Variation in the carbon and oxygen isotope composition of plant biomass and its relationship to water‐use efficiency at the leaf‐ and ecosystem‐scales in a northern Great Plains grassland

Measurements of the carbon (δ¹³C_m) and oxygen (δ¹⁸O_m) isotope composition of C₃ plant tissue provide important insights into controls on water‐use efficiency. We investigated the causes of seasonal and inter‐annual variability in water‐use efficiency in a grassland near Lethbridge, Canada using stable isotope (leaf‐scale) and eddy covariance measurements (ecosystem‐scale). The positive relationship between δ¹³C_m and δ¹⁸O_m values for samples collected during 1998–2001 indicated that variation in stomatal conductance and water stress‐induced changes in the degree of stomatal limitation of net photosynthesis were the major controls on variation in δ¹³C_m and biomass production during this time. By comparison, the lack of a significant relationship between δ¹³C_m and δ¹⁸O_m values during 2002, 2003 and 2006 demonstrated that water stress was not a significant limitation on photosynthesis and biomass production in these years. Water‐use efficiency was higher in 2000 than 1999, consistent with expectations because of greater stomatal limitation of photosynthesis and lower leaf c_i/ca during the drier conditions of 2000. Calculated values of leaf‐scale water‐use efficiency were 2–3 times higher than ecosystem‐scale water‐use efficiency, a difference that was likely due to carbon lost in root respiration and water lost during soil evaporation that was not accounted for by the stable isotope measurements.     

Reconstructing the δ18O of atmospheric water vapour via the CAM epiphyte Tillandsia usneoides: seasonal controls on δ18O in the field and large‐scale reconstruction of δ18Oa

Using both oxygen isotope ratios of leaf water (δ¹⁸O_L) and cellulose (δ¹⁸O_C) of Tillandsia usneoides in situ, this paper examined how short‐ and long‐term responses to environmental variation and model parameterization affected the reconstruction of the atmospheric water vapour (δ¹⁸O_a). During sample‐intensive field campaigns, predictions of δ¹⁸O_L matched observations well using a non‐steady‐state model, but the model required data‐rich parameterization. Predictions from the more easily parameterized maximum enrichment model (δ¹⁸O_L–M) matched observed δ¹⁸O_L and observed δ¹⁸O_a when leaf water turnover was less than 3.5 d. Using the δ¹⁸O_L–M model and weekly samples of δ¹⁸O_L across two growing seasons in Florida, USA, reconstructed δ¹⁸O_a was ?12.6 ± 0.3‰. This is compared with δ¹⁸O_a of ?12.4 ± 0.2‰ resolved from the growing‐season‐weighted δ¹⁸O_C. Both of these values were similar to δ¹⁸O_a in equilibrium with precipitation, ?12.9‰. δ¹⁸O_a was also reconstructed through a large‐scale transect with δ¹⁸O_L and the growing‐season‐integrated δ¹⁸O_C across the southeastern United States. There was considerable large‐scale variation, but there was regional, weather‐induced coherence in δ¹⁸O_a when using δ¹⁸O_L. The reconstruction of δ¹⁸O_a with δ¹⁸O_C generally supported the assumption of δ¹⁸O_a being in equilibrium with precipitation δ¹⁸O (δ¹⁸O_ppt), but the pool of δ¹⁸O_ppt with which δ¹⁸O_a was in equilibrium – growing season versus annual δ¹⁸O_ppt – changed with latitude.     

Interpreting species‐specific variation in tree‐ring oxygen isotope ratios among three temperate forest trees

Although considerable variation has been documented in tree‐ring cellulose oxygen isotope ratios (δ¹⁸O_cell) among co‐occurring species, the underlying causes are unknown. Here, we used a combination of field measurements and modelling to investigate the mechanisms behind variations in late‐wood δ¹⁸O_cell (δ¹⁸O_lc) among three co‐occurring species (chestnut oak, black oak and pitch pine) in a temperate forest. For two growing seasons, we quantified among‐species variation in δ¹⁸O_lc, as well as several variables that could potentially cause the δ¹⁸O_lc variation. Data analysis based on the δ¹⁸O_cell model rules out leaf water enrichment (Δ¹⁸O_lw) and tree‐ring formation period (Δt), but highlights source water δ¹⁸O (δ¹⁸O_sw) as an important driver for the measured difference in δ¹⁸O_lc between black and chestnut oak. However, the enriched δ¹⁸O_lc in pitch pine relative to the oaks could not be sufficiently explained by consideration of the above three variables only, but rather, we show that differences in the proportion of oxygen exchange during cellulose synthesis (p_ex) is most likely a key mechanism. Our demonstration of the relevance of some species‐specific features (or lack thereof) to δ¹⁸O_cell has important implications for isotope based ecophysiological/paleoclimate studies.     

