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

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.

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

10.5194/egusphere-egu21-12207 ◽

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

&#160;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 (&#948;2H and &#948;18O) of plant and source water. From these studies, we extracted the &#948;2H and &#948;18O 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 &#948;2H 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.&#160;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.&#160;

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Stem water cryogenic extraction biases estimation in deuterium isotope composition of plant source water

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.2014422117 ◽

2020 ◽

Vol 117 (52) ◽

pp. 33345-33350

Author(s):

Yongle Chen ◽

Brent R. Helliker ◽

Xianhui Tang ◽

Fang Li ◽

Youping Zhou ◽

...

Keyword(s):

Isotope Composition ◽

Experimental System ◽

Root Water Uptake ◽

Source Water ◽

Significant Discrepancy ◽

Plant Source ◽

Stem Water ◽

Hydrogen Isotope Ratio ◽

Dynamic Exchange ◽

Plant Water Sources

The hydrogen isotope ratio of water cryogenically extracted from plant stem samples (δ2Hstem_CVD) is routinely used to aid isotope applications that span hydrological, ecological, and paleoclimatological research. However, an increasing number of studies have shown that a key assumption of these applications—that δ2Hstem_CVD is equal to the δ2H of plant source water (δ2Hsource)—is not necessarily met in plants from various habitats. To examine this assumption, we purposedly designed an experimental system to allow independent measurements of δ2Hstem_CVD, δ2Hsource, and δ2H of water transported in xylem conduits (δ2Hxylem) under controlled conditions. Our measurements performed on nine woody plant species from diverse habitats revealed a consistent and significant depletion in δ2Hstem_CVD compared with both δ2Hsource and δ2Hxylem. Meanwhile, no significant discrepancy was observed between δ2Hsource and δ2Hxylem in any of the plants investigated. These results cast significant doubt on the long-standing view that deuterium fractionation occurs during root water uptake and, alternatively, suggest that measurement bias inherent in the cryogenic extraction method is the root cause of δ2Hstem_CVD depletion. We used a rehydration experiment to show that the stem water cryogenic extraction error could originate from a dynamic exchange between organically bound deuterium and liquid water during water extraction. In light of our finding, we suggest caution when partitioning plant water sources and reconstructing past climates using hydrogen isotopes, and carefully propose that the paradigm-shifting phenomenon of ecohydrological separation (“two water worlds”) is underpinned by an extraction artifact.

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

10.1101/2020.06.18.160002 ◽

2020 ◽

Cited By ~ 1

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.

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

Hydrology and Earth System Sciences ◽

10.5194/hess-23-2129-2019 ◽

2019 ◽

Vol 23 (4) ◽

pp. 2129-2146 ◽

Cited By ~ 23

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

10.5194/hess-2018-631 ◽

2019 ◽

Cited By ~ 1

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 soil water relatively deeper 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 largely 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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Hydrogen isotope fractionation affects the identification and quantification of tree water sources in a riparian forest

10.5194/hess-2018-402 ◽

2018 ◽

Cited By ~ 3

Author(s):

Adrià Barbeta ◽

Sam P. Jones ◽

Laura Clavé ◽

Lisa Wingate ◽

Teresa E. Gimeno ◽

...

Keyword(s):

Stable Isotopes ◽

Soil Water ◽

Isotopic Fractionation ◽

Water Sources ◽

Water Isotopes ◽

Mixing Models ◽

Source Water ◽

Plant Water ◽

Isotope Mixing Models ◽

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 isotopes. The stable isotopes of water are a powerful tracer of water fluxes in the soil-plant-atmosphere continuum. It is generally assumed that no isotopic fractionation occurs during root water uptake, and that xylem water isotopes effectively reflect source water isotopes. However, recent studies showed that under certain conditions the isotopes in plant water do not reflect any of the potential sources considered. Highly resolved datasets covering a range of environmental conditions could shed light on possible plant-soil fractionations processes. In this study, the hydrogen (δ2H) and oxygen (δ18O) isotope compositions of all potential tree water sources and xylem water were measured bi-weekly over an entire growing season. Using Bayesian isotope mixing models (MixSIAR), we then quantified the contribution of the considered sources to xylem water of 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 soil water relatively deeper 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 any other possible water source. We argue that an isotopic fractionation in the unsaturated zone and/or within the plant tissues could underlie this unexpected δ2H depletion 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 isotope mixing models was largely affected by this isotopic δ2H depletion. A better understanding of what causes this isotopic separation between xylem and source water is urgently needed.

