Selenium Partitioning and Stable Isotope Ratios in Urban Topsoils

Abstract

A combination of sequential extraction with stable isotope ratio measurements of Se might offer new insights into biogeochemical processes governing Se turnover in soils. Therefore, we determined the Se partitioning among three operationally defined sequential extracts (0.1 mol L−1 K2HPO4–KH2PO4 at pH 7, 0.05 mol L−1 NaOH, conc. HNO3) and the stable isotope ratios of total Se (δ82/76Se values) in 10 topsoils under 5 different land uses (alluvial grasslands, forests, house gardens, parks, and roadside grassland) from the city of Bayreuth (ca. 73,000 inhabitants) in Germany. Furthermore, we determined S and SO42− concentrations and stable isotope ratios of total S (δ34S values) to support our interpretation of the Se concentrations and isotope ratios because of the chemical similarity of Se and S. All topsoils had low total Se concentrations (0.09–0.52 mg kg−1). The largest contribution to total Se was extracted with NaOH comprising up to 42%,which is thought to be associated with organic matter and metal oxides. The δ82/76Se values of total Se in the topsoils were close to the bulk Earth composition with an average δ82/76Se value of −0.03 ± SD 0.38‰ suggesting that there was no or little Se isotope fractionation in soil. We attribute the small isotope fractionation to the low bioavailability of Se as a consequence of the presence of Fe oxides (adsorbing the dominating Se(IV) forms strongly), organic matter, and SO42− (prevents biouptake of the Se(IV) forms) in the study soils. Small Se isotope fractionations of −0.59 to −0.35‰ in mainly forest soils and of 0.26 to 0.45‰ in mainly alluvial soils were presumably caused by soil/plant‐recycling and Se contamination by river water, respectively. In spite of the similarities in the assimilation of S and Se by organisms, the total S and Se isotope ratios in soil were not correlated. Our results demonstrate that Se in urban soils developed from Se‐poor substrates is minimally cycled through the biosphere likely because of low bioavailability and competition with SO42−.