Paleosalinity in a brackish lake during the Holocene based on stable oxygen and carbon isotopes of shell carbonate in Nakaumi Lagoon, southwest Japan

Based on the relationship between salinity and δ¹⁸O and δ¹³C of modern shells in the Lake Nakaumi-Shinji lagoon system (southwestern Japan), where the salinity changes regularly from ca. 1 PSU to 34 PSU, a paleosalinity record for Nakaumi Lagoon during the Holocene has been derived from bulk mollusk shell δ¹⁸O and δ¹³C data. The robust relationships between the salinity and modern shell δ¹⁸O_ar and δ¹³C_ar (aragonite) were used to calibrate the paleosalinity reconstruction. The salinity relationships are expressed by the regressions:

Salinity (PSU)=3.86 δ¹⁸O_ar(‰ VPDB)+33.9 (n=18, r=0.978)     

Oxygen isotope fractionation between crocodilian phosphate and water

Oxygen isotope compositions of phosphate (δ¹⁸O_p) were measured in tooth enamel from captive and wild individuals of 8 crocodilian species. A rough linear correlation is observed between the δ¹⁸O_p of all the studied species and the oxygen isotope composition of ambient water (δ¹⁸O_w). Differences in mean air temperature, diet and physiology could contribute significantly to the large scatter of δ¹⁸O_p values. The combination of these parameters results in a fractionation equation for which the slope (0.82) is lower than that expected (≥ 1) from predictive model equations that assume temperature and diet as fixed parameters. Taking into account large uncertainties, the observed oxygen isotope fractionation between phosphate and ambient water does not statistically differ from that formerly established for aquatic turtles. Case studies show that δ¹⁸O_p values of fossil crocodile tooth enamel can be used to discriminate between marine and freshwater living environments within a precision of about ± 2‰ only.     

Oxygen and carbon isotope compositions of middle Cretaceous vertebrates from North Africa and Brazil: Ecological and environmental significance

In order to investigate mid-Cretaceous terrestrial climates of low paleolatitudes, Moroccan, Tunisian and Brazilian vertebrate apatites have been analyzed for their oxygen and carbon isotope compositions of phosphates (δ¹⁸O_p) and carbonates (δ¹⁸O_c, δ¹³C_c). At each site, coexisting theropod dinosaurs, titanosaurid sauropods, pterosaurs, crocodilians, turtles and fish have distinct δ¹⁸O_p and δ¹³C_c values reflecting their ecologies, diets and foraging environments. Oxygen isotope compositions of surface waters (δ¹⁸O_w) estimated from turtle and crocodile δ¹⁸O_p values range from − 5.0 ± 1.0‰ to − 2.4 ± 1.0‰, which do not differ from mean annual rainwater values occurring today under inter-tropical sub-arid to arid climates. High water temperatures ranging from 21 ± 6 °C to 34 ± 2 °C deduced from fish δ¹⁸O_p values are in agreement with those published for mid-Cretaceous low latitudes. Temporary or seasonal droughts are inferred from high δ¹⁸O_p values of lungfish teeth, even though lower reptile δ¹⁸O_p values suggest the use of distinct and most likely larger or regularly renewed bodies of water. Environmental conditions of the studied low latitude regions during the Aptian-Cenomanian interval were somewhat similar to those experienced today under semi-arid to arid tropical or equatorial climates, but with higher mean surface temperatures than present-day ones.     

