Foliar δ<Superscript>13</Superscript>C values of nine dominant species in the Loess Plateau of China

The foliar stable carbon isotope compositions (δ¹³C) of nine dominant species in seven sites, Yangling, Yongshou, Tongchuan, Fuxian, Ansai, Mizhi, and Shenmu, standing from the south to the north in the Loess Plateau of China were studied. The results showed that foliar δ¹³C values ranged from −22.61 to −30.73 ‰ with an average of −27.23 ‰ in 141 C₃ plant samples collected from the Loess Plateau. Foliar δ¹³C values varied significantly (p<0.001) among the nine C₃ species, which were Pinus tabulaeformis Carr., Robinia pseudoacacia L., Zizyphus jujuba Mill. var. spinosus Hu., Rubus parvifolius L., Hippophae rhamnoides L., Caragana korshinskii Kom., Lespedeza davurica (Laxm.) Schindl., Artemisia sacrorum Ledeb. var. incana Mattf., and Agropyron cristatum Gaertn. Comparatively, R. pseudoacacia, H. rhamnoides, and C. korshinskii had much higher δ¹³C values than the other six species, while A. sacrorum had the lowest δ¹³C value. There was no significant difference in foliar δ¹³C value among five species, P. tabulaeformis, Z. jujuba, R. parvifolius, L. davurica, and A. cristatum. Considering the life forms categorized from nine C₃ species, trees and shrubs had significantly higher δ¹³C values than herbs (p<0.001). The deciduous tree R. pseudoacacia had much higher δ¹³C value than the evergreen tree P. tabulaeformis (p<0.01). Among the four shrubs, foliar δ¹³C values in H. rhamnoides and C. korshinskii were markedly higher (p<0.01) than those in Z. jujuba and R. parvifolius. Among the three herbs, L. davurica and A. cristatum had significantly higher δ¹³C values than A. sacrorum (p<0.01). Leguminous species such as R. pseudoacacia, C. korshinskii, and L. davurica as well as a non-leguminous species with nitrogen-fixation capacity, H. rhamnoides, had higher δ¹³C values than other non-leguminous species with same life-form. The mean δ¹³C value increased by about 7 % from Yangling in the south to Shenmu in the north as climatic drought increased, and foliar δ¹³C values differed much (p<0.001) among the seven sites. For nine species in the Loess Plateau, foliar δ¹³C values were significantly and negatively (p<0.001) correlated with the mean annual precipitation, moreover, an increase of 100 mm in annual precipitation would result in a decrease of 1.2 ‰ in δ¹³C value.     

The δ13C changes in four plant species of the Loess Plateau over the last 70 years

The relative abundance of carbon isotope (δ¹³C) was measured in four C₃ species (Sophora viccifolia, Quercus liaotungensis, Ostryopsis davidiana and Zizyphus jujuba var. spinosa) of the Loess Plateau in China from the 1930’s to 2002. The results showed that the δ¹³C values in the four species varied from −25.05‰ to −29.75‰ with their a average at −27.04‰. A decrease in the δ¹³C value with time was found in all the four species, which indicating that the water use efficiencies (WUEs) of all the measured species declined during 70 years. However, the decrease in δ¹³C value differed among the four species with its significant decreases measured in two of the species, Sophora viciifolia and Quercus liaotungensis, its relatively significant decrease found in Ostryopsis davidiana, and its slight decrease appearing in Zizyphus jujuba var. spinosa. in the δ¹³C values in the four species decreased by 14.65 ‰, 14.46‰, 11.99‰ and 2.44‰, respectively. The different species were shown to have different sensitivities to climatic change, and Zizyphus jujuba var. spinosa was found to be the most drought-tolerant species of the four, which had a high WUE.     

Photosynthetic pathway of dominant plant species in the Qaidam Basin: evidence from foliar stable carbon isotope measurement

Foliar δ¹³C values of Calligogum kozlovi and Haloxylon ammodendron ranged from −13.13 to −15.11 ‰, while those of the rest 11 species were in the range of −22.22 to −27.73 ‰. This indicates that two of 13 dominant plant species in the Qaidam Basin possess a C₄ photosynthetic pathway. Significant differences were observed for the average foliar δ¹³C values between C₃ or C₄ plant communities, between grass and shrub communities, even between the same species derived from different sites. Precipitation accounted for the major part of the differences.     

