Escalating Carbon Export from High-Elevation Rivers in a Warming Climate

High-elevation mountains have experienced disproportionately rapid warming, yet the effect of warming on the lateral export of terrestrial carbon to rivers remains poorly explored and understood in these regions. Here, we present a long-term data set of dissolved inorganic carbon (DIC) and a more detailed, short-term data set of DIC, δ13CDIC, and organic carbon from two major rivers of the Qinghai-Tibetan Plateau, the Jinsha River (JSR) and the Yalong River (YLR). In the higher-elevation JSR with ∼51% continuous permafrost coverage, warming (>3 °C) and increasing precipitation coincided with substantially increased DIC concentrations by 35% and fluxes by 110%. In the lower-elevation YLR with ∼14% continuous permafrost, such increases did not occur despite a comparable extent of warming. Riverine concentrations of dissolved and particulate organic carbon increased with discharge (mobilization) in both rivers. In the JSR, DIC concentrations transitioned from dilution (decreasing concentration with discharge) in earlier, colder years to chemostasis (relatively constant concentration) in later, warmer years. This changing pattern, together with lighter δ13CDIC under high discharge, suggests that permafrost thawing boosts DIC production and export via enhancing soil respiration and weathering. These findings reveal the predominant role of warming in altering carbon lateral export by escalating concentrations and fluxes and modifying export patterns.

The Dynamic Evolution Model of the Chemical and Carbon Isotopic Composition of C1-3 during the Hydrocarbon Generation Process

A new approach is presented in this paper for the dynamic modeling of the chemical and isotopic evolution of C1-3 during the hydrocarbon generation process. Based on systematic data obtained from published papers for the pyrolysis of various hydrocarbon sources (type I kerogen/source rock, type II kerogen/source rock, type III kerogen/source rock, crude oil, and asphalt, etc.), the empirical evolution framework of the chemical and isotopic composition of C1-3 during the hydrocarbon generation process was built. Although the empirical framework was built only by fitting a large amount of pyrolysis data, the chemical and isotopic composition of C1-3 derived from the pyrolysis experiments all follow evolution laws, convincing us that it is applicable to the thermal evolution process of various hydrocarbon sources. Based on the simplified formula of the isotopic composition of mixed natural gas at different maturities (δ13Cmixed), δ13Cmixed = X×niA×δ13CiA+Y×niB×δ13CiBX×niA+Y×niB, it can be derived that the cumulative isotopic composition of alkane generated in a certain maturity interval can be expressed by the integral of the product of the instantaneous isotopic composition and instantaneous yield at a certain maturity point, and then divided by the cumulative yield of alkane generated in the corresponding maturity interval. Thus, the cumulative isotopic composition (A(X)), cumulative yield (B(X)), instantaneous isotope (C(X)), and instantaneous yield (D(x)) in the dynamic model, comply with the following formula during the maturity interval of (X0~X). A(X) = ∫X0XCX×DXdxB(X), where A(X) and B(X) can be obtained by the fitting of pyrolysis data, and D(x) can also be obtained from the derivation of B(X). The dynamic model was applied on the pyrolysis data of Pingliang Shale to illustrate the quantitative evolution of the cumulative yield, instantaneous yield, cumulative isotope, and instantaneous isotope of C1-3 with increasing maturity. The dynamic model can quantify the yield of methane, ethane, and propane, as well as δ13C1, δ13C2, and δ13C3, respectively, during the hydrocarbon generation process. This model is of great significance for evaluating the natural gas resources of hydrocarbon source rock of different maturities and for identifying the origin and evolutionary process of hydrocarbons by chemical and isotopic data. Moreover, this model provides an approach to study the dynamic evolution of the isotope series of C1-3 (including reversed isotopic series), which is promising for revealing the mechanism responsible for isotopic reversal when combined with post-generation studies.

Linking water use efficiency with water use strategy from leaves to communities

Limitations and utility of three measures of water use characteristics were evaluated: water use efficiency (WUE), intrinsic WUE and marginal water cost of carbon gain ( ∂ E / ∂ A ) estimated, respectively, as ratios of assimilation (A) to transpiration (E), of A to stomatal conductance (gs ) and of sensitivities of E and A with variation in gs . Only the measure ∂ E / ∂ A estimates water use strategy in a way that integrates carbon gain relative to water use under varying environmental conditions across scales from leaves to communities. This insight provides updated and simplified ways of estimating ∂ E / ∂ A and adds depth to understanding ways that plants balance water expenditure against carbon gain, uniquely providing a mechanistic means of predicting water use characteristics under changing environmental scenarios.

Leaf carbon isotope tracks the facilitation pattern of legume shrubs shaped by water availability and species replacement along a large elevation gradient in Trans-Himalayas

BACKGROUND AND AIMS

Understanding patterns and mechanisms of nurse plant facilitation is important to predict the resilience of arid/semi-arid ecosystems to climate change. We investigate whether water availability and nurse species turnover interact to shape the facilitation pattern of widespread legume shrubs along a large elevation gradient. We also investigate whether leaf δ13C of nurse plants can track the facilitation pattern.

METHODS

We measured the relative interaction index (RII) of the number of species within and outside the canopy of two widespread legume shrub species (Caragana gerardiana and Caragana versicolor) alternatively distributed along a large elevation gradient in the Trans-Himalayas. We also assessed the proportional increase of species richness (ISR) at the community level using the paired plot data. To determine site-specific water availability, we measured the leaf δ13C of nurse shrubs and calculated the Thornthwaite moisture index (MI) for each elevation site.

KEY RESULTS

Elevational variations in RII, ISR and δ13C were mainly explained by the MI when the effects of soil nitrogen and plant traits (leaf nitrogen and shrub size) were controlled. Variations in RII and ISR across the two nurse species were explained better by δ13C than by smoothly changing climatic factors along elevation. At the transition zone between the upper limit of C. gerardiana (4100 m) and the lower limit of C. versicolor (4200 m), RII and ISR were much higher in C. versicolor than in C. gerardiana under a similar MI. Such an abrupt increase in facilitation induced by nurse species replacement was well tracked by the variation of δ13C.

CONCLUSIONS

Water availability and nurse species replacement are crucial to shaping facilitation patterns by legume shrubs along a large elevation gradient in dry mountainous regions, such as the Trans-Himalayas. Turnover in nurse species under global change might significantly alter the pattern of nurse plant facilitation associated with water availability, which can be well tracked by leaf δ13C.

The Carbon Isotope Composition of Epiphytes Depends Not Only on Their Layers, Life Forms, and Taxonomical Groups but Also on the Carbon and Nitrogen Indicators of Host Trees

The carbon isotopic composition of plant tissues is a diagnostic feature of a number of physiological and ecological processes. The most important of which is the type of photosynthesis. In epiphytes, two peaks of δ13C values are known to correspond to C3 and CAM photosynthesis and some variants of transitional forms between them. But the diagnosis of δ13C may not be limited to the type of photosynthesis. This makes it necessary to study trends in the distribution of δ13C in a broader ecological context. In this study, we present trends in the distribution of δ13C epiphytes and other structurally dependent plants and their relationship with other isotopic and elemental parameters (δ15N, C%, N%, and C/N) and with life forms of epiphytes, taxonomic or vertical groups in crowns (synusia), and the parameters of the trees themselves. In all communities except for the moss forest, δ13C in epiphyte leaves was significantly higher (less negative) than in phorophyte leaves. In general, δ13C in epiphytes in mountain communities (pine forest and moss forest) was more negative than in other communities due to the absence of succulents with CAM. δ13C in the leaves of all epiphytes was negatively related to the percentage of carbon and δ15N in the leaves of the phorophyte. When considering the Gaussian distributions of δ13C with the method of modeling mixtures, we observe the unimodal, bimodal, and trimodal nature of the distribution.

Distinct Radiocarbon Ages Reveal Two Black Carbon Pools Preserved in Large River Estuarine Sediments

Black carbon (BC), a group of environmentally concentrated organic pollutants, is widely distributed in marine sediments via riverine run-off and atmospheric deposition. The fate of BC transformation and cycling in marine sediments, however, has not been well studied. Here, we present radiocarbon measurements for sedimentary solid-phase BC (SBC) and porewater-dissolved BC (DBC) in surface sediments collected from the Yangtze and Yellow River estuaries and their adjacent coastal regions. Radiocarbon results revealed that two distinct BC pools in the sediments of the SBC had ancient radiocarbon ages (7110-15,850 years BP) that were 5370-14,935 years older than the ¹⁴C ages of porewater DBC. Using a radiocarbon mass balance model, we calculated that modern biomass-derived BC contributed 77-97% of the DBC pool and that fossil material-produced BC accounted for 61-87% of the SBC pools. This discrepancy between modern and dead BC contributions was associated with the BC budget after particulate BC (PBC) deposition; 38 ± 13% of the PBC was transferred to porewater as DBC and 62 ± 13% was sequestrated as SBC in sediments, serving as an important CO₂ sink in marine sediments. We also provide evidence suggesting that DBC likely comprises some very fine particulate forms that are not completely dissolved as molecules. The nature and transformation mechanisms of DBC in natural aquatic systems need to be further studied.