Reconstruction of source water using the δ18O of tree ring phenylglucosazone: A potential tool in paleoclimate studies

The oxygen isotope ratios of tree ring cellulose have a great potential as proxy for the oxygen isotope ratios of source water, which is related to climate. However, source water isotopic signatures can be masked by plant physiological and biochemical processes during cellulose synthesis. To minimize biochemical effects in the recording of source water, we modified the cellulose molecule to phenylglucosazone, which only has oxygen attached to carbon 3–6 (O_C3–6) of the cellulose glucose moieties, thus eliminating the oxygen attached to carbon 2 of the cellulose glucose moieties (O_C-2). Here we developed a method to use small amounts of inter and intra-annual tree ring cellulose for phenylglucosazone synthesis. Using this new method we tested if the oxygen isotope ratios of source water reconstructed from tree ring phenylglucosazone (δ¹⁸O_sw^PG) and the observed source water (δ¹⁸O_sw^obs) would have a better agreement than those reconstructed from the tree ring cellulose molecule. Annual tree ring samples were obtained from Pinus sylvestris (1997–2003) (Finland) and Picea abies (1971–1992) (Switzerland) and intra-annual tree ring samples were obtained from Pinus radiata (October 2004–March 2006) (New Zealand), each near a meteorological station where precipitation and relative humidity (RH) were measured periodically. The δ¹⁸O of tree ring cellulose and tree ring phenylglucosazone for each of the three species were then used to back calculate the δ¹⁸O of source water according to a previous published empirical equation. As expected, the δ¹⁸O of tree ring phenylglucosazone was superior than cellulose in the reconstruction of source water available to the plant. Deviation between δ¹⁸O_sw^PG and δ¹⁸O_sw^obs was in part correlated with variation in atmospheric relative humidity (RH) which was not observed for the cellulose molecule. We conclude that this new method can be applicable to inter and intra-annual tree ring studies and that the use of the tree ring phenylglucosazone will significantly improve the quality of paleoclimate studies.     

Oxygen isotope fractionations across individual leaf carbohydrates in grass and tree species

Almost no δ¹⁸O data are available for leaf carbohydrates, leaving a gap in the understanding of the δ¹⁸O relationship between leaf water and cellulose. We measured δ¹⁸O values of bulk leaf water (δ¹⁸O_LW) and individual leaf carbohydrates (e.g. fructose, glucose and sucrose) in grass and tree species and δ¹⁸O of leaf cellulose in grasses. The grasses were grown under two relative humidity (rH) conditions. Sucrose was generally ¹⁸O‐enriched compared with hexoses across all species with an apparent biosynthetic fractionation factor (ε_bio) of more than 27‰ relative to δ¹⁸O_LW, which might be explained by isotopic leaf water and sucrose synthesis gradients. δ¹⁸O_LW and δ¹⁸O values of carbohydrates and cellulose in grasses were strongly related, indicating that the leaf water signal in carbohydrates was transferred to cellulose (ε_bio = 25.1‰). Interestingly, damping factor p_exp_x, which reflects oxygen isotope exchange with less enriched water during cellulose synthesis, responded to rH conditions if modelled from δ¹⁸O_LW but not if modelled directly from δ¹⁸O of individual carbohydrates. We conclude that δ¹⁸O_LW is not always a good substitute for δ¹⁸O of synthesis water due to isotopic leaf water gradients. Thus, compound‐specific δ¹⁸O analyses of individual carbohydrates are helpful to better constrain (post‐)photosynthetic isotope fractionation processes in plants.     

Tracing plant source water dynamics during drought by continuous transpiration measurements: An in-situ stable isotope approach

The isotopic composition of xylem water (δ_X) is of considerable interest for plant source water studies. In-situ monitored isotopic composition of transpired water (δ_T) could provide a nondestructive proxy for δ_X-values. Using flow-through leaf chambers, we monitored 2-hourly δ_T-dynamics in two tropical plant species, one canopy-forming tree and one understory herbaceous species. In an enclosed rainforest (Biosphere 2), we observed δ_T-dynamics in response to an experimental severe drought, followed by a ²H deep-water pulse applied belowground before starting regular rain. We also sampled branches to obtain δ_X-values from cryogenic vacuum extraction (CVE). Daily flux-weighted δ¹⁸O_T-values were a good proxy for δ¹⁸O_X-values under well-watered and drought conditions that matched the rainforest's water source. Transpiration-derived δ¹⁸O_X-values were mostly lower than CVE-derived values. Transpiration-derived δ²H_X-values were relatively high compared to source water and consistently higher than CVE-derived values during drought. Tracing the ²H deep-water pulse in real-time showed distinct water uptake and transport responses: a fast and strong contribution of deep water to canopy tree transpiration contrasting with a slow and limited contribution to understory species transpiration. Thus, the in-situ transpiration method is a promising tool to capture rapid dynamics in plant water uptake and use by both woody and nonwoody species.     