Carbon nutrition of mature green orchid <Emphasis Type="Italic">Serapias strictiflora</Emphasis> and its mycorrhizal fungus <Emphasis Type="Italic">Epulorhiza</Emphasis> sp.

We studied the nutritional modes of the orchid Serapias strictiflora and its mycorrhizal fungus Epulorhiza sp. using the differences in carbon isotopic composition (δ¹³C) of C₃ orchid and C₄ maize tissues. We found that if cultivated in substrate lacking any organic compounds, the mycorrhizal extraradical mycelia (δ¹³C = −26.3 ± 0.2 ‰) developed well, despite being fully dependent on nutrition from orchid roots (δ¹³C = −28.6 ± 0.1 ‰). If the mycorrhizal fungus had additional access to and colonized decaying maize roots (δ¹³C = −14.6 ± 0.1 ‰), its isotopic composition (δ¹³C = −21.6 ± 0.4 ‰) reflected a mixture of biotrophy and saprotrophy. No statistically significant differences in δ¹³C of new storage tubers were found between Epulorhiza-associated orchids with (δ¹³C = -28.2 ± 0.1 ‰) and without access to maize roots (δ¹³C = −28.6 ± 0.2 ‰). We conclude that autotrophy is the predominant nutritional mode of mature S. strictiflora plants and that they supply their mycorrhizal fungus with substantial amount of carbon (69 ± 3 % of the fungus demand), even if the fungus feeds saprotrophically.     

Characterization of photosynthetic pathway of plant species growing in the eastern Tibetan plateau using stable carbon isotope composition

The photosynthetic pathway of plant species collected at Menyuan, Henan, and Maduo sites, east of Tibetan Plateau, China, during the growing season were studied using stable carbon isotopes in leaves. The 232 samples leaves analyzed belonged to 161 species, 30 families, and 94 genera. The δ¹³C values (from −24.6 to − 29.2 ‰) indicated that all the considered species had a photosynthetic C₃ pathway. The absence of plant species with C₄ photosynthetic pathway might be due to the extremely low air temperature characterizing the Tibetan Plateau. The average δ¹³C value was significantly (p<0.05) different between annuals and perennials at the three considered study sites. Hence the longer-lived species had greater water-use efficiency (WUE) than shorter-lived species, that is, longer-lived species are better adapted to the extreme environmental conditions of the Tibetan Plateau.     

Carbon isotope ratio and photosynthetic activity of phytoplankton in the eutrophic Mikawa Bay,Japan

Primary production by phytoplankton in the eutrophic Mikawa Bay, Japan, was studied by simultaneous measurements of natural carbon isotope ratio (δ ¹³C) and short-term carbon uptake rates (¹³C tracer study) of size-fractionated nannoplankton (<10 μm) and net plankton (>10 μm) samples. Short-term photosynthetic rates, which represent the physiological state of algae, were variable regardless of standing stock sizes. Theδ ¹³C values of particulate organic carbon (POC) in June and July displayed horizontal variations for both the net plankton fraction (−19.8 to −12.7‰) and the nannoplankton fraction (−22.0 to −12.8‰). For both fractions, low concentrations of POC had more negativeδ ¹³C values (−22 to −18‰). Highδ ¹³C values for the net plankton were found when POC concentrations were much higher, due to red tide. This suggests that the increase in algal standing crop for the net plankton fraction resulted from accelerated photosynthetic activity. However the nannoplankton fractions with higher POC values have relatively lowδ ¹³C values.     

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.     

Stable carbon and nitrogen incorporation in blood and fin tissue of the catfish <Emphasis Type="Italic">Pterygoplichthys disjunctivus</Emphasis> (Siluriformes,Loricariidae)

