Isotope Balance Method Research Articles

Widespread root-zone water bypass for various climates and species: Implications for the ecohydrological separation understanding

Root-zone water storage is a core component of many hydrological systems, and a critical variable influencing water transport in the soil-vegetation-atmosphere continuum and biogeochemical processes. However, the dynamics of water bypass and replenishment in root-zone during precipitation infiltration processes have been rarely investigated. Here we employed a piecewise isotope balance method to estimate the proportion of root-zone water replenished by precipitation, and conducted a meta-analysis to check the patterns of water bypass and replenishment in root-zone for various species across five climate zones and examined the potential influencing factors. The results show that high proportion of root-zone water replenishment is likely to appear during the transition period from dry season/soil to wet season/soil. Root-zone water bypass is widespread under various climates, which is likely to occur by downward drainage under wet soil condition or via evaporation loss under high atmospheric demand. Antecedent soil water condition and rooting depth are found to be the two key factors regulating dynamics of root-zone water bypass and replenishment. These findings provide useful insight into our understanding of ecohydrological modeling (e.g., non-well-mixed water pool in soils) and plant responses to precipitation pulses.

When to conduct an isotopic survey for lake water balance evaluation in highly seasonal climates

AbstractOne‐time or short‐term lake water isotopic surveys are often employed to evaluate regional lake water balance. However, it can be difficult to determine the optimal time‐window for sampling to obtain a representative long‐term perspective of lake water balance in settings influenced by seasonal variations in precipitation, evaporative loss, glacial/snow meltwater, and larger seasonal shifts in isotopic composition of precipitation. This is especially true for areas of the Tibetan Plateau that are influenced by the summer Indian monsoon. Although high‐frequency sampling is always preferred as the most rigorous approach to characterize the water budget of lakes or watersheds, this may be impractical in remote regions and over large spatial scales. To assess the potential sensitivity of isotope balance characterization to seasonal variability, we used a weekly lake water isotope data set acquired over a period of 3 years on the Tibetan Plateau to evaluate the potential inaccuracies that might have arisen from using isotopic data collected during narrower time‐windows. For this assessment, we use weekly isotopic data collected during the study and assume that these sampling events were stand‐alone one‐time surveys. We then demonstrate the sensitivity of the isotope balance method in this setting, particularly for the rainy season that significantly underestimated the evaporation/inflow. In contrast, isotopic composition of the lake water was found to be more representative of long‐term conditions when sampled in October on the Tibetan Plateau. To broaden our evaluation of seasonality effects over a range of climatic zones, published high‐frequency isotopic data were also compiled, and a similar assessment was carried out for selected regions of the world. The synthesized data and model outputs, which confirm pronounced variations in lake water isotopic composition and evaporation/inflow across a range of seasonal climates, were used to determine optimal sampling windows for these specific regions.

Some results of tritium researches of natural waters in Russia (1960–2010)

Original data of tritium (1 H3 HO, or HTO) studying (1959–2010) of both natural, and artificial (technogenic) origin in natural waters on the country territory and in the seas of the World Ocean are presented. Entry sources of cosmogenic and technogenic HTO in hydrosphere of the Earth from atmosphere and its carrying over to various links of hydrological cycle are considered. The basic laws of the space-temporal HTO distribution in surface water of the northern part of the Eurasian continent, in the seas washing Russia and in separate water areas of the World Ocean, and also in atmospheric precipitation and Antarctica’s firn snow are studied. Prevailing influence of a molecular exchange on the HTO content in the top layer of lakes, the seas and oceans and in atmospheric moisture over the ocean is shown. The theory of a multicomponent isotope balance method is developed for studying of dynamic characteristics of water objects. Priority results of tritium use in the industry (on oil deposits, collieries, mines etc.) and in researches under national and international programs (WESTPAK, SOUTHERN OCEAN, “Isotopes in deposits”, the Soviet Arctic and Antarctic expeditions) are given. Systems of national and Far East regional monitoring HTO pollution of natural waters are created. Complexes for HTO measuring with record levels of threshold sensitivity (in the late seventies sensitivity of complex POI equaled to 4 × 10−6 Bq/ml) have been developed.

