Partitioning evapotranspiration fluxes from a Colorado grassland using stable isotopes: Seasonal variations and ecosystem implications of elevated atmospheric CO2

The stable isotopic composition of soil water is controlled by precipitation inputs, antecedent conditions, and evaporative losses. Because transpiration does not fractionate soil water isotopes, the relative proportions of evaporation and transpiration can be estimated using a simple isotopic mass balance approach. At our site in the shortgrass steppe in semi-arid northeastern Colorado, ¹⁸O values of soil water were almost always more enriched than those of precipitation inputs, owing to evaporative losses. The proportion of water lost by evaporation (E/ET) during the growing season ranged from nil to about 40% (to >90% in the dormant season), and was related to the timing of precipitation inputs. The sum of transpiration plus evaporation losses estimated by isotopic mass balance were similar to actual evapotranspiration measured from a nearby Bowen ratio system. We also investigated the evapotranspiration response of this mixed C₃/C₄ grassland to doubled atmospheric [CO₂] using Open-Top Chambers (OTC). Elevated atmospheric [CO₂] led to increased soil-water conservation via reduced stomatal conductance, despite greater biomass growth. We used a non-invasive method to measure the ¹⁸O of soil CO₂ as a proxy for soil water, after establishing a strong relationship between ¹⁸O of soil CO₂ from non-chambered control (NC) plots and ¹⁸O of soil–water from an adjacent area of native grassland. Soil–CO₂ ¹⁸O values showed significant treatment effects, particularly during a dry summer: values in ambient chambers (AC) were more enriched than in NC and elevated chamber (EC) plots. During the dry growing season of 2000, transpiration from the EC treatment was higher than from AC and lower than from NC treatments, but during 2001, transpiration was similar on all three treatments. Slightly higher evaporation rates from AC than either EC or NC treatments in 2000 may have resulted from increased convection across the soil surface from the OTC blowers, combined with lower biomass and litter cover on the AC treatment. Transpiration-use efficiency, or the amount of above-ground biomass produced per mm water transpired, was always greatest on EC and lowest on NC treatments.     

Passage kinetics of concentrates in dairy cows measured with carbon stable isotopes

Fractional passage rates form a fundamental element within modern feed evaluation systems for ruminants, but knowledge on feed-specific fractional passage is largely lacking. Commonly applied tracer techniques based on externally applied markers, such as chromium-mordanted neutral detergent fibre (Cr-NDF), have been criticised for behaving differently to feed particles. This study describes the use of the carbon stable isotope ratio (¹³C : ¹²C) as an internal digesta marker to quantify the fractional passage rate of concentrates through the digestive tract of dairy cows. In a crossover study, five dairy cows were fed low (24.6%) and high (52.6%) levels of concentrates (dry matter (DM) basis) and received a pulse-dosed Cr-NDF and ¹³C isotopes. The latter was administered orally by exchanging part of the dietary concentrates of low ¹³C natural abundance with a pulse dose of maize bran-based concentrates of high ¹³C natural abundance. Fractional passage rates from the rumen (K₁) and from the large intestine (K₂) were determined from faecal marker concentrations of Cr-NDF and of ¹³C in the DM (¹³C-DM), NDF (¹³C-NDF) and neutral detergent soluble (¹³C-NDS). No differences in K₁ estimates were found for the two concentrate levels fed but significant differences between markers (P<0.001) were observed. Faecal Cr-NDF excretions gave lower K₁ estimates (0.037–0.039/h) than ¹³C-DM (0.054–0.056/h) and ¹³C-NDF (0.061–0.063/h). The ¹³C-NDS was calculated by the difference of ¹³C in the DM and NDF, and K₁ values (0.039–0.043/h) were comparable to Cr-NDF. Total mean retention time was considerably higher for Cr-NDF (40.9–42.0 h) as compared to ¹³C-DM and ¹³C-NDF (32.0–33.5 h; P<0.001). The accuracy of the curve fits for Cr-NDF and ¹³C-DM and ¹³C-NDF was overall good (mean prediction error of 9.9–13.9%). Fractional passage rate of Cr-NDF was comparable to studies where this marker was assumed to represent the fractional passage of roughages. However, K₁ estimates based on the ¹³C : ¹²C ratio varied considerably from studies based on external markers. Our results suggest that the use of ¹³C isotopes as digesta passage markers can provide feed component-specific K₁ estimates for concentrates and provides new insight into passage kinetics of NDF from technologically treated compound feed.     