A feeding trial was performed in the laboratory with the catfish species Pterygoplichthys disjunctivus to determine stable carbon (¹³C) and nitrogen (¹⁵ N) turnover rates and discrimination factors in non-lethally sampled tissues (red blood cells, plasma solutes, and fin). A second feeding trial was conducted to determine what P. disjunctivus could assimilate from low-quality wood-detritus—refractory polysaccharides (e.g., cellulose), or soluble wood-degradation products inherent in wood-detritus. This was performed by feeding the fish an artificial wood-detritus diet with fibrous (δ¹³C = −26.36‰; δ¹⁵  N = 2.13‰) and soluble portions (δ¹³C = −11.82‰; δ¹⁵  N = 3.39‰) that had different isotopic signatures and monitoring the dynamics of isotopic incorporation in the different tissues over time. Plasma solutes turned over more quickly than red blood cells for ¹³C and ¹⁵ N. However, in contrast to previous studies of juvenile fishes, C and N incorporation was primarily driven by catabolic tissue turnover as opposed to growth rate. Tissue-diet discrimination factors for ¹⁵ N varied from 4.08 to 5.17‰, whereas they were <2‰ for ¹³C (and less than 0.3‰ for plasma and red blood cells). The results of trial two suggested that P. disjunctivus could not assimilate refractory polysaccharides. Moreover, the δ¹³C and δ¹⁵ N signatures of wild-caught P. disjunctivus from Florida confirmed their detrital trophic standing in Floridian aquatic ecosystems.     

Biological and physical influences on the carbon isotope content of CO<Subscript>2</Subscript> in a subalpine forest snowpack,Niwot Ridge,Colorado

Considerable research has recently been devoted to understanding biogeochemical processes under winter snow cover, leading to enhanced appreciation of the importance of many winter ecological processes. In this study, a comprehensive investigation of the stable carbon isotope composition (δ¹³C) of CO₂ within a high-elevation subalpine forest snowpack was conducted. Our goals were to study the δ¹³C of biological soil respiration under snow in winter, and to assess the relative importance of diffusion and advection (ventilation by wind) for gas transport within snow. In agreement with other studies, we found evidence of an active microbial community under a roughly 1-m deep snowpack during winter and into spring as it melted. Under-snow CO₂ mole fractions were observed up to 3,500 μmol mol⁻¹, and δ¹³C of CO₂ varied from ~−22 to ~−8‰. The δ¹³C of soil respiration calculated from mixing relationships was −26 to −24‰, and although it varied in time, it was generally close to that of the bulk organic horizon (−26.0‰). Subnivean CO₂ and δ¹³C were quite dynamic in response to changes in soil temperature, liquid water availability, and wind events. No clear biologically-induced isotopic changes were observed during periods when microbial activity and root/rhizosphere activity were expected to vary, although such changes cannot be eliminated. There was clear evidence of isotopic enrichment associated with diffusive transport as predicted by theory, but simple diffusive enrichment (4.4‰) was not observed. Instead, ventilation of the snowpack by sustained wind events in the forest canopy led to changes in the diffusively-enriched gas profile. The isotopic influence of diffusion on gases in the snowpack and litter was greatest at greater depths, due to the decreased relative contribution of advection at depth. There were highly significant correlations between the apparent isotopic content of respiration from the soil with wind speed and pressure. In summary, physical factors influencing gas transport substantially modified and potentially obscured biological factors in their effects on δ¹³C of CO₂ within this subalpine forest snowpack.     

Patterns of stable carbon isotope turnover in gag, <Emphasis Type="Italic">Mycteroperca microlepis</Emphasis>, an economically important marine piscivore determined with a non-lethal surgical biopsy procedure

To determine the feasibility of using stable isotopes to track diet shifts in wild gag, Mycteroperca microlepis, populations over seasonal timescales, we conducted a repeated measures diet-shift experiment on four adult gag held in the laboratory. Fish were initially fed a diet of Atlantic mackerel, Scomber scombrus, (mean δ¹³C = −21.3‰ ± 0.2, n = 20) for a period of 56 days and then shifted to a diet of pinfish, Lagodon rhomboids, (mean δ¹³C = −16.6‰ ± 0.6, n = 20) for the 256 day experiment. We developed a non-lethal surgical procedure to obtain biopsies of the muscle, liver, and gonad tissue monthly from the same four fish. We then determined the δ¹³C value of each tissue by isotope ratio mass spectrometry. For the gonad tissue we used the relationship between C/N and lipid content to correct for the influence of lipids on δ¹³C value. We observed a significant shift in the δ¹³C values of all of the tissues sampled in the study. Carbon turnover rates varied among the three tissues, but the shift in diet from mackerel to pinfish was clearly traceable through analysis of δ¹³C values. The turnover rates for muscle tissue were 0.005‰ day⁻¹, and for gonad tissue was 0.009‰ day⁻¹. Although it is generally thought that tissue turnover rates in ectotherms are driven primarily by growth, we found that metabolic rate can be a major factor driving tissue turnover in adult gag.     

