A global meta-analysis reveals a significant offset in δ2H between plant water and its sources

2021 ◽  
Author(s):  
Javier de la Casa ◽  
Adrià Barbeta ◽  
Asun Rodriguez-Uña ◽  
Lisa Wingate ◽  
Jérôme Ogeé ◽  
...  
Keyword(s):  
Isotopic Composition ◽  
Soil Water ◽  
Meta Analysis ◽  
Water Evaporation ◽  
Source Water ◽  
Plant Water ◽  
Water Line ◽  
Plant Source ◽  
Sampling Campaign ◽  
The Mean

<p> </p><p>Long-standing ecological theory establishes that the isotopic composition of the plant water reflects that of the root-accessed sources, at least in non-saline or non-xeric environments. However, a growing number of studies challenge this assumption by reporting plant-source offsets in water isotopic composition, for a wide range of ecosystems. We conducted a global meta-analysis to systematically quantify the magnitude of this plant-source offset in water isotopic composition and its potential explanatory factors. We compiled 108 studies reporting dual water isotopic composition (δ<sup>2</sup>H and δ<sup>18</sup>O) of plant and source water. From these studies, we extracted the δ<sup>2</sup>H and δ<sup>18</sup>O of both plant and source waters for 223 plant species from artic to tropical biomes. For each species and sampling campaign, within each study, we calculated the mean line conditioned excess (LC-excess), with the slope and intercept of the local meteoric water line, and the mean soil water line conditioned excess (SWL-excess), from the slope and intercept of the soil water evaporation line. For each study site and sampling campaign, we obtained land surface temperature and volumetric soil water from the ERA5 database. For each study species, we recorded the functional type, leaf habit and for those available wood density. We found, on average, a significantly negative SWL-excess: plant water was systematically more depleted in δ<sup>2</sup>H than soil water. In > 90% of the cases with significantly negative SWL-excess, we also found negative LC-excess values, meaning that access to sources alternative to soil water was unlikely to explain negative SWL-excess values. </p><p>Calculated SWL-excess was affected by temperature and humidity: there were larger mismatches between plant and source water in isotopic composition in colder and wetter sites. Angiosperms, broadleaved and deciduous species exhibited more negative SWL-excess values than gymnosperms, narrow-leaved and evergreen species. Our results suggest that when using the dual isotopic approach, potential biases in the adscription of plant water sources are more likely in broadleaved forests in humid, and cold regions. Potential underlying mechanism for these isotopic mismatches will be discussed.</p><p> </p>

scholarly journals Revealing a significant isotopic offset between plant water and its sources using a global meta-analysis

2021 ◽  
Author(s):  
Javier de la Casa ◽  
Adrià Barbeta ◽  
Asun Rodríguez-Uña ◽  
Lisa Wingate ◽  
Jérôme Ogée ◽  
...  
Keyword(s):  
Soil Water ◽  
Plant Traits ◽  
Isotope Composition ◽  
Meta Analysis ◽  
Source Water ◽  
Plant Water ◽  
Water Line ◽  
Plant Source ◽  
Stem Water ◽  
Plant Water Sources

Abstract. Isotope-based approaches to study plant water sources rely on the assumption that root water uptake and within-plant water transport are non-fractionating processes. However, a growing number of studies have reported offsets between plant and source water stable isotope composition, for a wide range of ecosystems. These isotopic offsets can result in the erroneous attribution of source water used by plants and potential overestimations of groundwater uptake by the vegetation. We conducted a global meta-analysis to quantify the magnitude of these plant-source water isotopic offsets and explore whether their variability could be explained by either biotic or abiotic factors. Our database compiled 112 studies, spanning arctic to tropical biomes that reported the dual water isotope composition (δ2H and δ18O) of plant (stem) and source water, including soil water. We calculated 2H offsets in two ways: a line conditioned excess (LC-excess) that describes the 2H deviation from the local meteoric water line, and a soil water line conditioned excess (SW-excess), that describes the deviation from the soil water line, for each sampling campaign within each study. We tested for the effects of climate (air temperature and soil water content), soil class and plant traits (growth form, leaf habit, wood density and parenchyma fraction and mycorrhizal habit) on LC-excess and SW-excess. Globally, stem water was more depleted in 2H than soil water (SW-excess < 0) by 3.02 ± 0.65 ‰. In 95 % of the cases where SW-excess was negative, LC-excess was negative, indicating that the uptake of water from mobile pools was unlikely to explain the observed soil-plant water isotopic offsets. SW-excess was more negative in cold and wet sites, whereas it was more positive in warm sites. Soil class and plant traits did not have any significant effect on SW-excess. The climatic effects on SW-excess suggest that methodological artefacts are unlikely to be the sole cause of observed isotopic offsets. Instead, our results support the idea that these offsets are caused by isotopic heterogeneity within plant stems whose relative importance will depend on soil and plant water status and evaporative demand. Our results would imply that plant-source water isotopic offsets may lead to inaccuracies when using the isotopic composition of bulk stem water as a proxy to infer plant water sources.

