The determining factors of hydrogen isotope offsets between plants and their source waters.

A fundamental assumption when using hydrogen and oxygen stable isotopes to understand ecohydrological processes is that no isotope fractionation occurs during plant water uptake/transport/redistribution. A growing body of evidence has indicated that hydrogen isotope fractionation occurs in certain environments or for certain plant species. However, whether the plant water source hydrogen isotope offset (δ2 H offset) is a common phenomenon and how it varies among different climates and plant functional types remains unclear. Here, we demonstrated the presence of positive, negative, and zero offsets based on extensive observations of 12 plant species of 635 paired stable isotopic compositions along a strong climate gradient within an inland river basin. Both temperature and relative humidity affected δ2 H offsets. In cool and moist environments, temperature mainly affected δ2 H offsets negatively due to its role in physiological activity. In warm and dry environments, relative humidity mainly affected δ2 H offsets, likely by impacting plant leaf stomatal conductance. These δ2 H offsets also showed substantial linkages with leaf water 18 O enrichment, an indicator of transpiration and evaporative demand. Further studies focusing on the ecophysiological and biochemical understanding of plant δ2 H dynamics under specific environments are essential for understanding regional ecohydrological processes and for conducting paleoclimate reconstructions.

Climate and industrial pollution determine the seasonal and spatial mercury variations in the China's Weihe River.

Natural processes and human activities impact mercury (Hg) pollution in rivers. Investigating the individual contributions and interactions of factors affecting variations in Hg concentrations, particularly under climate change, is crucial for safeguarding watershed ecosystems and human health. We collected 381 water samples from China's Weihe River Basin (WRB) during dry and wet seasons to assess the total Hg (THg) concentration. Results revealed high Hg concentrations in the WRB (0.1-2200.9 ng/L, mean 126.2 ± 335.5 ng/L), with higher levels during the wet season (wet season: 249.1 ± 453.5 ng/L, dry season: 12.7 ± 14.0 ng/L), particularly in the mainstream and southern tributaries of the Weihe River. Industrial pollution (contributing 26.2 %) and precipitation (contributing 33.5 %) drove spatial heterogeneity in THg concentrations during the dry and wet seasons, respectively. Notably, combined explanatory power increased to 47.9 % when interaction was considered, highlighting the amplifying effect of climate change, particularly precipitation, on the impact of industrial pollution. The middle and downstream of the Weihe River, especially the Guanzhong urban agglomeration, were identified as high-risk regions for Hg pollution. With ongoing climate change the risk of Hg exposure in the WRB is expected to escalate. This study lays a robust scientific foundation for the effective management of Hg pollution in analogous river systems worldwide.

Extreme precipitation stable isotopic compositions reveal unexpected summer monsoon incursions in the Qilian Mountains.

Isotope composition and moisture sources of precipitation are important for understanding water cycles and reconstructing paleoclimate. Based on 15-years' precipitation stable Isotope composition (δ18O and δ2H) from four stations of the Qilian Mountains, we found unique δ18O and δ2H features associated with the incursion of the summer monsoon over the Qilian Mountains, northwestern China. In 12 of the 15 years, similar seasonal variations of δ18O and δ2H confirmed a dominant source of moisture from Westerly circulation, and higher intercepts of the local meteoric water line (LMWL) indicated strong recycling of continental moisture. However, in August 2016 and 2018, extremely low slopes and intercepts of the LMWL, and more negative δ18O and δ2H revealed substantial contributions of the Asian summer monsoon to precipitation of the Qilian Mountains, with extremely heavy precipitation in August 2016. The column moisture flux, land-sea thermal contrast, correlations of precipitation δ18O with East Asian Summer Monsoon Index and Westerlies Index, HYSPLIT modeling results and precipitation δ18O along backward trajectories confirmed incursions of the summer monsoon in August 2016 and 2018. Our redefining of the boundary of the summer monsoon region confirmed the summer monsoon incursion zone can extend to the west of longitude 96°E and north of latitude 40°N in strong monsoon years, corresponding to boundaries of monsoon incursions in the mid-Holocene. Temperature correlated with precipitation δ18O at monthly and shorter time scales, but not for whole seasons or at yearly scale, revealing that summer monsoon incursions are therefore more likely than changing temperature to explain the multi-year cycles in the Qilian Mountains ice archives. Continent-scale shifts in atmospheric circulation strongly influence water resources in the Qilian mountains, and may change in frequency as climate warms. This study therefore has important implications for understanding water resources in the Qilian mountains in the past and into the future.