Foliar <Superscript>15</Superscript>N natural abundance indicates phosphorus limitation of bog species

Foliar δ¹⁵N, %N and %P in the dominant woody and herbaceous species across nutrient gradients in New Zealand restiad (family Restionaceae) raised bogs revealed marked differences in plant δ¹⁵N correlations with P. The two heath shrubs, Leptospermum scoparium (Myrtaceae) and Dracophyllum scoparium (Epacridaceae), showed considerable isotopic variation (−2.03 to −15.55‰, and −0.39 to −12.06‰, respectively) across the bogs, with foliar δ¹⁵N strongly and positively correlated with P concentrations in foliage and peat, and negatively correlated with foliar N:P ratios. For L. scoparium, the isotopic gradient was not linked to ectomycorrhizal (ECM) fractionation as ECMs occurred only on higher nutrient marginal peats where ¹⁵N depletion was least. In strong contrast, restiad species (Empodisma minus Sporadanthus ferrugineus, S. traversii) showed little isotopic variation across the same nutrient gradients. Empodisma minus and S. traversii had δ¹⁵N levels consistently around 0‰ (means of −0.12‰ and +0.15‰ respectively), and S. ferrugineus, which co-habited with E. minus, was more depleted (mean −4.97‰). The isotopic differences between heath shrubs and restiads were similar in floristically dissimilar bogs and may be linked to contrasting nutrient demands, acquisition mechanisms, and root morphology. Leptospermum scoparium shrubs on low nutrient peats were stunted, with low tissue P concentrations, and high N:P ratios, suggesting they were P-limited, which was probably exacerbated by markedly reduced mycorrhizal colonisations. The coupling of δ¹⁵N depletion and %P in heath shrubs suggests that N fractionation is promoted by P limitation. In contrast, the constancy in δ¹⁵N of the restiad species through the N and P gradients suggests that these are not suffering from P limitation.     

Carbon isotopic fractionation in subtropical brazilian grassland soils. Comparison with tropical forest soils

The natural relationship¹³C/¹²C determined in three soil profiles under grass vegetation indicated a depletion in organic¹³C at depth: theδ ¹³C was between −18‰ and −15‰ in the A horizons and ranged from −18 to −22‰ at depth. Previous work showed that in forest soils, whereδ ¹³C was near −28‰ in the upper horizon, there was, on the contrary, a relative enrichment of the lower strata. This meant thatδ ¹³C, initially different in the various topsoils, became more equal at depth. Comparison between dark, deep horizons (sombric horizons), which are certainly of illuvial origine, would confirm this:δ ¹³C of grassland and a forest sombric horizon were almost equal at around −22‰. These results might mean that, in natural ecosystems, the isotopic carbon composition of the soil underlying humus would be independent of the vegetation type. This would have practical implications for the use of¹³C as a tracer for soil organic matter studies.     

Contributions of Different Organic Carbon Sources to <Emphasis Type="Italic">Daphnia</Emphasis> in the Pelagic Foodweb of a Small Polyhumic Lake: Results from Mesocosm DI<Superscript>13</Superscript>C-Additions

Abstract Freshwater ecosystems derive organic carbon from both allochthonous and autochthonous sources. We studied the relative contributions of different carbon sources to zooplankton in a small, polyhumic, steeply stratified lake, using six replicate surface-to-sediment enclosures established during summer and autumn 2004. We added ¹³C-enriched bicarbonate to the epilimnion of half the enclosures for three weeks during each season and monitored carbon stable isotope ratios of DIC, DOC, POC and Daphnia, along with physical, chemical and biological variables. During summer, ¹³C-enriched DIC (δ¹³C up to 44 ± 7.2‰) was soon taken up by phytoplankton (δ¹³C up to −5.1 ± 13.6‰) and was transmitted to Daphnia (δ¹³C up to −1.7 ± 7.2‰), demonstrating consumption of phytoplankton. In contrast, during autumn, ¹³C-enriched DIC (δ¹³C up to 56.3 ± 9.8‰) was not transmitted to Daphnia, whose δ¹³C became progressively lower (δ¹³C down to −45.6 ± 3.3‰) concomitant with decreasing methane concentration. Outputs from a model suggested phytoplankton contributed 64–84% of Daphnia diet during summer, whereas a calculated pelagic carbon mass balance indicated only 30–40% could have come from phytoplankton. Although autumn primary production was negligible, zooplankton biomass persisted at the summer level. The model suggested methanotrophic bacteria contributed 64–87% of Daphnia diet during autumn, although the calculated carbon mass balance indicated a contribution of 37–112%. Thus methanotrophic bacteria could supply virtually all the carbon requirement of Daphnia during autumn in this lake. The strongly ¹³C-depleted Daphnia values, together with the outputs from the models and the calculated carbon mass balance showed that methanotrophic bacteria can be a greater carbon source for Daphnia in lakes than previously suspected.     