Isotopes in geobiochemistry: tracing metabolic pathways in microorganisms of environmental relevance with stable isotopes

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Compiled records of atmospheric CO2 concentrations and stable carbon isotopes to reconstruct climate and derive plant ecophysiological indices from tree rings

The stable carbon isotopic composition (δ¹³C) measured in tree rings is a standard proxy for paleoclimate reconstructions and is increasingly being used as a paleophysiological proxy. To fully exploit the potential of tree ring δ¹³C proxy, atmospheric CO₂ concentration and δ¹³C (δ¹³CO₂) data are required to correct tree ring δ¹³C from the declining trend of δ¹³CO₂ due to fossil fuel burning since 1850 CE, and to derive physiological parameters using biochemical models that link photosynthesis to δ¹³C. These atmospheric data are available from direct measurements or can be inferred from indirect proxies such as ice cores covering the Common Era (CE) at variable temporal resolutions. For almost two decades, tree-ring researchers have relied on a dataset derived from fitted linear regressions of ice core measurements available through the seminal McCarroll and Loader (2004) article for the 1850−2003 CE period. However, new calibrations and compilations of ice core data and direct measurements are now available as part of Earth System Modelling efforts which remain overlooked by the tree ring research community.Here, we present an overview of the new and freely available datasets and provide recommendations for their use in ecophysiology and paleoclimate research, that we expect will stimulate cross-disciplinary collaborations.     

Use of stable carbon and nitrogen isotopes in insect trophic ecology

Insects are the most diverse organisms and often the most abundant animals in some ecosystems. Despite the importance of their functional roles and of the knowledge for conservation, the trophic ecology of many insect species is not fully understood. In this review, I present how stable carbon (C) and nitrogen (N) isotopes have been used to reveal the trophic ecology of insects over the last 30 years. The isotopic studies on insects have used differences in C isotope ratios between C₃ and C₄ plants, along vertical profiles of temperate and tropical forest stands, and between terrestrial and aquatic resources. These differences enable exploration of the relative importance of the food resources, as well as movement and dispersal of insects across habitats. The ¹³C‐enrichment (approximately 3‰) caused by saprotrophic fungi can allow the estimation of the importance of fungi in insect diets. Stable N isotopes have revealed food resource partitioning across diverse insect species above and belowground. Detritivorous insects often show a large trophic enrichment in ¹³C (up to 3‰) and ¹⁵N (up to 10‰) relative to the food substrates, soil organic matter. These values are greater than those commonly used for estimation of trophic level. This enrichment likely reflects the prevalence of soil microbial processes, such as fungal development and humification, influencing the isotopic signatures of diets in detritivores. Isotope analysis can become an essential tool in the exploration of insect trophic ecology in terms of biogeochemical C and N cycles, including trophic interactions, plant physiological and soil microbial processes.     

Fractionation of the stable isotopes of inorganic carbon by seagrasses

The δ¹³C values for seagrasses collected along the Texas Gulf Coast range from −10.9 to −11.4‰. These values are similar to the δ¹³C values of terrestrial C₄ plants, but seagrasses lack bundle sheath cells which are important in determining the δ¹³C values of C₄ plants. This work attempts to explain the reason the δ¹³C values of seagrasses resemble the δ¹³C values of C₄ plants.     

羊肚菌营养方式的稳定碳同位素研究

采用稳定碳同位素分析方法对羊肚菌的营养方式进行了研究.发现山西省关帝山庞泉沟自然保护区羊肚菌生境中的δ13C=-24‰为菌根营养型/腐生营养型分界线;菌盖呈黑色、有纵脉的羊肚菌为腐生营养型,菌盖呈污黄色的羊肚菌为菌根菌营养型;云南、四川、北京等地羊肚菌的营养方式呈现相似规律.     