Carbon Isotope Fractionation Characteristics of Normally Pressured Shale Gas from the Southeastern Margin of the Sichuan Basin; Insights into Shale Gas Storage Mechanisms

Since the development of shale gas in the Wufeng-Longmaxi Formation in the Sichuan Basin, China's shale gas production and reserves have increased rapidly. The southeastern margin of the Sichuan Basin is located in a normally pressured transition zone, where single well gas production varies greatly under complex geological structures. In order to reveal the shale gas enrichment mechanism and favorable shale gas regions, shale gas samples from production wells were collected from different structures, with the formation pressure coefficient ranging between 0.98 and 1.35. The gas components and carbon isotope characteristics of normally pressured shale gas were investigated. The carbon isotope characteristics of the Wufeng-Longmaxi shale gas from the basin scale was mainly controlled using thermal maturity; as the thermal maturity increased, heavier carbon isotopes were found, in addition to drier shale gas. For normally pressured shale gas, the composition of δ¹³C₁ and δ¹³C₂ becomes heavier, and the dryness coefficient decreases with the decreasing pressure coefficient; this is not consistent with the results from thermal evolution. By comparing possible influencing factors, it is evident that the change in geological structure destroys the original shale gas reservoir, which leads to the escape of some gases, and it may be the main factor that contributes to the gas geochemical characteristics of the normally pressured shale gas. Compared with the geological parameters of the shale samples, such as mineral composition, organic abundance, organic pore distribution, and gas content, the carbon isotope characteristics of normally pressured shale gas show a higher efficiency, thus indicating favorable sweet spot evaluations for shale gas in the studied areas.

Explicitly accounting for needle sugar pool size crucial for predicting intra-seasonal dynamics of needle carbohydrates δ18 O and δ13 C

We explore needle sugar isotopic compositions (δ¹⁸ O and δ¹³ C) in boreal Scots pine (Pinus sylvestris) over two growing seasons. A leaf-level dynamic model driven by environmental conditions and based on current understanding of isotope fractionation processes was built to predict δ¹⁸ O and δ¹³ C of two hierarchical needle carbohydrate pools, accounting for the needle sugar pool size and the presence of an invariant pinitol pool. Model results agreed well with observed needle water δ¹⁸ O, δ¹⁸ O and δ¹³ C of needle water-soluble carbohydrates (sugars + pinitol), and needle sugar δ¹³ C (R²  = 0.95, 0.84, 0.60, 0.73, respectively). Relative humidity (RH) and intercellular to ambient CO₂ concentration ratio (C_i /C_a ) were the dominant drivers of δ¹⁸ O and δ¹³ C variability, respectively. However, the variability of needle sugar δ¹⁸ O and δ¹³ C was reduced on diel and intra-seasonal timescales, compared to predictions based on instantaneous RH and C_i /C_a , due to the large needle sugar pool, which caused the signal formation period to vary seasonally from 2 d to more than 5 d. Furthermore, accounting for a temperature-sensitive biochemical ¹⁸ O-fractionation factor and mesophyll resistance in ¹³ C-discrimination were critical. Interpreting leaf-level isotopic signals requires understanding on time integration caused by mixing in the needle sugar pool.

Atmospheric Particles Are Major Sources of Aged Anthropogenic Organic Carbon in Marginal Seas

Deposition of atmospheric particulates is a major pathway for transporting materials from land to the ocean, with important implications for climate and nutrient cycling in the ocean. Here, we report the results of year-round measurements of particulate organic carbon (POC) and black carbon (BC) in atmospheric aerosols collected on Tuoji Island in the coastal Bohai-Yellow Sea of China (2019-2020) and during a cruise in the western North Pacific. Aerosol POC contents ranged from 1.9 to 11.9%; isotope values ranged from -18.8 to -29.0‰ for δ¹³C and -150 to -892‰ for Δ¹⁴C, corresponding to ¹⁴C ages of 1,235 to 17,780 years before present (BP). Mass balance calculations indicated that fossil carbon contributed 19-66% of the POC, with highest values in winter. BC produced from fossil fuel combustion accounted for 18-54% of the POC. "Old" BC (mean 6,238 ± 740 yr BP) was the major contributor to POC, and the old ages of aerosol POC were consistent with the ¹⁴C ages of total OC preserved in surface sediments of the Bohai-Yellow Sea and East China Sea. We conclude that atmospheric deposition is an important source of aged OC sequestered in marginal sea sediments and thus represents an important sink for carbon dioxide from the atmosphere.

Carbon isotope and soluble metabolites reflect physiological status among contrasting faba bean genotypes in response to water deficit

Identification and validation of biomarkers and bioindicators to select genotypes with superior tolerance to water deficit (WD) under field conditions are paramount to plant breeding programs. However, the co-occurrence of different abiotic stresses such as WD, heat, and radiation makes it difficult to develop generalized protocols to monitor the physiological health of the plant system. The study assessed the most abundant carbohydrates and sugar alcohols in five faba bean (Vicia faba) genotypes under field conditions and the abundance of naturally occurring carbon isotopes in bulk leaf material to predict water use efficiency (WUE). Plant water status and biomass accumulation were also assessed. Among the accumulated sugars, inter-specific variation in glucose was most prevalent and was found at a higher concentration (8.52 mg g^-1 leaf) in rainfed trial. myo-Inositol concentrations followed that of glucose accumulation in that the rainfed trial had higher amounts compared to the irrigated trial. WUE calculated from carbon isotope abundance was consistently offset with measured WUE from measurements of leaf gas exchange. All genotypes demonstrated significant relationships between predicted and measured WUE (p < 0.05) apart from control variety PBA Warda. Thus, bulk leaf-level carbon isotope abundance can be used to calculate WUE and used as an effective selection criterion for improving WUE in faba bean breeding programs under field conditions.

Natural variation identifies new effectors of water-use efficiency in Arabidopsis

Water-use efficiency (WUE) is the ratio of biomass produced per unit of water consumed; thus, it can be altered by genetic factors that affect either side of the ratio. In the present study, we exploited natural variation for WUE to discover loci affecting either biomass accumulation or water use as factors affecting WUE. Genome-wide association studies (GWAS) using integrated WUE measured through carbon isotope discrimination (δ¹³C) of Arabidopsis thaliana accessions identified genomic regions associated with WUE. Reverse genetic analysis of 70 candidate genes selected based on the GWAS results and transcriptome data identified 25 genes affecting WUE as measured by gravimetric and δ¹³C analyses. Mutants of four genes had higher WUE than wild type, while mutants of the other 21 genes had lower WUE. The differences in WUE were caused by either altered biomass or water consumption (or both). Stomatal density (SD) was not a primary cause of altered WUE in these mutants. Leaf surface temperatures indicated that transpiration differed for mutants of 16 genes, but generally biomass accumulation had a greater effect on WUE. The genes we identified are involved in diverse cellular processes, including hormone and calcium signaling, meristematic activity, photosynthesis, flowering time, leaf/vasculature development, and cell wall composition; however, none of them had been previously linked to WUE. Thus, our study successfully identified effectors of WUE that can be used to understand the genetic basis of WUE and improve crop productivity.

Carbon Isotope Fractionation during the Formation of CO2 Hydrate and Equilibrium Pressures of 12CO2 and 13CO2 Hydrates

Knowledge of carbon isotope fractionation is needed in order to discuss the formation and dissociation of naturally occurring CO₂ hydrates. We investigated carbon isotope fractionation during CO₂ hydrate formation and measured the three-phase equilibria of ¹²CO₂–H₂O and ¹³CO₂–H₂O systems. From a crystal structure viewpoint, the difference in the Raman spectra of hydrate-bound ¹²CO₂ and ¹³CO₂ was revealed, although their unit cell size was similar. The δ¹³C of hydrate-bound CO₂ was lower than that of the residual CO₂ (1.0–1.5‰) in a formation temperature ranging between 226 K and 278 K. The results show that the small difference between equilibrium pressures of ~0.01 MPa in ¹²CO₂ and ¹³CO₂ hydrates causes carbon isotope fractionation of ~1‰. However, the difference between equilibrium pressures in the ¹²CO₂–H₂O and ¹³CO₂–H₂O systems was smaller than the standard uncertainties of measurement; more accurate pressure measurement is required for quantitative discussion.