Does Bienertia cycloptera with the single-cell system of C(4) photosynthesis exhibit a seasonal pattern of delta (13)C values in nature similar to co-existing C (4) Chenopodiaceae having the dual-cell (Kranz) system?

Family Chenopodiaceae is an intriguing lineage, having the largest number of C₄ species among dicots, including a number of anatomical variants of Kranz anatomy and three single-cell C₄ functioning species. In some previous studies, during the culture of Bienertia cycloptera Bunge ex Boiss., carbon isotope values (δ¹³C values) of leaves deviated from C₄ to C₃−C₄ intermediate type, raising questions as to its mode of photosynthesis during growth in natural environments. This species usually co-occurs with several Kranz type C₄ annuals. The development of B. cycloptera morphologically and δ¹³C values derived from plant samples (cotyledons, leaves, bracts, shoots) were analyzed over a complete growing season in a salt flat in north central Iran, along with eight Kranz type C₄ species and one C₃ species. For a number of species, plants were greenhouse-grown from seeds collected from the site, in order to examine leaf anatomy and C₄ biochemical subtype. Among the nine C₄ species, the cotyledons of B. cycloptera, and of the Suaeda spp. have the same respective forms of C₄ anatomy occurring in leaves, while cotyledons of members of tribe Caroxyloneae lack Kranz anatomy, which is reflected in the δ¹³C values found in plants grown in the natural habitat. The nine C₄ species had average seasonal δ¹³C values of −13.9‰ (with a range between species from −11.3 to −15.9‰). The measurements of δ¹³C values over a complete growing season show that B. cycloptera performs C₄ photosynthesis during its life cycle in nature, similar to Kranz type species, with a seasonal average δ¹³C value of −15.2‰. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

An isotopic method for testing the influence of leaf litter quality on carbon fluxes during decomposition

During microbial breakdown of leaf litter a fraction of the C lost by the litter is not released to the atmosphere as CO₂ but remains in the soil as microbial byproducts. The amount of this fraction and the factors influencing its size are not yet clearly known. We performed a laboratory experiment to quantify the flow of C from decaying litter into the soil, by means of stable C isotopes, and tested its dependence on litter chemical properties. Three sets of ¹³C-depleted leaf litter (Liquidambar styraciflua L., Cercis canadensis L. and Pinus taeda L.) were incubated in the laboratory in jars containing ¹³C-enriched soil (i.e. formed C4 vegetation). Four jars containing soil only were used as a control. Litter chemical properties were measured using thermogravimetry (Tg) and pyrolysis–gas chromatography/mass spectrometry–combustion interface–isotope ratio mass spectrometry (Py–GC/MS–C–IRMS). The respiration rates and the δ¹³C of the respired CO₂ were measured at regular intervals. After 8 months of incubation, soils incubated with both L. styraciflua and C. canadensis showed a significant change in δ¹³C (δ¹³C_final = −20.2 ± 0.4‰ and −19.5 ± 0.5‰, respectively) with respect to the initial value (δ¹³C_initial = −17.7 ± 0.3‰); the same did not hold for soil incubated with P. taeda (δ¹³C_final:−18.1 ± 0.5‰). The percentages of litter-derived C in soil over the total C loss were not statistically different from one litter species to another. This suggests that there is no dependence of the percentage of C input into the soil (over the total C loss) on litter quality and that the fractional loss of leaf litter C is dependent only on the microbial assimilation efficiency. The percentage of litter-derived C in soil was estimated to be 13 ± 3% of total C loss.     