Spatial variations on the hydrochemistry, controls, and solute sources of surface water in the Weihe River Basin, China.

The Weihe River Basin (WRB), the largest tributary of the Yellow River Basin (YRB), is located in the northwest of China. As the "mother river" of the Guanzhong Plain, it plays an important role in the development of Guanzhong City Group. Based on pH, electric conductivity (EC), total dissolved solids (TDS), and major ionic concentrations of 227 samples collected from the main stream (MS), northern tributaries (NT), and southern tributaries (ST) of the WRB, we explored spatial differential characteristics of hydrochemistry and their controlling factors, solute sources, and water quality of surface water. The results revealed mildly alkaline pH and much higher TDS values than the global average with mean values of 7.9 and 1037.7 mg L-1, respectively. Except NO3-, the concentrations of major ions in the MS and NT were higher than those of the ST, with similar spatial distribution patterns of Ca2+, Na+, Mg2+, SO42-, and Cl-. Na+ and SO42- were the most dominant cation and anion in the MS and NT controlled by both rock weathering and evaporation-crystallization processes. Ca2+ and HCO3- were the most dominant cation and anion in the ST mainly controlled by rock weathering process. Evaporite dissolution contributed the most to dissolved solutes in the MS and NT, while carbonate weathering dominated dissolved solutes in the ST. These findings were confirmed by the results of correlation matrix, principal component analysis (PCA), stoichiometric plots, and different water types identified as Na-SO4·Cl in the MS and NT, and Ca-HCO3 in the ST. Atmospheric and anthropogenic inputs had a minor effect on the surface water chemistry. However, human activities could not be ignored in the ST accounting for 10.9% of the total dissolved solutes, mainly because of the fertilizer application. And the surface water of the ST was more suitable for irrigation and drinking purposes than that of the MS and NT. Knowledge of our findings could contribute new insights to the solute geochemistry and sustainable management of water resources in the lithologically distinct segments of the WRB and other similar areas.

Do 2 H and 18 O in leaf water reflect environmental drivers differently?

We compiled hydrogen and oxygen stable isotope compositions (δ2 H and δ18 O) of leaf water from multiple biomes to examine variations with environmental drivers. Leaf water δ2 H was more closely correlated with δ2 H of xylem water or atmospheric vapour, whereas leaf water δ18 O was more closely correlated with air relative humidity. This resulted from the larger proportional range for δ2 H of meteoric waters relative to the extent of leaf water evaporative enrichment compared with δ18 O. We next expressed leaf water as isotopic enrichment above xylem water (Δ2 H and Δ18 O) to remove the impact of xylem water isotopic variation. For Δ2 H, leaf water still correlated with atmospheric vapour, whereas Δ18 O showed no such correlation. This was explained by covariance between air relative humidity and the Δ18 O of atmospheric vapour. This is consistent with a previously observed diurnal correlation between air relative humidity and the deuterium excess of atmospheric vapour across a range of ecosystems. We conclude that 2 H and 18 O in leaf water do indeed reflect the balance of environmental drivers differently; our results have implications for understanding isotopic effects associated with water cycling in terrestrial ecosystems and for inferring environmental change from isotopic biomarkers that act as proxies for leaf water.

Long-term observations of the chemical composition, fluxes and sources of atmospheric wet deposition at an urban site in Xi'an, Northwest China.