scholarly journals Evidence for distinct isotopic composition of sap and tissue water in tree stems: consequences for plant water source identification

2020 ◽  
Author(s):  
Adrià Barbeta ◽  
Régis Burlett ◽  
Paula Martín-Gómez ◽  
Bastien Fréjaville ◽  
Nicolas Devert ◽  
...  
Keyword(s):  
Isotopic Composition ◽  
Soil Water ◽  
Water Cycle ◽  
Isotopic Fractionation ◽  
Water Source ◽  
Source Water ◽  
Plant Water ◽  
Stem Water ◽  
Micrometer Scale ◽  
Water Isotope

AbstractFor decades, theory has upheld that plants do not fractionate water isotopes as they move across the soil-root interface or along plant stems. This theory is now being challenged by several recent studies reporting that the water held in woody stems has an isotopic composition that cannot be attributed to any potential water source. Isotopic offsets between stem and source water still need to be explained, as they prevent identifying unambiguously tree water’s origin from water isotope measurements. Here we show that isotopic offsets between stem and source water can be explained by micrometer-scale water isotope heterogeneity within woody stems and soil micropores. Using a novel technique to extract sap water in xylem conduits separately from the water held in other xylem tissues, we show that these non-conductive xylem tissues are more depleted in deuterium than sap water. We also report that, in cut stems and well-watered potted plants, the isotopic composition of sap water reflects well that of irrigation water, demonstrating that no isotopic fractionation occurs during root water uptake or the sap water extraction process. Previous studies showed that isotopic heterogeneity also exists in soils at the pore scale where water adsorbed onto soil particles is more depleted than capillary/mobile soil water. Data collected at a beech (Fagus sylvatica) forest indicate that sap water matches best the capillary/mobile soil water from deep soil horizons, indicating that micrometer-scale water isotope heterogeneity in soils and stems must be accounted for to unambiguously identify where trees obtain their water within catchments.Significance StatementForests are prime regulators of the water cycle over land. They return, via transpiration, a large fraction of precipitation back to the atmosphere, influence surface runoff, groundwater recharge or stream flow, and enhance the recycling of atmospheric moisture inland from the ocean. The isotopic composition of water in woody stems can provide unique information on the role forests play in the water cycle only if it can be unambiguously related to the isotopic composition of source water. Here, we report a previously overlooked isotopic fractionation of stem water whereby non-conductive tissues are more depleted in deuterium than sap water, and propose a new technique to extract sap water separately from bulk stem water to unambiguously identify plant water sources.

scholarly journals Isotopes of oxygen-18 and deuterium in precipitation in Slovakia / Izotopy kyslíka-18 A deutéria v zrážkach na Slovensku

2012 ◽  
Vol 60 (4) ◽  
pp. 265-276 ◽  
Author(s):  
Ladislav Holko ◽  
Michal Dóša ◽  
Juraj Michalko ◽  
Martin Šanda
Keyword(s):  
Isotopic Composition ◽  
Air Temperature ◽  
Meteoric Water ◽  
Precipitation Amount ◽  
Data Series ◽  
Water Line ◽  
Temperature And Precipitation ◽  
Meteoric Water Line ◽  
The Mean ◽  
Available Information

The article synthesizes available information on isotopic composition of precipitation in Slovakia (the Western Carpathians). Monthly δ18O data from eleven stations and period 1988-1997 were used to investigate correlations among the stations, altitude, air temperature and precipitation amount effects. The mean annual altitude and air temperature gradients of δ18O in precipitation were 0.21‰/100 m and 0.36‰/1°C, respectively. Maps of spatial distribution of mean annual δ18O in precipitation based on both gradients were constructed. The two maps do not significantly differ for the majority of Slovakia. δ2H data were available for only three stations. Local meteoric water line derived for the station with the longest data series (δ2H = = 7.86δ18O + 6.99) was close to the Global Meteoric Water line. Its parameters in periods 1991-1993 and 1991-2008 did not change. The study indicates that a more detailed monitoring of isotopic composition of precipitation in mountains should be carried out in the future. The highest station exhibited very small seasonal variability of δ18O in precipitation compared to other Slovak stations. The second highest mountain station had significantly higher deuterium excess than the neighboring stations located in the valley. In some analyses the data from the nearest stations situated abroad (Vienna, Krakow) were used.

