Radiocarbon evidence for the mining of organic nitrogen from soil by mycorrhizal fungi

Organic nitrogen use by mycorrhizal fungi and associated plants could fuel productivity in nitrogen-limited systems. To test whether fungi assimilated soil-derived organic nitrogen, we compared the ¹⁴C signal (expressed as Δ¹⁴C) from 1950s to 1960s thermonuclear testing in protein and structural carbon of ectomycorrhizal fungi. As expected, structural carbon had Δ¹⁴C similar to recent photosynthesis; however, protein Δ¹⁴C was either higher or lower than structural carbon depending on the fungal taxa. This suggests that some protein carbon derived from uptake of organic nitrogen with different Δ¹⁴C signals. Specimens from two taxa (Lactarius and Russula) adapted to taking up soluble nutrients had protein higher than structural carbon in Δ¹⁴C, indicating uptake of young, post-bomb organic nitrogen, whereas two taxa (Cortinarius and Leccinum) adapted to using insoluble, complex organic nutrients had protein lower than structural carbon in Δ¹⁴C, indicating uptake of old, pre-bomb organic nitrogen. Tuber, a genus common in mineral soil, was also consistently lower in Δ¹⁴C for protein than for structural carbon, with an estimated 10 % of protein carbon originating from old, deep organic nitrogen for this taxon. Our results indicate that radiocarbon can provide evidence of organic nitrogen use in ectomycorrhizal fungi and reflects the exploration depth of different taxa.     

Effects of body size and environment on diet-tissue δC fractionation in fishes

δ¹³C data are often used in trophodynamic research where diet-tissue fractionation (Δδ¹³C) is assumed to be 0-1‰ per trophic level and unaffected by the size of animals or their environment. Variation in Δδ¹³C will influence conclusions about food sources, energy pathways and trophic level. To assess the effects of body size, age and environmental conditions on Δδ¹³C, European sea bass (Dicentrarchus labrax) were reared on constant diets of dab (Limanda limanda) or (Ammodytes marinus) for 2years under natural environmental regimes. Bass were sampled approximately monthly to determine Δδ¹³C for muscle, heart and liver tissue and were 1.66‰, − 0.18‰, − 1.77‰ (sandeel diet) and 1.34‰, − 1.18‰, − 1.75‰ (dab diet) respectively. Arithmetic lipid correction increased Δδ¹³C to > 2‰ for muscle and liver. Δδ¹³C was dependent on body mass and experimental duration (age) and generally declined with weight or time even after correction for lipid content. For liver, increasing temperature increased Δδ¹³C. The Δδ¹³C estimates from this study were compared with all available published Δδ¹³C estimates for fish. Bass muscle Δδ¹³C was similar to previous estimates for fish white muscle Δδ¹³C (1.56 ± 1.10‰) and whole body Δδ¹³C (1.52 ± 1.13‰). Fractionations derived in this study, combined with those from the literature, support the use of diet-tissue fractionation values of between 1‰-2‰ for δ¹³C, rather than the commonly used 0‰ − 1‰. For muscle Δδ¹³C, 1.5‰ is appropriate.     

Investigations of Δ 14C, δ 13C, and δ 15N in vertebrae of white shark (Carcharodon carcharias) from the eastern North Pacific Ocean

The white shark, Carcharodon carcharias, has a complex life history that is characterized by large scale movements and a highly variable diet. Estimates of age and growth for the white shark from the eastern North Pacific Ocean indicate they have a slow growth rate and a relatively high longevity. Age, growth, and longevity estimates useful for stock assessment and fishery models, however, require some form of validation. By counting vertebral growth band pairs, ages can be estimated, but because not all sharks deposit annual growth bands and many are not easily discernable, it is necessary to validate growth band periodicity with an independent method. Radiocarbon (¹⁴C) age validation uses the discrete ¹⁴C signal produced from thermonuclear testing in the 1950s and 1960s that is retained in skeletal structures as a time-specific marker. Growth band pairs in vertebrae, estimated as annual and spanning the 1930s to 1990s, were analyzed for Δ¹⁴C and stable carbon and nitrogen isotopes (δ¹³C and δ¹⁵N). The aim of this study was to evaluate the utility of ¹⁴C age validation for a wide-ranging species with a complex life history and to use stable isotope measurements in vertebrae as a means of resolving complexity introduced into the ¹⁴C chronology by ontogenetic shifts in diet and habitat. Stable isotopes provided useful trophic position information; however, validation of age estimates was confounded by what may have been some combination of the dietary source of carbon to the vertebrae, large-scale movement patterns, and steep ¹⁴C gradients with depth in the eastern North Pacific Ocean.     