Temperature record in the oxygen stable isotopes of Pacific sardine otoliths: Experimental vs. wild stocks from the Southern California Bight

Pacific sardines (Sardinops sagax) are commercially fished in Canada, USA, and Mexico along approximately 5000 km of coastal waters that experience a wide range of temperatures. Trinational management of the species can be problematic because the connectivity between spawning, recruitment, stock residency, and migration in some years may not be well predicted. Oxygen isotopic value of otoliths (δ¹⁸O_otolith) has been used to infer stock residency and movement of fish populations within regions, but few studies have used laboratory data to establish a predictive temperature model to validate δ¹⁸O_otolith values of wild fish. We conducted a growth experiment with juveniles at different temperatures using Southern California Bight (SCB) seawater to test the assumption that Pacific sardine otoliths accurately record environmental water temperature in the presence of constant salinity. Sardine δ¹⁸O_otolith values were significantly and negatively correlated with temperature according to the linear model:

δ¹⁸Ootolith(‰)−δ¹⁸Owater(‰)=−0.132(±0.003 SE)×Temperature(°C)+2.455(±0.043 SE)     

Stable isotope analysis of modern human hair collected from Asia (China,India, Mongolia,and Pakistan)

We report isotopic data (δ²H, δ¹⁸O n = 196; δ¹³C, δ¹⁵N n = 142; δ³⁴S n = 85) from human hair and drinking water (δ²H, δ¹⁸O n = 67) collected across China, India, Mongolia, and Pakistan. Hair isotope ratios reflected the large environmental isotopic gradients and dietary differences. Geographic information was recorded in H and O and to a lesser extent, S isotopes. H and O data were entered into a recently developed model describing the relationship between the H and O isotope composition of human hair and drinking water in modern USA and pre‐globalized populations. This has anthropological and forensic applications including reconstructing environment and diet in modern and ancient human hair. However, it has not been applied to a modern population outside of the USA, where we expect different diet. Relationships between H and O isotope ratios in drinking water and hair of modern human populations in Asia were different to both modern USA and pre‐globalized populations. However, the Asian dataset was closer to the modern USA than to pre‐globalized populations. Model parameters suggested slightly higher consumption of locally producedfoods in our sampled population than modern USA residents, but lower than pre‐globalized populations. The degree of in vivo amino acid synthesis was comparable to both the modern USA and pre‐globalized populations. C isotope ratios reflected the predominantly C₃‐based regional agriculture and C₄ consumption in northernChina. C, N, and S isotope ratios supported marine food consumption in some coastal locales. N isotope ratios suggested a relatively low consumption of animal‐derived products compared to western populations. Am J Phys Anthropol 2010. © 2009 Wiley‐Liss, Inc.     

Correlating δ13C and δ18O in cellulose of trees

We measured the carbon and oxygen isotopic composition of stem cellulose of Pinus sylvestris, Picea abies, Fagus sylvatica and Fraxinus excelsior. Several sites along a transect of a small valley in Switzerland were selected which differ in soil moisture conditions. At every site, six trees per species were sampled, and a sample representing a mean value for the period from 1940 to 1990 was analysed. For all species, the mean site δ¹³C and δ¹⁸O of stem cellulose are related to the soil moisture availability, whereby higher isotope ratios are found at drier sites. This result is consistent with isotope fractionation models when assuming enhanced stomatal resistance (thus higher δ¹³C of incorporated carbon) and increased oxygen isotope enrichment in the leaf water (thus higher δ¹⁸O) at the dry sites. δ¹⁸ O-δ¹³C plots reveal a linear relationship between the carbon and oxygen isotopes in cellulose. To interpret this relationship we developed an equation which combines the above-mentioned fractionation models. An important new parameter is the degree to which the leaf water enrichment is reflected in the stem cellulose. In the combined model the slope of the δ¹⁸O-δ¹³C plot is related to the sensitivity of the p_i/p_a of a plant to changing relative humidity.     

