Monitoring water stable isotopic composition in soils using gas‐permeable tubing and infrared laser absorption spectroscopy

[1] In soils, the isotopic composition of water (δ2H and δ18O) provides qualitative (e.g., location of the evaporation front) and quantitative (e.g., evaporation flux and root water uptake depths) information. However, the main disadvantage of the isotope methodology is that contrary to other soil state variables that can be monitored over long time periods, δ2H and δ18O are typically analyzed following destructive sampling. Here we present a nondestructive method for monitoring soil liquid water δ2H and δ18O over a wide range of water availability conditions and temperatures by sampling water vapor equilibrated with soil water using gas-permeable polypropylene tubing and a cavity ring-down laser absorption spectrometer. By analyzing water vapor δ2H and δ18O sampled with the tubing from a fine sand for temperatures ranging between 8°C and 24°C, we demonstrate that our new method is capable of monitoring δ2H and δ18O in soils online with high precision and after calibration, also with high accuracy. Our sampling protocol enabled detecting changes of δ2H and δ18O following nonfractionating addition and removal of liquid water and water vapor of different isotopic compositions. Finally, the time needed for the tubing to monitor these changes is compatible with the observed variations of δ2H and δ18O in soils under natural conditions.

[1]  C. Scrimgeour.  Measurement of plant and soil water isotope composition by direct equilibration methods , 1995 .

[2]  L. Sternberg,et al.  The use of stable isotopes to study ecosystem gas exchange , 2000, Oecologia.

[3]  Jeffrey J. McDonnell,et al.  High‐frequency field‐deployable isotope analyzer for hydrological applications , 2009 .

[4]  X. Lee,et al.  Determining the Oxygen Isotope Composition of Evapotranspiration Using Eddy Covariance , 2010 .

[5]  M. Majoube.  Fractionnement en oxygène 18 et en deutérium entre l’eau et sa vapeur , 1971 .

[6]  Jake F. Weltzin,et al.  Dynamics of transpiration and evaporation following a moisture pulse in semiarid grassland: A chamber-based isotope method for partitioning flux components , 2005 .

[7]  L. Sternberg,et al.  Using stable isotopes to determine sources of evaporated water to the atmosphere in the Amazon basin , 1996 .

[9]  Daniele Romanini,et al.  Water isotope ratio (δ2H and δ18O) measurements in atmospheric moisture using an optical feedback cavity enhanced absorption laser spectrometer , 2010 .

[10]  V. Haverd,et al.  Soil-Litter-Iso: A one-dimensional model for coupled transport of heat, water and stable isotopes in soil with a litter layer and root extraction , 2010 .

[11]  M. Wolfsberg,et al.  Further empirical testing of the suitability of a nonrandom integration method for classical trajectory calculations , 1978 .

[12]  Masahisa Kakiuchi,et al.  Direct measurements of D/H and 18O/16O fractionation factors between vapor and liquid water in the temperature range from 10 to 40°C , 1979 .

[13]  H. Meijer,et al.  Simultaneous determination of the (2)h/(1)h, (17)o/(16)o, and (18)o/(16)o isotope abundance ratios in water by means of laser spectrometry. , 1999, Analytical chemistry.

[14]  O. Chadwick,et al.  Oxygen isotopic composition of soil water: Quantifying evaporation and transpiration , 1998 .

[15]  Dongmei Han,et al.  A study of root water uptake of crops indicated by hydrogen and oxygen stable isotopes: A case in Shanxi Province, China , 2010 .

[16]  K. Dinsmore,et al.  Transfer of carbon dioxide and methane through the soil‐water‐atmosphere system at Mer Bleue peatland, Canada , 2009 .

[17]  N. Buchmann,et al.  A study of soil methane sink regulation in two grasslands exposed to drought and N fertilization , 2011, Plant and Soil.

[18]  Luiz Antonio Martinelli,et al.  Contribution of transpiration to forest ambient vapour based on isotopic measurements , 1997 .

[19]  E. Barkan,et al.  Fractionation of oxygen and hydrogen isotopes in evaporating water , 2009 .

[20]  Salah Er-Raki,et al.  Evapotranspiration components determined by stable isotope, sap flow and eddy covariance techniques , 2004 .

[21]  R. GONFIANTINI,et al.  Standards for stable isotope measurements in natural compounds , 1978, Nature.

[22]  R. Scott,et al.  Partitioning overstory and understory evapotranspiration in a semiarid savanna woodland from the isotopic composition of water vapor , 2003 .

[23]  K. Maseyk,et al.  Concentration effects on laser-based δ18 O and δ2 H measurements and implications for the calibration of vapour measurements with liquid standards. , 2010, Rapid communications in mass spectrometry : RCM.

[24]  M. Vauclin,et al.  Factors controlling the isotopic partitioning between soil evaporation and plant transpiration: Assessment using a multi-objective calibration of SiSPAT-Isotope under controlled conditions , 2012 .

[25]  M. Schmid,et al.  An experimental determination of the scale length of N2O in the soil of a grassland , 2000 .

[26]  A. Heymsfield,et al.  Water Isotope Ratios D/H, 18O/16O, 17O/16O in and out of Clouds Map Dehydration Pathways , 2003, Science.

[27]  J. Horita,et al.  Liquid-vapor fractionation of oxygen and hydrogen isotopes of water from the freezing to the critical temperature , 1994 .

[28]  Evert Slob,et al.  Remote Estimation of the Hydraulic Properties of a Sand Using Full‐Waveform Integrated Hydrogeophysical Inversion of Time‐Lapse, Off‐Ground GPR Data , 2009 .

[29]  M. Weiler,et al.  A porewater-based stable isotope approach for the investigation of subsurface hydrological processes , 2011 .

[30]  Markus Weiler,et al.  Continuous in situ measurements of stable isotopes in liquid water , 2012 .

[31]  B. Helliker,et al.  Novel Approaches for Monitoring of Water Vapor Isotope Ratios: Plants, Lasers and Satellites , 2010 .

[32]  H. Craig,et al.  Oxygen isotope fractionation between CO2 and water, and the isotopic composition of marine atmospheric CO2 , 1968 .

[33]  M. Vauclin,et al.  Partitioning evapotranspiration fluxes into soil evaporation and plant transpiration using water stable isotopes under controlled conditions , 2010 .

[34]  J. Gat,et al.  Stable isotope hydrology : deuterium and oxygen-18 in the water cycle , 1981 .