Atmospheric wet deposition (AWD) is closely related to air quality, and excessive deposition poses risks to ecological systems and human health. Seasonal and interannual variations in acidity, electric conductivity (EC), ionic composition, fluxes, sources, and atmospheric transport of AWD were analyzed at an urban site in Xi'an from 2016 to 2019. The annual volume-weighted mean (VWM) pH and EC values were 6.8 and 40.6 µS cm-1, respectively. NO3- (47%) was the most dominant anion, while Ca2+ (34%) was the most dominant cation. The analysis of fractional acidity (FA) and neutralization factors (NFs) showed that 96% of the acidity was neutralized by alkaline constituents, especially Ca2+ and NH4+. The annual AWD flux of total ions was 125.9 kg ha-1 year-1, and NO3-, NO2-, SO42- and NH4+ fluxes accounted for approximately 70%, indicating considerable sulfur (9.1 kg ha-1 year-1) and nitrogen (22.0 kg ha-1 year-1) deposition. Under dilution by precipitation, the EC and major ion concentrations were lower, while the pH and fluxes were higher, in summer and autumn, and the opposite results were observed in spring and winter. The source apportionment via by positive matrix factorization (PMF) revealed that the six sources of major ions were confirmed as follows: vehicular emissions (38.1%), agriculture (22.3%), fossil fuel combustion (13.8%), crust (12.9%), marine (9.6%), and biomass burning (3.3%). And on the basis of back trajectory analysis, the air masses of precipitation were primarily from the northwest in spring and winter, from the southeast in summer, and from various directions in autumn, and they transported different natural and anthropogenic pollutants along their paths, thereby affecting the chemical composition and fluxes of AWD.

Warming and CO2 enrichment modified the ecophysiological responses of Dahurian larch and Mongolia pine during the past century in the permafrost of northeastern China.

Tree-ring δ13C and δ18O of dominant Dahurian larch and Mongolia pine in the permafrost region of the northern Great Higgnan Mountains, China were used to elucidate species-specific ecophysiological responses to warming temperatures and increasing CO2 over the past century. Larch and pine stable carbon discrimination (Δ13C) 13C and δ18O in tree rings both showed synchronous changes during the investigated period (1901-2010), but with species-specific isotopic responses to atmospheric enriched CO2 and warming. Tree-ring Δ13C and δ18O were controlled by both maximum temperature and moisture conditions (precipitation, relative humidity and vapor pressure deficit), but with different growth periods (Δ13C in June-July and δ18O in July-August, respectively). In addition, stable isotopes of larch showed relatively greater sensitivity to moisture deficits than pine. Climatic conditions from 1920 to 1960 strongly and coherently regulated tree-ring Δ13C and δ18O through stomatal conductance. However, climatic-sensitivities of tree-ring Δ13C and δ18O recently diverged, implying substantial adjustments of stomatal conductance, photosynthetic rate and altered water sources over recent decades, which reveal the varied impacts of each factor on tree-ring Δ13C and δ18O over time. Based on expected changes in leaf gas-exchange, we isolated the impacts of atmospheric CO2 and climate change on intrinsic water-use efficiency (iWUE) over the past century. Higher intracellular CO2 in pine than larch from 1960 onwards suggests this species may be more resilient to severe droughts in the future. Our data also illustrated no weakening of the iWUE response to increasing CO2 in trees from this permafrost region. The overall pattern of CO2 enrichment and climate impacts on iWUE of pine and larch were similar, but warming increased iWUE of larch to a greater extent than that of pine over recent two decades. Taken together, our findings highlight the importance of considering how leaf gas-exchange responses to atmospheric CO2 concentration influence species-specific responses to climate and the alteration of the hydrological environment in forests growing in regions historically dominated by permafrost that will be changing rapidly in response to future warming and increased CO2.

Significant Difference in Hydrogen Isotope Composition Between Xylem and Tissue Water in Populus Euphratica.

Deuterium depletions between stem water and source water have been observed in coastal halophyte plants and in multiple species under greenhouse conditions. However, the location(s) of the isotope fractionation is not clear yet and it is uncertain whether deuterium fractionation appears in other natural environments. In this study, through two extensive field campaigns utilizing a common dryland riparian tree species Populus euphratica Oliv., we showed that no significant δ(18) O differences were found between water source and various plant components, in accord with previous studies. We also found that no deuterium fractionation occurred during P. euphratica water uptake by comparing the deuterium composition (δD) of groundwater and xylem sap. However, remarkable δD differences (up to 26.4‰) between xylem sap and twig water, root water and core water provided direct evidence that deuterium fractionation occurred between xylem sap and root or stem tissue water. This study indicates that deuterium fractionation could be a common phenomenon in drylands, which has important implications in plant water source identification, palaeoclimate reconstruction based on wood cellulose and evapotranspiration partitioning using δD of stem water.

Acupuncture Meridian of Traditional Chinese Medical Science: An Auxiliary Respiratory System.