[Sources and Fate of Nitrate in Groundwater in a Typical Karst Basin: Insights from Carbon, Nitrogen, and Oxygen Isotopes]

Multiple isotopes (C, N, and O) and hydrochemical data were used to trace the sources and fate of nitrate in ground and surface waters of the Babu subterranean stream watershed in Guizhou Province. The origin of nitrate in the water samples was also quantitatively analyzed by the SIAR model. Approximately 38% of the groundwater samples were not drinkable because the nitrate exceeded the drinking water standard, thereby indicating that the groundwater was seriously polluted by nitrate. The ranges of δ¹⁵N-NO₃, δ¹⁸O-NO₃, and δ¹⁸O-H₂O in groundwater were 2.30‰-30.33‰ (mean of 9.68‰), 2.65‰-13.73‰ (mean of 6.64‰), and -8.83‰﹣-7.37‰ (mean of -8.18‰), respectively. Based on the stable isotopic compositions (δ¹⁵N-NO₃, δ¹⁸O-NO₃, and δ¹⁸O-H₂O), nitrification was the dominant process in the basin. The nitric acid produced by nitrification promoted the dissolution of carbonate rocks, thereby leading to a significantly negative correlation (P<0.001) between the carbon isotope of dissolved inorganic carbon (δ¹³C_DIC) and δ¹⁵N-NO₃ and indicating that δ¹³C_DIC, combined with δ¹⁵N-NO₃, is effective in exploring the fate of nitrate in karst groundwater. The nitrate in the ground and surface waters mainly originated from soil N, manure and sewage, and ammonium nitrogen fertilizer. The results of the SIAR model showed that the contributions of soil N, manure and sewage, and ammonium nitrogen fertilizer were 36.19%, 33.71%, and 30.10% in groundwater, respectively, and 39.15%, 36.08%, and 24.77% in surface water, respectively. Therefore, it would be more effective to reduce the nitrate recharge flux in groundwater by simultaneously removing nitrate and ammonium nitrogen during wastewater treatment and by adopting scientific fertilization technology in agricultural areas.

The combined effects of water and nitrogen on the relationship between a native hemiparasite and its invasive host

Stem hemiparasites are dependent on their hosts for water and nitrogen. Most studies, however, have assessed the influence of one factor on parasite : host associations, thus limiting our mechanistic understanding of their performance in nature. We investigated the combined effects of water and nitrogen (N) availability on both a host (Ulex europaeus) and its parasite (Cassytha pubescens). Parasite infection significantly decreased host shoot biomass and shoot : root ratio more severely in high water than low water, irrespective of N supply. Parasite stem [N] was significantly higher in high water than low water treatments, regardless of N supply, but parasite biomass did not vary among treatments. Irrespective of water and N supply, infected plants had significantly lower total, root and nodule biomass, predawn and midday quantum yields, maximum electron transport rates, water potentials and nitrogen concentration [N]. Parasite δ¹³ C was significantly higher than that of the host. Our results suggested that stem hemiparasites can better extract resources from hosts when water availability is high, resulting in a greater impact on the host under these conditions. When hemiparasitic plants are being investigated as a biocontrol for invasive weeds, they may be more effective in wetter habitats than in drier ones.

[Concentration and Carbon Isotope Composition of DOC and DIC in the Miyun Reservoir Watershed in Summer]

This study aims to complement existing research on the carbon cycle of water in reservoirs, for the effective control of nutrient input in drinking water. The content and isotopic compositions of water dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) from the Miyun Reservoir watershed in Beijing were studied, and water from the Jingmi diversion canal was also studied as a baseline reference. The results showed that the content of DOC in the Miyun Reservoir watershed in summer varied from 1.07 to 5.19 mg·L^-1, with an average value of 2.61 mg·L^-1. DOC in tributaries was lower than that of most rivers in China and globally, while DOC in reservoirs, particularly in Miyun Reservoir, was high. The high water level might be the main reason for the high DOC in the Miyun Reservoir. In summer, the δ¹³C_DOC in the Miyun Reservoir watershed ranged from -27.4‰ to -24.3‰, with an average value of -25.8‰. Results from the isotopic analysis showed that the water in the Miyun Reservoir watershed was less affected by human sources, with the exception of some points. In summer, DOC in tributaries and in the Miyun Reservoir was mainly derived from soil organic matter and terrestrial C3 plants. In addition, endogenous substances also contributed to DOC in the watershed. DOC concentration increased along the Chao and Bai Rivers, and DOC from soil erosion and human sources was higher in the Chao River. The δ¹³C_DIC in the study area varied from -12.6‰ to 5.75‰, with a mean value of -9.44‰. The weathering of carbonate rocks accounted for the major river DIC, and DIC had been clearly assimilated by the phytoplankton by photosynthesis. DOC and DIC in the Miyun Reservoir were significantly higher than in the Jingmi diversion canal (P<0.01), and the δ¹³C_DIC in the Miyun Reservoir showed a clear positive deviation. This indicated that there were differences in carbon concentration and components in the two kinds of water, which might affect the denitrification capacity of the water in the Miyun Reservoir after mixing. In general, the dissolved carbon in the water of the Miyun Reservoir watershed is less affected by human sources. The conversion of DIC to DOC may also be a potential source of DOC in the study area.

Isotopic evidence for the timing of the dietary shift toward C4 foods in eastern African Paranthropus

New approaches to the study of early hominin diets have refreshed interest in how and when our diets diverged from those of other African apes. A trend toward significant consumption of C₄ foods in hominins after this divergence has emerged as a landmark event in human evolution, with direct evidence provided by stable carbon isotope studies. In this study, we report on detailed carbon isotopic evidence from the hominin fossil record of the Shungura and Usno Formations, Lower Omo Valley, Ethiopia, which elucidates the patterns of C₄ dietary utilization in the robust hominin Paranthropus The results show that the most important shift toward C₄ foods occurred at ∼2.37 Ma, within the temporal range of the earliest known member of the genus, Paranthropus aethiopicus, and that this shift was not unique to Paranthropus but occurred in all hominins from this fossil sequence. This uptake of C₄ foods by hominins occurred during a period marked by an overall trend toward increased C₄ grazing by cooccurring mammalian taxa from the same sequence. However, the timing and geographic patterns of hominin diets in this region differ from those observed elsewhere in the same basin, where environmental controls on the underlying availability of various food sources were likely quite different. These results highlight the complexities of dietary responses by hominins to changes in the availability of food resources.

Stable isotopes in hair reveal dietary protein sources with links to socioeconomic status and health

Carbon and nitrogen isotope ratios in hair sampled from 65 communities across the central and intermountain regions of the United States and more intensively throughout 29 ZIP codes in the Salt Lake Valley, Utah, revealed a dietary divergence related to socioeconomic status as measured by cost of living, household income, and adjusted gross income. Corn-fed, animal-derived proteins were more common in the diets of lower socioeconomic status populations than were plant-derived proteins, with individual estimates of animal-derived protein diets as high as 75%; United States towns and cities averaged 57%. Similar patterns were seen across the socioeconomic status spectrum in the Salt Lake Valley. It is likely that corn-fed animal proteins were associated with concentrated animal-feeding operations, a common practice for industrial animal production in the United States today. Given recent studies highlighting the negative impacts of animal-derived proteins in our diets, hair carbon isotope ratios could provide an approach for scaling assessments of animal-sourced foods and health risks in communities across the United States.

The impact of a native hemiparasite on a major invasive shrub is affected by host size at time of infection

Many studies have investigated the effect of parasitic plants on their hosts; however, few have examined how parasite impact is affected by host size. In a glasshouse experiment, we investigated the impact of the Australian native hemiparasitic vine, Cassytha pubescens, on a major invasive shrub, Ulex europaeus, of different sizes. Infected plants had significantly lower total, shoot, and root biomass, but the parasite's impact was more severe on small than on large hosts. When infected, small but not large hosts had significantly lower nodule biomass. Irrespective of size, infection significantly decreased the host shoot/root ratio, pre-dawn and midday quantum yields, maximum electron transport rates, and carbon isotope composition, and the host nodule biomass per gram of root biomass significantly increased in response to infection. Infection did not affect host foliar nitrogen concentration or midday shoot water potential. Parasite biomass was significantly lower on small relative to large hosts, but was similar when expressed on a per gram of host total biomass basis. Parasite stem nitrogen, phosphorus, and potassium concentrations were significantly greater when C. pubescens was growing on small than on large hosts. Our results clearly show that C. pubescens strongly decreases performance of this major invasive shrub, especially when hosts are small. This suggests that C. pubescens could be used most effectively as a native biocontrol when deployed on smaller hosts.

Temporal Variations of Water Chemistry in the Wet Season in a Typical Urban Karst Groundwater System in Southwest China

It is important to investigate temporal variations of water chemistry for the purpose of improving water quality in karst groundwater systems. Groundwater samples were collected daily under various land uses of Guiyang. Major ions and stable carbon isotope composition of dissolved inorganic carbon (δ¹³C_DIC) were analyzed to understand the biogeochemical processes. The water chemistry was dominated by Ca²⁺, Mg²⁺, HCO₃^-, and SO₄^2-, which mainly derived from the dissolution of carbonate rocks (limestone and dolomite) and oxidation of sulfide. The groundwater was defined as of the HCO₃-Ca Mg and HCO₃·SO₄-Ca·Mg type, according to its hydrochemical characteristics. Results suggested that hydrochemical concentrations changed quickly, in response to rainfall events. The fast response revealed that karst groundwater was easily impacted by rainfall and anthropogenic inputs according to temporal variation of water chemistry. The distribution of DIC (dissolved inorganic carbon) and δ¹³C_DIC showed that DIC is mainly sourced from soil CO₂(g) influx and carbonate dissolution. δ¹³C_DIC and major ions ratios suggested that carbonate minerals were dissolved by H₂SO₄ at groundwater in wooded area, contributing an important source for DIC due to the slight enrichment of heavy δ¹³C_DIC. More negative δ¹³C_DIC values were observed after rainfall reflected the fact that soil CO₂(g) and organic carbon oxidation influxes accounted for a large share during DIC formation. Various δ¹³C_DIC and hydrochemical patterns were observed under various land use and human activity conditions. Meanwhile, relative high nitrate loads were found in groundwater after rainfall, suggesting high anthropogenic inputs following rainwater as having side effects on water quality. This study suggests that water chemistry and isotopic proof provide a better understanding of water quality and carbon dynamics responding to rainfall events in the karst groundwater systems.