scholarly journals Borehole equilibration: testing a new method to monitor the isotopic composition of tree xylem water in situ

2020 ◽  
Author(s):  
John Marshall ◽  
Matthias Cuntz ◽  
Matthias Beyer ◽  
Maren Dubbert ◽  
Kathrin Kuehnhammer
Keyword(s):  
Water Vapor ◽  
Isotopic Composition ◽  
Theoretical Models ◽  
New Method ◽  
Data Sets ◽  
Source Water ◽  
Plant Water ◽  
Limited Time ◽  
Slow Moving

&lt;p&gt;Forest water use has been difficult to quantify. One promising approach is to measure the isotopic composition of plant water, e.g.&lt;br&gt;the transpired water vapor or xylem water, which often differs from that of other water vapor sources. Traditionally such&lt;br&gt;measurements have relied on the extraction of wood samples, which provide limited time resolution at great expense, and risk&lt;br&gt;possible artefacts. Utilizing a borehole drilled through a trees&amp;#8217; stem, we propose a new method based on the notion that water&lt;br&gt;vapor in a slow-moving airstream approaches equilibration with the much greater mass of liquid water in the xylem. We present&lt;br&gt;two empirical data sets showing that the method can work in practice. We then present theoretical models estimating the&lt;br&gt;equilibration times and exploring the limits at which the approach will fail. Given long enough boreholes and slow enough flows,&lt;br&gt;the method provides a simple, cheap, and accurate means of continuously estimating the isotopic composition of the source water&lt;br&gt;for transpiration.&lt;/p&gt;

scholarly journals Soil water migration in the unsaturated zone of semiarid region in China from isotope evidence

2017 ◽  
Vol 21 (3) ◽  
pp. 1757-1767 ◽  
Author(s):  
Yonggang Yang ◽  
Bojie Fu
Keyword(s):  
Isotopic Composition ◽  
Soil Water ◽  
Loess Plateau ◽  
Unsaturated Zone ◽  
Soil Layer ◽  
Scientific Basis ◽  
Plant Water ◽  
Water Migration ◽  
Isotopic Tracers ◽  
Caragana Korshinskii

Abstract. Soil water is an important driving force of the ecosystems, especially in the semiarid hill and gully region of the northwestern Loess Plateau in China. The mechanism of soil water migration in the reconstruction and restoration of Loess Plateau is a key scientific problem that must be solved. Isotopic tracers can provide valuable information associated with complex hydrological problems, difficult to obtain using other methods. In this study, the oxygen and hydrogen isotopes are used as tracers to investigate the migration processes of soil water in the unsaturated zone in an arid region of China's Loess Plateau. Samples of precipitation, soil water, plant xylems and plant roots are collected and analysed. The conservative elements deuterium (D) and oxygen (18O) are used as tracers to identify variable source and mixing processes. The mixing model is used to quantify the contribution of each end member and calculate mixing amounts. The results show that the isotopic composition of precipitation in the Anjiagou River basin is affected by isotopic fractionation due to evaporation. The isotopic compositions of soil waters are plotted between or near the local meteoric water lines, indicating that soil waters are recharged by precipitation. The soil water migration is dominated by piston-type flow in the study area and rarely preferential flow. Water migration exhibited a transformation pathway from precipitation to soil water to plant water. δ18O and δD are enriched in the shallow (< 20 cm depth) soil water in most soil profiles due to evaporation. The isotopic composition of xylem water is close to that of soil water at the depth of 40–60 cm. These values reflect soil water signatures associated with Caragana korshinskii Kom. uptake at the depth of 40–60 cm. Soil water from the surface soil layer (20–40 cm) comprised 6–12 % of plant xylem water, while soil water at the depth of 40–60 cm is the largest component of plant xylem water (ranging from 60 to 66 %), soil water below 60 cm depth comprised 8–14 % of plant xylem water and only 5–8 % is derived directly from precipitation. This study investigates the migration process of soil water, identifies the source of plant water and finally provides a scientific basis for identification of model structures and parameters. It can provide a scientific basis for ecological water demand, ecological restoration, and management of water resources.