Stable carbon isotope characteristics of desert plants in the Junggar Basin, China

Desert plants have unique strategies for survival and growth to cope with the limited water availability in arid regions. The stable carbon isotope (δ ¹³C) provides an integrated measurement of internal plant physiological and external environmental properties affecting photosynthetic gas exchange and water use efficiency. The δ ¹³C values of 84 species in the Junggar Basin were categorized into two groups (ranged from −30.1 to −23.3‰ for C₃ and −14.9 to −9.9‰ for C₄ species, respectively). No life form differences in δ ¹³C values were detected in C₃ (p = 0.78) and C₄ plants (p = 0.63). Small differences among life forms were observed in δ ¹³C values in C₄ species with shrubs slightly depleted (−13.3‰) relative to perennials (−13.1‰) and annuals (−12.5‰). These differences suggested that δ ¹³C value could not represent a plant functional group classification based on life forms in C₄ plants in extremely arid regions. Ephemerals are all using C₃ photosynthetic pathway and no significant differences (p = 0.92) in δ ¹³C values were observed between annuals (−26.5‰) and perennials (−26.4‰). The δ ¹³C values of Tulipa iliensis (an important ephemeral species distributed widely in the Junggar Basin) among nine natural populations were positively correlated with leaf (r ² = 0.46, p = 0.046) and soil (r ² = 0.67, p = 0.007) total nitrogen content, and negatively correlated with leaf (r ² = 0.48, p = 0.039) and soil (r ² = 0.79, p = 0.001) water content. This indicated that the variation in δ ¹³C values of T. iliensis was probably caused by both water availability associated stomatal openness and nitrogen availability associated photosynthetic capacity. T. iliensis is very sensitive to water and nitrogen availability in soil.     

Nutrient cycling relative to δ<Superscript>15</Superscript>N and δ<Superscript>13</Superscript>C natural abundance in a coastal wetland with long-term nutrient additions

Because nitrogen and phosphorus are primary resources for plant, algal, and microbial production, increases in nutrient inputs can markedly alter aquatic ecosystems. Coastal wetland plots at Belle W. Baruch Marine Field Laboratory (South Carolina, USA) have been amended with nitrogen and phosphorus for ~20 years to determine the effects of nutrient loading on coastal wetlands. We conducted a survey of δ¹⁵N and δ¹³C natural abundance in coastal wetland organic pools (sediment, vegetation) with long-term nutrient amendments (control, no addition; nitrogen; phosphorus; and nitrogen + phosphorus additions). Additionally, we conducted laboratory assays to quantify pore water nutrient availability and nitrification rates. Marsh vegetation (Spartina alterniflora) had enriched δ¹³C values (mean −14‰) relative to bulk sediment samples (mean −18‰). Nitrogen-amended plots (alone and in combination with phosphorus) had enriched δ¹³C values in the surface sediment (0–5 cm; mean −16.1‰) relative to control (mean −16.5‰) and phosphorus-amended plots (mean −16.8‰). Nitrogen-amended plots also had depleted δ¹⁵N in S. alterniflora leaf tissues (−3.3‰) and surface sediment samples (mean 2.1‰) relative to leaf tissues (mean 2.1‰) or sediment samples (mean 5.8‰) from control or phosphorus-only amended plots. Nitrate availability (as increased pore water concentration) was higher in N-amended plots although ammonium availability did not differ. Phosphorus availability was higher only in phosphorus-only amended plots. Overall, we found that long-term nutrient amendments to coastal wetlands significantly altered nutrient availability and uptake rates as well as natural abundance of δ¹³C and δ¹⁵N in multiple organic matter sources.     

Nitrogen availability patterns in white-sand vegetations of Central Brazilian Amazon

Addressing spatial variability in nitrogen (N) availability in the Central Brazilian Amazon, we hypothesized that N availability varies among white-sand vegetation types (campina and campinarana) and lowland tropical forests (dense terra-firme forests) in the Central Brazilian Amazon, under the same climate conditions. Accordingly, we measured soil and foliar N concentration and N isotope ratios (δ¹⁵N) throughout the campina-campinarana transect and compared to published dense terra-firme forest results. There were no differences between white-sand vegetation types in regard to soil N concentration, C:N ratio and δ¹⁵N across the transect. Both white-sand vegetation types showed very low foliar N concentrations and elevated foliar C:N ratios, and no significant difference between site types was observed. Foliar δ¹⁵N was depleted, varying from −9.6 to 1.6‰ in the white-sand vegetations. The legume Aldina heterophylla had the highest average δ¹⁵N values (−1.5‰) as well as the highest foliar N concentration (2.1%) while the non-legume species had more depleted δ¹⁵N values and the average foliar N concentrations varied from 0.9 to 1.5% among them. Despite the high variation in foliar δ¹⁵N among plants, a significant and gradual ¹⁵N-enrichment in foliar isotopic signatures throughout the campina–campinarana transect was observed. Individual plants growing in the campinarana were significantly enriched in ¹⁵N compared to those in campina. In the white-sand N-limited ecosystems, the differentiation of N use seems to be a major cause of variations observed in foliar δ¹⁵N values throughout the campina–campinarana transect.     

