Isotope ratios of nonexchangeable hydrogen in soils from different climate zones

Abstract

The fractionation of H isotopes in global precipitation leads to a continuous decrease in 2H concentrations of precipitation with increasing latitude, altitude and continentality. Thus, the local precipitation, soil and ground water used by plants for photosynthesis or consumed during neoformation of pedogenic clay minerals often have a spatially diagnostic δ 2H value. We hypothesize that the δ 2H value of isotopically nonexchangeable H in bulk soil (composed of C-bonded H in soil organic matter (SOM) and the nonexchangeable fraction of O-bonded H in pedogenic clay minerals) is correlated with the mean δ 2H value of local precipitation water at the place and time of biomass production and clay mineral neoformation, respectively. We equilibrated soil samples with water of known isotopic composition prior to δ 2H analysis with an Elemental Analyzer-Isotope Ratio Mass Spectrometer (EA-IRMS) to determine the δ 2H value of isotopically nonexchangeable H in soil. We analyzed 12 topsoil samples from different climate zones (temperate zone, subarctic zone and tropics). The δ 2H values of nonexchangeable H in soil ranged from − 75‰ to − 167‰. The δ 2H values of nonexchangeable H in soil were highly correlated with predicted average annual δ 2H values of local precipitation at the sampling sites ( r = 0.89, p < 0.001). The δ 2H values of nonexchangeable H in soils from Siberia were up to 80‰ depleted in 2H compared to soils from Central Europe, which directly corresponds to the difference in average annual δ 2H values of precipitation from Central Europe and Siberia. Our study supports the hypothesis that the average H isotope composition of local precipitation is the primary control of the isotopic composition of nonexchangeable H in bulk soils. Since δ 2H values of nonexchangeable H in SOM and pedogenic clay minerals are both primarily determined by δ 2H values of global precipitation, δ 2H analyses of bulk soil samples are meaningful irrespective of the precise fractions of SOM and clay minerals in soil.