Reproductive phenology and physiological traits in the red mangrove hybrid complex (Rhizophora mangle and R. racemosa) across a natural gradient of nutrients and salinity

Factors modulating introgressive hybridization between the red mangrove species Rhizophora mangle and R. racemosa in spatially defined sites are poorly understood. To investigate this, we evaluated the reproductive phenology and the nutrient and physiological traits in those two species and their F₁ hybrids genotyped with microsatellite data across a natural hybrid zone from the Pacific coast of Panama. We found no evidence that reproductive phenology represents a barrier to gene flow, because R. mangle and the F₁ hybrids produced flowers and propagules throughout the annual cycle, while R. racemosa flowered only in the dry season. Soil nutrient concentrations decreased landward, while soil salinity varied only slightly. Foliar nutrients and δ¹⁵N signatures varied according to the soil nutrient gradient, but only foliar phosphorus and carbon varied among species. In contrast, two structural variables (height and trunk diameter) and leaf variables related to salinity tolerance (Na, Cl:Na, K:Na, cation:anion) and water-use efficiency (i.e., δ¹³C) differed among species, suggesting higher salinity tolerance for R. mangle and F₁ hybrids compared with R. racemosa. We conclude that parental species and F₁ hybrids differ in salinity tolerance and water-use efficiency, which could be associated with adaptive evolution of the red mangrove hybrid complex.     

δ18O‐derived incubation temperatures of oviraptorosaur eggs

In order to determine the incubation temperature of eggs laid by non‐avian dinosaurs, we analysed the oxygen isotope compositions of both eggshell carbonate (δ¹⁸O_c) and embryo bone phosphate (δ¹⁸O_p) from seven oviraptorosaur eggs with preserved in ovo embryo bones. These eggs come from the Upper Cretaceous Nanxiong Formation of Jiangxi Province, China. Oviraptorosaur theropods were selected because of their known brooding behaviour as evidenced by preserved adult specimens fossilized in brooding posture on their clutch. Incubation temperature of these embryos was estimated based on the following considerations: eggshell δ¹⁸O_c value reflects the oxygen isotope composition of egg water fluid; embryo bones precipitate from the same egg fluid; and oxygen isotope fractionation between phosphate and water is controlled by the egg temperature. A time‐dependent model predicting the δ¹⁸O_p evolution of the embryo skeleton during incubation as a function of egg temperature was built, and measured δ¹⁸O_c and δ¹⁸O_p values used as boundary conditions. According to the model outputs, oviraptorosaurs incubated their eggs within a 35–40°C range, similar to extant birds and compatible with the known active brooding behaviour of these theropod dinosaurs. Provided that both eggshell and embryo bones preserved their original oxygen isotope compositions, this method could be extended to investigate some reproductive traits of other extinct groups of oviparous amniotes.     

Water sources of riparian plants during a rainy season in Taihu Lake Basin,China: a stable isotope study

In this study, we investigated water sources of three typical plant species, i.e., Ginkgo biloba (Ginkgo biloba L.), Green soybean (Glycine max (L) Merr.), and Mulberry tree (Morus alba L.) in a rainy season by using a dual stable isotope approach (δ¹⁸O and δ²H). Potential water sources were divided into direct or internal (i.e. soil water at different depths) and indirect or external water sources (i.e. precipitation, river water and groundwater). The results indicated that the surface soil water δ¹⁸O and δ²H is enriched probably due to evaporation. Ginkgo biloba and Green soybean prefer using soil water from the upper soil layer (0–60 cm) and precipitation, and the Mulberry tree mainly used deep soil water (120-150 cm) and groundwater. The different water use strategies of the three plant species are likely to be determined by their different root distribution at the above correspondent soil depths.     

