title: Quantifying unfrozen water in frozen soil by high-field 2H NMR.
abstract: To understand wintertime controls of biogeochemical processes in high latitude soils it is essential to distinguish between direct temperature effects and the effects of changes in water availability mediated by freezing. Efforts to separate these controls are hampered by a lack of adequate methods to determine the proportion of unfrozen water. In this study we present a high-field 2H2O NMR method for quantifying unfrozen water content in frozen soil. The experimental material consisted of the humic layer of a boreal spruce forest soil mixed with varying proportions of quartz sand and humidified with deuterium-enriched water. The relative standard deviation of unfrozen water content (measured as NMR signal integral) was less than 2% for repeated measurements on a given sample and 3.5% among all samples, based on a total of 16 measurements. As compared to 1H NMR, this 2H NMR method was found to be superior for several reasons: it is less sensitive to field inhomogeneity and paramagnetic impurities, it gives a bigger line shape difference between the ice and liquid signal, it shows a sharper response to water fusion, and it excludes the possibility of hydrogen in the organic material interfering with the measurement.
authors.affiliation: Department of Medical Biochemistry and Biophysics, Umeå University, SE-901 87 Umeå, Sweden. tobias.sparrman@chem.umu.se
authors.forename: Tobias
authors.initials: T
authors.lastname: Sparrman
authors.forename: Mats
authors.initials: M
authors.lastname: Oquist
authors.forename: Leif
authors.initials: L
authors.lastname: Klemedtsson
authors.forename: Jürgen
authors.initials: J
authors.lastname: Schleucher
authors.forename: Mats
authors.initials: M
authors.lastname: Nilsson
mesh_terms: D003903:Deuterium; D005615:Freezing; D006812:Humic Substances; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D012987:Soil / Q000032:analysis*; D013548:Sweden; D014867:Water / Q000032:analysis*
chemical_list: D006812:Humic Substances; D012987:Soil; D014867:Water; D003903:Deuterium