Expression of meadowfoam Des5 and FAE1 genes in yeast and in transgenic soybean somatic embryos,and their roles in fatty acid modification

Meadowfoam (Limnanthes spp.) species are unique in that their seeds are rich in the unusual fatty acids Δ⁵-eicosenoic acid (C20:1^Δ5) and the diene, C22:2^{Δ5, Δ13}. Previously the cloning of Δ⁵ desaturase (Des5) and fatty acid elongase 1 (FAE1) meadowfoam genes and their expression in soybean were reported. Here, we present the first successful expression of the Limnanthes Des5 in yeast, resulting in the desaturation of C16:0, C18:0 and C20:0 to their corresponding cis Δ⁵ isomers. In soybean (Glycine max L.), Limnanthes Des5/FAE1 double transformant somatic embryos fed with radiolabeled C14:0 or C16:0 could elongate these substrates to C18:0, C20:0 and C22:0 and C24:0. However, radiolabeled C18:1^Δ9 or C20:1^Δ11 were not elongated to their respective monounsaturated very long-chain products, confirming that the cloned Limnanthes FAE1 homolog gene product was specific for elongating saturated fatty acids. To understand better the biosynthetic pathway for C22:2^{Δ5, Δ13}, soybean somatic embryos transformed with the Des5 cDNA were fed in culture with 〚1-¹⁴C〛C 22:1^Δ13 fatty acid, which resulted in the biosynthesis of 〚1-¹⁴C〛-labeled C22:2^{Δ5, Δ13}. Cell-free preparations enriched with detergent-solubilized Δ⁵ desaturase activity extracted from both developing meadowfoam seeds and from Des5 transgenic soybean embryos, produced ¹⁴C-22:2^{Δ5, Δ13} when supplied with 〚1-¹⁴C〛 C22:1-CoA. Thus, both the in vivo and in vitro experiments showed that the biosynthesis of C22:2^{Δ5, Δ13} can occur in somatic soybean embryos transformed with the Limnanthes Des5 cDNA, and confirmed that the pathway for C22:2 biosynthesis in meadowfoam involves further desaturation of erucoyl-CoA by a Δ⁵-regiospecific desaturase.     

Stereospecificity of sterol biosynthesis in Calendula officinalis flowers

Flowers of Calendula officinalis were incubated with mevalonic acid doubly labelled with ¹⁴C in position 2 and ³H in positions 2R, 2S, 4R or 5R,S and the [³H/¹⁴C] ratios determined in squalene β-sitosterol, stigmasterol, Δ⁷-sterols and stigmastan-3 β-ol. The results indicated that in the biosynthesis of these sterols: formation of the Δ⁷ double bond is associated with elimination of hydrogen from the 7β position, formation of the Δ⁵ double bond with elimination of hydrogens from the 5 and 6α positions, and formation of the Δ²² double bond with elimination of the 22-pro-S and 23 hydrogens. Demethylation in position 4 is associated with elimination of hydrogen from the 3α position whereas demethylation in position 14 occurs without hydrogen loss from position 15. Alkylation in position 24 is associated with hydrogen elimination from this position.     

