In situ unsaturated zone water stable isotope (&amp;lt;sup&amp;gt;2&amp;lt;/sup&amp;gt;H and &amp;lt;sup&amp;gt;18&amp;lt;/sup&amp;gt;O) measurements  in semi-arid environments: a soil water balance

Gaj, Marcel; Beyer, Matthias; Koeniger, Paul; Wanke, Heike; Hamutoko, Josefina; Himmelsbach, Thomas

doi:10.5194/hess-20-715-2016

Cited by 94 publications

(115 citation statements)

References 66 publications

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“…4) One can expect that the isotope composition of the soil water taken up by the roots will impact 17 O-excess Phyto . In tropical dry and humid areas, evaporative kinetic fractionation can lead to a 18 O-enrichment of the soil water of several ‰, in the first dm depth (e.g., Gaj et al, 2016;Liu et al, 2010). Spatial variability in the composition of the rainfall feeding the upper soil water may also intervene.…”

Section: Imprint Of Changes In Atmospheric Rh On the 17 O-excess Of Pmentioning

confidence: 99%

The triple oxygen isotope composition of phytoliths as a proxy of continental atmospheric humidity: insights from climate chamber and climate transect calibrations

et al. 2018

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Abstract. Continental atmospheric relative humidity (RH) is a key climate parameter. Combined with atmospheric temperature, it allows us to estimate the concentration of atmospheric water vapor, which is one of the main components of the global water cycle and the most important gas contributing to the natural greenhouse effect. However, there is a lack of proxies suitable for reconstructing, in a quantitative way, past changes of continental atmospheric humidity. This reduces the possibility of making model-data comparisons necessary for the implementation of climate models. Over the past 10 years, analytical developments have enabled a few laboratories to reach sufficient precision for measuring the triple oxygen isotopes, expressed by the 17 O-excess ( 17 O-excess = ln (δ 17 O + 1) -0.528 × ln (δ 18 O + 1)), in water, water vapor and minerals. The 17 O-excess represents an alternative to deuterium-excess for investigating relative humidity conditions that prevail during water evaporation. Phytoliths are micrometric amorphous silica particles that form continuously in living plants. Phytolith morphological assemblages from soils and sediments are commonly used as past vegetation and hydrous stress indicators. In the present study, we examine whether changes in atmospheric RH imprint the 17 O-excess of phytoliths in a measurable way and whether this imprint offers a potential for reconstructing past RH. For that purpose, we first monitored the 17 O-excess evolution of soil water, grass leaf water and grass phytoliths in response to changes in RH (from 40 to 100 %) in a growth chamber experiment where transpiration reached a steady state. Decreasing RH from 80 to 40 % decreases the 17 Oexcess of phytoliths by 4.1 per meg/% as a result of kinetic fractionation of the leaf water subject to evaporation. In order to model with accuracy the triple oxygen isotope fractionation in play in plant water and in phytoliths we recommend direct and continuous measurements of the triple isotope composition of water vapor. Then, we measured the 17 O-excess of 57 phytolith assemblages collected from top soils along a RH and vegetation transect in inter-tropical West and Central Africa. Although scattered, the 17 O-excess of phytoliths decreases with RH by 3.4 per meg/%. The similarity of the trends observed in the growth chamber and nature supports that RH is an important control of 17 O-excess of phytoliths in the natural environment. However, other parameters such as changes in the triple isotope composition of the soil water or phytolith origin in the plant may come into play. Assessment of these parameters through additional growth chambers experiments and field campaigns will bring us closer to an accurate proxy of changes in relative humidity.

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Section: Imprint Of Changes In Atmospheric Rh On the 17 O-excess Of Pmentioning

confidence: 99%

The triple oxygen isotope composition of phytoliths as a proxy of continental atmospheric humidity: insights from climate chamber and climate transect calibrations

et al. 2018

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“…This clearly indicates that adapting the water uptake to soil water availability plays a role, but probably on longer timescales than what we observed during the 10-day experiment. The development of membrane-based in situ methods of soil water (Gaj et al, 2016;Rothfuss et al, 2015;Volkmann et al, 2016a) and xylem sap sampling (Volkmann et al, 2016b) and transpiration (Dubbert et al, 2014a(Dubbert et al, , 2017 will advance the studies of dynamic changes in ecohydrological soil-vegetation feedbacks in the future. Furthermore, the coupling of isotope laser spectroscopes to gas-exchange chambers and soil or xylem equilibration probes overcomes the costly and time-consuming classical destructive sampling methods.…”

Section: Dynamic Responses Of Event Water Use and Plasticity Of Watermentioning

confidence: 99%

Quantification of dynamic soil–vegetation feedbacks following an isotopically labelled precipitation pulse