The δ18O and δ2H enrichment of Ethiopian lakes

Up-to-date overview of 18O and deuterium isotope composition of Ethiopian (low latitude) lakes is presented and the suitability of the isotope balance method in water balance estimations of these lakes is assessed. To determine the water budgets of the lakes and to investigate the factors that influence isotopic enrichment in these lakes we used a method whereby the calculated (theoretical) local evaporation line (CLEL) is compared with the evaporation line derived from fitting of the measured isotopic compositions of the lakes in the region. The CLEL is specific to a given region and is determined from representative relative humidity, the isotopic composition of the ambient moisture and the isotopic composition of inflow waters. The comparison between CLEL and the measured isotopic composition of the lakes, and the departures of the isotopic compositions of these lakes from the calculated values, were interpreted in terms of the lakes water budget and their catchment characteristics. This comparison revealed how far the existing empirical parameters in the isotope balance method predict evaporative enrichment in lakes under low latitude setting. Significant discrepancies were noted when CLEL was compared with the measured isotope enrichments of lakes. In this regard the low latitude lakes show similar enrichment behavior to that of other lakes in high latitude settings. Satisfactory agreement between the calculated and the measured local evaporation lines was obtained through adjusting the theta value which accounts for humidity build-up over the evaporating lake. All the investigated lakes are evaporation-dominated systems, with evaporative water losses exceeding 50% of total water loss.

Effects of chronic glucagon administration on cholesterol and bile acid metabolism

Male adult Wistar rats received daily, at 9 a.m. and 5 p.m., 10 /gmg of Zn-protamine glucagon for 21 days by subcutaneous injections. The blood glucose level was not significantly modified. Cholesterol and triacylglycerol levels were decreased by 40 and 70% in plasma but not in the liver. The rates of cholesterol turnover processes were determined in vivo with an isotope balance method. Internal secretion of cholesterol (13.8 ± 0.5 mg/day per rat in control rats and 22.4 ± 0.9 mg/day per rat in glucagon-treated rats) and cholesterol transformation into bile acids were strikingly increased by chronic administration of glucagon. Biliary secretion rates of bile acids measured by a wash-out method were increased by 139%, while the intestinal bile acid pool was not changed. The enterohepatic cycle number was increased from five per day in control rats to nine per day in glucagon-treated rats. An increased turnover rate of the exchangeable cholesterol would explain the hypocholesterolemic effect of glucagon.

On the environmental isotope method for determining the water balance components of some lakes

The paper deals with the isotope balance method applied to lakes, which can be assumed as being under steady-state conditions. It is shown that the long-term averages and non-normalized temperatures can be used in the balance equations. The applicability of the proposed approach was tested by the interpretation or reinterpretation of several known case studies (Chala, Titicaca, Waid, Burdur, Beysehir and Egridir lakes). An isotope study of a small artificial lake (1.2 · 10 6 m 3) is presented and compared with whole-body tracing by K 3 60 Co(CN) 6. This artificial tracer does not appear useful for long-term tracing of surface waters. The water balance equation for each of the lakes considered was solved with the aid of the isotope balance equation of 18O. Applying the calculated components of the water balance, the isotope balance equation of deuterium was solved for finding the values of kinetic enrichment for deuterium. These values, which give the fit of the evaporation lines to the experimental data, do not agree with the values known from direct laboratory experiments, thus they have to be treated as apparent values, which may be useful for future field work.

Sources of error in the isotopic cholesterol balance method in African green monkeys consuming a cholesterol-free diet

Six African green monkeys were labeled intravenously with [1,2-(3)H]cholesterol while consuming a cholesterol-free liquid formula diet. The plasma cholesterol specific activity was compared with the specific activity of the biliary cholesterol and bile acids and with the fecal neutral steroids in order to determine whether the traditional isotopic balance method was valid for the calculation of endogenous cholesterol excretion. The specific activity of biliary cholesterol and bile acids averaged 10-15% lower than plasma cholesterol specific activity. Fecal cholesterol and coprostanone specific activities were similar to that of the biliary cholesterol, but the specific activity of fecal coprostanol was approximately 25% lower. This suggests that biliary cholesterol and bile acids were derived from a pool of hepatic cholesterol that did not completely equilibrate with the whole body exchangeable cholesterol pool. In addition, there was further reduction in the specific activity of coprostanol, the major fecal neutral steroid, presumably by cholesterol synthesized in the lower intestine and preferentially converted to coprostanol. As a result, the traditional isotopic balance procedure underestimated endogenous neutral steroid excretion by 46% and bile acid excretion by 31% in African green monkeys fed the cholesterol-free diet. Within 7 days after the addition of 1 mg cholesterol/kcal to the diet, the specific activities of plasma and biliary cholesterol and biliary bile acids were identical and there was no difference in the specific activities of the individual fecal neutral steroids. Thus, the traditional isotopic balance procedure (DPM fecal neutral steroids + bile acids/specific activity [DPM/mg] plasma cholesterol) can be used for calculation of endogenous cholesterol excretion in cholesterol-fed animals during the nonsteady state when plasma cholesterol concentrations are rapidly increasing, as well as after a new steady state has been achieved.-Henderson, G. R., and R. W. St. Clair. Sources of error in the isotopic cholesterol balance method in African green monkeys consuming a cholesterol-free diet.