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

10.5194/egusphere-egu21-11487 ◽

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

Isotopic tracing is de rigueur&#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 Salix viminalis 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 &#948;2H phloem water was 2.05 &#8240; and &#948;18O phloem water was 0.66 &#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 (&#948;2H r = 0.89; &#948;18O 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 &#948;2H difference between phloem and xylem increased with the increase in daily use of phloem water storages, while &#948;18O 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.

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Drivers of spatial and temporal soil water isotope variability in a sub-arctic catchment

10.5194/egusphere-egu21-14845 ◽

2021 ◽

Author(s):

Filip Muhic ◽

Pertti Ala-Aho ◽

Hannu Marttila ◽

Björn Klöve

Keyword(s):

Soil Water ◽

Water Use ◽

Isotope Composition ◽

Growing Season ◽

Snow Accumulation ◽

Plant Water ◽

Catchment Scale ◽

Data Set ◽

Plant Water Use ◽

Water Isotope

Research on ecohydrological separation and plant water use have been increasing in the last few years, and various studies indicate that trees can use winter precipitation as a dominant water source during the growing season. Such studies are of great importance to northern regions, where soil water recharge timing is predicted to be significantly altered due to climate change. In order to assess plant water use in sub-arctic environment, it is necessary to understand how soil water pools under different land covers evolve throughout the year and how cryogenic processes alter the isotope input signal. This field study was conducted from May 2019 to June 2020 in Pallas catchment, located in sub-arctic conditions in Finnish Lapland. Soil cores up to 1 meter depth with 5&#160;cm increments and xylem water of dominant tree species were collected in 4 locations, ranging from forest to shrubland/forest transitional area, and to forested peatland. All locations are positioned on a snow survey, in the vicinity of previously installed groundwater wells and snow lysimeters, and within 2 kilometers of rain gauge. Additional spatial samples of topsoil and xylem water were collected throughout the catchment during 2019 growing season. Relative proportions of tree source water were calculated by Bayesian mixing model MixSIAR. We produce new data set that displays plot and catchment scale soil water heterogeneities in a snow dominated environment, and examine: i) How soil properties affect isotopic composition of soil water?; ii) What is the effect of rising groundwater level on soil water isotope composition?; and iii) How snowpack thickness and melt timing modify soil water isotope patterns? We analyze if these varying pools of water are reflected in tree xylem water. Soil water isotope dynamics under deep snowpack, during and after snowmelt reveal how snow accumulation and melt timing and magnitude influence plant available water for growing season.

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Parsimonious representation of plant water use strategies via non-dimensional parameter groups

10.5194/egusphere-egu21-3966 ◽

2021 ◽

Author(s):

Maoya Bassiouni ◽

Stefano Manzoni ◽

Giulia Vico

Keyword(s):

