Fractionation of stable boron isotopes in alfisols: Application of a novel tool for tracing boron behavior in two soil profiles

Available boron (AB) and total boron (TB) in soil have distinct isotopic values, which are mainly influenced by the pH, organic matter and clay minerals, and can be used to explore the chemical behaviors of boron (B) in adsorption, weathering and eluviation processes. The B isotopic compositions of AB and TB in brown soil (slightly acidic) and cinnamon soil (neutral to slightly alkaline) profiles were investigated by multicollector inductively coupled plasma mass spectrometry. Variations in pH value and organic matter in the two soils directly affected the adsorption of B isotopes (dissolved 11B(OH)3 or 10B(OH)4- ) in the aqueous solution, as shown by AB having an average negative delta 11B value of -1%o and a great range of -10.83%o to +8.32%o in the cinnamon soil. Furthermore, the long-term effects of adsorption and transportation processes were evidenced by the large range of delta 11BTB values (-24.22%o to +21.07%o) in the two soils. The topsoil layers with more organic matter and higher delta 11B values indicated that B isotope fractionation occurred as a result of the input of rainfall, recycling of plant litter and the adsorption of organic matter and clay minerals. The lower delta 11B values, the higher B contents and the element ratios in the underlying layers resulted from the weathering of different minerals and the adsorption of AB in the leaching process. This finding is supported by the relationship between AB and the clay/ silt ratio, with more negative delta 11B values being associated with finer clay particles via the adsorption of 10B(OH)4in a weakly alkaline soil solution. The delta 11B values in soils may be a useful tool for distinguishing geochemical cycling and processes and characterizing long-term processes that are difficult to assess.