The role of leaf water potential in the temperature response of mesophyll conductance

Variation in temperature (T) is usually accompanied by changes in leaf water potential (Ψ_leaf ), which may influence mesophyll conductance (g_m ). However, the effects of Ψ_leaf on g_m have not yet been considered in models of the g_m response to temperature. Temperature responses of g_m and Ψ_leaf and the response of g_m to Ψ_leaf were studied in rice (Oryza sativa) and wheat (Triticum aestivum), and then an empirical model of Ψ_leaf was incorporated into an existing g_m -T model. In wheat, Ψ_leaf was dramatically decreased with increasing T, whereas in rice Ψ_leaf was less sensitive or insensitive to T. Without taking Ψ_leaf into account, g_m for wheat showed no response to T. However, at a given Ψ_leaf , g_m was significantly higher at high temperature compared with low. After incorporating the function of Ψ_leaf into the g_m -T model, we suggest that the g_m -T relationship can be influenced by the activation and deactivation energy for membrane permeability, Ψ_leaf gradient between temperatures, and the sensitivity of g_m to Ψ_leaf , below a threshold (Ψ_leaf,0 ). The data presented here suggest that Ψ_leaf plays an important role in the g_m -T relationship and should be considered in future studies related to the temperature response of g_m and photosynthesis.

[Geochemical Characteristics and Ecological Significance of Carbon Isotopes in Groundwater Under the Influence of Different Land Use Types in Karst Areas]

Based on the seasonal characteristics of groundwater hydrochemistry and the carbon isotopes (δ¹³C) of dissolved inorganic carbon (DIC) in the Hongjiadu Basin, Guizhou Province, this paper discusses the natural processes and anthropogenic factors affecting the characteristics of δ¹³C_DIC in karst groundwaters under different land use types. The results show that the main sources of DIC in groundwater are carbonate weathering and soil CO₂. In winter, the δ¹³C_DIC values for groundwater ranged from -14.8‰ to -4.1‰ with an average of -10.1‰ and, in summer, ranged from -14.5‰ to -6.3‰ with an average of -10.2‰. Sulfuric acid from sulfide oxidation in coal-bearing strata and acid rain is involved in carbonate weathering, resulting in the enrichment of groundwater with heavy carbon isotopes. Due to the soil CO₂ effect, the δ¹³C_DIC values of woodland groundwater experiencing less disturbance from human activities are lower in summer than in winter. The degradation of organic matter input from residential areas is a significant contributor of DIC to groundwater. The average values of δ¹³C_DIC in winter and summer were -11.9‰ and -11.6‰, respectively, and the seasonal difference was relatively small in residential areas. During different seasons and for different types of land use, human activities could lead to differences in groundwater δ¹³C_DIC values and hydrochemistry. Therefore, δ¹³C_DIC can reflect the impact of human activities on karst aquifers, which has important ecological significance.

Associations of plasma, RBCs, and hair carbon and nitrogen isotope ratios with fish, meat, and sugar-sweetened beverage intake in a 12-wk inpatient feeding study

BACKGROUND

Naturally occurring carbon and nitrogen stable isotope ratios [13C/12C (CIR) and 15N/14N (NIR)] are promising dietary biomarkers. As these candidate biomarkers have long tissue residence times, long-term feeding studies are needed for their evaluation.

OBJECTIVE

Our aim was to evaluate plasma, RBCs, and hair CIR and NIR as biomarkers of fish, meat, and sugar-sweetened beverage (SSB) intake in a 12-wk dietary intervention.

METHODS

Thirty-two men (aged 46.2 ± 10.5 y; BMI: 27.2 ± 4.0 kg/m2) underwent a 12-wk inpatient dietary intervention at the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) in Phoenix, Arizona. The effects of fish, meat, and SSB intake on CIR and NIR were evaluated using a balanced factorial design, with each intake factor at 2 levels (present/absent) in a common, background diet (50% carbohydrate, 30% fat, 20% protein). Fasting blood samples were taken biweekly from baseline, and hair samples were collected at baseline and postintervention. Data were analyzed using multivariable regression.

RESULTS

The postintervention CIR of plasma was elevated when diets included meat (β = 0.89, 95% CI: 0.73,1.05) and SSBs (β = 0.48, 95% CI: 0.32, 0.64). The postintervention NIR of plasma was elevated when diets included fish (β = 0.85, 95% CI: 0.64, 1.05) and meat (β = 0.61, 95% CI: 0.42, 0.8). Results were similar for RBCs and hair. Postintervention RBC CIR and NIR had strong associations with baseline, suggesting that turnover to the intervention diets was incomplete after 12 wk. Estimates of isotopic turnover rate further confirmed incomplete turnover of RBCs.

CONCLUSIONS

CIR was associated with meat and SSBs, and more strongly with meat. NIR was associated with fish and meat, and more strongly with fish. Overall, CIR and NIR discriminated between dietary fish and meat, and to a lesser extent SSBs, indicating their potential utility as biomarkers of intake in US diets. Approaches to make these biomarkers more specific are needed. This trial was registered at clinicaltrials.gov as NCT01237093.

Maintenance of photosynthesis and the antioxidant defence systems have key roles for survival of Halopeplis perfoliata (Amaranthaceae) in a saline environment

Coastal salt marsh plants employ various combinations of morphological and physiological adaptations to survive under saline conditions. Little information is available on salinity tolerance mechanisms of Halopeplis perfoliata, a C3 stem succulent halophyte. We investigated the growth, photosynthesis and antioxidant defence mechanisms of H. perfoliata under saline conditions (0, 150, 300 and 600 mM NaCl) in an open greenhouse. Optimal shoot succulence, projected shoot area and relative growth rate were obtained in the low (150 mm NaCl) salinity treatment, while growth was inhibited at the highest salinity (600 mm NaCl). The CO₂ compensation point and carbon isotope composition of biomass confirmed C₃ photosynthesis. Increases in salinity did not affect the photosynthetic pigment content or maximum quantum efficiency of PSII of H. perfoliata. Assimilation of CO₂ (A) also remained unaffected by salinity. A modest effect on some gas exchange and photochemistry parameters was observed at 600 mm NaCl. With increasing salinity, there was a continual increase in respiration, suggesting utilisation of energy to cope with saline conditions. Under 300 and 600 mm NaCl, there was an increase in H₂ O₂ and MDA with a concomitant rise in AsA, GR content and CAT activity. Hence, H. perfoliata appears to be an obligate halophyte that can grow up to seawater salinities by modulating photosynthetic gas exchange, photochemistry and the antioxidant defence systems.

Competition Between Chemolithotrophic Acetogenesis and Hydrogenotrophic Methanogenesis for Exogenous H2/CO2 in Anaerobically Digested Sludge: Impact of Temperature

Anaerobic digestion is a widely applied technology for sewage sludge treatment. Hydrogen and CO₂ are important degradation products, which serve as substrates for both hydrogenotrophic methanogenesis and chemolithotrophic acetogenesis. In order to understand the competition between these processes for H₂/CO₂, sludge samples were incubated under H₂/CO₂ headspace at different temperatures, and analyzed with respect to turnover of H₂, CO₂, CH₄ and acetate including their δ¹³C values. At 15°C, ¹³C-depleted acetate (δ¹³C of -41 to -43‰) and transient acetate accumulation were observed under H₂/CO₂, and CH₄ accumulated with δ¹³C values increasing from -53 to -33‰. The copy numbers of the fhs gene, which is characteristic for acetogenic bacteria, were at 15°C one order of magnitude higher in the H₂/CO₂ incubations than the N₂ control. At 30°C, however, acetate did not accumulate in the H₂/CO₂ incubation and the δ¹³C of CH₄ was very low (-100 to -77‰). At 50°C, isotopically enriched acetate was transiently formed and subsequently consumed followed by the production of ¹³C-depleted CH₄. Collectively, the results indicate a high contribution of chemolithotrophic acetogenesis to H₂/CO₂ utilization at 15°C and 50°C, while H₂/CO₂ was mainly consumed by hydrogenotrophic methanogenesis at 30°C. Fermentative production and methanogenic consumption of acetate were active at 50°C.

[Relationship between tree ring δ13C and net primary productivity of Pinus koraiensis in Changbai Mountain, China]

Carbon isotope in tree ring is an effective indicator of climate and environmental change. However, few studies have analyzed the indication effect of tree ring carbon isotope on net primary productivity (NPP) of forests. Based on meteorological factors of growing seasons, we analyzed the variation trend and the relationship between the tree ring δ¹³C chronosequence of Korean pine (Pinus koraiensis) and net primary productivity (NPP) of Korean pine in Changbai Mountain. We found that before 1970, the change of Korean pine tree ring δ¹³C and NPP was synchronous, with a highly significant linear positive correlation between them, indicating that tree ring δ¹³C recorded the impacts of climate change on NPP. After 1970, tree ring δ¹³C was negatively correlated with NPP but not statistically significant, meaning that other environmental factors such as severe droughts reduced the sensitivity of tree ring δ¹³C to climate change and the recording of NPP by tree ring δ¹³C. The δ¹³C of the current year was also correlated with the corresponding NPP in the following year, which indicated that the current year's environmental conditions were of great significance to the growth of Korean pine in the following year. This study showed that tree ring δ¹³C was a good indicator of the NPP of Korean pine in Changbai Mountain and that tree ring δ¹³C had the potential to reconstruct long-term changes of forest NPP in the history.