Application of CO2 carbon stable isotope analysis to ant trophic ecology

Stable isotope analysis of animal tissues is commonly used to infer diet and trophic position. However, it requires destructive sampling. The analysis of carbon isotopes from exhaled CO₂ is non-invasive and can provide useful ecological information because isotopic CO₂ signatures can reflect the diet and metabolism of an animal. However, this methodology has rarely been used on invertebrates and never on social insects. Here, we first tested whether this method reflects differences in δ¹³C-CO₂ between workers of the Mediterranean ant Crematogaster scutellaris (Olivier) (Hymenoptera: Formicidae, Crematogastrini) fed with sugar from beet (C3; Beta vulgaris L., Amaranthaceae) or cane (C4; Saccharum officinarum L., Poaceae). We found that a significant difference can be obtained after 24 h. Consequently, we used this technique on wild co-occurring ant species with different feeding preferences to assess their reliance on C3 or C4 sources. For this purpose, we sampled workers of C. scutellaris, the invasive garden ant Lasius neglectus (van Loon et al.) (Lasiini), and the harvester ant Messor capitatus (Latreille) (Stenammini). No significant differences in their carbon isotopic signatures were recorded, suggesting that in our study site no niche partitioning occurs based on the carbon pathway, with all species sharing similar resources. However, further analysis revealed that M. capitatus, a seed-eating ant, can be regarded as a C3 specialist, whereas L. neglectus and C. scutellaris are generalists that rely on both C3 and C4 pathways, though with a preference for the former. Our results show that this methodology can be applied even to small animals such as ants and can provide useful information on the diets of generalist omnivores.     

Disentangling drought-induced variation in ecosystem and soil respiration using stable carbon isotopes

Combining C flux measurements with information on their isotopic composition can yield a process-based understanding of ecosystem C dynamics. We studied the variations in both respiratory fluxes and their stable C isotopic compositions (δ¹³C) for all major components (trees, understory, roots and soil microorganisms) in a Mediterranean oak savannah during a period with increasing drought. We found large drought-induced and diurnal dynamics in isotopic compositions of soil, root and foliage respiration (δ¹³C_res). Soil respiration was the largest contributor to ecosystem respiration (R _eco), exhibiting a depleted isotopic signature and no marked variations with increasing drought, similar to ecosystem respired δ¹³CO₂, providing evidence for a stable C-source and minor influence of recent photosynthate from plants. Short-term and diurnal variations in δ¹³C_res of foliage and roots (up to 8 and 4‰, respectively) were in agreement with: (1) recent hypotheses on post-photosynthetic fractionation processes, (2) substrate changes with decreasing assimilation rates in combination with increased respiratory demand, and (3) decreased phosphoenolpyruvate carboxylase activity in drying roots, while altered photosynthetic discrimination was not responsible for the observed changes in δ¹³C_res. We applied a flux-based and an isotopic flux-based mass balance, yielding good agreement at the soil scale, while the isotopic mass balance at the ecosystem scale was not conserved. This was mainly caused by uncertainties in Keeling plot intercepts at the ecosystem scale due to small CO₂ gradients and large differences in δ¹³C_res of the different component fluxes. Overall, stable isotopes provided valuable new insights into the drought-related variations of ecosystem C dynamics, encouraging future studies but also highlighting the need of improved methodology to disentangle short-term dynamics of isotopic composition of R _eco.     