Univalent ions in phospholipid model membranes: Thermodynamic and hydration aspects

By means of differential scanning calorimetry, effects of systematic series of Group I and VII ions on the phase state of model multibilayer dimyristoylphosphatidylcholine (di(14:0)PC) membranes have been studied at a lipid/ion molar ratio of 3/1. The sign-changing correlations between the ionic radii of cations and temperature shifts of di(14:0)PC phase transition were obtained. For cosmotropic Li⁺ and Na⁺, the observed shifts were positive (LiCl: ΔT _m = 0.6°C; ΔT _p = 1.9°C), whereas chaotropic K⁺ and Rb⁺ presence resulted in negative shifts (RbCl: ΔT _m = ?0.3°C; ΔT _p = ?2.5°C). The anions (Cl^?, Br^?, I^?) showed a similar effect increasing with the ion chaotropicity. An essentially weaker effect of Cs⁺ as compared to other alkali metal ions (CsCl: ΔT _m ≈ 0°C; ΔT _p = ?0.1°C) can be one of the reasons of its accumulation in living organisms. Generalization of all available data allowed us to specify some important factors of lipid-ion interactions that should be taken into account in further investigations in this field.     

Partitioning sources of soil respiration in boreal black spruce forest using radiocarbon

Separating ecosystem and soil respiration into autotrophic and heterotrophic component sources is necessary for understanding how the net ecosystem exchange of carbon (C) will respond to current and future changes in climate and vegetation. Here, we use an isotope mass balance method based on radiocarbon to partition respiration sources in three mature black spruce forest stands in Alaska. Radiocarbon (Δ¹⁴C) signatures of respired C reflect the age of substrate C and can be used to differentiate source pools within ecosystems. Recently‐fixed C that fuels plant or microbial metabolism has Δ¹⁴C values close to that of current atmospheric CO₂, while C respired from litter and soil organic matter decomposition will reflect the longer residence time of C in plant and soil C pools. Contrary to our expectations, the Δ¹⁴C of C respired by recently excised black spruce roots averaged 14‰ greater than expected for recently fixed photosynthetic products, indicating that some portion of the C fueling root metabolism was derived from C storage pools with turnover times of at least several years. The Δ¹⁴C values of C respired by heterotrophs in laboratory incubations of soil organic matter averaged 60‰ higher than the contemporary atmosphere Δ¹⁴CO₂, indicating that the major contributors to decomposition are derived from a combination of sources consistent with a mean residence time of up to a decade. Comparing autotrophic and heterotrophic Δ¹⁴C end members with measurements of the Δ¹⁴C of total soil respiration, we calculated that 47–63% of soil CO₂ emissions were derived from heterotrophic respiration across all three sites. Our limited temporal sampling also observed no significant differences in the partitioning of soil respiration in the early season compared with the late season. Future work is needed to address the reasons for high Δ¹⁴C values in root respiration and issues of whether this method fully captures the contribution of rhizosphere respiration.     

Use of 14C/C ratios to provide insights into the magnitude of carbon turnover in the crustose saxicolous lichen Caloplaca trachyphylla

Abstract:¹⁴C/C ratios in samples from radial transects across individual thalli ofCaloplaca trachyphylla collected at two sites were measured and the results used to investigate whether ¹⁴C/C data might provide some insight into the magnitude of carbon turnover in this lichen species. The ¹⁴C/C data suggest that significant internal recycling/translocation of carbon is unlikely in the sampled thalli. However, converting the¹⁴ C/C data for the larger intact thalli sampled at each site to calendar years, using the atmospheric ¹⁴C record, does not yield constant or even monotonically varying growth rates. Since crustose lichen growth rates are constant or decrease with thallus size, and since the ¹⁴C/C data from these larger thalli show a relatively small spread in¹⁴ C/C data values compared to the Northern Hemisphere atmospheric ¹⁴C record over the past 50 years, the¹⁴ C/C data suggest that carbon turnover may be occurring. Carbon turnover was modelled starting with the atmospheric¹⁴ C record. Turnover was incorporated so that for each year in the record a constant percentage of the total carbon was lost annually and replaced by new photosynthetically fixed carbon with a ¹⁴C/C ratio equal to that of the contemporary atmosphere. The ¹⁴C/C data from the radial samples were then converted to a calendar year using the model record. Constant annual carbon turnover values of 0, 0·5, 1, 1·5, 2, 2·5, 3, 3·5, 4, 4·5, 5, 5·5, 6, 7, 8, 9, 10, 15, 20, 25 and 50% were modelled. Carbon turnover values between 3 and 6% created ¹⁴C model records that when applied to ¹⁴C/C data from the thalli produced constant radial growth rates that were: (1) identical for all lichens at a given site, and (2) independent of lichen size at a given site. The ¹⁴C/C data further indicate that annual carbon turnover in this species of lichen is <10%, independent of the nature of thallus radial growth. The data and modelling suggest that carbon turnover might provide a simple explanation for the ¹⁴C/C data from the thalli and might explain the discrepancies between the standard atmospheric ¹⁴C record and the ¹⁴C/C ratios observed in C. trachyphylla.     