Cholesterol metabolism in a patient with familial Hyper-betalipoproteinaemia during periods of high and low cholesterol intake

The metabolism of cholesterol was estimated in a man with familial hyper-betalipoproteinaemia in the heterozygous form during periods of high and low cholesterol intake. [ 14C]Cholesterol of constant specific activity was fed daily for several weeks at the beginning of each period. Faecal excretion of total steroids was estimated by the isotopic balance method during each period when all unabsorbed dietary [ 14C]cholesterol had been excreted. Absorption of dietary cholesterol during each period was estimated from the cumulative total of endogenous 14C excreted in the faeces. Cholesterol synthesis was estimated as the difference between faecal excretion of endogenous steroids and absorption of dietary cholesterol, on the assumption that the amount of cholesterol in the exchangeable pool remained constant during each period. Absorption of dietary cholesterol was about 100 mg/day more on the high than on the low intake. Excretion of bile acids in the faeces was higher on the high than on the low intake, but the difference was not statistically significant. Cholesterol synthesis was not significantly lower on the high than on the low cholesterol intake. It is concluded that increased absorption of dietary cholesterol did not lead to inhibition of cholesterol synthesis in this patient.

Effect of neomycin on cholesterol metabolism in the germ-free pig.

Abstract The effect of neomycin sulphate in an oral dose of 1.5 g daily was studied in germ‐free piglets given a preliminary intravenous dose of labelled cholesterol. Combined urinary and faecal pools were assayed for radioactivity and the faecal excretion of neutral sterols and bile acids was calculated by the isotopic balance method. During the administration of neomycin there was only a slight and transient decrease in serum cholesterol, but the faecal excretion of endogenous neutral sterols was more than doubled. There was also a significant rise in faecal fat excretion, although bile acid excretion remained unchanged. Examination of the morphology of the small intestinal mucosa by both light and electron microscopy showed no evidence that neomycin had caused any damage to either absorptive or crypt cells. The only abnormality found was the presence in macrophages in the lamina propria of dense inclusions which exhibited a paracrystalline structure at very high magnification; they were not found in control germ‐free animals. These findings demonstrate that neomycin exerts an effect on lipid metabolism which is independent of its antibiotic actions and which is not secondary to mucosal damage.

Effect of dietary fat on the fecal excretion of cholesterol and its degradation products in man.

Fecal bile acid and neutral sterol excretion rates were determined in five healthy young men when serum cholesterol changes were induced by isocaloric substitution of an unsaturated (safflower oil) for a saturated fat (butter). The isotope balance method was used after the intravenous injection of cholesterol-4-(14)C. A feces extraction method is presented which permits essentially complete separation of fecal neutral sterols and bile acids. There was a significant increase in the total excretion of the fecal end products of cholesterol metabolism from 966 +/- 42 mg/day on saturated fat to 1147 +/- 45 mg/day on unsaturated fat, and the increase was equally distributed between the neutral sterol and bile acid fractions. With the substitution of dietary fats, regardless of the sequence of their feeding, there was a 28% reduction in serum cholesterol concentration during ingestion of the unsaturated fat. There were reciprocal changes in serum cholesterol levels and fecal steroid excretion with the substitution of one type of fat for the other. The changes in plasma cholesterol content were more than adequately balanced by the reciprocal changes in fecal cholesterol end product excretion. The findings in this study agree with several previous reports in supporting the hypothesis that the hypocholesteremic action of dietary unsaturated fatty acids is associated with an increase in the fecal loss of bile acids and neutral sterols.