Differences in nitrogen and carbon stable isotopes between planktonic and benthic microalgae

We compiled published data on the nitrogen and carbon stable isotope ratios of phytoplankton and benthic microalgae from lentic systems and explored the primary factors determining the isotope values among systems. Also, we investigated seasonal changes in nitrogen stable isotope ratios of phytoplankton and benthic microalgae in the strongly acidic lake, Lake Katanuma, which has only two dominant species, Pinnularia acidojaponica as a benthic diatom and Chlamydomonas acidophila, a planktonic green alga. From the published dataset, it may be concluded that δ¹³C of benthic diatoms were more enriched than those of phytoplankton at the same sites, although the nitrogen isotope of phytoplankton and benthic microalgae were similar. This differences in δ¹³C between benthic microalgae and phytoplankton could be explained by the boundary layer effect. On the other hand, nitrogen isotope values of both benthic microalgae and phytoplankton were primarily controlled by the same environmental factor, and boundary layer effects are not the primary factor determining the nitrogen isotope values of microalgae. Also, we showed temporal dynamics in nitrogen isotopes of benthic and planktonic microalgae species in Lake Katanuma, and the trends of nitrogen isotopes are similar between benthic and planktonic microalgae, as concluded from the published dataset.     

四川裂腹鱼耳石碳、氧稳定同位素特征

为探讨耳石碳(δ¹³C)、氧(δ¹⁸O)稳定同位素在淡水鱼类群体识别中的作用,本研究以养殖条件下不同年龄组四川裂腹鱼为对象,采用稳定同位素质谱仪进行碳、氧同位素测定,揭示耳石中碳、氧稳定同位素特征,探讨其与环境间的关系. 结果表明:1⁺龄四川裂腹鱼δ¹³C和δ¹⁸O值均与耳石质量无显著相关关系,但在微耳石和星耳石之间存在显著差异;不同年龄四川裂腹鱼微耳石δ¹³C和δ¹⁸O平均值分别为(-9.58±0.06)‰、(-8.33±0.17)‰,其在雌雄个体之间均无显著性差异,但在不同年龄组间存在显著差异. 耳石δ¹⁸O和δ¹³C的关联分析能有效区分四川裂腹鱼不同养殖年龄群体,可作为一种识别淡水鱼类养殖群体的手段.     

Biogeochemistry of the stable isotopes of hydrogen and carbon in salt marsh biota

Deuterium to hydrogen ratios of 14 plant species from a salt marsh and lagoon were 55‰ depleted in deuterium relative to the environmental water. Carbon tetrachloride-extractable material from these plants was another 92‰ depleted in deuterium. This gave a fractionation factor from water to CCl₄ extract of 1.147. This over-all fractionation was remarkably constant for all species analyzed. Plants also discriminate against ¹³C, particularly in the lipid fraction. Data suggest that different mechanisms for carbon fixation result in different fractionations of the carbon isotopes. Herbivore tissues reflected the isotopic ratios of plants ingested. Apparently different metabolic processes are responsible for the different degrees of fractionation observed for hydrogen and carbon isotopes.     

Global pattern of carbon stable isotopes of suspended particulate organic matter in lakes

Carbon stable isotopes (??¹³C) of particulate organic matter (POM) are useful indicators for tracking the sources of organic matter, CO₂ concentrations, primary productivity and the trophic base in lakes. Here we provide a synthesis of literature data from 228 lakes around the world to assess the distribution pattern of ??¹³C_POM along latitudinal, morphometric and biogeochemical gradients, and the feasibility of utilizing ??¹³C_POM as an indicator for lake metabolism. Results from this analysis revealed a large variation in ??¹³C_POM among lakes with a range from ?46.2 to ?13.0?? and a median of ?29.7??. The ??¹³C_POM generally decreased from low to high latitude along with the decreases in total phosphorus (TP), pH, lake size and the increases in partial pressure of CO₂. The combination of these factors may play a significant role in shaping the pattern of ??¹³C_POM distribution. A multiple regression model using matching data (n?=?92 lakes) indicated that latitude, lake size and TP concentration were the important factors determining ??¹³C_POM, although only 25% of the variance in ??¹³C_POM was explained by the model. Compared to the average ??¹³C value (?27??) of terrestrial plants, 165 lakes (72%) in this analysis were isotopically depleted in ??¹³C_POM, with a mean of ?31.5??, which is indicative of an allochthonous contribution of terrestrial organic matter. This finding is consistent with the view that the majority of lakes in the world receive a terrestrial subsidy of carbon and are sources of CO₂ to the atmosphere.     

