magnitude of carbon inputs, and total ecosystem carbon storage displayed inconsistent responses to long-term livestock grazing along this gradient. C₄ plants [primarily Bouteloua gracilis (H.B.K.) Lag ex Steud.] dominated the long-term grazed site in the shortgrass community, whereas the ungrazed site was co-dominated by C₃ and C₄ species; functional group composition did not differ between grazed and ungrazed sites in the mid- and tallgrass communities. Above-ground biomass was lower, but the relative proportion of fine root biomass was greater, in grazed compared to ungrazed sites at all three locations. The grazed site of the shortgrass community had 24% more whole-ecosystem carbon storage compared to the ungrazed site (4022 vs. 3236 g C m⁻²). In contrast, grazed sites at the mid- and tallgrass communities had slightly lower (8%) whole-ecosystem carbon storage compared to ungrazed sites (midgrass: 7970 vs. 8683 g C m⁻²; tallgrass: 8273 vs. 8997 g C m⁻²). Differential responses between the shortgrass and the mid- and tallgrass communities with respect to grazing and whole-ecosystem carbon storage are likely a result of: (1) maintenance of larger soil organic carbon (SOC) pools in the mid- and tallgrass communities (7476–8280 g C m⁻²) than the shortgrass community (2517–3307 g C m⁻²) that could potentially buffer ecosystem carbon fluxes, (2) lower root carbon/soil carbon ratios in the mid- and tallgrass communities (0.06–0.10) compared to the shortgrass community (0.20–0.27) suggesting that variation in root organic matter inputs would have relatively smaller effects on the size of the SOC pool, and (3) the absence of grazing-induced variation in the relative proportion of C₃ and C₄ functional groups in the mid- and tallgrass communities. We hypothesize that the magnitude and proportion of fine root mass within the upper soil profile is a principal driver mediating the effect of community composition on the biogeochemistry of these grassland ecosystems.