Constraining carbon sources and growth rates of freshwater microbialites in Pavilion Lake using 14C analysis

This study determined the natural abundance isotopic compositions (¹³C, ¹⁴C) of the primary carbon pools and microbial communities associated with modern freshwater microbialites located in Pavilion Lake, British Columbia, Canada. The Δ¹⁴C of dissolved inorganic carbon (DIC) was constant throughout the water column and consistent with a primarily atmospheric source. Observed depletions in DIC ¹⁴C values compared with atmospheric CO₂ indicated effects due either to DIC residence time and/or inputs of ¹⁴C‐depleted groundwater. Mass balance comparisons of local and regional groundwater indicate that groundwater DIC could contribute a maximum of 9–13% of the DIC. ¹⁴C analysis of microbial phospholipid fatty acids from microbialite communities had Δ¹⁴C values comparable with lake water DIC, demonstrating that lake water DIC was their primary carbon source. Microbialite carbonate was also primarily derived from DIC. However, some depletion in microbialite carbonate ¹⁴C relative to lake water DIC occurred, due either to residence time or mixing with a ¹⁴C‐depleted carbon source. A detrital branch covered with microbialite growth was used to estimate a microbialite growth rate of 0.05 mm year^?1 for the past 1000 years, faster than previous estimates for this system. These results demonstrate that the microbialites are actively growing and that the primary carbon source for both microbial communities and recent carbonate is DIC originating from the atmosphere. While these data cannot conclusively differentiate between abiotic and biotic formation mechanisms, the evidence for minor inputs of groundwater‐derived DIC is consistent with the previously hypothesized biological origin of the Pavilion Lake microbialites.     

Observations on the biosynthesis of 24-methylcholesterol and 24-ethylcholesterol by Zea mays

Examination of the sterols of Zea mays shoots has established that the 24-ethylcholesterol is predominately the 24α-epimer, sitosterol, but the 24-methylcholesterol is a mixture of the 24α- and 24β-epimers. After incubation of Z. mays shoots with [2-¹⁴C, (4R)4-³H₁]mevalonic acid the sitosterol had a ³H: ¹⁴C atomic ratio of 2.09:5 which is consistent with previous results indicating that a Δ²⁴⁽²⁵⁾ -sterol is implicated in its biosynthesis. By contrast, the 24α- and 24β-methylcholesterol mixture had a higher ³H: ¹⁴C atomic ratio of 2.82:5. This can be explained by the operation of two routes for the elaboration of the 24-methylcholesterol side chain. One may proceed via Δ²⁴⁽²⁵⁾- and Δ²⁴⁽²⁵⁾-sterols to produce the 24α-methylcholesterol with a ³H: ¹⁴C atomic ratio of 2:5. The other route may involve reduction of either a Δ²⁴⁽²⁸⁾-, a Δ²³- or a Δ²⁵-sterol intermediate to give the 24β1-methylcholesterol with a ³H: ¹⁴C atomic ratio of 3:5. The proportion of these two labelled compounds in the mixture then determines the observed ³H: ¹⁴C atomic ratio (2.82:5). Some evidence for the formation of a Δ²⁵-compound, cyclolaudenol, by Z. mays shoots was provided by incorporation studies employing either [2-¹⁴C]mevalonic acid or [Me-¹⁴C]methionine as the sterol precursor.     

