Algal bloom could drastically influence the nutrient cycling in lakes. To understand how the internal nutrient release responds to algal bloom decay, water and sediment columns were sampled at 22 sites from four distinct regions of China's eutrophic Lake Taihu and incubated in the laboratory to examine the influence of massive algal bloom decay on nutrient release from sediment. The column experiment involved three treatments: (1) water and sediment (WS); (2) water and algal bloom (WA); and (3) water, sediment, and algal bloom (WSA). Concentrations of dissolved oxygen (DO), total nitrogen (TN), total phosphorus (TP), ammonium (NH (4) (+) -N), and orthophosphate (PO (4) (3-) -P) were recorded during incubation. The decay of algal material caused a more rapid decrease in DO than in the algae-free controls and led to significant increases in NH (4) (+) -N and PO (4) (3-) -P in the water. The presence of algae during the incubation had a regionally variable effect on sediment nutrient profiles. In the absence of decaying algae (treatment WS), sediment nutrient concentrations decreased during the incubation. In the presence of blooms (WSA), sediments from the river mouth released P to the overlying water, while sediments from other regions absorbed surplus P from the water. This experiment showed that large-scale algal decay will dramatically affect nutrient cycling at the sediment-water interface and would potentially transfer the function of sediment as "container" or "supplier" in Taihu, although oxygen exchange with atmosphere in lake water was stronger than in columns. The magnitude of the effect depends on the physical-chemical character of the sediments.
We investigated the moisture origin and contribution of different water sources to surface runoff entering the headwaters of the Heihe River basin on the basis of NECP/NCAR (National Centers for Environmental Prediction/National Center for AtmosphericResearch) re-analysis data and variations in the stable hydrogen and oxygen isotope ratios (δ D and δ
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.
Abstract. Stable isotopologues of water are widely used to derive relative root water
uptake (RWU) profiles and average RWU depth in lignified plants. Uniform
isotope composition of plant xylem water (δxyl) along the stem
length of woody plants is a central assumption of the isotope tracing
approach which has never been properly evaluated. Here we evaluate whether strong variation in δxyl within woody
plants exists using empirical field observations from French Guiana,
northwestern China, and Germany. In addition, supported by a mechanistic
plant hydraulic model, we test hypotheses on how variation in δxyl can develop through the effects of diurnal variation in RWU, sap
flux density, diffusion, and various other soil and plant parameters on the
δxyl of woody plants. The hydrogen and oxygen isotope composition of plant xylem water shows
strong temporal (i.e., sub-daily) and spatial (i.e., along the stem)
variation ranging up to 25.2 ‰ and
6.8 ‰ for δ2H and δ18O,
respectively, greatly exceeding the measurement error range in all evaluated
datasets. Model explorations predict that significant δxyl variation
could arise from diurnal RWU fluctuations and vertical soil water
heterogeneity. Moreover, significant differences in δxyl emerge
between individuals that differ only in sap flux densities or are monitored
at different times or heights. This work shows a complex pattern of δxyl transport in the
soil–root–xylem system which can be related to the dynamics of RWU by
plants. These dynamics complicate the assessment of RWU when using stable
water isotopologues but also open new opportunities to study drought
responses to environmental drivers. We propose including the monitoring of sap
flow and soil matric potential for more robust estimates of average RWU
depth and expansion of attainable insights in plant drought strategies and
responses.
Abstract:We used hydrochemistry and environmental isotope data (υ 18 O, υD, tritium, and 14 C) to investigate the characteristics of river water, groundwater, and groundwater recharge in China's Heihe River basin. The river water and groundwater could be characterized as Ca 2C -Mg 2C -HCO 3 -SO 4 2 and Na C -Mg 2C -SO 4 2 -Cl types, respectively. Hydrogeochemical modelling using PHREEQC software revealed that the main hydrogeochemical processes are dissolution (except for gypsum and anhydrite) along groundwater flow paths from the upper to middle Heihe reaches. Towards the lower reaches, dolomite and calcite tend to precipitate. The isotopic data for most of the river water and groundwater lie on the global meteoric water line (GMWL) or between the GMWL and the meteoric water line in northwestern China, indicating weak evaporation. No direct relationship existed between recharge and discharge of groundwater in the middle and lower reaches based on the isotope ratios, d-excess, and 14 C values. On the basis of tritium in precipitation and by adopting an exponential piston-flow model, we evaluated the mean residence time of shallow groundwater with high tritium activities, which was around 50 years (a). Furthermore, based on the several popular models, it is calculated that the deep groundwaters in piedmont alluvial fan zone of the middle reaches and in southern part of the lower reaches are modern water, whereas the deep groundwaters in the edge of the middle reaches and around Juyan Lake in the lower reaches of Heihe river basin are old water.
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