Soil Water ◽

Water Use ◽

Plant Traits ◽

Water Flux ◽

Climatic Conditions ◽

Model Parameters ◽

Plant Water ◽

Dimensional Parameter ◽

Water Limitation ◽

Plant Water Use

Process-based models are needed to improve estimates of water and carbon cycles in variable climatic conditions. Yet, their utility is often limited by our inability to directly measure plant stomatal and hydraulic traits at scales suitable to quantify characteristics of whole ecosystems. Inferring such parameters from ecosystem-scale data with parsimonious models offers an avenue to address this limitation. To this aim, we use a simple representation of the water flux through the soil-plant-atmosphere continuum (SPAC) and derive a parameterization of Feddes-type soil water-limitation constraints on transpiration (expressed via a soil moisture dependent function &#946;). This parameterization explicitly accounts for community-effective plant eco-physiological traits as encoded in the SPAC model parameters. We express analytically the fractional loss of conductivity in well-watered conditions and the soil saturation thresholds at which transpiration is down-regulated from its well-watered rate and at which transpiration ceases, as a function of non-dimensional parameter groups. These non-dimensional groups combine plant stomatal and hydraulic traits, soil texture and climate. We implement the theoretical &#946; function into a soil water balance and infer distributions of plant traits which best-match FLUXNET observations in a range of biomes. Finally, we analyze the resulting non-dimensional groups to explore patterns in plant water use strategies. Our results indicate that non-dimensional groups reflect combinations of plant traits which are adapted to growing season environmental conditions and these groups may be more meaningful model parameters than individual traits at ecosystem scales. Additionally, using non-dimensional groups instead of focusing on individual parameters reduces risks of equifinality and provides future opportunities to exploit satellite data to quantify robust ecosystem-scale parameters. This analysis provides a parsimonious and functionally accurate alternative to account for ecosystem hydraulic controls and feedbacks and can help overcome limitations of commonly used empirical water-limitation constraints.

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The Effect of Irrigation Rate on the Water Relations of Young Citrus Trees in High-Density Planting

Sustainability ◽

10.3390/su13041759 ◽

2021 ◽

Vol 13 (4) ◽

pp. 1759

Author(s):

Said A. Hamido ◽

Kelly T. Morgan

Keyword(s):

Water Potential ◽

Soil Water ◽

Soil Salinity ◽

Planting Density ◽

Stem Water Potential ◽

Irrigation Rate ◽

Biological Stress ◽

Stem Water ◽

Water Contents ◽

Soil Water Contents

The availability and proper irrigation scheduling of water are some of the most significant limitations on citrus production in Florida. The proper volume of citrus water demand is vital in evaluating sustainable irrigation approaches. The current study aims to determine the amount of irrigation required to grow citrus trees at higher planting densities without detrimental impacts on trees’ water relation parameters. The study was conducted between November 2017 and September 2020 on young sweet orange (Citrus sinensis) trees budded on the ‘US-897’ (Cleopatra mandarin x Flying Dragon trifoliate orange) citrus rootstock transplanted in sandy soil at the Southwest Florida Research and Education Center (SWFREC) demonstration grove, near Immokalee, Florida. The experiment contained six planting densities, including 447, 598, and 745 trees per ha replicated four times, and 512, 717, and 897 trees per ha replicated six times. Each density treatment was irrigated at 62% or 100% during the first 15 months between 2017 and 2019 or one of the four irrigation rates (26.5, 40.5, 53, or 81%) based on the calculated crop water supplied (ETc) during the last 17 months of 2019–2020. Tree water relations, including soil moisture, stem water potential, and water supplied, were collected periodically. In addition, soil salinity was determined. During the first year (2018), a higher irrigation rate (100% ETc) represented higher soil water contents; however, the soil water content for the lower irrigation rate (62% ETc) did not represent biological stress. One emitter per tree regardless of planting density supported stem water potential (Ψstem) values between −0.80 and −0.79 MPa for lower and full irrigation rates, respectively. However, when treatments were adjusted from April 2019 through September 2020, the results substantially changed. The higher irrigation rate (81% ETc) represented higher soil water contents during the remainder of the study, the lower irrigation rate (26.5% ETc) represents biological stress as a result of stem water potential (Ψstem) values between −1.05 and −0.91 MPa for lower and higher irrigation rates, respectively. Besides this, increasing the irrigation rate from 26.5% to 81%ETc decreased the soil salinity by 33%. Although increasing the planting density from 717 to 897 trees per hectare reduced the water supplied on average by 37% when one irrigation emitter was used to irrigate two trees instead of one, applying an 81% ETc irrigation rate in citrus is more efficient and could be managed in commercial groves.

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