Validation of yellowedge grouper, <Emphasis Type="Italic">Epinephelus flavolimbatus</Emphasis>, age using nuclear bomb-produced radiocarbon

Age validation and estimates of longevity of yellowedge grouper (Epinephelus flavolimbatus) from the Gulf of Mexico (GOM) are needed to inform fishery management decisions. Yellowedge grouper sagittal otoliths (n = 100) were collected, aged using conventional means, and cores were submitted for radiocarbon (¹⁴C) measurement. Radiocarbon values of yellowedge grouper otoliths were compared to established radiocarbon chronologies in the region to validate the age and ageing methodology of this species. The yellowedge grouper chronology displayed a similar sigmoidal trend as previously published chronologies. In addition to the core analysis, multiple areas on otolith sections from eight specimens were analyzed for Δ¹⁴C to validate age estimates for fish born prior to the ¹⁴C increase. Our results indicate that yellowedge grouper live longer than previously reported (minimum of 40 years based on radiocarbon measurements). The validated ageing methodology supported an estimated maximum longevity of 85 years and established that yellowedge grouper have the longest lifespan currently known for any species of grouper in the GOM. Results also indicate a depth-age interaction in that material extracted from adult otolith sections assigned to post-bomb dates exhibited lower Δ¹⁴C values than cores (juvenile material) assigned to the same post-bomb dates. This finding is likely explained by lower ¹⁴C levels reported from water masses at deeper depths (>100 m) which are inhabited by adults.     

Gender-related traits in the dioecious shrub Empetrum rubrum in two plant communities in the Magellanic steppe

Following the theory on costs of reproduction, sexually dimorphic plants may exhibit several trade-offs in energy and resources that can determine gender dimorphism in morphological or physiological traits, especially during the reproductive period.In this study we assess whether the sexes of the dioecious species Empetrum rubrum differ in morphological and ecophysiological traits related to water economy and photochemical efficiency and whether these differences change in nearby populations with contrasting plant communities.We conducted physiological, morphological, sex ratio, and cover measurements in E. rubrum plants in the Magellanic steppe, North-Eastern part of Tierra del Fuego (Argentina), from two types of heathlands with differing community composition.We found differences between sites in soil pH and wind speed at the canopy level. E. rubrum plants exhibited lower photosynthetic height and higher LAI (leaf area index), lower RWC (relative water content) and higher water-use efficiency (lower Δ¹³C) in the heathland with harsher environmental conditions. Gender dimorphism in the physiological response was patent for photochemical efficiency and water use (RWC and Δ¹³C discrimination), with males showing a more conservative strategy in relation to females. Accordingly, male-biased sex ratio in the stress-prone community suggested a better performance of male plants under stressful environmental conditions. The integrated analysis of all variables (photochemical efficiency, RWC, leaf dry matter content (LDMC), pigments, and Δ¹³C) indicated an interaction between gender and heathland community effects in the physiological response. We suggest that female plants may exhibit compensatory mechanisms to face their higher reproductive costs.     

Characterization of three novel desaturases involved in the delta-6 desaturation pathways for polyunsaturated fatty acid biosynthesis from Phytophthora infestans

Phytophthora infestans is the causative agent of potato blight that resulted in the great famine in Ireland in the nineteenth century. This microbe can release large amounts of the C20 very long-chain polyunsaturated fatty acids arachidonic acid (ARA; 20:4Δ^{5, 8, 11, 14}) and eicosapentaenoic acid (EPA; 20:5Δ^{5, 8, 11, 14, 17}) upon invasion that is known to elicit a hypersensitive response to their host plant. In order to identify enzymes responsible for the biosynthesis of these fatty acids, we blasted the recently fully sequenced P. infestans genome and identified three novel putatively encoding desaturase sequences. These were subsequently functionally characterized by expression in Saccharomyces cerevisiae and confirmed that they encode desaturases with Δ12, Δ6 and Δ5 activity, designated here as PinDes12, PinDes6 and PinDes5, respectively. This, together with the combined fatty acid profiles and a previously identified Δ6 elongase activity, implies that the ARA and EPA are biosynthesized predominantly via the Δ6 desaturation pathways in P. infestans. Elucidation of ARA and EPA biosynthetic mechanism may provide new routes to combating this potato blight microbe directly or by means of conferring resistance to important crops.     