The isotopic composition of phloem water and its relations to xylem water at daily and sub-daily resolutions

2021 ◽  
Author(s):  
Magali F. Nehemy ◽  
Benettin Paolo ◽  
Andrea Rinaldo ◽  
Jeffrey J. McDonnell
Keyword(s):  
Isotopic Composition ◽  
Water Transport ◽  
Isotope Composition ◽  
Water Status ◽  
Three Dimensional ◽  
Plant Water Status ◽  
Source Water ◽  
Plant Water ◽  
Xylem Water Potential ◽  
Dual Isotope

&lt;p&gt;Isotopic tracing is de rigueur&amp;#160;in ecohydrology and for quantifying tracing water sources that contribute to xylem water. But, tree transpiration is not a one dimensional process from roots to leaves. Three dimensional storages actively participate in water transport within the stem complicating in unknown ways, the otherwise straightforward tracing from source to xylem. Phloem is the largest elastic storage and works as a hydraulic capacitor, and as such is of great importance to tree water transport and functioning. Water stored in phloem moves into xylem vessels buffering changes in xylem water potential and sustaining tree hydraulic integrity. Although phloem water is of great importance to transpiration, we lack understanding about the relationship between xylem and phloem water isotopic composition. Assessing the isotopic composition of phloem is a needed next step to fully comprehend patterns of tree water use and improve understanding about isotopic offset between xylem and source water. Here we show daily and sub-daily dual-isotope measurements of phloem water in relation to xylem and leaf water in &lt;em&gt;Salix viminalis&lt;/em&gt; along with high-resolution measurements of plant water status and transpiration rates in a large lysimeter. We found that phloem was more depleted in heavier isotopes than xylem and leaves. On average &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H phloem water was 2.05 &amp;#8240; and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O phloem water was 0.66 &amp;#8240; more negative than xylem water. The largest difference observed between phloem and xylem isotopic composition occurred at night during a period of tree water deficit. Although, there was variability in the observed difference between xylem and phloem throughout the experiment, xylem and phloem isotopic composition were highly correlated (&amp;#948;&lt;sup&gt;2&lt;/sup&gt;H r = 0.89; &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O r = 0.75). Our sub daily measurements showed that xylem and phloem differences decreased during predawn and morning compared to previous evening and midday measurements. We observed that the &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H difference between phloem and xylem increased with the increase in daily use of phloem water storages, while &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O difference between phloem and xylem increased with transpiration rate. Our results show that xylem and phloem isotope composition are in sync and that observed differences can be related to changes in plant water status and possible fractionation associated with transport within phloem-xylem. Further studies are necessary to understand how phloem affects source water interpretations across different tree species and larger trees, where phloem contribution to daily transpiration may be larger.&lt;/p&gt;

scholarly journals Soil water migration in the unsaturated zone of semi-arid region in China from isotope evidence

2016 ◽  
Author(s):  
Yonggang Yang ◽  
BoJie Fu
Keyword(s):  
Isotopic Composition ◽  
Soil Water ◽  
Loess Plateau ◽  
Unsaturated Zone ◽  
Arid Region ◽  
Scientific Basis ◽  
Plant Water ◽  
Water Migration ◽  
Isotopic Tracers ◽  
Semi Arid

Abstract. Abstract. Soil water is an important driving force of the ecosystems, especially in the semi-arid hilly and gully region of northwestern Loess Plateau in China. The mechanism of soil water migration in the reconstruction and restoration of Loess Plateau is a key scientific problem that must be solved. Isotopic tracers can provide valuable information associated with complex hydrological problems, which is difficult to obtain by other methods. In this study, the oxygen and hydrogen isotopes are used as tracers to investigate the migration processes of soil water in the unsaturated zone in the Loess Plateau of arid region in China. Samples of precipitation, soil water, plant xylems and plant roots are collected and analysed. The conservative elements D and 18O are used as tracers to identify variable source and mixing processes. The mixing model is used to quantify the contribution of each end member and calculate mixing amounts.The results show that the isotopic composition of precipitation in the Anjiagou River basin is affected by isotopic fractionation due to evaporation. The isotopic compositions of soil waters are plotted between or near the local meteoric water lines, indicating that soil waters are recharged by precipitation. The soil water migration is dominated by piston-type flow in the study area, but rarely preferential flow.Water migration exhibited a transformation pathway from precipitation to soil water to plant water. δ18O and δD are enrichment in the shallow (<20 cm depth) soil water in most soil profiles due to evaporation. The isotopic composition of xylem water is close to that of soil water at the depth of 40%–60 cm. These values reflect soil water signatures associated with caraganakorshinshiikom uptake at the depth of 40%–60 cm. Soil water from the surface soil lay (20%–40 cm) contributed to 6 %–12 % of plant xylem water, while soil water at the depth of 40%–60 cm is the largest component of plant xylem water (range from 60 % to 66 %), soil water below 60 cm depth contributed 8 %–14 % to plant xylem water, and only 5 %–8 % is derived directly from precipitation.This study investigates the migration process of soil water, and identifies the source of plant water, and finally provides a scientific basis for identification of model structure and parameter. It can provide a scientific basis for ecological water demand, ecological restoration, management of water resources and the improvement of water benefit on each scale.

