Determination of boron isotopic compositions in five fractions of soil boron: traces of the environmental behavior of boron in soil†

Boron in soil can be divided into five fractions: readily soluble, carbonate-adsorbed, organically bound, adsorbed-bound and residual boron. Regarding differences in physicochemical properties, the fractions of soil boron affect the distribution and processes of boron and its isotopes in soil. A stepwise extraction for five fractions of soil boron was successfully established to reveal the isotopic behavior of boron in soils, which was performed to eliminate the isotope interference of neighboring forms in soil boron as much as possible. Ten soil samples were used to evaluate the proposed method. A large difference of 57‰ (−31.9‰ to +25.8‰) in the δ¹¹B values occurred among the five fractions of soil boron. Residual boron had the highest content, which was hundreds of times greater than that in the other forms, and the lowest δ¹¹B values, ranging from −31.9‰ to −24.5‰. Among the other four fractions, readily soluble boron had a relatively low content and the most positive δ¹¹B values, adsorbed-bound boron had a relatively high content and less negative δ¹¹B values, and carbonate-adsorbed boron had the lowest content. The relationship between the content and δ¹¹B values revealed the scope of each fraction of soil boron, which may have resulted from the different processes of adsorption, transportation and weathering and the input of various provenances in the open soil medium. Further studies on the different types of soil influenced by different climates are expected to make important contributions to further knowledge of boron recycling in plant–soil systems with the proposed extraction method.