Kinetics and mechanism of malonate replacement in bis(malonato)oxovanadate(IV) by oxalate

The kinetics of malonate replacement in bis- (malonato)oxovanadate(IV), [VO(mal)₂H₂O]²⁻(hereafter water molecule will be omitted), by oxalate has been studied by the stopped-flow method. The reaction was found to consist of two consecutive steps (k₁ and k₂: first-order rate constants) passing through a mixed ligand complex, [VO(mal)(ox)]²⁻. The rates for each step depended linearly on the concentrations of free oxalate species, Hox⁻ and ox²⁻. The second-order rate constants for the replacement by ox²⁻ were much larger in the k₁ step than in the k₂ step and the activation parameters were determined as follows: ΔH^≠= 43.5 ± 5.6 kJ mol⁻¹, ΔS±-53 ± 19 J K⁻¹ mol⁻¹ and ΔH≠= 43.6 ± 0.5 kJ mol⁻¹, δS≠ = -62 ± 2 J K^−l mol⁻¹ for the k₁ and k₂ steps, respectively. The volume of activation was determined to be -0.65 ± 0.75 cm³ mol⁻¹ at 20.2 °C by the high-pressure stopped-flow method for the apparent rate constants.     

Variability in carbon uptake and (re)cycling in Antarctic cryptoendolithic microbial ecosystems demonstrated through radiocarbon analysis of organic biomarkers

Cryptoendolithic lichens and cyanobacteria living in porous sandstone in the high‐elevation McMurdo Dry Valleys are purported to be among the slowest growing organisms on Earth with cycles of death and regrowth on the order of 10³–10⁴ years. Here, organic biomarker and radiocarbon analysis were used to better constrain ages and carbon sources of cryptoendoliths in University Valley (UV; 1,800 m.a.s.l) and neighboring Farnell Valley (FV; 1,700 m.a.s.l). Δ¹⁴C was measured for membrane component phospholipid fatty acids (PLFA) and glycolipid fatty acids, as well as for total organic carbon (TOC). PLFA concentrations indicated viable cells comprised a minor (<0.5%) component of TOC. TOC Δ¹⁴C values ranged from ?272‰ to ?185‰ equivalent to calibrated ages of 1,100–2,550 years old. These ages may be the result of fractional preservation of biogenic carbon and/or sudden large‐scale community death and extended period(s) of inactivity prior to slow recolonization and incorporation of ¹⁴C‐depleted fossil material. PLFA Δ¹⁴C values were generally more modern than the corresponding TOC and varied widely between sites; the FV PLFA Δ¹⁴C value (+40‰) was consistent with modern atmospheric CO₂, while UV values ranged from ?199‰ to ?79‰ (calibrated ages of 1,665–610 years). The observed variability in PLFA Δ¹⁴C depletions is hypothesized to reflect variations in the extent of fixation of modern atmospheric CO₂ and the preservation and recycling of older organic carbon by the community in various stages of sandstone recolonization. PLFA profiles and microbial community compositions as determined by molecular genetic characterizations and microscopy differed between the two valleys (e.g., predominance of biomarker 18:2 [>50%] in FV compared to UV), representing microbial communities that may reflect distinct stages of sandstone recolonization and/or environmental conditions. It is thus proposed that Dry Valley cryptoendolithic microbial communities are faster growing than previously estimated.     

Using FAME Analysis to Compare, Differentiate, and Identify Multiple Nematode Species

We have adapted the Sherlock^® Microbial Identification system for identification of plant parasitic nematodes based on their fatty acid profiles. Fatty acid profiles of 12 separate plant parasitic nematode species have been determined using this system. Additionally, separate profiles have been developed for Rotylenchulus reniformis and Meloidogyne incognita based on their host plant, four species and three races within the Meloidogyne genus, and three life stages of Heterodera glycines. Statistically, 85% of these profiles can be delimited from one another; the specific comparisons between the cyst and vermiform stages of H. glycines, M. hapla and M. arenaria, and M. arenaria and M. javanica cannot be segregated using canonical analysis. By incorporating each of these fatty acid profiles into the Sherlock^® Analysis Software, 20 library entries were created. While there was some similarity among profiles, all entries correctly identified the proper organism to genus, species, race, life stage, and host at greater than 86% accuracy. The remaining 14% were correctly identified to genus, although species and race may not be correct due to the underlying variables of host or life stage. These results are promising and indicate that this library could be used for diagnostics labs to increase response time.     

