Continuous in situ measurements of water stable isotopes in soils, tree trunk and root xylem: Field approval

Abstract

Rationale: New methods to measure stable isotopes of soil and tree water directly in the field enable us to increase the temporal resolution of obtained data and advance our knowledge on the dynamics of soil and plant water fluxes. Only few field applications exist. However, these are needed to further improve novel methods and hence exploit their full potential. Methods:We tested the borehole equilibration method in the field and collected in situ and destructive samples of stable isotopes of soil, trunk and root xylem water over a 2.5-month experiment in a tropical dry forest under natural abundance conditions and following labelled irrigation. Water from destructive samples was extracted using cryogenic vacuum extraction. Isotope ratios were determined with IRIS instruments using cavity ring-down spectroscopy both in the field and in the laboratory.Results: In general, timelines of both methods agreed well for both soil and xylem samples. Irrigation labelled with heavy hydrogen isotopes clearly impacted the isotope composition of soil water and one of the two studied tree species. Intermethod deviations increased in consequence of labelling, which revealed their different capabilities to cover spatial and temporal heterogeneities. Conclusions:We applied the novel borehole equilibration method in a remote field location. Our experiment reinforced the potential of this in situ method for measuring xylem water isotopes in both tree trunks and roots and confirmed the reliability of gas permeable soil probes. However, in situ xylem measurements should be further developed to reduce the uncertainty within the range of natural abundance and hence enable their full potential. | INTRODUCTIONPlant transpiration fuels the hydrological cycle by returning 35% to 90% of water from land surfaces to the atmosphere. 1,2 Therefore, transpiration greatly influences our climate and impacts water availability in consequence of land use and climate change. 3,4 However, why is the quantification of this essential water flux so uncertain? This comes down to knowledge gaps in the mechanistic functioning of root water uptake (RWU) as well as plant rooting depth that persist despite its crucial role in predicting the future of one of our most important resources. 5 A major constraint is the practical difficulty in observing below-ground processes. Moreover, the magnitude and location of RWU are the results of multiple influencing factors, such as extent of the root system and its hydraulic properties,

Technical note: Conservative storage of water vapour: a key to practical measurements of water stable isotopes in tree stems and soils

“…40 cm aboveground, where the tree diameter was 21.6 cm. Because this method has now been tested several times (Marshall et al, 2020;Kühnhammer et al, 2021), we used the calibrated in-situ data as our "true isotopic composition" of the trees' xylem. The calibration for the insitu data was conducted as described in Marshall et al (2020).…”

“…A recently developed method based on direct vapour equilibration reduces the co-extraction of organic compounds and increases sample throughput (Millar et al, 2018;Wassenaar et al, 2008). One of the biggest advantages of in-situ equilibration techniques is that water from plants and soils can be sampled at high temporal resolution without altering their physiology or physical properties (Kühnhammer et al, 2021). Therefore, in-situ measurements of water stable isotopes have gained popularity and have been proposed a way forward to disentangle isotopic processes in the critical zone or the soilvegetation-atmosphere continuum (Rothfuss and Javaux, 2017).…”

“…In-situ measurement systems are based on direct inferences of liquid water isotopic composition from equilibrated water vapour from the soil or the plant (for a detailed review see Beyer et al, (2020). The vapour is collected either using a gaspermeable membrane (the utility of which was proven by Herbstritt et al, (2012) buried in the soil (Rothfuss et al, 2013;Volkmann et al, 2016b;Volkmann and Weiler, 2014;Kübert et al, 2020) or in the xylem of woody species (Volkmann et al, 2016a, b;Seeger and Weiler, 2021), or drawing equilibrated water vapour from a borehole in the xylem directly (Marshall et al, 2020;Kühnhammer et al, 2021). Additionally, it is possible to measure the isotopic composition of plant transpiration and evapotranspiration in-situ, using gas exchange chambers in the lab (Simonin et al, 2013;Dubbert et al, 2017), as well as in the field (Kübert et al, 2019;Dubbert et al, 2013;Wang et al, 2013).…”

Technical note: Conservative storage of water vapour – practical in situ sampling of stable isotopes in tree stems

“…A recently developed method based on direct vapour equilibration reduces the co-extraction of organic compounds and increases sample throughput (Millar et al, 2018;Wassenaar et al, 2008). One of the biggest advantages of in situ equilibration techniques is that water from plants and soils can be sampled at high temporal resolution without altering their physiology or physical properties (Kühnhammer et al, 2021). This is particularly noticeable when repeatedly sampling the same tree for cores, as water transport is repeatedly disrupted, whereas when using the in situ approach this only happens once.…”

“…In situ measurement systems are based on direct inferences of liquid water isotopic composition from equilibrated water vapour from the soil or the plant (for a detailed review see Beyer et al, 2020). The vapour is collected using a gas-permeable membrane (the utility of which was proven by Herbstritt et al (2012) buried in the soil (Rothfuss et al, 2013;Volkmann et al, 2016b;Volkmann and Weiler, 2014;Kübert et al, 2020), or in the xylem of woody species (Volkmann et al, 2016a, b;Seeger and Weiler, 2021), or drawing equilibrated water vapour from a borehole in the xylem directly (Marshall et al, 2020;Kühnhammer et al, 2021). Additionally, it is possible to measure the isotopic composition of plant transpiration and evapotranspiration in situ, using gas exchange chambers in the lab (Simonin et al, 2013;Dubbert et al, 2017), as well as in the field (Kübert et al, 2019;Dubbert et al, 2013;Wang et al, 2013).…”