[Soil organic carbon mineralization and priming effects in the topsoil and subsoil under no-tillage black soil.]

Priming effect is one of the important mechanisms regulating soil organic matter decomposition. However, the variation of priming effects in different soil layers remains unclear. In this study, we conducted a 30-day incubation experiment using no-tillage black soil from northeastern China. ¹³C-glucose and dynamic CO₂ trapping methods were employed to investigate soil organic carbon (SOC) mineralization rates and the priming effect of the added ¹³C-glucose in the upper soil layer (0-10 cm) and the lower soil layer (30-40 cm). Our results showed that the cumulative SOC-specific mineralization rate in the upper layer was similar to that in the lower layer soil without glucose addition. Glucose addition significantly altered the mineralization rates in both layers, resulting in a positive priming effect (36.7%) in the upper layer but a negative priming effect (-12.4%) in the lower layer. The cumulative priming effect during the 30-day incubation was 3.24 mg C·g^-1 SOC for the upper layer soil and -1.24 mg C·g^-1 SOC for the lower layer soil. There was still a net SOC increase, even with positive priming effects in the upper layer soil. This was due to considerable amount of added glucose-C remained un-mineralized in the soil which would compensate the carbon loss from priming effects. Overall, our results demonstrated that the magnitude and direction of priming effects might differ between soil layers. Our findings contribute to a better understanding of the effects of conservation tillage practices (no-tillage and straw incorporation) on soil organic matter dynamics in agroecosystems.

Consistent Differences in Field Leaf Water-Use Efficiency among Soybean Cultivars

High intrinsic water-use efficiency (WUEi), the ratio of leaf photosynthesis to stomatal conductance, may be a useful trait in adapting crops to water-limited environments. In soybean, cultivar differences in stomatal response to vapor pressure deficit have not consistently translated into differences in WUEi in the field. In this study, six cultivars of soybeans previously shown to differ in WUEi in indoor experiments were grown in the field in Beltsville, Maryland, and tested for mid-day WUEi on nine clear days during the mid-seasons of two years. Measurement dates were chosen for diverse temperatures, and air temperatures ranged from 21 to 34 °C on the different dates. Air saturation deficits for water vapor ranged from 0.9 to 2.2 kPa. Corrected carbon isotope delta values for 13C (CID) were determined on mature, upper canopy leaves harvested during early pod filling each year. WUEi differed among cultivars in both years and the differences were consistent across measurement dates. Correlations between mean WUEi and CID were not significant in either year. It is concluded that consistent cultivar differences in WUEi exist in these soybean cultivars under field conditions, but that carbon isotope ratios may not be useful in identifying them because of cultivar differences in mesophyll conductance.

Climate warming and tree carbon use efficiency in a whole-tree 13 CO2 tracer study

Autotrophic respiration is a major driver of the global C cycle and may contribute a positive climate warming feedback through increased atmospheric concentrations of CO₂ . The extent of this feedback depends on plants' ability to acclimate respiration to maintain a constant carbon use efficiency (CUE). We quantified respiratory partitioning of gross primary production (GPP) and CUE of field-grown trees in a long-term warming experiment (+3°C). We delivered a ¹³ C-CO₂ pulse to whole tree crowns and chased that pulse in the respiration of leaves, whole crowns, roots, and soil. We also measured the isotopic composition of soil microbial biomass and the respiration rates of leaves and whole crowns. We documented homeostatic respiratory acclimation of foliar and whole-crown respiration rates; the trees adjusted to experimental warming such that leaf-level respiration rates were not increased. Experimental warming had no detectable impact on respiratory partitioning or mean residence times. Of the ¹³ C label acquired by the trees, aboveground respiration consumed 10%, belowground respiration consumed 40%, and the remaining 50% was retained. Experimental warming of +3°C did not alter respiratory partitioning at the scale of entire trees, suggesting that complete acclimation of respiration to warming is likely to dampen a positive climate warming feedback.

[Stable carbon isotopic characteristics of plant-litter-soil continuum along a successional gradient of broadleaved Korean pine forests in Changbai Mountain, China.]

Stable carbon isotope composition can accurately indicate ecosystem carbon cycling and provide key information for the study of the influence of forest succession on the carbon cycling and carbon sequestration potential. We measured the δ¹³C values and carbon and nitrogen contents of leaf, trunk, root, litter, and soil along a forest successional gradient in Changbai Mountain, which included a middle-aged poplar-birch secondary forest, a mature poplar-birch secondary forest, and an old-growth broad-leaved Korean pine forest. The results showed that leaf δ¹³C reduced with their position from the upper canopy to lower canopy, bark δ¹³C was less than xylem, fine root δ¹³C was less than course root. In contrast to the secondary forests, δ¹³C of the undecomposed litter layer was less than that of the semi-decomposed layer and decomposed litter layer in the broad-leaved Korean pine forest. Soil δ¹³C increased with depth. The ascending order of mean δ¹³C was leaf, litter, root, trunk, and soil, indicating that there is obvious fractionation among different organs of plants and among different parts of a specific organ. In addition, plant δ¹³C first decreased and then increased with the succession process, but soil δ¹³C increased with the succession processes. The different patterns of the changes of plant and soil δ¹³C along forest succession could be explained by the relationship between nitrogen content and carbon isotope fractionation effect, indicating that carbon isotope fractionation was affected by the change of dominant tree species and the variation of carbon turnover rate.

Remobilization of Old Permafrost Carbon to Chukchi Sea Sediments During the End of the Last Deglaciation

Climate warming is expected to destabilize permafrost carbon (PF-C) by thaw-erosion and deepening of the seasonally thawed active layer and thereby promote PF-C mineralization to CO2 and CH4. A similar PF-C remobilization might have contributed to the increase in atmospheric CO2 during deglacial warming after the last glacial maximum. Using carbon isotopes and terrestrial biomarkers (Δ14C, δ13C, and lignin phenols), this study quantifies deposition of terrestrial carbon originating from permafrost in sediments from the Chukchi Sea (core SWERUS-L2-4-PC1). The sediment core reconstructs remobilization of permafrost carbon during the late Allerød warm period starting at 13,000 cal years before present (BP), the Younger Dryas, and the early Holocene warming until 11,000 cal years BP and compares this period with the late Holocene, from 3,650 years BP until present. Dual-carbon-isotope-based source apportionment demonstrates that Ice Complex Deposit-ice- and carbon-rich permafrost from the late Pleistocene (also referred to as Yedoma)-was the dominant source of organic carbon (66 ± 8%; mean ± standard deviation) to sediments during the end of the deglaciation, with fluxes more than twice as high (8.0 ± 4.6 g·m-2·year-1) as in the late Holocene (3.1 ± 1.0 g·m-2·year-1). These results are consistent with late deglacial PF-C remobilization observed in a Laptev Sea record, yet in contrast with PF-C sources, which at that location were dominated by active layer material from the Lena River watershed. Release of dormant PF-C from erosion of coastal permafrost during the end of the last deglaciation indicates vulnerability of Ice Complex Deposit in response to future warming and sea level changes.

Competition Between Chemolithotrophic Acetogenesis and Hydrogenotrophic Methanogenesis for Exogenous H2/CO2 in Anaerobically Digested Sludge: Impact of Temperature

Anaerobic digestion is a widely applied technology for sewage sludge treatment. Hydrogen and CO₂ are important degradation products, which serve as substrates for both hydrogenotrophic methanogenesis and chemolithotrophic acetogenesis. In order to understand the competition between these processes for H₂/CO₂, sludge samples were incubated under H₂/CO₂ headspace at different temperatures, and analyzed with respect to turnover of H₂, CO₂, CH₄ and acetate including their δ¹³C values. At 15°C, ¹³C-depleted acetate (δ¹³C of -41 to -43‰) and transient acetate accumulation were observed under H₂/CO₂, and CH₄ accumulated with δ¹³C values increasing from -53 to -33‰. The copy numbers of the fhs gene, which is characteristic for acetogenic bacteria, were at 15°C one order of magnitude higher in the H₂/CO₂ incubations than the N₂ control. At 30°C, however, acetate did not accumulate in the H₂/CO₂ incubation and the δ¹³C of CH₄ was very low (-100 to -77‰). At 50°C, isotopically enriched acetate was transiently formed and subsequently consumed followed by the production of ¹³C-depleted CH₄. Collectively, the results indicate a high contribution of chemolithotrophic acetogenesis to H₂/CO₂ utilization at 15°C and 50°C, while H₂/CO₂ was mainly consumed by hydrogenotrophic methanogenesis at 30°C. Fermentative production and methanogenic consumption of acetate were active at 50°C.