The thermodynamics of the complexation of lanthanides by 1-hydroxy-4,7-disulfo-2-naphthoic acid

The thermodynamic parameters, log β, ΔH and ΔS, for formation of lanthanide-1-hydroxy-4,7- disulfo-2-naphthoic acid complexes have been determined at 25 °C in 0.10 M NaClO₄ solutions by potentiometric and calorimetric titrations. Under the experimental conditions, the data can be explained with the formation of LnL⁻, LnL₂⁵⁻ and LnHL complexes (H₂L²⁻ = 1-hydroxy-4,7-disulfo-2- naphthoic acid anion). At pH < 3 the LnHL complex is the major species, whereas by increasing pH the formation of LnL_n³⁻⁴ⁿ complexes becomes more important. The data are compared to the comparable data for complexing by aromatic carboxylic acids.     

Drivers of radial growth and carbon isotope discrimination of bur oak (Quercus macrocarpa Michx.) across continental gradients in precipitation,vapour pressure deficit and irradiance

Tree‐ring characteristics are commonly used to reconstruct climate variables, but divergence from the assumption of a single biophysical control may reduce the accuracy of these reconstructions. Here, we present data from bur oaks (Quercus macrocarpa Michx.) sampled within and beyond the current species bioclimatic envelope to identify the primary environmental controls on ring‐width indices (RWIs) and carbon stable isotope discrimination (Δ¹³C) in tree‐ring cellulose. Variation in Δ¹³C and RWI was more strongly related to leaf‐to‐air vapour pressure deficit (VPD) at the centre and western edge of the range compared with the northern and wettest regions. Among regions, Δ¹³C of tree‐ring cellulose was closely predicted by VPD and light responses of canopy‐level Δ¹³C estimated using a model driven by eddy flux and meteorological measurements (R² = 0.96, P = 0.003). RWI and Δ¹³C were positively correlated in the drier regions, while they were negatively correlated in the wettest region. The strength and direction of the correlations scaled with regional VPD or the ratio of precipitation to evapotranspiration. Therefore, the correlation strength between RWI and Δ¹³C may be used to infer past wetness or aridity from paleo wood by determining the degree to which carbon gain and growth have been more limited by moisture or light.     

Large 13C/12C and small 15N/14N isotope fractionation in an experimental detrital foodweb (litter–fungi–collembolans)

Correctly estimating the trophic fractionation factors (Δ¹⁵N and Δ¹³C) in controlled laboratory conditions is essential for the application of stable isotope analysis in studies on the trophic structure of soil communities. Laboratory experiments usually suggest large ¹⁵N/¹⁴N and small ¹³C/¹²C trophic fractionation, but in field studies litter-dwelling microarthropods and other invertebrates are consistently enriched in ¹³C relative to plant litter. In the present study, we report data from two laboratory experiments investigating both fungi–collembolans and litter–fungi–collembolans systems. In the fungi–collembolans system, Δ¹⁵N and Δ¹³C averaged 1.4 ± 0.1 and 1.0 ± 0.2 ‰, respectively. In microcosms with fungi-inoculated litter, the difference in δ¹⁵N between collembolans and plant litter averaged 1.5 ± 0.2 ‰, confirming the relatively small ¹⁵N/¹⁴N trophic fractionation at the basal level of detrital foodwebs reported in numerous field studies. In full agreement with field observations, the difference in δ¹³C between bulk litter and collembolans in laboratory microcosms averaged 3.6 ± 0.1 ‰ and only little depended on collembolan species identities or the presence of water-soluble compounds in the litter. We conclude that increased δ¹³C values typical of litter-dwelling decomposers are largely determined by an increased ¹³C content in saprotrophic microorganisms.