Piayda¹,

Dubbert

Siegwolf

et al. 2017

Biogeosciences

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Abstract. The presence of vegetation alters hydrological cycles of ecosystems. Complex plant-soil interactions govern the fate of precipitation input and water transitions through ecosystem compartments. Disentangling these interactions is a major challenge in the field of ecohydrology and a pivotal foundation for understanding the carbon cycle of semiarid ecosystems. Stable water isotopes can be used in this context as tracer to quantify water movement through soilvegetation-atmosphere interfaces.The aim of this study is to disentangle vegetation effects on soil water infiltration and distribution as well as dynamics of soil evaporation and grassland water use in a Mediterranean cork oak woodland during dry conditions. An irrigation experiment using δ 18 O labelled water was carried out in order to quantify distinct effects of tree and herbaceous vegetation on the infiltration and distribution of event water in the soil profile. Dynamic responses of soil and herbaceous vegetation fluxes to precipitation regarding event water use, water uptake depth plasticity, and contribution to ecosystem soil evaporation and transpiration were quantified.Total water loss to the atmosphere from bare soil was as high as from vegetated soil, utilizing large amounts of unproductive evaporation for transpiration, but infiltration rates decreased. No adjustments of main root water uptake depth to changes in water availability could be observed during the experiment. This forces understorey plants to compete with adjacent trees for water in deeper soil layers at the onset of summer. Thus, understorey plants are subjected to chronic water deficits faster, leading to premature senescence at the onset of drought. Despite this water competition, the presence of cork oak trees fosters infiltration and reduces evapotranspirative water losses from the understorey and the soil, both due to altered microclimatic conditions under crown shading. This study highlights complex soilplant-atmosphere and inter-species interactions controlling rain pulse transitions through a typical Mediterranean savannah ecosystem, disentangled by the use of stable water isotopes.

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“…However, the temporal variability of the isotopic fractionation in the field has not yet been studied, despite recent technical developments that enabled easier analysis of stable isotopes in soil water (see review by Sprenger et al, 2015a). While Rothfuss et al (2015) sampled the soil water isotopes in a soil column undergoing evaporation in the laboratory, field studies are usually limited to a few sampling campaigns or a few days (Twining et al, 2006;Gaj et al, 2016;Volkmann et al, 2016). Only recently, Oerter and Bowen (2017) applied in situ soil water isotope measurements to cover almost 1 year, but their study was limited to one particular location.…”

Section: Introductionmentioning

confidence: 99%

Soil water stable isotopes reveal evaporation dynamics at the soil–plant–atmosphere interface of the critical zone

Sprenger

Tetzlaff

Soulsby

2017

Hydrol. Earth Syst. Sci.

160

165

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Abstract. Understanding the influence of vegetation on water storage and flux in the upper soil is crucial in assessing the consequences of climate and land use change. We sampled the upper 20 cm of podzolic soils at 5 cm intervals in four sites differing in their vegetation (Scots Pine (Pinus sylvestris) and heather (Calluna sp. and Erica Sp)) and aspect. The sites were located within the Bruntland Burn longterm experimental catchment in the Scottish Highlands, a low energy, wet environment. Sampling took place on 11 occasions between September 2015 and September 2016 to capture seasonal variability in isotope dynamics. The pore waters of soil samples were analyzed for their isotopic composition (δ 2 H and δ 18 O) with the direct-equilibration method. Our results show that the soil waters in the top soil are, despite the low potential evaporation rates in such northern latitudes, kinetically fractionated compared to the precipitation input throughout the year. This fractionation signal decreases within the upper 15 cm resulting in the top 5 cm being isotopically differentiated to the soil at 15-20 cm soil depth. There are significant differences in the fractionation signal between soils beneath heather and soils beneath Scots pine, with the latter being more pronounced. But again, this difference diminishes within the upper 15 cm of soil. The enrichment in heavy isotopes in the topsoil follows a seasonal hysteresis pattern, indicating a lag time between the fractionation signal in the soil and the increase/decrease of soil evaporation in spring/autumn. Based on the kinetic enrichment of the soil water isotopes, we estimated the soil evaporation losses to be about 5 and 10 % of the infiltrating water for soils beneath heather and Scots pine, respectively. The high sampling frequency in time (monthly) and depth (5 cm intervals) revealed high temporal and spatial variability of the isotopic composition of soil waters, which can be critical, when using stable isotopes as tracers to assess plant water uptake patterns within the critical zone or applying them to calibrate traceraided hydrological models either at the plot to the catchment scale.

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In situ unsaturated zone water stable isotope (<sup>2</sup>H and <sup>18</sup>O) measurements in semi-arid environments: a soil water balance

Abstract: Abstract. Stable isotopes (deuterium, 2 H, and oxygen-18, 18

Cited by 94 publications

References 66 publications

The triple oxygen isotope composition of phytoliths as a proxy of continental atmospheric humidity: insights from climate chamber and climate transect calibrations

The triple oxygen isotope composition of phytoliths as a proxy of continental atmospheric humidity: insights from climate chamber and climate transect calibrations

Quantification of dynamic soil–vegetation feedbacks following an isotopically labelled precipitation pulse

Soil water stable isotopes reveal evaporation dynamics at the soil–plant–atmosphere interface of the critical zone

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