Minimum pulses of stable and radioactive carbon isotopes to detect belowground carbon transfer between plants

Methodological problems, such as unwanted shifts in plant carbon allocation patterns following large isotopic labeling pulses, have hindered accurate quantification of belowground carbon movement in plant–soil systems. These problems must be addressed before we can understand the factors regulating carbon movement between plants and soils and the importance of this movement to the global carbon cycle. We studied the effects of pulse-label size on carbon allocation and transfer between ectomycorrhizal paper birch (Betula papyrifera Marsh.) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings using increasing pulse levels of either ¹³C or ¹⁴C in two separate laboratory experiments. Our specific objectives were: (1) to determine the minimum pulse of ¹³CO₂ or ¹⁴CO₂ for detecting carbon movement between plants through belowground transfer pathways, (2) to determine whether carbon allocation patterns within these plants change when exposed to short pulses of elevated carbon dioxide and, (3) to determine whether carbon allocation patterns are similar when using two different carbon isotopes. We detected carbon movement between plants at each ¹³C and ¹⁴C pulse level. There was a tendency for the amount of interplant carbon transfer to increase with increasing ¹³C pulse level, but the same amount of transfer occurred at all ¹⁴C pulses between 0.19 and 0.56 MBq. Carbon allocation patterns did not change with pulse level but they were affected by the choice of carbon isotope. We conclude that at least 8 ml of ¹³C or 0.19 MBq of ¹⁴C is sufficient to detect belowground carbon transfer in small seedlings growing in close proximity.     

Effects of nutritional restriction on nitrogen and carbon stable isotopes in growing seabirds

When using stable isotopes as dietary tracers it is essential to consider effects of nutritional state on isotopic fractionation. While starvation is known to induce enrichment of ¹⁵N in body tissues, effects of moderate food restriction on isotope signatures have rarely been tested. We conducted two experiments to investigate effects of a 50–55% reduction in food intake on δ¹⁵N and δ¹³C values in blood cells and whole blood of tufted puffin chicks, a species that exhibits a variety of adaptive responses to nutritional deficits. We found that blood from puffin chicks fed ad libitum became enriched in ¹⁵N and ¹³C compared to food-restricted chicks. Our results show that ¹⁵N enrichment is not always associated with food deprivation and argue effects of growth on diet–tissue fractionation of nitrogen stable isotopes (Δ¹⁵N) need to be considered in stable isotope studies. The decrease in δ¹³C of whole blood and blood cells in restricted birds is likely due to incorporation of carbon from ¹³C-depleted lipids into proteins. Effects of nutritional restriction on δ¹⁵N and δ¹³C values were relatively small in both experiments (δ¹⁵N: 0.77 and 0.41‰, δ¹³C: 0.20 and 0.25‰) compared to effects of ecological processes, indicating physiological effects do not preclude the use of carbon and nitrogen stable isotopes in studies of seabird ecology. Nevertheless, our results demonstrate that physiological processes affect nitrogen and carbon stable isotopes in growing birds and we caution isotope ecologists to consider these effects to avoid drawing spurious conclusions.     

Soil organic carbon stabilization in permafrost peatlands

In permafrost peatlands, the degradation of permafrost soil can raise soil temperature and alter moisture conditions, which increases the rate of loss of soil organic carbon (SOC). Here we selected three typical permafrost types that have very different active layer thicknesses but with soil originating from the same vegetation and which exist under comparable climatic conditions in the Da Xing’an mountain range: continuous permafrost, island permafrost, and island melting permafrost. To quantify the relative importance of control elements on SOC stabilization in these different permafrost types, we used correlation analysis to assess the relationship between organic carbon, physical and chemical properties and microorganisms, and explored the contribution of these factors to the accumulation of organic carbon. This study shows that the interaction between clay or silt, iron oxides and microorganisms have an important influence on the stability of organic carbon in permafrost peatlands.