Stable isotope tracers as diagnostic tools in studying water sources in a humid bamboo watershed during the plum rainfall events

Water Policy ◽  
2019 ◽  
Vol 21 (2) ◽  
pp. 368-381
Author(s):  
Jianfeng Gou ◽  
Simin Qu ◽  
Peng Shi ◽  
Dachen Li ◽  
Xueqiu Chen ◽  
...  
Keyword(s):  
Stable Isotope ◽  
Isotopic Composition ◽  
Temporal Variation ◽  
Soil Water ◽  
Meteoric Water ◽  
Water Sources ◽  
Rainfall Events ◽  
Water Line ◽  
Isotopic Variation ◽  
Meteoric Water Line

Abstract This study investigates the temporal variation of stable isotopic composition in precipitation, soil water, and streamflow water during the plum rainfall events in an upland headwater watershed which is mainly covered with bamboo. The results show that the isotopic composition of various water sources exhibit significant temporal variation. The local meteoric water line is established by using the relationship between the stable isotope of oxygen-18 and deuterium, which is slightly different from the meteoric water line of China. The isotopic temporal variation of precipitation is closely related to exchange effect between raindrops and environmental vapor, evaporation fractionation and rainfall intensity. The isotopic variation of shallow soil water is mainly determined by canopy interception, ground evaporation and the mixing with pre-event water; as for the isotopic variation of deep soil water, it is virtually influenced by pre-event water. The most enriched isotopic composition of streamflow and deuterium excess (d-excess) differences between streamflow and rainfall both indicate that streamflow is recharged not only by event water but also by pre-event water. Hence, a better understanding of precipitation formation and the hydrological response under the plum rainfall system may be instructive for the management of water resources in humid watersheds in southern China.

scholarly journals Unexplained hydrogen isotope offsets complicate the identification and quantification of tree water sources in a riparian forest

2019 ◽  
Vol 23 (4) ◽  
pp. 2129-2146 ◽  
Author(s):  
Adrià Barbeta ◽  
Sam P. Jones ◽  
Laura Clavé ◽  
Lisa Wingate ◽  
Teresa E. Gimeno ◽  
...  
Keyword(s):  
Soil Water ◽  
Riparian Forest ◽  
Stream Water ◽  
Isotopic Fractionation ◽  
Growing Season ◽  
Water Sources ◽  
Source Water ◽  
Plant Water ◽  
Deep Soil ◽  
Plant Water Sources

Abstract. We investigated plant water sources of an emblematic refugial population of Fagus sylvatica (L.) in the Ciron river gorges in south-western France using stable water isotopes. It is generally assumed that no isotopic fractionation occurs during root water uptake, so that the isotopic composition of xylem water effectively reflects that of source water. However, this assumption has been called into question by recent studies that found that, at least at some dates during the growing season, plant water did not reflect any mixture of the potential water sources. In this context, highly resolved datasets covering a range of environmental conditions could shed light on possible plant–soil fractionation processes responsible for this phenomenon. In this study, the hydrogen (δ2H) and oxygen (δ18O) isotope compositions of all potential tree water sources and xylem water were measured fortnightly over an entire growing season. Using a Bayesian isotope mixing model (MixSIAR), we then quantified the relative contribution of water sources for F. sylvatica and Quercus robur (L.) trees. Based on δ18O data alone, both species used a mix of top and deep soil water over the season, with Q. robur using deeper soil water than F. sylvatica. The contribution of stream water appeared to be marginal despite the proximity of the trees to the stream, as already reported for other riparian forests. Xylem water δ18O could always be interpreted as a mixture of deep and shallow soil waters, but the δ2H of xylem water was often more depleted than the considered water sources. We argue that an isotopic fractionation in the unsaturated zone and/or within the plant tissues could underlie this unexpected relatively depleted δ2H of xylem water, as already observed in halophytic and xerophytic species. By means of a sensitivity analysis, we found that the estimation of plant water sources using mixing models was strongly affected by this δ2H depletion. A better understanding of what causes this isotopic separation between xylem and source water is urgently needed.

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