[Chemical Constitution and Carbon Isotopic Compositions of PM2.5 in the Northern Suburb of Nanjing in Spring]

PM_2.5 is an important atmospheric pollution component and has a complicated composition. The chemical constitution of PM_2.5 in Nanjing northern region during March 2016 was analyzed using the Dinoex ICS-3000 and ICS-2000 ion chromatograph and DRI Model 2001A thermal/optical carbon analyzer, and the carbon isotopic compositions were analyzed using EA-IRMS from Thermo Scientific in order to explore pollution behaviors and source apportionment of PM_2.5. The results showed that the mean concentration of atmospheric PM_2.5 was (106.16±48.70) μg·m^-3, which equated with heavy pollution. Meanwhile, 88% of the samples exhibited the presence of the secondary organic pollutants. The average concentration of secondary organic carbon (SOC) was (3.58±2.78) μg·m^-3 and this high concentration was attributed to the reaction of O₃ with atmospheric hydrocarbons under ultraviolet light on sunny days. In addition, δ¹³C_TC in PM_2.5 ranged from -26.56‰ to -23.75‰ and the mean was (-25.47‰±0.63‰). Combining the various analyses, we concluded that atmospheric PM_2.5 for the study area was mainly derived from coal combustion, vehicle exhaust, geology (natural sources) and biomass burning.

Ecological and cultural shifts of hunter-gatherers of the Jomon period paralleled with environmental changes

OBJECTIVES

Holocene hunter-gatherers adapted to climatic and environmental changes over time. Carbon and nitrogen stable isotope analysis of human skeletal remains from the Inariyama shell mound of the Final Jomon period have revealed large dietary variations in the population. This study analyzed radiocarbon dates of these individuals to test temporal changes in diet and its relationship with tooth ablation.

MATERIALS AND METHODS

Twenty-nine human skeletal remains from Inariyama were included in this study. Extracted bone collagen samples were purified to graphite. Then, radiocarbon dating of these samples was performed using the accelerator mass spectrometer.

RESULTS

The radiocarbon ages of Inariyama ranged about, 3,230-2,140 cal BP and showed three peaks of occupation. In the early and late phases, terrestrial resource consumption and incisor extraction were observed, while marine resource consumption and canine extraction were observed in the middle phase.

DISCUSSION

These temporal changes of diet and tooth ablation types occurred in parallel with climatic cooling and environmental change and help reveal how Holocene hunter-gatherers adapted to the changing environments.

A comparison of substrate oxidation during prolonged exercise in men at terrestrial altitude and normobaric normoxia following the coingestion of 13C glucose and 13C fructose

This study compared the effects of coingesting glucose and fructose on exogenous and endogenous substrate oxidation during prolonged exercise at altitude and sea level, in men. Seven male British military personnel completed two bouts of cycling at the same relative workload (55% W_max) for 120 min on acute exposure to altitude (3375 m) and at sea level (~113 m). In each trial, participants ingested 1.2 g·min⁻¹ of glucose (enriched with ¹³C glucose) and 0.6 g·min⁻¹ of fructose (enriched with ¹³C fructose) directly before and every 15 min during exercise. Indirect calorimetry and isotope ratio mass spectrometry were used to calculate fat oxidation, total and exogenous carbohydrate oxidation, plasma glucose oxidation, and endogenous glucose oxidation derived from liver and muscle glycogen. Total carbohydrate oxidation during the exercise period was lower at altitude (157.7 ± 56.3 g) than sea level (286.5 ± 56.2 g, P = 0.006, ES = 2.28), whereas fat oxidation was higher at altitude (75.5 ± 26.8 g) than sea level (42.5 ± 21.3 g, P = 0.024, ES = 1.23). Peak exogenous carbohydrate oxidation was lower at altitude (1.13 ± 0.2 g·min⁻¹) than sea level (1.42 ± 0.16 g·min⁻¹, P = 0.034, ES = 1.33). There were no differences in rates, or absolute and relative contributions of plasma or liver glucose oxidation between conditions during the second hour of exercise. However, absolute and relative contributions of muscle glycogen during the second hour were lower at altitude (29.3 ± 28.9 g, 16.6 ± 15.2%) than sea level (78.7 ± 5.2 g (P = 0.008, ES = 1.71), 37.7 ± 13.0% (P = 0.016, ES = 1.45). Acute exposure to altitude reduces the reliance on muscle glycogen and increases fat oxidation during prolonged cycling in men compared with sea level.

Changes in vegetation phenology are not reflected in atmospheric CO2 and 13 C/12 C seasonality

Northern terrestrial ecosystems have shown global warming-induced advances in start, delays in end, and thus increased lengths of growing season and gross photosynthesis in recent decades. The tradeoffs between seasonal dynamics of two opposing fluxes, CO₂ uptake through photosynthesis and release through respiration, determine the influence of the terrestrial ecosystem on the atmospheric CO₂ and ¹³ C/¹² C seasonality. Here, we use four CO₂ observation stations in the Northern Hemisphere, namely Alert, La Jolla, Point Barrow, and Mauna Loa Observatory, to determine how changes in vegetation productivity and phenology, respiration, and air temperature affect both the atmospheric CO₂ and ¹³ C/¹² C seasonality. Since the 1960s, the only significant long-term trend of CO₂ and ¹³ C/¹² C seasonality was observed at the northern most station, Alert, where the spring CO₂ drawdown dates advanced by 0.65 ± 0.55 days yr^-1 , contributing to a nonsignificant increase in length of the CO₂ uptake period (0.74 ± 0.67 days yr^-1 ). For Point Barrow station, vegetation phenology changes in well-watered ecosystems such as the Canadian and western Siberian wetlands contributed the most to ¹³ C/¹² C seasonality while the CO₂ seasonality was primarily linked to nontree vegetation. Our results indicate significant increase in the Northern Hemisphere soil respiration. This means, increased respiration of ¹³ C depleted plant materials cancels out the ¹² C gain from enhanced vegetation activities during the start and end of growing season. These findings suggest therefore that parallel warming-induced increases both in photosynthesis and respiration contribute to the long-term stability of CO₂ and ¹³ C/¹² C seasonality under changing climate and vegetation activity. The summer photosynthesis and the soil respiration in the dormant seasons have become more vigorous which lead to increased peak-to-through CO₂ amplitude. As the relative magnitude of the increased photosynthesis in summer months is more than the increased respiration in dormant months, we have the increased overall carbon uptake rates in the northern ecosystems.

The fate of recently fixed carbon after drought release: towards unravelling C storage regulation in Tilia platyphyllos and Pinus sylvestris

Carbon reserves are important for maintaining tree function during and after stress. Increasing tree mortality driven by drought globally has renewed the interest in how plants regulate allocation of recently fixed C to reserve formation. Three-year-old seedlings of two species (Tilia platyphyllos and Pinus sylvestris) were exposed to two intensities of experimental drought during ~10 weeks, and ¹³ C pulse labelling was subsequently applied with rewetting. Tracking the ¹³ C label across different organs and C compounds (soluble sugars, starch, myo-inositol, lipids and cellulose), together with the monitoring of gas exchange and C mass balances over time, allowed for the identification of variations in C allocation priorities and tree C balances that are associated with drought effects and subsequent drought release. The results demonstrate that soluble sugars accumulated in P. sylvestris under drought conditions independently of growth trends; thus, non-structural carbohydrates (NSC) formation cannot be simply considered a passive overflow process in this species. Once drought ceased, C allocation to storage was still prioritized at the expense of growth, which suggested the presence of 'drought memory effects', possibly to ensure future growth and survival. On the contrary, NSC and growth dynamics in T. platyphyllos were consistent with a passive (overflow) view of NSC formation.

Isotope signals and anatomical features in tree rings suggest a role for hydraulic strategies in diffuse drought-induced die-back of Pinus nigra

The 2003 and 2012 summer seasons were among the warmest and driest of the last 200 years over southeastern Europe, and in particular in the Karst region (northeastern Italy). Starting from winter-spring 2013, several black pines (Pinus nigra J.F. Arnold) suffered crown die-back. Declining trees occurred nearby individuals with no signs of die-back, raising hypotheses about the occurrence of individual-specific hydraulic strategies underlying different responses to extreme drought. We investigated possible processes driving black pine decline by dendrochronological and wood anatomical measurements, coupled with analysis of tree-ring carbon (δ13C) and oxygen (δ18O) isotopic composition in healthy trees (H) and trees suffering die-back (D). Die-back trees showed higher growth rates than H trees at the beginning of the last century, but suffered important growth reduction following the dry summers in 2003 and 2012. After the 2012 drought, D trees produced tracheids with larger diameter and greater vulnerability to implosion than H ones. Healthy trees had significantly higher wood δ13C than D trees, reflecting higher water-use efficiency for the surviving trees, i.e., less water transpired per unit carbon gain, which could be related to lower stomatal conductance and a more conservative use of water. Relatively high δ18O for D trees indicates that they were strongly dependent on shallow water sources, or that they sustained higher transpiration rates than H trees. Our results suggest that H trees adopted a more conservative water-use strategy under drought stress compared with D trees. We speculate that this diversity might have a genotypic basis, but other possible explanations, like different rooting depth, cannot be ruled out.