The influence of seawater carbonate ion concentration [CO32−] on the stable carbon isotope composition of the planktic foraminifera species Globorotalia inflata

We sampled the upper water column for living planktic foraminifera along the SW-African continental margin. The species Globorotalia inflata strongly dominates the foraminiferal assemblages with an overall relative abundance of 70–90%. The shell δ¹⁸O and δ¹³C values of G. inflata were measured and compared to the predicted oxygen isotope equilibrium values (δ¹⁸O_eq) and to the carbon isotope composition of the total dissolved inorganic carbon (δ¹³C_DIC) of seawater. The δ¹⁸O of G. inflata reflects the general gradient observed in the predicted δ¹⁸O_eq profile, while the δ¹³C of G. inflata shows almost no variation with depth and the reflection of the δ¹³C_DIC in the foraminiferal shell seems to be covered by other effects. We found that offsets between δ¹⁸O_shell and predicted δ¹⁸O_eq in the surface mixed layer do not correlate to changes in seawater [CO₃²⁻].To calculate an isotopic mass balance of depth integrated growth, we used the oxygen isotope composition of G. inflata to estimate the fraction of the total shell mass that is grown within each plankton tow depth interval of the upper 500 m of the water column. This approach allows us to calculate the Δδ¹³C_{interval added-DIC}; i.e. the isotopic composition of calcite that was grown within a given depth interval. Our results consistently show that the Δδ¹³C_IA-DIC correlates negatively with in situ measured [CO₃²⁻] of the ambient water. Using this approach, we found Δδ¹³C_IA-DIC/[CO₃²⁻] slopes for G. inflata in the large size fraction (250–355 μm) of − 0.013‰ to 0.015‰ (μmol kg^− 1)^− 1 and of − 0.013‰ to 0.017‰ (μmol kg^− 1)^− 1 for the smaller specimens (150–250 μm). These slopes are in the range of those found for other non-symbiotic species, such as Globigerina bulloides, from laboratory culture experiments. Since the Δδ¹³C_IA-DIC/[CO₃²⁻] slopes from our field data are nearly identical to the slopes established from laboratory culture experiments we assume that the influence of other effects, such as temperature, are negligibly small. If we correct the δ¹³C values of G. inflata for a carbonate ion effect, the δ¹³C_shell and δ¹³C_DIC are correlated with an average offset of 2.11.     

Kangaroo tooth enamel oxygen and carbon isotope variation on a latitudinal transect in southern Australia: implications for palaeoenvironmental reconstruction

Tooth enamel apatite carbonate carbon and oxygen isotope ratios of modern kangaroos (Macropus spp.) collected on a 900-km latitudinal transect spanning a C₃–C₄ transition zone were analysed to create a reference set for palaeoenvironmental reconstruction in southern Australia. The carbon isotope composition of enamel carbonate reflects the proportional intake of C₃ and C₄ vegetation, and its oxygen isotope composition reflects that of ingested water. Tooth enamel forms incrementally, recording dietary and environmental changes during mineralisation. Analyses show only weak correlations between climate records and latitudinal changes in δ¹³C and δ¹⁸O. No species achieved the δ¹³C values (~?1.0 ‰) expected for 100 % C₄ grazing diets; kangaroos at low latitudes that are classified as feeding primarily on C₄ grasses (grazers) have δ¹³C of up to ?3.5 ‰. In these areas, δ¹³C below ?12 ‰ suggests a 100 % C₃ grass and/or leafy plant (browse) diet while animals from higher latitude have lower δ¹³C. Animals from semi-arid areas have δ¹⁸O of 34–40 ‰, while grazers from temperate areas have lower values (~28–30 ‰). Three patterns with implications for palaeoenvironmental reconstruction emerge: (1) all species in semi-arid areas regularly browse to supplement limited grass resources; (2) all species within an environmental zone have similar carbon and oxygen isotope compositions, meaning data from different kangaroo species can be pooled for palaeoenvironmental investigations; (3) relatively small regional environmental differences can be distinguished when δ¹³C and δ¹⁸O data are used together. These data demonstrate that diet–isotope and climate–isotope relationships should be evaluated in modern ecosystems before application to the regional fossil record.     