The acupuncture meridian system (AMS) is the key concept of Traditional Chinese Medical Science (TCMS). It is a natural network formed by the tissue space that connects human viscera and skin. In this article, a new hypothesis that the AMS is an auxiliary respiratory system is presented. The AMS collects the CO2 that is produced by tissue supersession and that cannot be excreted via blood circulation, and discharges the CO2 through the body's pores, thus preventing a pressure increase in the internal environment. Thus, local blood circulation will not be blocked, and the body will remain healthy. In addition to neuroregulation and humoral regulation, AMS regulation is an important method of physiological regulation. Furthermore, the pathological principle of the AMS, therapies of TCMS, and the excellent future of the AMS are discussed.

Origins of groundwater inferred from isotopic patterns of the Badain Jaran Desert, northwestern China.

There are many viewpoints about the sources of groundwater in the Badain Jaran Desert (BJD), such as precipitation and snowmelt from the Qilian Mountains (the upper reaches [UR] of the Heihe River Basin [HRB]) and precipitation from the BJD and the Yabulai Mountains. To understand the source of the groundwater of the BJD and their possible associations with nearby bodies of water, we analyzed variations of stable isotope ratios (δD and δ(18) O) and the deuterium excess (d-excess) of groundwater and precipitation in the BJD, of groundwater, precipitation, river and spring water in the UR, and of groundwater and river water in the middle and lower reaches (MR and LR) of the HRB. In addition, the climatic condition under which the groundwater was formed in the BJD was also discussed. We found obvious differences in δD, δ(18) O, and d-excess among groundwater in the BJD, nearby water bodies and the HRB. The groundwater δD-δ(18) O equation for the BJD was δD = 4.509δ(18) O-30.620, with a slope and intercept similar to that of nearby areas (4.856 and -29.574), indicating a strong evaporation effect in the BJD and its surrounding areas. The equation's slope of the BJD was significantly lower than those of HRB groundwater (6.634), HRB river water (6.202), precipitation in the BJD and Youqi (7.841), and the UR of the HRB (7.839). The d-excess (-17.5‰) of the BJD was significantly lower than those of nearby groundwater (-7.4‰), HRB groundwater (12.1‰), precipitation in the BJD (5.7‰) and in the UR of the HRB (15.2‰), and HRB river water (14.4‰). The spatial patterns of δ(18) O and d-excess values in the BJD suggest mixing and exchange of groundwater between the BJD and neighboring regions, but no hydraulic relationship between the BJD groundwater and water from more distant regions except Outer Mongolia, which is north of the BJD. Moreover, we conclude that there is little precipitation recharge to groundwater because of the obvious d-excess difference between groundwater and local precipitation, low precipitation, and high evaporation rates. The abnormally negative d-excess values in groundwater of the BJD indicate that this water was formed in the past under higher relative humidity and lower temperatures than modern values.

Detailed assessment of isotope ratio infrared spectroscopy and isotope ratio mass spectrometry for the stable isotope analysis of plant and soil waters.

As an alternative to isotope ratio mass spectrometry (IRMS), the isotope ratio infrared spectroscopy (IRIS) approach has the advantage of low cost, continuous measurement and the capacity for field-based application for the analysis of the stable isotopes of water. Recent studies have indicated that there are potential issues of organic contamination of the spectral signal in the IRIS method, resulting in incorrect results for leaf samples. To gain a more thorough understanding of the effects of sample type (e.g., leaf, root, stem and soil), sample species, sampling time and climatic condition (dry vs. wet) on water isotope estimates using IRIS, we collected soil samples and plant components from a number of major species at a fine temporal resolution (every 2 h for 24-48 h) across three locations with different climatic conditions in the Heihe River Basin, China. The hydrogen and oxygen isotopic compositions of the extracted water from these samples were measured using both an IRMS and an IRIS instrument. The results show that the mean discrepancies between the IRMS and IRIS approaches for δ(18) O and δD, respectively, were: -5.6‰ and -75.7‰ for leaf water; -4.0‰ and -23.3‰ for stem water; -3.4‰ and -28.2‰ for root water; -0.5‰ and -6.7‰ for xylem water; -0.06‰ and -0.3‰ for xylem flow; and -0.1‰ and 0.3‰ for soil water. The order of the discrepancy was: leaf > stem ≈ root > xylem > xylem flow ≈ soil. In general, species of the same functional types (e.g., woody vs. herbaceous) within similar habitats showed similar deviations. For different functional types, the differences were large. Sampling at nighttime did not remove the observed deviations.