[Variation in δ13C and water use efficiency of plant leaf at different slopes in an alpine mea-dow]

Through the systematical measurements of δ¹³C values of samples representing 25 families and 86 C3 herb species along the slope aspects in an alpine meadow, the responses of δ¹³C va-lues and water use efficiency (WUE) for C3 plants to slope aspect changes and influence of environmental factors were analyzed and main environmental factors related to plant δ¹³C values change were revealed. Along the north-facing to south-facing slope, soil water content decreased gradually, the soil temperature and light intensity increased gradually, and plant community structure also had corresponding change. The δ¹³C values of C3 plant species in 5 slope aspects were from -31.19‰ to -21.8‰, with an average value of (-27.18±0.13)‰. The average δ¹³C value was the highest along the south-facing slope, followed by that along the southwest slope, west slope and northwest slope, with the lowest along the north slope during the whole growing season. The difference of δ¹³C values among the different slope aspects were caused by discrepancy in soil water content, soil temperature and light intensity. Soil water content was the main limiting factor. From north slope to south slope, plant δ¹³C value increased with the increasing soil temperature, light intensity and the reducing soil water content, which suggested that different species had different strategies to adapt to environmental changes of drought stress in different slope aspects, and water use efficiency was improved gradually in order to adapt to the environment of drought stress.

[Molecular and Carbon Isotopic Compositions of n-Alkanoic Acids in Smoke from Maize Straw Combustion]

Four cultivars of maize straw were burned under flaming and smoldering conditions. Smoke samples were colleted and analyzed by GC/MS and GC/C/IRMS to determine molecular and stable carbon isotopic compositions of n-alkanoic acids in them. The results showed that n-alkanoic acids in flaming smoke were composed of C₇ to C₃₄, with a mean total content of 13895.0 mg·kg^-1. The average ratio values of the content for homologues with lower carbon number (≤C₁₆) to that for ones with higher carbon number (>C₁₆) (L/H), C₁₈/C₁₆, C₂₄/C₁₆, C₂₄/C₁₈, and C₂₄/(C₂₂+C₂₆) (CAR) were 1.1, 0.33, 0.17, 0.50, and 1.2, respectively. Moreover, the acids exhibited a bimodal profile with peaks at C₁₆ and C₂₄, respectively. They had evident even to odd carbon number predominance (mean CPI: 4.5). In smoldering smoke the acids were consisted of C₆ to C₃₄, with an average total content of 50183.7 mg·kg^-1. The mean ratios of L/H, C₁₈/C₁₆, C₂₄/C₁₆, C₂₄/C₁₈, and CAR were 1.3, 0.33, 0.20, 0.60, and 1.6, respectively. In addition, the homologues had the same distribution pattern as in flaming smoke and even to odd carbon number predominance (mean CPI=6.1). The mean carbon isotopic ratio (δ¹³ C) values for individual n-alkanoic acids (C₁₄ to C₂₆) in flaming smoke ranged from -21.0‰ to -24.8‰, the overall average of which was -23.5‰. The difference between mean δ¹³ C values of n-fatty acids in flaming smoke and the straw (Δ¹³ C) was -0.7‰. In smoldering smoke, the mean δ¹³ C values for the compounds varied from -21.8‰ to -25.4‰, with an overall average of -23.3‰. The Δ¹³ C value was up to -0.5‰. The molecular and carbon isotopic compositions of n-alkanoic acids in smoke were significantly different from those in straw matter. L/H, C₂₄/C₁₆, C₂₄/C₁₈, CAR, and δ¹³ C might be useful proxies to discriminate the organic pollutants in atmospheric aerosols derived from maize straw combustion.

Landscape effects on the abundance and larval diet of the polyphagous pest Helicoverpa armigera in cotton fields in North Benin

BACKGROUND

The noctuid Helicoverpa armigera is one of the key cotton pests in the Old World. One possible pest regulation method may be the management of host crop in the landscapes. For polyphagous pests such as H. armigera, crop diversity and rotations can offer sequential and alternate resources that may enhance abundance. We explore the impact of landscape composition and host crop diversity on the abundance and natal host plant use of H. armigera in northern Benin.

RESULTS

Host plant diversity at the largest scale examined (500 m diameter) was positively correlated with H. armigera abundance. Host plant diversity and the cover of tomato crops were the most important variables in relation to high abundance of H. armigera. Host plant (cotton, maize, tomato, sorghum) proportions and C3 versus C4 plants did not consistently correlate positively with H. armigera abundance. Moth proportion derived from cotton-fed larvae was low, 15% in 2011 and 11% in 2012, and not significantly related to H. armigera abundance.

CONCLUSION

Cotton crop cover was not significantly related to H. armigera abundance and may be considered as a sink crop. Landscape composition and sequential availability of host plants should be considered as keys factors for further studies on H. armigera regulation. © 2015 Society of Chemical Industry.

Effects of prescribed burning on ecophysiological, anatomical and stem hydraulic properties in Pinus pinea L

Prescribed burning (PB) is a widespread management technique for wildfire hazard abatement. Understanding PB effects on tree ecophysiology is key to defining burn prescriptions aimed at reducing fire hazard in Mediterranean pine plantations, such as Pinus pinea L. stands. We assessed physiological responses of adult P. pinea trees to PB using a combination of dendroecological, anatomical, hydraulic and isotopic analyses. Tree-ring widths, xylem cell wall thickness, lumen area, hydraulic diameter and tree-ring δ(13)C and δ(18)O were measured in trees on burned and control sites. Vulnerability curves were elaborated to assess tree hydraulic efficiency or safety. Despite the relatively intense thermal treatment (the residence time of temperatures above 50 °C at the stem surface ranged between 242 and 2239 s), burned trees did not suffer mechanical damage to stems, nor significant reduction in radial growth. Moreover, the PB did not affect xylem structure and tree hydraulics. No variations in (13)C-derived water use efficiency were recorded. This confirmed the high resistance of P. pinea to surface fire at the stem base. However, burned trees showed consistently lower δ(18)O values in the PB year, as a likely consequence of reduced competition for water and nutrients due to the understory burning, which increased both photosynthetic activity and stomatal conductance. Our multi-approach analysis offers new perspectives on post-fire survival strategies of P. pinea in an environment where fires are predicted to increase in frequency and severity during the 21st century.

A Pleistocene palaeovegetation record from plant wax biomarkers from the Nachukui Formation, West Turkana, Kenya

Reconstructing vegetation at hominin fossil sites provides us critical information about hominin palaeoenvironments and the potential role of climate in their evolution. Here we reconstruct vegetation from carbon isotopes of plant wax biomarkers in sediments of the Nachukui Formation in the Turkana Basin. Plant wax biomarkers were extracted from samples from a wide range of lithologies that include fluvial-lacustrine sediments and palaeosols, and therefore provide a record of vegetation from diverse depositional environments. Carbon isotope ratios from biomarkers indicate a highly dynamic vegetation structure (ca 5-100% C4 vegetation) from 2.3 to 1.7 Ma, with an overall shift towards more C4 vegetation on the landscape after about 2.1 Ma. The biomarker isotope data indicate ca 25-30% more C4 vegetation on the landscape than carbon isotope data of pedogenic carbonates from the same sequence. Our data show that the environments of early Paranthropus and Homo in this part of the Turkana Basin were primarily mixed C3-C4 to C4-dominated ecosystems. The proportion of C4-based foods in the diet of Paranthropus increases through time, broadly paralleling the increase in C4 vegetation on the landscape, whereas the diet of Homo remains unchanged. Biomarker isotope data associated with the Kokiselei archaeological site complex, which includes the site where the oldest Acheulean stone tools to date were recovered, indicate 61-97% C4 vegetation on the landscape.This article is part of the themed issue 'Major transitions in human evolution'.

Changes in northeast African hydrology and vegetation associated with Pliocene-Pleistocene sapropel cycles

East African climate change since the Late Miocene consisted of persistent shorter-term, orbital-scale wet-dry cycles superimposed upon a long-term trend towards more open, grassy landscapes. Either or both of these modes of palaeoclimate variability may have influenced East African mammalian evolution, yet the interrelationship between these secular and orbital palaeoclimate signals remains poorly understood. Here, we explore whether the long-term secular climate change was also accompanied by significant changes at the orbital-scale. We develop northeast African hydroclimate and vegetation proxy data for two 100 kyr-duration windows near 3.05 and 1.75 Ma at ODP Site 967 in the eastern Mediterranean basin, where sedimentation is dominated by eastern Sahara dust input and Nile River run-off. These two windows were selected because they have comparable orbital configurations and bracket an important increase in East African C4 grasslands. We conducted high-resolution (2.5 kyr sampling) multiproxy biomarker, H- and C-isotopic analyses of plant waxes and lignin phenols to document orbital-scale changes in hydrology, vegetation and woody cover for these two intervals. Both intervals are dominated by large-amplitude, precession-scale (approx. 20 kyr) changes in northeast African vegetation and rainfall/run-off. The δ(13)Cwax values and lignin phenol composition record a variable but consistently C4 grass-dominated ecosystem for both intervals (50-80% C4). Precessional δDwax cycles were approximately 20-30‰ in peak-to-peak amplitude, comparable with other δDwax records of the Early Holocene African Humid Period. There were no significant differences in the means or variances of the δDwax or δ(13)Cwax data for the 3.05 and 1.75 Ma intervals studied, suggesting that the palaeohydrology and palaeovegetation responses to precessional forcing were similar for these two periods. Data for these two windows suggest that the eastern Sahara did not experience the significant increase in C4 vegetation that has been observed in East Africa over this time period. This observation would be consistent with a proposed mechanism whereby East African precipitation is reduced, and drier conditions established, in response to the emergence of modern zonal sea surface temperature gradients in the tropical oceans between 3 and 2 Ma.This article is part of the themed issue 'Major transitions in human evolution'.