Species-specific differences in temporal and spatial variation in δ13C of plant carbon pools and dark-respired CO2 under changing environmental conditions

Stable carbon isotope signatures are often used as tracers for environmentally driven changes in photosynthetic δ¹³C discrimination. However, carbon isotope signatures downstream from carboxylation by Rubisco are altered within metabolic pathways, transport and respiratory processes, leading to differences in δ¹³C between carbon pools along the plant axis and in respired CO₂. Little is known about the within-plant variation in δ¹³C under different environmental conditions or between species. We analyzed spatial, diurnal, and environmental variations in δ¹³C of water soluble organic matter (δ¹³C_WSOM) of leaves, phloem and roots, as well as dark-respired δ¹³CO₂ (δ¹³C_res) in leaves and roots. We selected distinct light environments (forest understory and an open area), seasons (Mediterranean spring and summer drought) and three functionally distinct understory species (two native shrubs—Halimium halimifolium and Rosmarinus officinalis—and a woody invader—Acacia longifolia). Spatial patterns in δ¹³C_WSOM along the plant vertical axis and between respired δ¹³CO₂ and its putative substrate were clearly species specific and the most δ¹³C-enriched and depleted values were found in δ¹³C of leaf dark-respired CO₂ and phloem sugars, ~?15 and ~?33 ‰, respectively. Comparisons between study sites and seasons revealed that spatial and diurnal patterns were influenced by environmental conditions. Within a species, phloem δ¹³C_WSOM and δ¹³C_res varied by up to 4 ‰ between seasons and sites. Thus, careful characterization of the magnitude and environmental dependence of apparent post-carboxylation fractionation is needed when using δ¹³C signatures to trace changes in photosynthetic discrimination.     

The oxygen isotopic signature of soil‐ and plant‐derived sulphate is controlled by fertilizer type and water source

The oxygen isotope signature of sulphate (δ¹⁸O_sulphate) is increasingly used to study nutritional fluxes and sulphur transformation processes in a variety of natural environments. However, mechanisms controlling the δ¹⁸O_sulphate signature in soil–plant systems are largely unknown. The objective of this study was to determine key factors, which affect δ¹⁸O_sulphate values in soil and plants. The impact of an ¹⁸O‐water isotopic gradient and different types of fertilizers was investigated in a soil incubation study and a radish (Raphanus sativus L.) greenhouse growth experiment. Water provided 31–64% of oxygen atoms in soil sulphate formed via mineralization of organic residues (green and chicken manures) while 49% of oxygen atoms were derived from water during oxidation of elemental sulphur. In contrast, δ¹⁸O_sulphate values of synthetic fertilizer were not affected by soil water. Correlations between soil and plant δ¹⁸O_sulphate values were controlled by water δ¹⁸O values and fertilizer treatments. Additionally, plant δ³⁴S data showed that the sulphate isotopic composition of plants is a function of S assimilation. This study documents the potential of using compound‐specific isotope ratio analysis for investigating and tracing fertilization strategies in agricultural and environmental studies.     

Nitrogen fertilization and δ18O of CO2 have no effect on 18O‐enrichment of leaf water and cellulose in Cleistogenes squarrosa (C4) – is VPD the sole control?

The oxygen isotope composition of cellulose (δ¹⁸O_Cel) archives hydrological and physiological information. Here, we assess previously unexplored direct and interactive effects of the δ¹⁸O of CO₂ (δ¹⁸O_CO2), nitrogen (N) fertilizer supply and vapour pressure deficit (VPD) on δ¹⁸O_Cel, ¹⁸O‐enrichment of leaf water (Δ¹⁸O_LW) and cellulose (Δ¹⁸O_Cel) relative to source water, and p_exp_x, the proportion of oxygen in cellulose that exchanged with unenriched water at the site of cellulose synthesis, in a C₄ grass (Cleistogenes squarrosa). δ¹⁸O_CO2 and N supply, and their interactions with VPD, had no effect on δ¹⁸O_Cel, Δ¹⁸O_LW, Δ¹⁸O_Cel and p_exp_x. Δ¹⁸O_Cel and Δ¹⁸O_LW increased with VPD, while p_exp_x decreased. That VPD‐effect on p_exp_x was supported by sensitivity tests to variation of Δ¹⁸O_LW and the equilibrium fractionation factor between carbonyl oxygen and water. N supply altered growth and morphological features, but not ¹⁸O relations; conversely, VPD had no effect on growth or morphology, but controlled ¹⁸O relations. The work implies that reconstructions of VPD from Δ¹⁸O_Cel would overestimate amplitudes of VPD variation, at least in this species, if the VPD‐effect on p_exp_x is ignored. Progress in understanding the relationship between Δ¹⁸O_LW and Δ¹⁸O_Cel will require separate investigations of p_ex and p_x and of their responses to environmental conditions.