Effect of C3–C4 Vegetation Change on δ13C and δ15N Values of Soil Organic Matter Fractions Separated by Thermal Stability

Carbon isotopic composition of soils subjected to C₃–C₄ vegetation change can be used to estimate C turnover in bulk soil and in soil organic matter (SOM) pools with fast and intermediate turnover rates. We hypothesized that the biological availability of SOM pools is inversely proportional to their thermal stability, so that thermogravimetry can be used to separate SOM pools with contrasting turnover rates. Soil samples from a field plot cultivated for 10.5 years with the perennial C₄ plant Miscanthus×gigantheus were analyzed by thermogravimetry coupled with differential scanning calorimetry (DSC). Three SOM fractions were distinguished according to the differential weight losses and exothermic or endothermic reactions measured by DSC. The δ¹³C and δ¹⁵N values of these three fractions obtained by gradual soil heating were measured by IRMS. The weight losses up to 190 °C mainly reflected water evaporation because no significant C and N losses were detected and δ¹³C and δ¹⁵N values of the residual SOM remained unchanged. The δ¹³C values (−16.4‰) of SOM fraction decomposed between 190 and 390 °C (containing 79% of total soil C) were slightly closer to that of the Miscanthus plant tissues (δ¹³C = −11.8‰) compared to the δ¹³C values (−16.8‰) of SOM fraction decomposed above 390 °C containing the residual 21% of SOM. Thus, the C turnover in the thermally labile fraction was faster than that in thermally stable fractions, but the differences were not very strong. Therefore, in this first study combining TG-DSC with isotopic analysis, we conclude that the thermal stability of SOM was not very strongly related to biological availability of SOM fractions. In contrast to δ¹³C, the δ¹⁵N values strongly differed between SOM fractions, suggesting that N turnover in the soil was different from C turnover. More detailed fractionation of SOM by thermal analysis with subsequent isotopic analysis may improve the resolution for δ¹³C.     

Mechanism for the small-scale movement of carbon among estuarine habitats: organic matter transfer not crab movement

In theory, carbon is highly mobile in aquatic systems. Recent evidence from carbon stable isotopes of crabs (Parasesarma erythrodactyla and Australoplax tridentata), however, shows that in subtropical Australian waters, measurable carbon movement between adjacent mangrove and saltmarsh habitats is limited to no more than a few metres. We tested whether the pattern in crab δ¹³C values across mangrove and saltmarsh habitats was explained by crab movement, or the movement of particulate organic matter. We estimated crab movement in a mark–recapture program using an array of pitfall traps on 13 transects (a total of 65 traps) covering an area of 600 m² across the interface of these two habitats. Over a 19-day period, the majority of crabs (91% for P. erythrodactyla, 93% for A. tridentata) moved <2 m from the place of initial capture. Crab movement cannot, therefore, explain the patterns in δ¹³C values of crabs. δ¹³C values of detritus collected at 2-m intervals across the same habitat interface fitted a sigmoidal curve of a similar form to that fitting the δ¹³C values of crabs. δ¹³C values of detritus were 2–4‰ more depleted in saltmarsh (−18.5±0.6‰), and 4–7‰ more depleted in mangroves (−25.9±0.1‰) than δ¹³C values of crabs recorded previously in each habitat. Assimilation by crabs of very small detrital fragments or microphytobenthos, more enriched in ¹³C, may explain the disparity in δ¹³C values. Nevertheless, the pattern in δ¹³C values of detritus suggests that crabs obtain their carbon from up to several metres away, but without themselves foraging more then a metre or so from their burrow. Such detailed measurements of carbon movement in estuaries provide a spatially explicit understanding of the functioning of food webs in saltmarsh and mangrove habitats.