Neogene biomarker record of vegetation change in eastern Africa

The evolution of C4 grassland ecosystems in eastern Africa has been intensely studied because of the potential influence of vegetation on mammalian evolution, including that of our own lineage, hominins. Although a handful of sparse vegetation records exists from middle and early Miocene terrestrial fossil sites, there is no comprehensive record of vegetation through the Neogene. Here we present a vegetation record spanning the Neogene and Quaternary Periods that documents the appearance and subsequent expansion of C4 grasslands in eastern Africa. Carbon isotope ratios from terrestrial plant wax biomarkers deposited in marine sediments indicate constant C3 vegetation from ∼24 Ma to 10 Ma, when C4 grasses first appeared. From this time forward, C4 vegetation increases monotonically to present, with a coherent signal between marine core sites located in the Somali Basin and the Red Sea. The response of mammalian herbivores to the appearance of C4 grasses at 10 Ma is immediate, as evidenced from existing records of mammalian diets from isotopic analyses of tooth enamel. The expansion of C4 vegetation in eastern Africa is broadly mirrored by increasing proportions of C4-based foods in hominin diets, beginning at 3.8 Ma in Australopithecus and, slightly later, Kenyanthropus This continues into the late Pleistocene in Paranthropus, whereas Homo maintains a flexible diet. The biomarker vegetation record suggests the increase in open, C4 grassland ecosystems over the last 10 Ma may have operated as a selection pressure for traits and behaviors in Homo such as bipedalism, flexible diets, and complex social structure.

Response of Quercus velutina growth and water use efficiency to climate variability and nitrogen fertilization in a temperate deciduous forest in the northeastern USA

Nitrogen (N) deposition and changing climate patterns in the northeastern USA can influence forest productivity through effects on plant nutrient relations and water use. This study evaluates the combined effects of N fertilization, climate and rising atmospheric CO2on tree growth and ecophysiology in a temperate deciduous forest. Tree ring widths and stable carbon (δ(13)C) and oxygen (δ(18)O) isotopes were used to assess tree growth (basal area increment, BAI) and intrinsic water use efficiency (iWUE) ofQuercus velutinaLamb., the dominant tree species in a 20+ year N fertilization experiment at Harvard Forest (MA, USA). We found that fertilized trees exhibited a pronounced and sustained growth enhancement relative to control trees, with the low- and high-N treatments responding similarly. All treatments exhibited improved iWUE over the study period (1984-2011). Intrinsic water use efficiency trends in the control trees were primarily driven by changes in stomatal conductance, while a stimulation in photosynthesis, supported by an increase in foliar %N, contributed to enhancing iWUE in fertilized trees. All treatments were predominantly influenced by growing season vapor pressure deficit (VPD), with BAI responding most strongly to early season VPD and iWUE responding most strongly to late season VPD. Nitrogen fertilization increasedQ. velutinasensitivity to July temperature and precipitation. Combined, these results suggest that ambient N deposition in N-limited northeastern US forests has enhanced tree growth over the past 30 years, while rising ambient CO2has improved iWUE, with N fertilization and CO2having synergistic effects on iWUE.

Dietary options and behavior suggested by plant biomarker evidence in an early human habitat

The availability of plants and freshwater shapes the diets and social behavior of chimpanzees, our closest living relative. However, limited evidence about the spatial relationships shared between ancestral human (hominin) remains, edible resources, refuge, and freshwater leaves the influence of local resources on our species' evolution open to debate. Exceptionally well-preserved organic geochemical fossils--biomarkers--preserved in a soil horizon resolve different plant communities at meter scales across a contiguous 25,000 m(2) archaeological land surface at Olduvai Gorge from about 2 Ma. Biomarkers reveal hominins had access to aquatic plants and protective woods in a patchwork landscape, which included a spring-fed wetland near a woodland that both were surrounded by open grassland. Numerous cut-marked animal bones are located within the wooded area, and within meters of wetland vegetation delineated by biomarkers for ferns and sedges. Taken together, plant biomarkers, clustered bone debris, and hominin remains define a clear spatial pattern that places animal butchery amid the refuge of an isolated forest patch and near freshwater with diverse edible resources.

High water availability increases the negative impact of a native hemiparasite on its non-native host

Environmental factors alter the impacts of parasitic plants on their hosts. However, there have been no controlled studies on how water availability modulates stem hemiparasites' effects on hosts. A glasshouse experiment was conducted to investigate the association between the Australian native stem hemiparasite Cassytha pubescens and the introduced host Ulex europaeus under high (HW) and low (LW) water supply. Cassytha pubescens had a significant, negative effect on the total biomass of U. europaeus, which was more severe in HW than LW. Regardless of watering treatment, infection significantly decreased shoot and root biomass, nodule biomass, nodule biomass per unit root biomass, F v/F m, and nitrogen concentration of U. europaeus. Host spine sodium concentration significantly increased in response to infection in LW but not HW conditions. Host water potential was significantly higher in HW than in LW, which may have allowed the parasite to maintain higher stomatal conductances in HW. In support of this, the δ(13)C of the parasite was significantly lower in HW than in LW (and significantly higher than the host). C. pubescens also had significantly higher F v/F m and 66% higher biomass per unit host in the HW compared with the LW treatment. The data suggest that the enhanced performance of C. pubescens in HW resulted in higher parasite growth rates and thus a larger demand for resources from the host, leading to poorer host performance in HW compared with LW. C. pubescens should more negatively affect U. europaeus growth under wet conditions rather than under dry conditions in the field.

In situ 13CO2 pulse labelling of field-grown eucalypt trees revealed the effects of potassium nutrition and throughfall exclusion on phloem transport of photosynthetic carbon

Potassium (K) is an important limiting factor of tree growth, but little is known of the effects of K supply on the long-distance transport of photosynthetic carbon (C) in the phloem and of the interaction between K fertilization and drought. We pulse-labelled 2-year-old Eucalyptus grandis L. trees grown in a field trial combining K fertilization (+K and -K) and throughfall exclusion (+W and -W), and we estimated the velocity of C transfer by comparing time lags between the uptake of (13)CO2 and its recovery in trunk CO2 efflux recorded at different heights. We also analysed the dynamics of the labelled photosynthates recovered in the foliage and in the phloem sap (inner bark extract). The mean residence time of labelled C in the foliage was short (21-31 h). The time series of (13)C in excess in the foliage was affected by the level of fertilization, whereas the effect of throughfall exclusion was not significant. The velocity of C transfer in the trunk (0.20-0.82 m h(-1)) was twice as high in +K trees than in -K trees, with no significant effect of throughfall exclusion except for one +K -W tree labelled in the middle of the drought season that was exposed to a more pronounced water stress (midday leaf water potential of -2.2 MPa). Our results suggest that besides reductions in photosynthetic C supply and in C demand by sink organs, the lower velocity under K deficiency is due to a lower cross-sectional area of the sieve tubes, whereas an increase in phloem sap viscosity is more likely limiting phloem transport under drought. In all treatments, 10 times less (13)C was recovered in inner bark extracts at the bottom of the trunk when compared with the base of the crown, suggesting that a large part of the labelled assimilates has been exported out of the phloem and replaced by unlabelled C. This supports the 'leakage-retrieval mechanism' that may play a role in maintaining the pressure gradient between source and sink organs required to sustain high velocity of phloem transport in tall trees.

Examining the response of needle carbohydrates from Siberian larch trees to climate using compound-specific δ(13) C and concentration analyses

Little is known about the dynamics of concentrations and carbon isotope ratios of individual carbohydrates in leaves in response to climatic and physiological factors. Improved knowledge of the isotopic ratio in sugars will enhance our understanding of the tree ring isotope ratio and will help to decipher environmental conditions in retrospect more reliably. Carbohydrate samples from larch (Larix gmelinii) needles of two sites in the continuous permafrost zone of Siberia with differing growth conditions were analysed with the Compound-Specific Isotope Analysis (CSIA). We compared concentrations and carbon isotope values (δ(13) C) of sucrose, fructose, glucose and pinitol combined with phenological data. The results for the variability of the needle carbohydrates show high dynamics with distinct seasonal characteristics between and within the studied years with a clear link to the climatic conditions, particularly vapour pressure deficit. Compound-specific differences in δ(13) C values as a response to climate were detected. The δ(13) C of pinitol, which contributes up to 50% of total soluble carbohydrates, was almost invariant during the whole growing season. Our study provides the first in-depth characterization of compound-specific needle carbohydrate isotope variability, identifies involved mechanisms and shows the potential of such results for linking tree physiological responses to different climatic conditions.